document: surveys with bolometer arrays at millimetre ( mm ) and submillimetre ( submm ) wavelengths are potentially sensitive to dusty objects at extreme redshifts , galaxies that drop out of surveys at shorter and longer wavelengths due to obscuration and unfavourable @xmath11 corrections . 
   the first cosmological surveys using scuba ( holland et al . 
   1999 ) and mambo ( kreysa et al .  1998 ) quickly and radically changed the accepted picture of galaxy formation and evolution , moving away from the optocentric view of the last century . 
   the discovery of so - called ` scuba galaxies ' ( smail , ivison & blain 1997 ) was greeted with surprise due to the remarkable evolution in the dusty , starburst galaxy population implied by such a large source density at the flux levels accessible to the first generation of bolometer arrays ( blain et al . 
   excitement was replaced by pessimism with the first efforts to study smgs at optical and infrared ( ir ) wavelengths : early reports , backed up with a study in the hubble deep field north by hughes et al . 
   ( 1998 ) , suggested that the majority of the submm population had no plausible optical counterparts . 
   attention was diverted to various redshift engines and broadband photometric techniques ( e.g.townsend et al . 
   2001 ; aretxaga et al .  2003 ; wiklind 2003 ) . as a result , only a handful of detailed studies were attempted , often for extreme and possibly unrepresentative galaxies ( e.g.  knudsen et al . 
   2004 ) . 
   recent progress has largely been the result of radio imaging of submm survey fields . 
   early radio follow - up detected roughly half of the submm sources observed ( smail et al . 
   2000 ; ivison et al .  2002 
   hereafter i02 ) , with an astrometric precision of @xmath30.3@xmath12 and , combined with the submm flux density , provide a rough estimate of redshift ( carilli & yun 1999 ) . 
   radio data also enabled some refinement of submm samples ( i02 ) , increasing the detection fraction to two thirds of smgs at 0.85-mm flux density levels in excess of @xmath35mjy . with positions in hand , 
   these bright smgs were found to be a diverse population  some quasar - like , with broad lines and x - ray detections ( e.g.  ivison et al . 
   1998 ) , some morphologically complex ( e.g.  ivison et al .  2000 
   downes & solomon 2003 ; smail , smith & ivison 2005 ) , some extremely red ( e.g.  smail et al.1999 ; gear et al . 
   2000 ; i02 ; webb et al .  2003b 
   ; dunlop et al.2004 ) , some with the unmistakable signatures of obscured active nuclei and/or superwinds ( e.g.  smail et al . 
   2003 ) . 
   spectroscopic redshifts have been difficult to determine . the first survey based on a submm / radio sample 
   was undertaken by chapman et al.(2003 , 2005  hereafter c03 , c05 ) : the median redshift was found to be @xmath32.2 for @xmath13-mjy galaxies selected using scuba and pinpointed at 1.4ghz . 
   the accurate redshifts reported by c03 and c05 facilitated the first systematic measurements of molecular gas mass for smgs ( @xmath310@xmath14m@xmath15 ) via observations of co ( neri et al . 
   2003 ; greve et al . 
   2005 ) , as well as constraints on gas reservoir size and dynamical mass ( tacconi et al.2005 ) . 
   the data suggest smgs are massive systems and provide some of the strongest tests of galaxy - formation models to date ( greve et al.2005 ) .    in spite of this progress , 
   a detailed understanding of smgs remains a distant goal . 
   confusion currently limits our investigations to the brightest smgs ( although surveys through lensing clusters have provided a handful of sources more typical of the faint population that dominates the cosmic background  smail et al . 
   2002 ; kneib et al . 
   2004 ; borys et al . 
   we must also recall that selection biases have potentially skewed our understanding : around half of all known smgs remain undetected in the radio ( due simply to the lack of sufficiently deep radio data , which do not benefit from the same @xmath11 correction as submm data ) and the radio - undetected fraction remains largely untargeted by existing spectroscopic campaigns . 
   these is also only limited coverage of red and ir wavelengths in spectroscopic surveys .    here , we present a robust sample of bright smgs selected using scuba and mambo in one of the ` 8-mjy survey ' regions : the lockman hole ( see scott et al . 
   2002 ; fox et al . 
   2002 ; i02 ; greve et al .  2004 ; mortier et al . 
   our goal is to provide a bright sample which we would expect to detect in well - matched radio imaging ( @xmath16 ) whilst minimising , so far as is practicable , the possibility that sources are spurious or anamalously bright . 
   we may thus determine the true fraction of radio drop - outs amongst smgs ( potentially lying at very high redshift , @xmath7 ) , as well as practical information such as the intrinsic positional uncertainty for smgs in the absence of radio / ir counterparts . 
   throughout we adopt a cosmology , with @xmath17 , @xmath18 and @xmath19km@xmath20mpc@xmath5 . 
   existing surveys have typically employed a snr threshold of 3.03.5 . at these 
   snrs , false detections are dominated by ` flux boosting ' (  2.2 ) , possibly at the 1040 per cent level ( scott et al .  2002 
   ; laurent et al .  2005 ) . 
   our goal is to provide a highly reliable submm source catalogue , free from concerns about contamination by spurious or artificially bright sources . 
   this issue has limited our ability to address the true recovery fraction in the radio , and hence the corrections that must be made to the redshift distributions that are used to determine star - formation histories and galaxy - formation models .    to achieve this 
   we have combined independent submm and mm maps of the lockman hole , constructing a single , reliable catalogue that is several times larger than would have been realised by simply adopting a high snr threshold in the individual submm and mm maps . 
   greve et al . 
   ( 2004 ) argued that several maps with low signal - to - noise ratio ( snr ) of the same region , with only marginal differences in frequency , produce several visualisations of essentially the same sky , tracing the same population of luminous , dust - enshrouded galaxies and we have adopted the same philosophy in the present work . 
   our maps came from the survey of greve et al . 
   ( 2004 ) who presented a 1.2-mm map of the lockman hole region ( as well as data on the elais n2 region ) , centred on the coordinates mapped at 0.85 mm by scuba in the ` 8-mjy survey ' ( scott et al .  2002 , 2004 ) . 
   mortier et al . 
   ( 2005 ) have recently present a refined analysis of scuba data which lies within the lockman hole mambo map ; we began with a 2-@xmath1 mambo catalogue , extracted as described by greve et al . 
   ( 2004 ) , and looked for 0.85-mm sources within 14arcsec ( roughly the area of five beams ) in the mortier et al . 
   ( 2005 ) sample , applying a correction for separation and checking for a combined significance above our initial threshold , 4.5@xmath1 . 
   we included sources that exceed 4.5@xmath1 in either dataset , as long as there is a valid reason why the source is not seen in the other image . 
   smgs from scott et al . 
   ( 2002 ) were substituted where blends were evident in the mortier et al.catalogue .    [ cols="<,^,^,^,^,^,^,^,^,^,^ " , ]     notes : @xmath21 sources in parentheses lack robust radio identifications . 
   + @xmath22 values in bold are for sources identified in the radio ( @xmath23 ) . 
   + @xmath24 values in bold are for redshifts that we consider to be most robust . 
   [ speccat ] 
   table  3 lists the mag_best magnitudes of our sample in @xmath25 , measured using sextractor . fig . 
   7 shows a histogram of @xmath25 magnitudes compared with those from the surveys by smail et al.(2002 ) , i02 , clements et al . 
   ( 2004 ) and pope et al . 
   the various surveys broadly agree , with median @xmath25-band magnitudes ranging between @xmath2626 and a wide range in magnitude within each sample . 
   the clements et al . 
   ( 2004 ) sample has a bright tail which is less evident in the other surveys . 
   our sample appears to show the smallest dispersion , although the presence of upper - limits in all five samples make this statement hard to quantify . 
   nevertheless , from this comparison we can state that the bright , radio - identified smg population , spanning barely a magnitude in submm flux density , covers over 10 orders of magnitude in rest - frame ultraviolet flux . 
   the optical magnitudes of the radio - identified smgs discussed in this paper span around @xmath2726 , with a faint tail . 
   the median magnitude is @xmath28 , comparable to the sample analysed by c05 , @xmath29 , and @xmath30.6mag fainter than the spectroscopically - identified sample of c05 . 
   as expected , a similar bias in the spectroscopically identified subset exists within our own sample , which is @xmath30.9mag brighter than our complete catalogue . 
   the redshift distribution determined here inevitably suffers from spectroscopic incompleteness , but our sample should be otherwise unbiased . concentrating on cases where our spectroscopy failed to secure a robust redshift 
   the failures tend to coincide with the faintest optical counterparts , @xmath30 , as one might expect . 
   looking at the long - wavelength flux ratios ( fig . 
   5 ) for those targets where we failed to obtain identifications or redshifts , e.g.lh1200.002 or .004 , we see no indication that their ( sub)mm / radio photometric properties differ from those of the sample as a whole . it seems unlikely therefore that they lie at substantially higher redshifts than the subset of smgs for which we have obtained redshifts . 
   the exceptions are lh1200.022 , and especially lh1200.007 , which have relatively low 0.85-/1.2-mm flux density ratios and fairly high 1.2-mm/1.4-ghz flux density ratios . 
   this suggests they may lie at high redshift , or be particularly cold . 
   however , as discussed in  3.2 , these two sources are not particularly secure ( although that may simply reflect their general faintness in all bands due to their high redshifts ) . 
   reliable identification of these two sources is therefore only achievable through higher resolution ( interferometric ) ( sub)mm observations ( e.g.  lutz et al . 
   2001 ) . 
   for the six smgs where we have a single radio identification ( and hence an unambigious counterpart ) and for which we have measured robust redshifts ( table  3 ) , we determine a median of @xmath31 ( where the scatter is estimated from bootstrap re - sampling ) . 
   this rises slightly to @xmath10 if we include the four sources with robust redshifts for at least one radio counterpart and the five smgs where we have less secure redshifts . 
   these figures are slightly lower than , but entirely consistent with , the spectroscopic redshift distribution determined by c05 ( _ z _ = 2.2 ) . 
   this suggests that the statistical properties of the c05 sample has not been strongly biased by the modest significance @xmath323@xmath1 ) of some of the submm data used in their analysis . 
   the median redshift we derive is in reasonable agreement with that predicted by the galform semi - analytic model ( baugh et al.2005 ) , which gives a median redshift of @xmath33 at our flux limit , with a quartile range of @xmath340.9 and only 1020 per cent of the population at @xmath35 . 
   this again suggests that much of the activity in the bright smg population is amenable to study via the precise positions derived from their radio counterparts . 
   this provides a much - needed route to identify the true far - ir - luminous source within these frequently morphologically complex and crowded fields . 
   we illustrate in fig . 
   8 the distribution of 1.2-mm/1.4-ghz and 0.85-/1.2-mm flux ratios for our sample versus our spectroscopic redshifts . 
   the former show a trend to higher flux ratios at higher redshifts , in line with predictions from sed modelling ( e.g.  carilli & yun 1999 ) , although with a large scatter ; the latter flux ratio , however , is essentially a scatter plot . 
   we conclude that significantly more reliable flux measurements ( in terms of both absolute calibration and overall snr ) will be needed to use the 0.85-/1.2-mm flux ratio for astrophysical analysis .    at face value , both our spectroscopic redshift distribution and that of c05 are inconsistent with the redshift distribution ( _ z _ @xmath32 3 ) claimed by eales et al . 
   ( 2003 ) using the @xmath36 flux density ratio ( fig . 
   the eales et al.sample contained 23 sources with @xmath37 = 3.16.5mjy ( cf . 
   19 sources , @xmath37 = 1.65.7mjy here ) , with a radio - detected fraction of 73 per cent ( cf . 
   79 per cent here ) . 
   thus , the two samples differ only in that the eales et al . 
   photometry targets were selected above 4@xmath1 at 1200@xmath38 m rather than above 5@xmath1 at a combination of 850 and 1200@xmath38 m . 
   we believe the disagreement in the mean redshifts derived by these studies most likely results from the fact that current photometric measurements in the mm and submm wavebands are insufficiently precise for typical smgs to allow them to be used as a reliable redshift indicator , particularly when some data do not comprise fully sampled images . 
   the disagreement may have been compounded by the lower significance of the eales et al . 
   targets , and the dual - wavelength extraction performed here could exclude the most distant starbursts ( though we note that the radio - detection of a substantial number of the eales et al.targets is hard to reconcile with the high median redshift claimed for that sample ; indeed , the radio - based estimates given in eales et al . , with _ z _ = 2.35 , are more consistent with the spectroscopic results ) . 
   we have developed and applied a dual - survey extraction technique to scuba and mambo images of the lockman hole , resulting in a robust sample of 19 smgs . 
   of these , 15 are detected securely by our deep radio imaging . those undetected at 1.4ghz can be explained by a combination of contamination by spurious sources ( 10 per cent ) and the large observed scatter in radio flux densities , which is probably due to a significant range in dust temperature . 
   we determine 15 spectroscopic redshifts , of which we consider ten to be secure . 
   the resulting redshift distribution ( _ z _ = 2.14 for the full spectroscopic sample ) is consistent with that determined for a much larger sample by c05 ( _ z _ = 2.2 ) . 
   our results thus support the conclusions of c05 , who modelled their incompleteness and estimated only a small shift ( @xmath39 ) in the median redshift as a result . 
   those galaxies for which our spectroscopy failed to determine redshifts are usually optically faint , @xmath30 , where the sample ranges from @xmath27 to @xmath40 with a median of 25.0 .    from the radio detections , and 
   the spectroscopy , is seems unlikely that a significant fraction of bright smgs lie at very high redshift ( @xmath41 ) and we conclude that the bright smg population is readily amenable to study via radio - selected samples , down to a 850-@xmath38 m flux density limit of @xmath37mjy . 
   " we note , however , that the dual - wavelength extraction performed here could potentially bias our sample against very - high - redshift sources which would only be detected at 1.2 mm .    an analysis of separations between smgs and their radio counterparts has allowed us to re - calibrate the rule - of - thumb relationship between positional accuracy , beam size and significance . the most secure smgs , at @xmath310@xmath1 , representative of those expected in upcoming , confusion - limited , wide - field ( tens of square degrees ) 0.85-mm surveys using scuba-2 ( audley et al . 
   2004 ) , with lower significance , simultaneous detections at 0.45 mm , will be located with a uncertainty of @xmath42arcsec , at which level the precision of the telescope pointing and scuba-2 flat - field may become important contributors to the positional error budget . assuming these sources of uncertainty can be minimised , the current requirement for deep radio coverage to identify counterparts and enable follow - up spectroscopy may not be as urgent , particularly if 3@xmath433-arcsec deployable integral - field units are employed for spectroscopic follow up ( e.g.  kmos  sharples et al . 
   2003 , 2004 ) . 
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   notes on our spectroscopic observations of the individual sources , referring to the slit positions shown in fig .  3 . 
   * lh1200.001 : * a complex field . 
   lris slits on several masks had been placed on a faint @xmath25-band source coincident with the most probable radio counterpart ( @xmath44 ) . 
   its spectrum , shown in fig . 
   a1 , is consistent with a redshift of 3.036 , with ly@xmath45 and hints of weak nv and several other lines . 
   we searched for [ oii ] and [ oiii ] using nirspec (  4.1 ) , yet we were not able to confirm or rule out @xmath46 with confidence . 
   a search for co(32 ) at iram was unsuccessful ( greve et al . 
   2005 ) , though this could have been due to insufficient velocity coverage . 
   the second radio counterpart , 4arcsec to the north and also with @xmath23 , is associated with an ero ( fig . 
   the nirspec longslit was positioned to cover this galaxy ; no lines were evident . 
   its closest neighbour was observed with gmos ( fig . 
   a2 ; [ 28 ] ) and found to have emission lines ( ly@xmath45 , possibly nv and siii ) consistent with @xmath47 ( the apparent continuum blueward of ly@xmath45 is merely an artifact of the reduction process ) . at the edge of the same slit , near the northern radio position 
   , there is tentative evidence for line emission from ly@xmath45 , nv and possibly civ at @xmath48 . the optically bright radio source 11arcsec se of the smg centroid ( fig . 
   a2 ; [ 26 ] ) is an agn , with broad ciii ] and mgii emission lines , this time at @xmath49 . 
   the optical galaxy 8arcsec sse of the smg centroid ( fig . 
   a2 ; [ 27 ] ) has [ oii ] at @xmath50 . 
   a further ero , overlooked by i02 , and detected at 3.5@xmath1 in our smoothed radio image , lies 7.5 arcsec nne of the smg centroid ( fig . 
   we conclude that the submm emission likely originates from the two central radio components , which most probably lie at @xmath51 . 
   * lh1200.002 : * two galaxies in this field were targeted spectroscopically with gmos . 
   one target , slit [ 25 ] , lies close to the faint radio emission near the smg centroid , but we were unable to identify its redshift . 
   the other target  probably unassociated with the smg  has a faint emission line which , if [ oii ] , indicates @xmath52 ( fig . 
   a2 ; [ 24 ] ) . 
   * lh1200.003 : * another complex field . 
   lh1200.003 may be a blend of 23 smgs , with mambo resolving the source into two , and the vla resolving three faint radio sources ( one marginally outside our radio search region ) . 
   slits had been placed on all likely counterparts . 
   the radio source to the se , targeted by slits on five lris masks at various position angles , is a starburst at @xmath53 . 
   the most probable ( central ) radio identification is associated with a galaxy at @xmath54 ( fig . 
   a1 ) . the galaxy to the nnw 
   possibly associated with faint mm emission , lh1200.213 in greve et al . 
   ( 2004 )  was targeted spectroscopically using lris and gmos and is an agn at @xmath55 , with broad civ emission , as well as narrower ly@xmath45 , heii , niv and nv ( fig . 
   a2 ; [ 15 ] ) . 
   * lh1200.004 : * from two possibilities , an lris slit had been placed on the galaxy with the marginally higher @xmath56 value ( 0.044 versus 0.033 ) . 
   the galaxy has a weak line in its spectrum at 558 nm , possibly ly@xmath45 , but we were unable to determine the redshift reliably .    * lh1200.005 : * lris slits were placed on a faint optical galaxy @xmath33 arcsec west of the obvious radio counterpart . 
   the galaxy is estimated to lie at @xmath57 from absorption lines in its optical spectrum ( c05 ) , a value tentatively confirmed via h@xmath45 in the @xmath11 band , although nirspec slit - rotation problems mean the line can not be recovered in the final spectrum or image . 
   although optically faint ( fig .  3 , @xmath58 ) , the extremely red object ( ero ) described by lutz et al.(2001 ) , seen in fig .  4 at the radio position , was targeted using lris and gmos . 
   no redshift was forthcoming . 
   the likelihood of finding a @xmath59 galaxy so close to the smg centroid is slim , so the two may well be associated , but we regard the redshift as tentative until h@xmath45 is detected unambiguously . 
   * lh1200.006 : * lris slits had been placed on by far the most probable radio counterpart in the region ( @xmath60 ) . its optical spectrum was identified by c05 as that of a starburst at @xmath61 ( fig . 
   this position was also observed by swinbank et al . 
   ( 2004 ) using nirspec , although only [ nii ] would have been accessible . 
   since the one - dimensional optical spectrum is not wholly convincing , this redshift can not be relied upon absolutely . the optically bright object to the nw 
   was targeted by gmos : a foreground galaxy ( @xmath62 ) with h@xmath63 and [ oiii ] emission lines evident ( fig . 
   a2 ; [ 220 ] ) . 
   * lh1200.007 : * there is no secure radio counterpart . 
   an lris slit was placed on a nearby _ xmm - newton _ x - ray source , which is marginally detected in our smoothed radio image . 
   it lies at @xmath64 ( fig . 
   a1 ) , and may contribute submm flux to what could well be a blended submm source . the faint , red galaxy to the north , barely seen in a noisy part of our @xmath11 image ( fig . 
   4 ) , was not targeted spectroscopically . 
   * lh1200.008 : * an lris slit had been placed on by far the most probable radio counterpart in the region ( @xmath65 ) , an @xmath66 galaxy amongst a dense ensemble of fainter objects ( fig . 
   3 ) , with absorption lines in its spectrum corresponding to @xmath67 ( fig .  a1 ) , with ciii ] and heii weakly in emission . 
   * lh1200.009 : * gmos slits were placed on the radio counterpart and on an optically bright galaxy to the nnw . 
   an unambiguous redshift could not be determined for either , although the nnw galaxy may show faint [ oii ] at @xmath68 ( fig . 
   a2 ; [ 231 ] ) . 
   lris slits had been placed on a faint @xmath25-band galaxy @xmath36arcsec ssw of the obvious radio counterpart . 
   this was due to an positional offset in an earlier version of the greve et al . 
   mambo catalogue . 
   the galaxy 3arcsec nw of the slit centre , 4arcsec sww of the radio source , appears to be a ly@xmath45 emitter at @xmath69 ( fig . 
   a1 ) , though no other lines are seen . as with lh1200.005 , the likelihood of finding a @xmath59 galaxy so close to the submm centroid is low , so it may be associated with the radio source and the smg . 
   the position of the radio source is blank to @xmath70 in our deep gemini / niri imaging ( fig . 
   the ly@xmath45 emitter is detected , barely , in @xmath11 . 
   we note that at least one similarly faint smg ( smmj14009 + 0252 , @xmath71 ) has been found at @xmath59 ( smith et al . 
   , in preparation ) . 
   * lh1200.010 : * lris slits had been placed on both of the radio - identified galaxies in this region with one of them ( and several other radio - quiet galaxies ) targeted by gmos . by far the most robust counterpart ( @xmath72 ) , morphologically complex in @xmath25 ( fig .  3 ) but relatively uncluttered in @xmath11 ( fig .  4 ) , 
   has the spectrum of a starburst at @xmath73 ( c05 , smmj105230.73 + 572209.5 ) , confirmed in h@xmath45 by swinbank et al . 
   ( 2005 ) and in nv by gmos ( ly@xmath45 falling between chips  fig . 
   a2 ; [ 210 ] ) , although undetected in co(32 ) at iram ( greve et al . 
   the lris spectrum of the radio - bright disk - like galaxy 10arcsec to the sw is also consistent with this redshift . 
   nothing was seen at the remaining gmos slit positions , [ 211 ] and [ 212 ] . 
   * lh1200.011 : * an lris slit had been placed on by far the most probable radio counterpart in the region ( @xmath74 ) , a compact source with a 5-arcsec - long tail visible in @xmath25 and @xmath11 ( figs 3 & 4 ) . 
   it has the spectrum of a starburst at @xmath75 ( c05 , smmj105158.02 + 571800.2 ) , confirmed convincingly in h@xmath45 and [ nii ] by swinbank et al . 
   the fainter radio source , 14arcsec to the nee , lies at @xmath76 . 
   simpson et al .  ( 2004 ) identified a @xmath77-band feature , presumably a noise spike , as [ oii ] at @xmath78 ( rest - frame 359 nm for @xmath79 ) , as well as a continuum break at 1.2@xmath38 m ( consistent with the balmer break at either redshift ) . 
   * lh1200.012 : * a complex field , with the robust radio identification ( @xmath80 ) lying between 34 galaxies visible in @xmath25 , and on top of a galaxy seen in the _ spitzer _ 
   3.6-@xmath38 m imaging of huang et al . 
   a ns - oriented lris slit had been placed on the brightest of the @xmath25-band galaxies , @xmath32arcsec sse , a starburst at @xmath81 ( fig . 
   a1 ; c05 , smmj105155.47 + 572312.7 ) . 
   the nirspec slit , centred on the same point , yielded continuum but no strong lines . 
   however , 2arcsec nw along the slit  where fuzz is visible in @xmath25 , 1arcsec sw of the radio centroid  there is more red continuum emission a line at 2.405@xmath38 m , corresponding to h@xmath45 at @xmath82 ( fig . 
   we view the likelihood of its association with the smg as high . another lris slit was placed on the radio source to the nw , associated with another _ spitzer _ galaxy , this time at @xmath83 ( @xmath84 in h@xmath45 according to swinbank et al.2005 ) . 
   gmos slits  [ 129 ] through [ 132 ]  were placed on all of the brightest optical knots , but nothing was seen in the spectra . 
   * lh1200.014 : * an interesting case , with strong similarities to lh1200.010 and .012 . 
   the brightest of the two radio counterparts  associated with faint @xmath11-band emission ( fig . 
   4 )  is the least likely identification , although by a small margin ( @xmath85 versus 0.022 ) . 
   lris slits were placed on the brightest @xmath25-band galaxy ( fig . 
   3 ) , just to the wsw , which displays [ oii ] in emission as well as ca h / k and several balmer lines in absorption at @xmath86 ( fig .  a1  lh1200.014a ; c05 , smmj105200.22 + 572420.2 ) . 
   its association with the smg , and with the radio - identified galaxies , is plausible , as is the possibility that it acts as a lens ( chapman et al . 
   the slit passed over the radio centroid which appears to share the same broad spectral features ( fig . 
   a1  lh1200.014b ) . 
   a gmos slit , [ 123 ] , was placed directly on the radio centroid , but the spectral coverage ( @xmath87550 nm in this case ) meant we were unable to confirm the presence of [ oii ] . 
   * lh1200.042 : * lris slits had been placed on by far the most probable radio counterpart in the region ( @xmath88 ) . although optically faint , it has an absorption - line spectrum consistent with @xmath89 ( fig .  a1  lh1200.042a ) . 
   the bright optical galaxy 3arcsec to the sse , targeted by both gmos [ 225 ] and lris , is associated with very faint radio emission and is tentatively consistent with @xmath90 ( fig . 
   a1  lh1200.042b ) . 
   * lh1200.096 : * again , lris and gmos slits had been placed on the most probable radio counterpart ( @xmath91 ) , an ero ( i02 ) visible out to 24@xmath38 m ( egami et al . 
   2004 ) , ignoring several brighter optical galaxies . 
   the lris spectrum was classified as a starburst at @xmath92 by c05 ( fig . 
   a1  smmj105151.69 + 572636.0 ) ; gmos saw nothing at this position . 
   this is the curious smg discussed by i02 , apparently associated with the steep - spectrum lobe of a radio galaxy , the flat - spectrum core of which lies to the west , with [ oii ] evident at @xmath93 in its spectrum ( fig . 
   a2 , [ 120 ] ) . 
   is this system a jet - triggered burst , a galaxy projected onto an unrelated radio lobe , or a faint , dusty , ultra - steep - spectrum radio galaxy ? 
   our picture of this system is muddled and contradictory . to confuse matters further , our niri @xmath11-band imaging ( fig . 
   4 ) reveals another ero , wsw of the radio emission , visible out to 8@xmath38 m in the _ spitzer _ imaging , and just missed by gmos slit [ 119 ] on a nearby , bluer galaxy . 
   * lh1200.104 : * despite the lack of radio detections in the vicinity , a bright _ spitzer _ counterpart described by ivison et al . 
   ( in preparation ) suggests this smg is not spurious . 
   a diffraction spike from the nearby star makes identification impossible in our optical imaging .
summary: the modest significance of most sources detected in current ( sub)millimetre surveys can potentially compromise some analyses due to the inclusion of spurious sources in catalogues typically selected at @xmath03.03.5@xmath1 . here , 
   we develop and apply a dual - survey extraction technique to scuba and mambo images of the lockman hole . cut above 5@xmath1 , our catalogue of submillimetre galaxies ( smgs ) is more robust than previous samples , with a reduced likelihood of real , but faint smgs ( beneath and around the confusion limit ) entering via superposition with noise . 
   our selection technique yields 19 smgs in an effective area of 165arcmin@xmath2 , of which we expect at most two to be due to chance superposition of scuba and mambo noise peaks . 
   the effective flux limit of the survey ( @xmath34mjy at @xmath31 mm ) is well matched to our deep 1.4-ghz image ( @xmath4jy beam@xmath5 ) . 
   the former is sensitive to luminous , dusty galaxies at extreme redshifts whilst the latter probes the @xmath6 regime . 
   a high fraction of our robust smgs ( @xmath380 per cent ) have radio counterparts which , given the @xmath310-per - cent contamination by spurious sources , suggests that very distant smgs ( @xmath7 ) are unlikely to make up more than @xmath310 per cent of the bright smg population . 
   this implies that almost all of the @xmath8mjy smg population is amenable to study via the deepest current radio imaging . 
   we use these radio counterparts to provide an empirical calibration of the positional uncertainty in smg catalogues . 
   we then go on to outline the acquisition of redshifts for radio - identified smgs , from sample selection in the submillimetre , to counterpart selection in the radio and optical / infrared , to slit placement on spectrograph masks . 
   we determine a median of @xmath9 from a sample of six secure redshifts for unambigious radio - identified submillimetre sources and @xmath10 when we include submillimetre sources with multiple radio counterparts and/or less reliable redshifts . 
   these figures are consistent with previous estimates , suggesting that our knowledge of the median redshift of bright smgs population has not been biased by the low significance of the source catalogues employed . 
   galaxies : starburst  galaxies : formation  cosmology : observations  cosmology : early universe