document: stephan s quintet ( hereafter sq ) is a strongly interacting compact group which has produced a highly disturbed intragroup medium ( igm ) @xcite through a complex sequence of interactions and harrassment @xcite . 
   this interplay has produced a large - scale intergalactic shock - wave , first observed as a narrow filament in the radio continuum  @xcite , and subsequently detected in the x - ray  @xcite . 
   the high - velocity ( @xmath61000 km  s@xmath13 ) collision of an intruder galaxy , ngc 7318b , with the intergalactic medium of the group  @xcite is believed to be responsible for the shock - heating of the x - ray emitting gas . 
   optical emission line ratios and observed broad linewidths provide evidence that the region is powered by strong shocks and not star formation ( xu et al . 2003 ; duc et al . 
   2010 , in preparation ) . 
   the main elements of stephan s quintet are shown in figure 1 . 
   central to the system is the primary shock , as defined by the 20 cm radio continuum emission . 
   ngc 7318b , the intruder galaxy , lies to the west of the shock , while the large seyfert 2 galaxy @xcite , lies to the east . 
   other members of the group are also indicated . 
   the peculiar extranuclear star formation region , named sq - a @xcite , lies at the extreme northern end of the main shock wave . 
   ngc 7318a ( west of ngc 7318b ) is also a strongly interacting group member , with ngc 7317 further away from the core . 
   the unexpected discovery of extremely powerful , pure - rotational h@xmath9 line emission from the center of the shock @xcite , using the _ spitzer space telescope _ 
   @xcite , has sparked intense interest in the sq system . 
   warm molecular hydrogen emission was found with a line luminosity exceeding the x - ray luminosity from the shock . 
   the mid - infrared ( mir ) h@xmath9 linewidth was resolved ( @xmath14 870 km s@xmath13 ) suggesting that the h@xmath9-emitting clouds carry a large bulk - kinetic energy , tapping a large percentage of the energy available in the shock . 
   a recent model of sq , involving the collision between two inhomogeneous gas flows , describes h@xmath9 formation out of the multiphase , shocked gas , and an efficient cooling channel for high - speed shocks as an alternative to x - ray emission @xcite    since the sq detection , several other systems exhibiting similarly strong h@xmath9 emission have been discovered . 
   @xcite find that a large - subset of the local 3cr radio galaxies have extremely dominant mir rotational h@xmath9 lines , often seen against a very weak thermal continuum . 
   the low agn and star formation power are insufficient to drive the mir h@xmath5  emission . mechanical heating driven by the radio jet interaction with the host galaxy ism is the favoured mechanism . 
   in addition , total h@xmath9 luminosities in the range @xmath15 ergs s@xmath13 have been detected in some central cluster galaxies out to @xmath16 ( * ? ? ? 
   * g. de messieres - university of virginia , private communication ) and in filaments in clusters @xcite . 
   the study of nearby prototypes may provide valuable insight into the nature of these more distant systems . the large scale ( @xmath6  30  kpc ) of the sq shock is well - suited for such a study .    in this paper 
   i , we extend the single pointing observations of @xcite to full spectral maps of sq using the irs instrument on the _ spitzer _ space telescope , hereafter _ spitzer_. in paper ii 
   , we will present detailed 2-dimensional excitation maps of the h@xmath5 emission across the face of the x - ray emitting shock , and compare them with models . 
   in addition , several other papers are being prepared by our team which will discuss the relationship between the uv / x - ray emission and emission from dust .    in section 2 
   , we present our observations and data reduction methods . in section 3 
   we discuss the mapping results for various detected lines . in sections 4 , 5 and 6 
   we present our discussion on the properties of the system and in section 7 the implication for galaxy formation . in section 8 
   we present our conclusions . 
   additional material is included as appendices , with a discussion on ngc 7319 in appendix a , and a reanalysis of the high resolution mir spectrum of the shock , as well as renanalysis of the x - ray data presented in appendix b and c , respectively .    throughout this paper , we assume a systemic velocity of @xmath17 for the group , corresponding to a distance of 94 mpc with @xmath18 . 
   mid - infrared spectroscopy of the shock region in sq was obtained using the irs instrument @xcite onboard _ 
   spitzer_. observations were done in low - resolution mapping mode , using the short - low ( @xmath19 ; @xmath20 ) and long - low ( @xmath21 ; @xmath22 ) modules and taken on january 11 2008 and december 10 2007 , respectively . 
   figure 1 displays the outline of the areas observed superimposed on a composite image of the group . 
   the sl spectral mapping consists of two separate , partially overlapping maps , centered north and south on the x - ray emission associated with the shock . 
   the map was constructed with 23 steps of 2.8(0.75 @xmath2 slit width ) perpendicular to the slit and one parallel step of 7.2 . 
   observations consisted of 60s integrations with 5 cycles per step . 
   the ll module was used to map an area of @xmath23 using 21 steps of 8.0  ( 0.75 @xmath2 slit width ) perpendicular to the slit and a parallel step of 24.0 . 
   an integration time of 120s was used with 3 cycles per step . 
   primary data reductions were done by the _ spitzer _ 
   science center ( ssc ) pipeline , version s17.0.4 and s17.2.0 for sl and ll respectively , which performs standard reductions such as ramp fitting , dark current subtraction and flat - fielding . 
   background subtraction for ll data was performed by subtracting dedicated off - source observations , with the same observing mode , taken shortly after the mapping sequence . in the case of sl , where for scheduling reasons the dedicated `` off '' observations were too far away in time to be optimal , backgrounds were generated from observations at the periphery of the map that contain no spectral line signatures . 
   examination of the pipeline products showed that the stray - light - corrected images ( that account for potential spillover from the peak - up arrays onto the sl1/2 spectral apertures ) contained wavelength - dependent over - corrections in some of the images , seemingly due to a high background of cosmic rays during the sl portion of the mapping . to negate this effect , the alternative flat - fielded images ( also available from the science pipeline ) that are uncorrected for crosstalk and straylight removal were used . 
   it was determined that after background subtraction , any uncorrected stray light in the sl spectral extraction areas was unmeasurable at the @xmath24% level . 
   thus stray - light correction was unneccesary , and the resulting basic calibrated data ( bcd ) images used were of better quality than the standard bcds .    for all modules , individual frames for each pointing 
   were median - combined and obvious `` bad '' pixels were replaced using customised software that allows for manual `` average '' replacement . 
   the spectra were assembled into spectral cubes for each module using the software tool , cubism @xcite . 
   further bad pixel removal was performed within cubism . 
   spectral maps were generated by making continuum maps on either side of a feature and subtracting the averaged continuum map from the relevant emission line map . 
   one dimensional spectra were further extracted from the data cubes using matched apertures . 
   broad - band images at 16 and 24@xmath25 m were obtained with the _ spitzer _ irs blue peak - up imager ( pui ) and the mips instrument . 
   the pui was obtained in a 5 x 5 map with 3 cycles of 30s duration each on 2007 december 10 . 
   the mips instrument @xcite on _ spitzer _ obtained 24  imaging of sq on 2008 july 29 , achieving a spatial resolution of @xmath66 . 
   primary data reduction was done by the _ spitzer _ 
   science center ( ssc ) science pipeline ( version s18.0.2 ) run through the mopex software , and for the mips image a smooth 2d polynomial background was further removed to correct for a large - scale background gradient . _ 
   spitzer _ irac 3.6 , 4.5 , 5.8 and 8.0  data of sq ( p.i . 
   houck ) were obtained from the ssc archive ; these were reduced using science pipeline version s18.0.2 . 
   the final mosaics have a pixel scale of 0.61 . 
   the pure rotational transitions of molecular hydrogen can be excited by several mechanisms . 
   these include fuv ( far ultraviolet ) induced pumping , and possible additional collisional heating , of the h@xmath5  in photodissociation regions associated with star formation ( e. g. black & van dishoeck 1987 , hollenbach & tielens 1997 ) , hard x - rays penetrating and heating regions within molecular clouds , which in turn excite h@xmath5  via collisions with electrons or hydrogen atoms ( lepp & mccray 1983 ; draine & woods 1992 ) and finally collisional excitation of h@xmath5  due to acceleration produced by shocks ( e.g. shull & hollenbach 1978 ) . 
   the pure rotational mir line ratios are not especially good diagnostics for distinguishing between these mechanisms since all three mechanisms discussed can lead to well thermalized level distributions of lower - level rotational states . 
   the rotational h@xmath5  emission lines do , however , allow us to trace gas at different temperatures and compare with model predictions ( this will be the main emphasis of paper ii ) . 
   higher level transitions 0 - 0 s(3)-s(5 ) tend to trace warmer gas , whereas the s(0 ) and s(1 ) lines are sensitive to the `` coolest '' warm h@xmath5 . 
   although the line ratios themselves can not be used directly as shock diagnostics , in stephan s quintet the distribution of large - scale x - ray and radio emission , plus optical emission line diagnostics , provide strong evidence that the giant filament seen in figure 1 is the result of a strong shock . in appleton et al . 
   ( 2006 ) this fact was used to reveal the surprising association of detected h@xmath5  emission with the shock . however , in this paper we can make a more definitive association of the emission with the shock by means of spectral maps . 
   the spectral cubes were used to extract maps of all the pure rotational emission lines of molecular hydrogen that were detected , namely the 0 - 0 s(0)28.22 , s(1)17.03 , s(2)12.28 , s(3)9.66 , s(4)8.03  and s(5)6.91  lines . 
   specific intensity contour maps of these lines are presented in figure 2 overlaid on a @xmath26-band image of the region from @xcite . 
   the s(0 ) and s(1 ) lines were mapped by the ll modules , while s(2 ) - s(5 ) transitions were mapped by the sl modules of the irs . as indicated in figure 1 , 
   the sl observations were concentrated on the main shock to provide high signal to noise ( s / n ) measurements there . as a result 
   , these maps do not fully cover sq - a or ngc 7319 . 
   we note that the s(4 ) line at 8.03  ( fig . 
   2e ) is faint , and also suffers from contamination from the pah bands at 7.7  and 8.6 . 
   the contours indicate powerful , widespread emission running north - south along the shock ridge ( see fig . 
   1 ) . in addition 
   we detect strong emission from the star forming region , sq - a , as well as associated with ngc 7319 . 
   we discuss further details of the agn - like mir emission lines from ngc  7319 in appendix a.    figures 2a - d reveal a new h@xmath5  structure running eastward from the `` main '' shock ridge . in what follows 
   , we refer to this feature as the `` bridge '' . 
   this structure is observed faintly in the _ chandra _ @xcite and xmm @xcite x - ray images and detected as faint h@xmath27 emission by @xcite , but is not strong in radio continuum images . 
   as is evident in figure 2 , there is distinct variation in the distribution of the warm h@xmath5  emission . the brightest 0 - 0  s(0 ) emission ( fig . 
   2a ) appears to be concentrated towards the north of the shock , whereas the 0 - 0  s(1 ) transition emission ( fig . 
   2b ) appears more concentrated towards the center . 
   the s(0 ) and s(1 ) maps demonstrate that the h@xmath5  in the bridge terminates in a large clump a few arcsecs west of the nucleus of the seyfert 2 galaxy ngc  7319 , and in a small detour to the north ( especially in the s(1 ) map which has the highest s / n ) . 
   the s(2 ) through s(5 ) lines clearly show that the warm h@xmath5  emission breaks into clumps in the shock . despite the limited coverage compared to the ll mapping , 
   the base of the `` bridge '' is visible and sq - a is partially covered . 
   sq - a is fully covered by the sl2 module ( because of fortuitous `` off observation '' coverage ) and hence the s(5 ) emission line reveals that in sq - a the h@xmath5  emission is also clumpy . 
   the molecular - line maps provide considerable information about the excitation of the h@xmath5  along and across the x - ray shock , but these discussions will be deferred to a full 2-d modelling of the h@xmath5  excitation in paper ii ( appleton et al . , in preparation ) . 
   instead we shall limit ourselves to global properties of the h@xmath5  here . in section [ spec ] 
   we shall present spectra of some selected regions of the emission and discuss a global excitation diagram for the shock .        to further demonstrate the close connection between the h@xmath5  emission and the main global shock - wave in sq 
   , we now consider the distribution of warm h@xmath5  in relation to the x - ray and radio emission . 
   figure [ fig : xray_rad]a and b show the s(0 ) and s(3 ) contours overlaid on an xmm - newton x - ray image @xcite of stephan s quintet . 
   the warm molecular hydrogen is distributed along the length of the main north - south ( ns ) x - ray shock ridge and along the `` bridge '' , demonstrating the remarkable projected coexistence of of hot x - ray plasma ( 10@xmath28 @xmath29 t @xmath29 10@xmath30 k ) and warm h@xmath5 ( 10@xmath31 @xmath29 t@xmath29 10@xmath32 k ) . 
   although the h@xmath5  appears to follow the x - ray , there are subtle differences . 
   the cooler s(0 ) line has emission concentrated to the north and follows the x - ray less closely compared to the warmer s(3 ) line . 
   the s(3 ) line exhibits a clear correspondence to the x - ray , notably at the center of the shock , where we find peaks at both wavelengths . 
   thus the region of greatest shock heating , as traced by the stronger x - ray emission , appears to correspond to the higher - j h@xmath5  transitions , perhaps implying a causal connection . 
   the intergalactic star formation region sq - a , is essentially absent in x - ray emission , as seen in figure [ fig : xray_rad]a , but is strongly detected in h@xmath9 . 
   we find a similar picture in the radio continuum ( fig . 
   [ fig : xray_rad]c & d ) with the s(0 ) line demonstrating correspondence with the main shock , but dominated by emission in the north where we observe less powerful radio emission . 
   the s(3 ) ( and s(2)see fig . 
   2c ) line presents a much tighter correlation with regions of the shock that are more radio luminous than the lower - j transitions . 
   the radio emission is quite likely sensitive to the most compressed regions of the shock where cosmic ray particles are accelerated more strongly @xcite , whereas the brightest x - ray patches are likely due to the fastest regions of the shock @xcite . 
   as already mentioned the `` bridge '' emission is detected only faintly in the x - ray and is weak or absent at radio continuum wavelengths . 
   this noticeable difference compared to the main shock likely implies that the conditions that give rise to strong synchrotron emission in the main shock are absent in the bridge . 
   emission from fine - structure lines provide key diagnostics that trace the interplay between the various constituents of the shocked interstellar medium . in @xcite 
   the spectra were limited to the very core of the shock and only weak emission was detected from all but two metal lines , namely @xmath10neii@xmath11$]12.81and @xmath10siii@xmath11$]34.82  ( this data has been reanalysed and is presented in appendix b ) . in this section 
   we discuss the spatial distribution of emission from the @xmath10feii@xmath11$]25.99 , @xmath10oiv@xmath11$]25.89 , @xmath10siii@xmath11$]33.48 , @xmath10siii@xmath11$]34.82 , @xmath10neii@xmath11$]12.81  and @xmath10neiii@xmath11$]15.56  lines .    in figure 
   [ fig : fs ] we present the specific intensity contours of the @xmath10feii@xmath11$]25.99  and @xmath10oiv@xmath11$]25.89  blend , @xmath10siii@xmath11$]33.48  and @xmath10siii@xmath11$]34.82  emission lines .    given the low spectral resolution of the sl and ll modules of _ 
   spitzer _ , we can not distinguish between emission from @xmath10fe ii@xmath11$]25.99  and @xmath10oiv@xmath11$]25.89 . 
   however , except in the direction of the seyfert ii galaxy ngc  7319 , the emission near 26  seen in figure [ fig : fs]a is likely to be pure @xmath10feii@xmath11 $ ] with little contamination from @xmath10oiv@xmath11 $ ] as there is no evidence from the spectra of high - excitation emission from the intragroup medium in sq . 
   for example , @xmath10oiv@xmath11 $ ] has an excitation potential of 56 ev ( compared to 7.9 ev of @xmath10feii@xmath11 $ ] ) , and yet @xcite have shown that the @xmath10neiii@xmath11$]/@xmath10neii@xmath11 $ ] ratio is low suggesting low - excitation conditions for the ions in the shock , further supported in section [ em ] . assuming that , apart from towards ngc  7319 , @xmath10feii@xmath11 $ ] dominates the @xmath10feii@xmath11$]+@xmath10oiv@xmath11 $ ] complex , we detect faint emission from @xmath10feii@xmath11$]25.99  at the location of the center of the shock ( as defined by the x - ray `` hotspot '' in fig . 
   [ fig : xray_rad]a ) . the energetic requirements for shocks to produce strong @xmath10feii@xmath11 $ ] emission are usually present in @xmath33 shocks while the ion abundance in @xmath34 shocks are low in comparison @xcite . 
   we will discuss the production of @xmath10feii@xmath11 $ ] in the shock in section 6.2 . 
   the @xmath10siii@xmath11$]33.48  distribution is presented in figure [ fig : fs]b . 
   this fine structure line acts as a strong tracer of hii  regions @xcite and we observe emission from sq - a and from other regions of star formation in the south ( see section [ sf ] ) . in the primary shock region the distributions of @xmath10feii@xmath11 $ ] and @xmath10siii@xmath11 $ ] are anti - correlated .    in strong contrast to the weak @xmath10feii@xmath11 $ ] emission , there is copious @xmath10siii@xmath11$]34.82  emission ( fig . 
   [ fig : fs]c ) , which follows the s(1 ) distribution closely with respect to the primary shock , as mapped by x - ray emission ( fig . 
   [ fig : fs]d ) . 
   although @xmath10siii@xmath11 $ ] is commonly found in normal hii  regions , we will demonstrate below that , apart from in sq - a , the strong silicon emission does not correlate with regions of strong pah emission ( tracing star formation ) in sq , but instead closely follows the h@xmath5  and x - ray ( shock ) distributions . @xmath10siii@xmath11 $ ] acts as an efficient coolant of x - ray - irradiated gas and is predicted to be one of the top four cooling lines under these circumstances @xcite . 
   we will discuss the excitation of @xmath10siii@xmath11 $ ] in the shock in section 6.2 . 
   @xmath10neii@xmath11$]12.81  ( with an ionisation potential of 21.6 ev ) is also represented in the shock , as shown in figure [ fig : ne]a , although it is also emitted from some hii  regions associated with star formation , such as sq - a and the star - forming region south of ngc 7318b ( see section [ sf ] ) . 
   the @xmath10neiii@xmath11$]15.56  contours ( with an ionisation potential of 41 ev ) are shown in figure [ fig : ne]b and are associated with excitation in the shock , as well as from regions of star formation ( see section [ sf ] ) 
   . the higher ionisation line of @xmath10neiii@xmath11 $ ] is much weaker in the shock ridge , but regions of emission correspond closely to peaks seen for the h@xmath5  s(3 ) line . the @xmath10neii@xmath11 $ ] emission , however , exhibits clearly extended emission with regions of greatest luminosity matching those seen for the h@xmath5  s(3 ) line . 
   the @xmath10neii@xmath11 $ ] emission suggests excitation from the shock , with @xmath10neiii@xmath11 $ ] found at the location of the center of the shock , similar to what is observed for @xmath10feii@xmath11 $ ] . 
   previous observations and spectroscopy of sq @xcite have determined that there are some regions of star formation associated with the spiral arms of the intruder galaxy ngc  7318b . 
   we discuss in this section how these regions , which have a different spatial distribution from the shocked gas , are correlated with the pah emission we detect in the irs spectra .    in figure 
   [ fig : dust]a there is a strong correlation between the 11.3  pah distribution ( from the irs cube ) superimposed on a near - uv image from _ galex _ @xcite , which maps the uv emission from hot stars associated with weak star formation from the system . 
   this correlation suggests that the pah molecules are excited by star formation . 
   a similar close correlation is shown in figure [ fig : dust]b where we overlay the 11.3  contours on the irac 8  band , which is dominated by the 7.7 and 8.6 @xmath25 m feature . 
   it is noticeable , however , that regions with strong 11.3  emission in the shock , do not appear similarly strong at 8 . 
   this point will be addressed in section 6.4 . 
   we compare the 11.3  pah map to the distribution of warm dust in figure [ fig : dust]c , using the mips 24@xmath25 m map of sq . 
   again , it is clear that there is a good correlation between the pah emission and the thermal dust , most of which seems only poorly correlated with the shock ridge . 
   the lack of conspicuous star formation in the ridge was also observed by @xcite . 
   the main point we emphasize here is that the dust , pah and uv emission appears to be associated with previously known star formation regions and no additional star formation is observed in the shock ; this can be seen in figure [ fig : dust]d showing the h@xmath5 0 - 0 s(1 ) emission overlaid on an irac 8  image . 
   there is little correspondence between the h@xmath5  emission in the shock and 8@xmath25 m ( hot dust plus pah ) image . 
   this is important because it implies that there is very little triggered star formation in the molecular gas associated with the shock - excited h@xmath5 . 
   figure [ fig : dust]c demonstrates that there is only faint dust emission at 24@xmath25 m from the shock ridge . 
   the presence of dust in the shock is required in the model of guillard et al . 
   ( 2009 ) to explain the formation of h@xmath5  behind the shock and we do observe evidence of depletion onto dust grains ( see section 6.2 ) . thus the faint 24  emission could be the result of destruction of very small grains ( vsgs ) , with only larger grains surviving , or indicate that the grains are cold and radiating more strongly at longer wavelengths , where _ spitzer _ has the least spatial resolution . 
   a more detailed description of the dust and faint pah emission in the sq group ( including results from mips 70  imaging ) is discussed in separate papers ( guillard et al . 2010 . ; natale et al . 
   2009 , in preparation ) . 
   guillard et al . 
   ( 2010 ) show that the ir emission in the shock is faint due to dust being heated by a relatively low intensity uv radiation field and determine a galactic pah / vsg abundance ratio in this region . 
   a more complete understanding of the likely existence of cool dust in the shock will require higher angular resolution and a broader wavelength coverage than that achieved by _ 
   spitzer_.    despite the faintness of emission from the main shock , the 24@xmath25 m map presents a new result , which was less obvious in previous studies , namely that the dominant regions of star formation in sq lie not in the galaxies themselves , but in two strikingly powerful , almost symmetrically disposed regions at either end of the shock . 
   the region to the north is the well studied sq - a , but the region to the south ( which we refer to as 7318b - south ) is also very powerful and both regions lie at the ends of the shock , as defined by the h@xmath5  distribution . 
   this may not be a coincidence , and we will discuss this further in section 6.1 . 
   the @xmath10neii@xmath11$]12.81 , @xmath10neiii@xmath11$]15.56  and @xmath10siii@xmath11$]33.48  fine - structure lines , as mentioned above , are also tracers of star formation in sq as these lines are often associated with hii  regions . 
   the @xmath10neii@xmath11$]12.81  emission appears to follow _ both _ the h@xmath5  and the star formation regions ( see fig . [ 
   fig : ne]a ) , appearing more extended in the south than the corresponding h@xmath5  emission and flaring out where star formation regions , especially 7318b - south , are observed optically , and through pah emission ( see fig . 
   [ fig : dust ] ) . 
   the warm molecular hydrogen in stephan s quintet follows the x - ray distribution in the main shock and in the `` bridge '' structure . 
   this might suggest that the molecular hydrogen is excited directly by the x - ray heating . 
   however , we will show that the h@xmath5  emission exceeds by at least a factor of 3 the x - ray luminosity from the various shocked filaments , thus ruling out direct x - ray excitation from the shock .    to measure the strength of the h@xmath5  emission , 
   we extract spectra from various rectangular sub - areas of sq which are defined in figure [ fig : ext ] . 
   the spectra were extracted from cubism cubes built from each irs module , and joined to make a continous spectrum 
   no scaling was necessary to join the spectra . 
   figures [ fig : ext ]  a ,  b and c indicate the spectral extraction regions of the main ns shock , a sub - region of the shock and a characteristic part of the `` bridge '' . the shock sub - region is chosen to be just north of the center of the shock , avoiding regions contaminated by star formation in the intruder . 
   spectra for these extractions are shown in figure [ fig : spectra ]  a , b and c. all three spectra , share the common property that they are dominated by molecular hydrogen emission . 
   the shock sub - region ( fig . 
   [ fig : spectra]b ) , unlike figure [ fig : spectra]a , is less contaminated by the star forming regions discussed in the previous section . the mir continuum of the main shock appears stronger than the shock sub - region indicating stronger emission from warm dust ; this is likely the result of contamination from star - forming regions in the main shock extraction . 
   the bridge exhibits a similarly weak continuum emission compared to the shock sub - region . 
   figure [ fig : spectra]b includes photometry from the irac bands , 16  peak - up image ( pui ) and mips-24  image superimposed on the irs spectrum . 
   these are useful to probe conditions in the shock , in particular star formation ( see section 6.4 ) . 
   stellar light in the extraction area , from an extended spiral arm of ngc 7318b , produces contamination of the shock spectrum , visible as continuum emission shortwards of 6 , in both figure 8a and b.    there exists a striking similarity between the mid - ir spectrum of all three regions , showing powerful h@xmath5  lines and low excitation weak emission from fine structure lines . also the pah emission observed in the spectra of the shock regions , and in the region of the `` bridge '' in the irac 8  image ( fig . 
   [ fig : dust]b ) , appears weak . 
   this confirms that these properties , observed in the @xcite observations of the shock core , extend to both the full extent of the main shock and the `` bridge '' . 
   this , and the fact that the `` bridge '' has similar x - ray properties to the main shock ( see later ) suggests that the bridge is a `` scaled - down '' version of the main shock . 
   the weaker radio continuum emission at this location is significant . 
   one possibility , that the `` bridge '' is older than the main shock , and the cosmic rays compressed in it have diffused away , will be discussed further in paper ii.thus the new irs observations seem to suggest that more than one large - scale group - wide shock is present in the group . 
   this could be the result of previous tidal interactions and imply multiple shock heating events have taken place in sq , consistent with what is seen in the x - ray @xcite . 
   figures [ fig : ext]d and [ fig : spectra]d present the extraction region and spectrum of sq - a , the extragalactic star - forming region . in this case , although h@xmath5  lines are still strong , a rising continuum and an increase in the strength of the metal lines relative to the h@xmath5  is consistent with a spectrum that is increasingly dominated by star formation  a result which is already known from previous optical observations @xcite . 
   line fluxes for all the h@xmath5  and metal lines in the spectra discussed above are presented in table  [ tableh2fluxes ] and table  [ tablemetalfluxes ] , respectively . 
   we estimate the luminosity emitted from the h@xmath5  lines in the main aperture shown in figure [ fig : spectra]a . 
   the emission from the 0 - 0 s(1 ) line alone can be calculated from table  [ tableh2fluxes ] ( for d = 94  mpc ) to be 2.3  @xmath210@xmath35 ergs@xmath13 . 
   summing the emission measured in the observed lines for the main shock ( 0 - 0 s(0 ) through s(5 ) lines ) and including an extra 28% emission from unobserved lines ( see model fit to excitation diagram below ) , yields a total h@xmath5  line luminosity from the main shock of 9.7  @xmath210@xmath35 ergs@xmath13 . 
   this phenomenal power in the molecular hydrogen lines dwarfs by a factor of ten the next brightest mid - ir line , which is @xmath10siii@xmath11$]34.82  with a line luminosity of l@xmath36 = 0.85 @xmath210@xmath35 ergs@xmath13 . 
   figure [ fig : excite ] presents the excitation diagram of the low - j 0 - 0 h@xmath5  transitions for the main shock extraction . 
   the points are well fit by a model including three temperature components ( t@xmath37 = 158@xmath38k , t@xmath9 = 412@xmath39k , and t@xmath40 = 1500@xmath41k ) . 
   it is likely that in reality , many different temperature components are present in the shock , and the three - temperature fit is only an approximation . 
   however , it does allow us to provide an estimate of the total mass of warm h@xmath5  of 5.0@xmath4210@xmath43m@xmath44 . 
   temperature t@xmath9 is more uncertain than formally represented by the fit because it depends on the value of the s(4 ) flux , which may be systematically too low due to pah contamination ( see fig . 
   [ fig : excite ] ) . in paper 
   ii we will present a more complete two - dimensional map of the excitation of the h@xmath5  in sq and explore variations in the shape of the excitation diagram along and across the shock in more detail . 
   our observations have shown that h@xmath5  is the dominant line coolant in the mir . 
   however , how does it compare with the most important coolant in high - speed shocks  namely the x - ray emission ? 
   @xcite suggested that the h@xmath5  emission was stronger than the x - ray emission at the shock center . 
   this can now be evaluated over much of the inner sq group . 
   we present a complete reanalysis of the xmm - newton observations of sq using the latest calibrations ( see appendix c for full details ) in order to determine the fluxes and luminosities of the x - ray emission to match our spectral extractions . 
   the results indicate the striking dominance of the h@xmath5  line luminosities compared with the x - ray emission from the same regions . for the main shock , the x - ray `` bolometric '' flux of l@xmath45 = 2.8  @xmath2  10@xmath46erg  s@xmath13@xmath47 corresponds to 2.95 @xmath2  10@xmath35  erg  s@xmath13 , or l(h@xmath5)/l@xmath45 = 2.9 . 
   this is a lower limit since we have not attempted to remove the contribution to the main shock aperture of an extended group - wide x - ray component upon which the emission from the shock lies . therefore it is likely that the h@xmath5  line luminosity dominates over the main shock x - ray gas by a factor @xmath48 3 . 
   similar calculations can be done for the other regions for which h@xmath5  spectra have been extracted . 
   for example , in the `` bridge '' region , which we have already indicated has many of the same characteristics as the main shock , we find l(h@xmath5)/l@xmath45  =  2.5 . 
   these values demonstrate that throughout the extended regions of sq , the molecular hydrogen cooling pathway dominates over the x - ray in this shocked system . 
   this is a very significant result , upturning the traditional view that x - ray emission always dominates cooling in the later stages of evolution in compact groups of galaxies . 
   the fine - structure flux ratios ( see table [ tablemetalfluxes ] ) can be used to probe the conditions within the extracted regions of sq . 
   the @xmath10siii@xmath11$]34.82/@xmath10siii@xmath11$]33.48  ratio provides an indication of the sources of excitation within the system . 
   as mentioned previously , @xmath10siii@xmath11 $ ] is mainly a tracer of hii  regions , whereas enhanced @xmath10siii@xmath11 $ ] emission can be generated via several mechanisms , including thermal excitation by x - rays ( xdrs ) , or in shocks . in the main shock , 
   the @xmath10siii@xmath11$]34.82/@xmath10siii@xmath11$]33.48  ratio of @xmath6 4.59 is high compared to , for example , both normal galaxies ( @xmath6 1.2 ) and agn ( @xmath6 2.9 ) in the sings sample @xcite . 
   however , this large aperture is contaminated by star formation emission from sq - a and the intruder galaxy . a better measure 
   is given by the smaller shock sub - region , where the @xmath10siii@xmath11$]34.82/@xmath10siii@xmath11$]33.48  ratio is @xmath49 . 
   thus it is clear that the @xmath10siii@xmath11 $ ] emission is well outside the normal range of values , even for local well - studied agn . using the upper limit found for @xmath10siii@xmath11$]33.48   in the `` bridge '' structure , we find a ratio of @xmath50  again values well outside the range of normal galaxy disk emission . 
   indeed , these high values are typical of galactic supernova remnants where shock excitation is well determined ( e.g. * ? ? ? 
   * ; * ? ? ? 
   we will argue in the next section that silicon is being ionised in regions experiencing fast shocks @xmath51km@xmath0 and depleted onto dust grains . 
   the @xmath10feii@xmath11$]26.0/@xmath10siii@xmath11$]34.82  ratio for the main shock and sub - region ( @xmath6 0.12 ) is in agreement with values found by @xcite for their sample of snr .    for the extragalactic star forming region sq - 
   a , @xmath10siii@xmath11$]34.82/@xmath10siii@xmath11$]33.48  is @xmath61.49 , only slightly higher than the average of @xmath6  1.2 found for star - forming regions in the sings sample @xcite ; another indication that sq - a is dominated by star formation .    in star - forming galaxies 
   the @xmath10neiii@xmath11$]15.56/@xmath10neii@xmath11$]12.81  ratio can be used as a measure of the hardness of the radiation field as it is sensitive to the effective temperature of the ionising sources . in the main shock , 
   we find a value of @xmath52 which would be considered typical compared to those found in starburst systems , which range from @xmath53 @xcite , and in supernovae remnants ranging from @xmath54 @xcite . 
   the shock subregion has a ratio of only @xmath55 indicating a lower intensity radiation field north of the shock center . 
   however , it is clear from its spatial distribution relative to the 8  and h@xmath5  emission , that most of the neon is not originating from star formation , and so shocks are an obvious source of excitation . 
   the @xmath10neii@xmath11$]12.81/@xmath10neiii@xmath11$]15.56  ratio can be used to estimate the shock velocity using the mappings shock model library of @xcite . in the main shock 
   this ratio corresponds to shock speeds of between 100 and 300kms@xmath13 ( using preshock densities of @xmath56 @xmath57 and magnetic parameter @xmath58 = 1 and 3.23  the nominal equipartition value ) . 
   we do , however , have contamination from starforming regions in the main shock and can not disentangle this emission from that produced in the shock . to address this we use the @xmath10neii@xmath11$]12.81  and @xmath10neiii@xmath11$]15.56  maps to mask areas associated with star formation ( see section 3.3 ) and determine a lower limit for the @xmath10neii@xmath11$]12.81/@xmath10neiii@xmath11$]15.56  ratio in the shock of @xmath6  4.54 . 
   this value corresponds to a shock velocity of @xmath6 150kms@xmath13 . 
   the average electron density is determined from the @xmath10siii@xmath11$]18.71/@xmath10siii@xmath11$]33.48  ( two lines of the same ionisation state ) ratio . for sq - 
   a we find a ratio of @xmath60.56 , and in the main shock @xmath60.41 . 
   this corresponds to an electron density of @xmath59 @xmath57 @xcite for both , i.e. in the low - density limit for this diagnostic @xcite . 
   our observations have shown that the molecular hydrogen and x - ray emitting plasma appear to follow a similar distribution , and we have ruled out the possibility that this is a consequence of x - rays heating the h@xmath5 , since the h@xmath5  has the dominant luminosity . 
   how then can we explain the similar distributions ? 
   are these results consistent with the hypothesis that the shock is formed where the intruding galaxy ngc 7318b collides with a pre - existing tidal filament of hi  drawn out of ngc 7319 in a previous interaction with another group member @xcite ? 
   this basic mechanical picture appears plausible as can be seen in figure [ fig : hi+h2 ] which shows that the h@xmath5  distribution `` fills in '' the gap in the hi  tidal tail as observed by the vla @xcite . 
   the implication is that the hi  has been converted into both a hot x - ray component and a warm h@xmath5  component by the collision of the intruder with the now missing hi . 
   part of the puzzle of how this high - speed ( @xmath60 @xmath61 kms@xmath13 ) shock can lead to both x - ray and very strong molecular line emission is presented in a model by @xcite . 
   the high - speed collision of ngc  7318b with the hi  filament ( assumed to be composed of a multiphase medium ) leads to multiple shocks passing through and compressing denser clumps ( which become dusty nucleation sites for h@xmath5  formation ) as opposed to the lower - density gas , which is shock - heated to x - ray tempertures . 
   the h@xmath5  therefore forms in denser clouds experiencing slower shocks . 
   thus the coexistence of both hot x - ray gas , and cooler molecular material is a natural consequence of the multiphase medium of the pre - shocked material . 
   modeling of the h@xmath5  excitation by @xcite demonstrates that the emission can be reproduced by low velocity ( @xmath62 ) magnetohydrodynamic shocks within the dense ( @xmath63 ) h@xmath5  gas . 
   the denser clouds survive long enough to be heated by turbulence in the hot - gas component , tapping into the large available kinetic energy of the shock . 
   this picture is consistent with both the broad h@xmath5  linewidth ( 870kms@xmath13 ) measured in irs high - resolution spectrometer observations of @xcite , and the velocity center of the warm h@xmath5  ( based on new irs spectral calibrations - see appendix b ) which places the gas at intermediate velocities between the intruder and the group igm . 
   both these measurements are consistent with h@xmath5  being accelerated in a turbulent post - shocked layer . 
   intermediate pre - shock densities and post - shock temperatures result in regions of hi  and hii  that have cooled , but where the dust content has been destroyed @xcite . 
   pre - existing giant molecular clouds ( gmcs ) which may have been embedded in the hi  gas , would be rapidly compressed and collapse quickly , thus forming stars.this mechanism , proposed for sq - a by @xcite , might also apply to 7318b - south . however , if this was the case , it would have to explain why two such gmcs happened to be positioned at the extreme ends of the current shock  an unlikely coincidence . 
   more probable , however , is that the geometry of the shock somehow favors the collapse of clouds at the ends of the shock  perhaps in regions where the turbulent heating is less efficient . 
   as outlined above , the combination of emission detected in the shock region of sq can be understood in terms of a spectrum of shock velocities . 
   the fastest shock velocities ( @xmath64 km@xmath0 ) are associated with the lowest density pre - shock regions and the post - shock x - ray emitting plasma . 
   these are fast @xmath33 shocks and represent a discontinous change of hydrodynamic variables and are often dissociative @xcite . 
   @xmath34 shocks have a broad transition region such that the transition from pre - shock to post - shock is continuous and are usually non - dissociative @xcite . 
   the lowest velocity shocks associated with the turbulent h@xmath5  emission are @xmath62 @xmath34 shocks @xcite . 
   @xcite find optical emission line ratios consistent with shock models that do not include a radiative precursor @xcite . 
   the @xmath10feii@xmath11$]25.99  emission associated with the shock region in sq is relatively weak , but coincides with the most energetic part of the shock as traced by the x - rays ( fig . 
   [ fig : fs ] ) . 
   we also detect abundant @xmath10siii@xmath11$]34.82  emission associated with the main shock . 
   silicon and iron have very similar first and second ionization potentials . 
   their first ionization potentials ( 7.9 and 8.15  ev for fe and si , respectively ) are lower than that of hydrogen but their second ionization potential is higher ( 16.19 and 16.35  ev ) . 
   the mir @xmath10feii@xmath11 $ ] and @xmath10siii@xmath11 $ ] line emission observed from the sq shock could thus arise from predominantly neutral , as well as ionized gas . 
   we discuss the contribution from the ionized gas using the @xmath10feii@xmath11$](@xmath65m)/@xmath10neii@xmath11$](@xmath66 m ) and @xmath10siii@xmath11$](@xmath67m)/@xmath10neii@xmath11$](@xmath66 m ) line ratios . 
   the high @xmath10neii@xmath11$](@xmath66m)/@xmath10neiii@xmath11$](@xmath68 m ) mid - ir line ratio ( see section 5 ) implies that @xmath10neii@xmath11 $ ] is the dominant ionization state of ne in the sq shock . unlike fe and si , 
   ne is not much depleted on dust @xcite . 
   because the @xmath10neii@xmath11$]@xmath66 m line has a high critical density ( @xmath69@xmath57 , ho and keto 2007 ) , the neon line strength scales with the emission measure of the ionised gas . 
   the optical line emission from the sq shock is discussed in detail in xu et al . 
   the high values of the @xmath10oi@xmath11$](6300 ) and @xmath10nii@xmath11$](6584 ) lines to h@xmath27 line ratios are evidence of shock ionization . 
   the optical @xmath10sii@xmath11$](6716/6731 ) line ratio as well as the mir @xmath10siii@xmath11 $ ] line ratio correspond to the low density limit ( see section 5 ) and comparison with the shock models of allen et al . 
   ( 2008 ) constrain the pre - shock gas density to be about @xmath70@xmath57 or smaller .    in figure 
   [ fig : sil_ne ] we indicate the region ( in grey ) corresponding to the observed mir ratios ( using the upper and lower limits determined for the @xmath10neii@xmath1112.81\mu$]m line emission as discussed in section 5 ) . 
   for comparison the expected emission from the shock models of @xcite are displayed for a pre - shock gas density of @xmath70@xmath57 and two values of the magnetic parameter , @xmath58 = 0.5 and 3 . 
   for clarity of the figure , only shock velocities from 100 to 300kms@xmath13 are used and higher velocities shocks which do not match the observed @xmath10neii@xmath11$]/@xmath10neiii@xmath11 $ ] line emission ratio ( see section 5 ) are discarded . 
   the observed @xmath10feii@xmath11$](@xmath65m)/@xmath10neii@xmath11$](@xmath66 m ) and @xmath10siii@xmath11$](@xmath67m)/@xmath10neii@xmath11$](@xmath66 m ) ratios are both smaller than the shock values . 
   the iron and silicon lines are not dominant cooling lines of ionizing shocks . 
   the gas abundances of these elements do not impact the thermal structure of the shock and the line intensities roughly scale with the gas phase abundances . 
   we thus interpret the offset between the irs observation and model values as evidence for fe and si depletion . 
   we consider the magnitude of the depletions indicated by the arrow ( @xmath650% and @xmath660% for fe and si , respectively ) in figure [ fig : sil_ne ] as lower limits since there could be a contribution to the @xmath10feii@xmath11 $ ] and @xmath10siii@xmath11 $ ] line emission from non - ionizing @xmath33-shocks into molecular gas @xcite . 
   such shocks could also contribute to the @xmath10oi@xmath11$](6300 ) line emission as discussed in guillard et al . 
   forthcoming observations of the h@xmath9 ro - vibrational line emission in the near - ir should allow us to estimate whether they may be significant . a contribution from non - ionizing shocks to the @xmath10feii@xmath11 $ ] and @xmath10siii@xmath11 $ ] line emission will raise the depletion of both fe and si as well as the fe / si depletion ratio because non - ionizing shocks do not produce @xmath10neii@xmath11 $ ] line emission and the @xmath10feii@xmath11$](@xmath71m)/@xmath10siii@xmath11$](@xmath72 m ) line emission ratio in @xmath33-shocks within dense gas is larger than for shocks plotted in figure [ fig : sil_ne ] .      a key aspect to understanding the emission we observe in sq , as well as a test of the proposed model of h@xmath5  excitation , is the amount and distribution of cold h@xmath5 , as measured by co. a reservoir of cold h@xmath5  associated with the warm gas observed in sq would provide key insight into conditions within the intragroup medium . in figure [ fig : co ] 
   we show the bima co ( 1 - 0 ) integrated intensity contours from @xcite , shown with the h@xmath5  s(0 ) and s(1 ) emission contours , overlaid on an optical image of sq . 
   these interferometric observations use the large primary beam of 110 , centered on the shock , to determine areas of high column densities in the group . 
   the co traces areas of known star formation seen at 24  and in the nuv ( see section [ sf ] ) , notably sq - a and 7318b - south . as observed by @xcite , the co around ngc 7319 
   is concentrated in two regions . 
   the dominant complex is north of the nucleus residing in a dusty tidal feature . 
   the nuclear co is elongated perpendicular to the stellar disk suggesting a deficiency of ongoing star formation in the disk . 
   the co distribution does not correlate with the location of warm h@xmath5  emission , particularly around ngc 7319 . 
   the cold h@xmath5  complexes are clearly offset from the concentrations of warm h@xmath9 . 
   even the s(0 ) emission line ( see fig . 
   [ fig : co]a ) , which follows the coldest warm h@xmath9 , does not have peaks corresponding to the strongest co detections . in a forthcoming paper ( guillard et al . 
   2010 , in preparation ) , we will report the recent detection of @xmath73co(1 - 0 ) and ( 2 - 1 ) emission , associated with the warm h@xmath9 in the sq shock , using the single - dish 30 m iram telescope . 
   these observations suggest that most of the co emission in the shock has been missed by interferometers ( because of the broad linewidth ) and show that the co emission is both present in the shock , and extends along the h@xmath9-emitting bridge and towards ngc 7319 . 
   the kinematics of the co gas lying outside of star - forming regions , in the new observations , appears to be highly disturbed with a broad linewidth in agreement with the appleton et al . 
   ( 2006 ) interpretation that the mir h@xmath5  lines were intrinsically very broad and resolved by the high resolution module of irs . 
   the co data also agrees with our re - analysis of the appleton et al . 
   ( 2006 ) data ( see appendix b ) using more recent and reliable wavelength calibration , which places the bulk of the h@xmath5  gas at velocities intermediate to that of the intruder and the group  supporting the idea that the h@xmath5  gas is accelerated in the shock . 
   section [ sf ] discussed the star forming regions observed in sq and noted that there was very little evidence for star formation in the shock associated with the warm h@xmath5  emission . 
   the main shock contains a large quantity of warm molecular hydrogen ( 5.0 @xmath210@xmath43m@xmath44 ) providing a reservoir of fuel for star formation once it cools ( see section [ spec ] ) . 
   we investigate star formation in the warm h@xmath9-dominated medium by considering the shock sub - region , chosen to avoid star forming regions in the intruder galaxy , but likely still subject to some contamination from these regions . 
   pah emission is a classical tracer of star formation , but the molecules are fragile and easily destroyed in hard radiation fields . in the spectra of the main shock and subregion ( fig . 
   [ fig : spectra]a and b ) the pah emission bands at 6.2 , 7.7  and 8.6  are far weaker compared to the 11.3  bands , which when strong are predominantly produced by neutral pah molecules @xcite . in the shock subregion we find an upper limit flux for the 6.2  pah of @xmath74 wm@xmath75 and fluxes of @xmath76 wm@xmath75 and @xmath77 wm@xmath75 for the 7.7 and 11.3  bands respectively . 
   this corresponds to a 7.7/11.3  pah ratio of 0.35 , very low compared to the median value found for the sings sample of 3.6 @xcite . 
   the suppression of the 7.7/11.3  pah ratio has been observed in agn environments ( e.g. low - luminosity agn in the sings sample of * ? ? ? * ) and is favored to be the result of selective destruction of pah molecules small enough to emit at 7.7 . 
   pah processing in the shock due to larger molecules being less fragile than smaller ones is discussed further in guillard et al . 
   an alternative explanation is that the pah molecules are chiefly large and neutral in the shock , producing enhanced 11.3  emission in comparison to the smaller pah molecules . 
   a detailed comparison of the dust and pah emission properties can be found in guillard et al . 
   ( 2010 ) . 
   dust emission in the mid- and far - infrared can be used to infer the amount of star formation taking place @xcite . 
   we measure a 24  flux in this region of 0.408 mjy , corresponding to a spectral luminosity ( @xmath78 ) of @xmath79 . 
   this low luminosity is consistent with the weak mir continuum ( fig . 
   [ fig : spectra]b ) , arising from emission from vsgs heated by the uv radiation field . 
   we can combine this with a measurement of the h@xmath27 emission in this region to obtain a star formation rate ( sfr ) , given that they are complimentary ( h@xmath27 tracing the young stellar population and 24  as a measure of dust - absorbed stellar light ) . 
   we find @xmath80 , but caution that h@xmath27 emission in sq is also the result of shock - excitation @xcite and must be considered an upper limit for measuring star formation . when we combine @xmath81 with @xmath82 , using the relation of @xcite , we find a sfr of @xmath290.05@xmath83 
   .    a further measure of star formation can be obtained from the pah strength . using the relation of @xcite , derived from the starburst sample of @xcite 
   , we can use the 7.7  flux density as a measure of star formation . 
   the shock subregion has a 7.7  flux density of 0.73 mjy which corresponds to a sfr of @xmath84 , in good agreement with our previous calculation , but also an upper limit as some pah emission is contamination from known star formation regions in the group . a point of caution 
   , however , is that this star formation indicator may be biased given that we detect suppressed 7.7  emission compared to the 11.3  pah . 
   comparing the @xmath85(7.7)/@xmath82 ratio to the galaxies in sings , we find that the value of @xmath86 is typical of star - forming galaxies @xcite , which suggests that both measures of star formation are low , but self - consistent . 
   the low _ upper limits _ for the sfr in the shock suggests that star formation is depressed in the shock apart from in the igm starburst , sq - a , which has a sfr of @xmath87 at the velocity of the group @xcite 
   . this would be consistent with a picture of molecular hydrogen being reheated by mhd shocks in the turbulent medium . 
   since the jeans mass increases with gas temperature and turbulence and shearing morions will prevent collapse , the cold molecular gas clouds may be too short - lived or undersized to facilitate collapse and produce significant star formation @xcite . 
   the extent of a cold reservoir of molecular gas in the shock is a key consideration in this scenario , as discussed in section [ co ] . 
   the present observations suggest that molecular line cooling in dense clumps dissipates a significant fraction of the kinetic energy available in high - speed shocks . for reference , and to provide a glimpse of what this group might look like at high redshift when filling a single beam , we present in figure [ fig : full ] the spectrum extracted for the entire group ( i.e. including the three neighboring galaxies and the shock ) . 
   the total h@xmath5  line emission from the whole group exceeds @xmath8 and is still the most dominant mir coolant . 
   indeed the luminosity of the rotational h@xmath5  lines is sufficient that it could be detected at high redshift with future far - infrared or sub - mm instrumentation like spica or safir ( see appleton et al . 
   how likely is it , however , that high - speed shocks play a role in the assembly of galaxies ? 
   there is growing evidence that galaxies at high redshift are turbulent @xcite and increasingly clumpy ( e.g. conselice et al . 
   2005 , elmegreen & elmegreen 2005 ) . 
   indeed bournaud & elmegreen ( 2009 ) discuss the importance of the growth instabilities in massive gas clumps in forming disks at z  @xmath88  1 , and favor at least a large fraction of the clump systems being formed in smooth flows , perhaps similar to those discussed by dekel et al . 
   to what extent the build - up of these disks is truly `` smooth '' is not yet clear since the medium is likely to be a multiphase one . in a more standard picture , gas flowing into the more massive dark halos will experience strong shocks , most likely in an inhomogenious medium ( e. g. greif et al . 
   2008 )  thus it begs the question of how important h@xmath9 cooling may be in these different cases . 
   models of the collapse of the first structures predict that strong metal lines soon dominate the cooling over molecular hydrogen when the first stars pollute the environment . 
   it is therefore usually assumed that , except at very early stages , molecular hydrogen is a minority coolant in gas that forms the first major structures ( e.g. bromm et al . 
   2001 , santoro and shull 2006 ) . 
   however , our observations show that , under the right conditions , even in high metallicity environments , molecular hydrogen can be extremely powerful  in this case dominating by a factor of ten over the usually powerful @xmath10siii@xmath1134.82 \mu$]m line in strong shocks . 
   if there are situations at high redshift where strong shocks propagate into a clumpy , multiphase medium , then our observations imply that molecular hydrogen cooling can not be assumed to be negligible . on the other hand 
   , this will not be a trivial problem . 
   our best model of stephan s quintet @xcite involves the formation of h@xmath5  in a complex multiphase turbulent medium in which shocks destroy dust in some places , but allow survival in others - thus encouraging h@xmath5  formation .    in the early universe , this enhanced cooling , 
   which has so far been neglected , will depend on the distribution and nature of the first dust grains , in concert with the formation , temperature and abundance of gas , and the feedback effects from the first stars and agn . 
   in this paper we have presented the results of the mid - infrared spectral mapping of the stephan s quintet system using the _ spitzer space telescope_. we highlight here our five main conclusions :    * the powerful h@xmath5  emission detected by @xcite surprisingly represents only a small fraction of the group - wide warm h@xmath5  ( with a lower limit luminosity of @xmath89 spread over @xmath90 kpc@xmath7 ) that dominates the mid - infrared emission of the system . 
   there is evidence for another shock - excited feature , the so - called h@xmath5  bridge between the main shock and ngc 7319 , which is likely a remnant of past tidal interactions within the group . 
   the spatial variation in the distribution of the h@xmath5  0 - 0 line ratios implies differences in temperature and excitation in the shocked system - this will be explored fully in paper ii . * 
   the global l(h@xmath5)/l@xmath91 ratio in the main shock is @xmath92 , and @xmath62.5 in the new `` bridge '' feature . 
   the results confirm that mir h@xmath9 lines are a stronger coolant than x - ray emission over the shock structures , indicating a new cooling pathway seen on a large scale in sq . 
   this modifies the traditional view that x - rays dominate cooling at all times in the later stages of compact group evolution . since h@xmath5  forms on the surfaces of dust grains , we expect dust emission associated with these regions , but a low intensity radiation field produces only weak emission at 24 . 
   * following earlier interpretations of nebular line ratios in the optical , we interpret infrared ionic lines within the framework of fast ( @xmath93 km@xmath0 ) ionizing shocks . 
   comparison between the @xmath10neii@xmath11 $ ] , @xmath10neiii@xmath11 $ ] , @xmath10siii@xmath11 $ ] and @xmath10feii@xmath11 $ ] line intensities implies that both silicon and iron are depleted onto dust . 
   this result implies that dust is not destroyed in the shock . 
   * star formation in sq is dominated by sq - a and 7318b - south , located at the extreme ends of the shock ridge seen at radio wavelengths , suggesting they are both shock triggered starbursts . 
   however , regions dominated by warm h@xmath5  emission exhibit very low star formation rates , consistent with a turbulent model where h@xmath5  is significantly reheated and cool clouds are too short - lived or undersized to collapse . * in sq we observe the projected coexistence of @xmath10siii@xmath11 $ ] and h@xmath5 being produced by @xmath6200km@xmath0 and @xmath620km@xmath0 velocity shocks , respectively . 
   our observational results are consistent with a model of a multiphase postshock medium produced by a galaxy - wide collision @xcite . 
   the cooling pathway of warm h@xmath5  emission we observe group - wide in sq is clearly a significant , albeit surprising , mechanism in shock systems . to determine the overall dominant cooling mechanism in sq 
   , we require an inventory of lines and continuum processes at all wavelengths . early shock models @xcite predict that , apart from the rotational emission from h@xmath5 , contributions from lines such as @xmath10oi@xmath11$]63.2 , @xmath10cii@xmath11$]157.7  and the thz spectrum of h@xmath50 @xcite could be significant . 
   we hope to explore this chemistry more fully , and the detailed distribution of cool dust , using the capabilities of the _ herschel _ space observatory . 
   in addition , we can not rule out strong uv line - cooling . 
   stephan s quintet provides the ideal laboratory for probing a mechanism potentially crucial in systems ranging from ulirgs to radio galaxies to supernova remnants . 
   mec is supported by nasa through an award issued by jpl / caltech under program 40142 . 
   we thank tom jarrett for use of his irs pixel cleaning software and irac / mips photometry software . 
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   in this section we discuss the results pertaining to ngc 7319 , a seyfert 2 galaxy @xcite lying to the east in the sq group ( see fig . 
   the specific intensity contours for the h@xmath5 s(0 ) and h@xmath5 s(1 ) lines ( fig . 2a and b ) show emission associated with the galaxy , seemingly connected to the rest of the group by the h@xmath5  `` bridge '' discussed in section [ molhy ] . figure [ fig : xray_rad ] shows that the nucleus of ngc 7319 produces strong x - ray emission @xcite and is prominent at radio wavelengths @xcite . 
   however , we observe an offset between the peak of the h@xmath5  emission near ngc 7319 and the seyfert nucleus , which suggests that the `` bridge '' is a separate structure and not being excited by the agn . 
   the agn in ngc 7319 does not have a well - collimated jet , but two extended lobes with compact hotspots , asymetrically distributed along the minor axis of the galaxy @xcite . 
   this structure runs ne / sw and its orientation compared to the h@xmath5  filament ( which runs ew ) is not consistent with causing the excitation of the h@xmath5  bridge . 
   this is also evident from the relatively weak power in the agn , as inferred by the emission line diagnostics discussed below , and in the x - ray where it is only a factor of @xmath62 greater than the emission associated with the main shock and `` bridge '' . in figure 
   [ fig : fs ] we present the specific intensity contours of the @xmath10feii@xmath11$]25.99  ( blended with @xmath10oiv@xmath11$]25.89@xmath25 m ) , @xmath10siii@xmath11$]33.48  and @xmath10siii@xmath11$]34.82  emission lines . given the low spectral resolution of the sl and ll modules of _ 
   spitzer _ , we can not distinguish between emission from @xmath10fe ii@xmath11 $ ] and @xmath10oiv@xmath11 $ ] . 
   the agn in ngc 7319 is likely to produce both , with @xmath10feii@xmath11 $ ] emission likely originating in x - ray dissociation regions ( xdrs ) surrounding the agn @xcite . 
   prominent emission from @xmath10siii@xmath11$]33.48  and @xmath10siii@xmath11$]34.82  are due to the high excitation conditions associated with the agn . 
   dense pdrs and x - ray dominated regions , powered by agn , show strong @xmath10siii@xmath11 $ ] emission at 34.82 , while @xmath10siii@xmath11 $ ] 33.48  emission acts as a tracer of hii  regions . 
   we now focus on the emission line properties of ngc 7319 . 
   figure [ fig : agn ] shows the spectrum extracted from the galaxy and the measured properties of the line ratios are listed in table [ tableh2fluxes ] and [ tablemetalfluxes ] . 
   for the first time in the sq group , we observe a spectrum which is no longer dominated by h@xmath5  emission , but instead the brightest lines are the high - excitation @xmath10oiv@xmath11$]25.91@xmath25 m and @xmath10feii@xmath11$]25.98  blended lines , as well as the @xmath10siii@xmath11$]34.81@xmath25 m line . @xmath10nev@xmath11 $ ] is prominent at both 14.32  and 24.30@xmath25 m  a line typically seen in agn . also , unlike the majority of the extended shocked regions , there is a rising thermal continuum more typical of a starforming galaxy than a classical seyfert galaxy , although as various studies have shown ( buchanan et al . 2006 ; deo et al . 2007 ) , seyfert ii galaxies exhibit a variety of mir spectral characteristics at long wavelengths . 
   ngc  7319 s rising continuum is similar to that seen in the seyfert ii galaxy ngc  3079 ( deo et al . 
   2007 ) , and likely represents a dominant starburst component in the far - ir . 
   spitzer imaging reveals not only a bright nucleus , but also extended emission regions in the galaxy . using the @xmath10siii@xmath11$]34.81@xmath25m/@xmath10siii@xmath11$]33.48  ratio as a probe of excitation sources 
   , we find a ratio of @xmath61.85 which is low compared to the average value found for agn galaxies ( @xmath94 ) in the _ spitzer _ infrared nearby galaxy sample ( sings ) , but high compared to star - forming regions ( @xmath95 ) in the same sample @xcite . 
   this suggests a relatively weak agn . the @xmath10neiii@xmath11$]15.56/@xmath10neii@xmath11$]12.81  ratio is a measure of radiation field strength and the value of 0.97 indicates a typical radiation field strength compared to other agn ( the sample of weedman et al . 
   2005 shows a range of @xmath60.17 to 1.9 ) . the average electron density , estimated from the @xmath10siii@xmath11$]18.71/@xmath10siii@xmath11$]33.48  ratio of @xmath60.56 , is @xmath59 @xmath57 @xcite , in the low - density limit for this diagnostic @xcite . 
   the properties of the h@xmath5  emission of ngc  7319 are limited to the two long wavelength lines observed by irs - ll ( the galaxy lies outside the region mapped by sl ) . as a result , 
   the excitation diagram contains only two points allowing only an approximate idea of the h@xmath5  mass , since without the sl wavelength coverage to provide information on possible warmer components , we are likely to overestimate the temperature of the h@xmath5  by fitting a straight line to the points . any warmer component would contribute to the flux of the 0 - 0 s(1 ) line thus leading to a reduction of the temperature ( and increase in h@xmath5  mass ) for any cooler component . however , to provide a guide , we estimate the temperature of h@xmath5  @xmath96171@xmath97 k ( assuming the gas is in thermal equilibrium and thus an ortho - para ratio of 2.65 ) and a total h@xmath5  mass of 3.0@xmath9810@xmath3m@xmath4 . 
   we consider this a lower limit to the total warm mass if ( as is likely ) more than one component is present . 
   gao & xu ( 2000 ) estimated the _ cold _ molecular hydrogen mass of ngc 7319 based on @xmath73co ( 1 - 0 ) observations as @xmath88 3.6  @xmath210@xmath99m@xmath4 , a factor of roughly ten greater than the warm h@xmath5  mass . 
   such a ratio is not atypical of large spiral galaxies @xcite . 
   the high resolution spectrum at the center of the shock in sq , obtained by @xcite , has been reanalysed using the latest calibrations available for the irs instrument ( ssc pipeline version s17 ) and is included as figure [ fig : hr ] . 
   this has shown that the h@xmath5  gas lies at a velocity of 6360(@xmath100100 ) km s@xmath13 , between the velocity of the group ( 6600 kms@xmath13 ) and the velocity of the intruder ( 5700 kms@xmath13 ) . 
   this is consistent with a model of gas being accelerated by the shock , as well as the turbulence demonstrated by the broad linewidth of the h@xmath5  in the shock ( 860 kms@xmath13 ) . 
   the new sh spectrum shows that with the improved calibrations , the 11.3  pah feature is detected , although faint . 
   this is consistent with what is found in the larger extractions that show enhanced 11.3  pah emission compared to ionised pah features emitting at 6.2  and 7.7 . 
   in this section we present the x - ray fluxes in the extraction regions shown in figure [ fig : ext ] . 
   the reanalysis of archival data is necessary to obtain accurate fluxes and luminosities for the x - ray emission over the various apertures matched to our spectral extractions . 
   we use the xmm - newton epic - pn data ( see * ? ? ? 
   * for observational details ) to obtain the most sensitive measurements . 
   a calibrated event file was generated and filtered using standard quality flags , and subsequently cleaned of background flares . 
   a 0.32  kev image was then extracted and corrected for instrumental response . 
   all point - like sources were masked out to @xmath101 in the analysis of diffuse emission , and the local background level was estimated within a @xmath102  arcmin@xmath7 rectangular region away from the group core . for each region in fig . 
   7 , resulting background - subtracted photon count rates were converted to 0.32  kev and  bolometric " ( 0.00110  kev ) x - ray fluxes assuming an absorbed thermal plasma model of metallicity 0.4 solar , with temperature as estimated from the map of osullivan et al . 
   ( 2009 ) , and an absorbing galactic hi column density of @xmath103  @xmath47 . for ngc7319 , which harbors an agn , an absorbed power - law spectrum of photon index @xmath104 was assumed instead . 
   results are listed in table [ x - ray ] . 
   lcccc + region & count rate ( 0.3 - 2 kev ) & flux ( 0.3 - 2 kev ) & bolometric flux ( 0.001 - 10 kev ) & adopted spectral model + & ( photonss@xmath105 ) & ergs@xmath13@xmath47 & ergs@xmath13@xmath47 & +   + main shock & 0.102 & 1.8 @xmath106 & 2.8 @xmath106 & t = 0.7 kev + shock sub - region & 0.017 & 3.3 @xmath107 & 5.1 @xmath107 & t = 0.8 kev + bridge & 0.017 & 3.0 @xmath107 & 4.5 @xmath107 & t = 0.6 kev + ngc  7319 & 0.051 & 1.1 @xmath106 & 3.7 @xmath106 & @xmath108 = 1.7 + sq - a & 0.013 & 2.4 @xmath107 & 3.7 @xmath107 & t = 0.6 kev +        we note that for the main shock ( with luminosity 1.9  @xmath210@xmath35ergs@xmath13 in the 0 - 3 - 2 kev band ) we are within a factor of 2 of the 0.5 - 2 kev luminosity @xmath63.1  @xmath210@xmath35ergs@xmath13 obtained for a similar , but larger extraction of the shock by @xcite . 
   @xcite obtain a 0.5 - 2 kev surface brightness of 0.07 l@xmath4pc@xmath75 in the main shock compared to our value of 0.1 l@xmath4pc@xmath75 for the 0.3 - 2 kev surface brightness . 
   lccccccc + target region & aperture & h@xmath5 0 - 0 s(0 ) & h@xmath5 0 - 0 s(1 ) & h@xmath5 0 - 0 s(2 ) & h@xmath5 0 - 0 s(3 ) & h@xmath5 0 - 0 s(4 ) & h@xmath5 0 - 0 s(5 ) + & ( arcsec@xmath7 ) & @xmath10928.22@xmath25 m & @xmath10917.03@xmath25 m & @xmath10912.28@xmath25 m & @xmath1099.66@xmath25 m & @xmath1098.03@xmath25 m & @xmath1096.91@xmath25 m +   + main shock & 2307 & 3.09@xmath1000.19 & 23.05@xmath1000.26 & 9.10@xmath1000.38 & 22.76@xmath1000.84 & 2.5@xmath1001.0 & 14.1@xmath1000.7 + shock sub - region & 242 & 0.36@xmath1000.03 & 3.13@xmath1000.02 & 1.47@xmath1000.08 & 3.60@xmath1000.08 & 1.04@xmath1000.15 & 2.37@xmath1000.30 + bridge & 413 & 0.72@xmath1000.04 & 4.94@xmath1000.09 & & & & + ngc  7319 & 1302 & 2.06@xmath1000.26 & 11.00@xmath1000.48 & & & & + sq - a & 671 & 1.83@xmath1000.05 & 8.8@xmath1000.13 & & & & 3.7@xmath1000.93 +      lcccccccc + target region & @xmath10neii@xmath11 $ ] & @xmath10nev@xmath11 $ ] & @xmath10neiii@xmath11 $ ] & @xmath10siii@xmath11 $ ] & @xmath10nev@xmath11 $ ] & @xmath10feii@xmath11$]+@xmath10oiv@xmath11 $ ] & @xmath10siii@xmath11 $ ] & @xmath10siii@xmath11 $ ] + & @xmath10912.81@xmath25 m & @xmath10914.32@xmath25 m & @xmath10915.56@xmath25 m & @xmath10918.71@xmath25 m & @xmath10924.32@xmath25 m & @xmath10925.99 + 25.89@xmath25 m & @xmath10933.48@xmath25 m & @xmath10934.82@xmath25 m +   + main shock & 5.31@xmath1000.30 & @xmath290.9 & 1.94@xmath1000.39 & 0.76@xmath1000.13 & @xmath290.14 & 1.07@xmath1000.10 & 1.87@xmath1000.15 & 8.59@xmath1000.27 + shock sub - region & 1.04@xmath1000.09 & @xmath290.18 & 0.15@xmath1000.06 & @xmath290.12 & @xmath290.14 & 0.15@xmath1000.03 & 0.13@xmath1000.02 & 1.23@xmath1000.04 + bridge & & & 0.39@xmath1000.04 & @xmath290.1 & @xmath290.15 & 0.29@xmath1000.04 & @xmath290.22 & 1.1@xmath1000.1 + ngc  7319 & & 2.89@xmath1000.64 & 9.47@xmath1000.29 & 2.81@xmath1000.40 & 1.56@xmath1000.39 & 10.9@xmath1000.27 & 5.0@xmath1000.42 & 9.24 @xmath1000.49 + sq - a & & @xmath290.8 & 1.29@xmath1000.16 & 0.87@xmath1000.06 & @xmath290.18 & @xmath290.17 & 1.54@xmath1000.10 & 2.3@xmath1000.16 +
summary: we present results from the mid - infrared spectral mapping of stephan s quintet using the _ spitzer space telescope_. a 1000km@xmath0 collision ( @xmath1yr ) has produced a group - wide shock and for the first time the large - scale distribution of warm molecular hydrogen emission is revealed , as well as its close association with known shock structures . in the main shock region alone 
   we find 5.0  @xmath210@xmath3m@xmath4 of warm h@xmath5  spread over @xmath6480kpc@xmath7 and additionally report the discovery of a second major shock - excited h@xmath5  feature , likely a remnant of previous tidal interactions . 
   this brings the total h@xmath5  line luminosity of the group in excess of @xmath8 . in the main shock , 
   the h@xmath5  line luminosity exceeds , by a factor of three , the x - ray luminosity from the hot shocked gas , confirming that the h@xmath9-cooling pathway dominates over the x - ray . 
   @xmath10siii@xmath11$]34.82emission , detected at a luminosity of 1/10th of that of the h@xmath9 , appears to trace the group - wide shock closely and in addition , we detect weak @xmath10feii@xmath11$]25.99  emission from the most x - ray luminous part of the shock . 
   comparison with shock models reveals that this emission is consistent with regions of fast shocks ( @xmath12km@xmath0 ) experiencing depletion of iron and silicon onto dust grains . 
   star formation in the shock ( as traced via ionic lines , pah and dust emission ) appears in the intruder galaxy , but most strikingly at either end of the radio shock . 
   the shock ridge itself shows little star formation , consistent with a model in which the tremendous h@xmath5  power is driven by turbulent energy transfer from motions in a post - shocked layer which suppresses star formation . 
   the significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy - scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies .