document: x - ray studies of fairly  normal " galaxies , with high - energy emission not obviously dominated by a luminous active galactic nucleus ( agn ) , have recently been extended to cosmologically interesting distances in the  deep field ( cdf ) surveys , which have now reached 1  ms of exposure ( cdf - n : hornschemeier et  al . 
   2001 , hereafter paper  ii ; brandt et  al . 
   2001b , hereafter paper  v ; cdf - s : tozzi et  al . 2001 ; p. 
   rosati et  al . , in prep . ) . 
   galaxies with @xmath8 are detected in appreciable numbers at 0.52  kev fluxes below @xmath9 erg  @xmath6  s@xmath7 ( e.g. , paper  ii ) ; the cdf - n survey goes almost two orders of magnitude fainter , detecting significant numbers of normal galaxies among the population of x - ray sources making the diffuse x - ray background ( xrb ; paper  ii ; a.j . 
   barger et al . , in prep . ) . 
   these normal galaxies contribute as much as 510% of the xrb flux in the 0.52  kev band . 
   the bulk of the energy density of the xrb is certainly explained by agn , but the investigation of the  typical " galaxy , whether its x - ray emission is dominated by a population of x - ray binaries , hot interstellar gas , or even a low - luminosity agn , is an equally important function of deep x - ray surveys . 
   normal galaxies are likely to be the most numerous extragalactic x - ray sources in the universe and are expected to dominate the number counts at 0.52  kev fluxes of @xmath10@xmath11 erg  @xmath6  s@xmath7 ( ptak et  al . 
   2001 ) . 
   the cdf - n has reached the depths necessary to detect individually many normal [ @xmath12 ; @xmath13 is from 0.52 kev ] @xmath14 galaxies to @xmath15 , corresponding to a look - back time of @xmath16  gyr ( @xmath17  km  s@xmath7 mpc@xmath7 , @xmath18 , and @xmath19 are adopted throughout this paper ) . 
   reaching larger look - back times presents the exciting possibility of detecting the bulk x - ray response to the heightened star - formation rate at @xmath203 ( e.g. , madau et al . 1996 ) . 
   one thus expects the x - ray luminosity per unit @xmath2-band luminosity to be larger at @xmath211 in the past due to the increased energy output of x - ray binary populations at @xmath203 ; this x - ray emission represents a  fossil record " of past epochs of star formation ( e.g. , ghosh & white 2001 ; ptak et al . 
   therefore , measurements of the x - ray luminosities of typical galaxies can constrain models of x - ray binary production in galaxies .    while x - ray emission from individual galaxies is not easily detected at @xmath22 , 
   it is possible to estimate the emission at their extremely faint flux levels using statistical methods such as stacking , a technique implemented successfully on the cdf - n survey data in several previous studies . 
   these include the detection of x - ray emission from the average @xmath21 bright ( @xmath23 ) galaxy in the hubble deep field - north ( ) described in brandt et  al . 
   ( 2001a , hereafter paper  iv ) and a study of x - ray emission from @xmath244 lyman break galaxies identified in the ( brandt et  al . 2001c , hereafter paper  vii ) . encouraged by the success of these analyses , 
   we extend here the study of normal galaxies to the entire plus  flanking fields region , now concentrating on galaxies at @xmath25 to complement the study of @xmath26 galaxies performed in paper  vii . 
   we focus on this redshift range due to the extensive spectroscopic redshift coverage ( cohen et  al . 2000 and references therein ) and superb  imaging which has allowed a comprehensive galaxy morphology study ( van den bergh , cohen , & crabbe 2001 ) . 
   the cdf - n data provide extremely deep x - ray coverage over this area ( see figure  7 of paper  v for the exposure map of this region ) ; the point - source detection limits in this region of the cdf - n survey in the 0.52  kev and 28  kev bands are @xmath27  erg  @xmath6  s@xmath7 and @xmath28  erg  @xmath6  s@xmath7 , respectively . 
   in this study , we place observational constraints on the evolution of the ratio of x - ray luminosity to @xmath2-band luminosity of  normal " spiral galaxies up to @xmath29 ; this ratio is an indicator of the current level of star formation in a galaxy ( e.g. , david , jones , & forman 1992 ; shapley et  al . 
   we also place constraints on the fraction of the diffuse xrb explained by galaxies lingering just below the cdf - n detection threshold , and thus the contribution to the xrb by normal galaxies . 
   spectroscopic redshifts for the galaxies are drawn from the catalogs of cohen et al . 
   ( 2000 ) , cohen ( 2001 ) , and dawson et al . 
   ( 2001 ) in the range @xmath30 . 
   spectroscopic redshift determination is difficult in the range @xmath31 due to the absence of strong features in the observed - frame optical band and the lack of the lyman break feature useful to identify higher redshift objects . 
   we have therefore used the deep photometric redshift catalog of fernndez - soto , lanzetta , & yahil ( 1999 ) for the redshift interval @xmath32 , which allows some overlap in redshift space with the spectroscopic catalogs for cross - checking . 
   the spectroscopic catalogs cover the entire hdf - n plus a substantial fraction of the  flanking fields region , whereas the photometric catalog only covers the hdf - n . 
   we shall refer to these two samples as the  spectroscopic sample " and the  photometric sample " throughout the rest of this letter .    for the spectroscopic sample , 
   the @xmath33-band magnitude was used to filter the sources by optical luminosity , as this is best matched to rest - frame @xmath2 over most of the redshift range under consideration here . 
   the @xmath33 magnitudes are those given in barger et al . 
   ( 1999 ) for the hawaii flanking fields area . for the photometric sample , the  f814w ( hereafter @xmath34 ) magnitudes of fernndez - soto et al . 
   ( 1999 ) were used . 
   we chose galaxies which had no x - ray detection within 40 in the 0.58  kev ( full ) , 0.52  kev ( soft ) and 28  kev ( hard ) bands down to a wavdetect ( freeman et al . 
   2002 ) significance threshold of @xmath35 in the restricted acis grade set of paper iv . 
   this low detection threshold ensures that our study does not include sources with x - ray emission just below the formal detection limits of paper  v.    we have attempted to construct a sample of galaxies similar to spiral galaxies in the local universe . to accomplish this , 
   we have used the morphological classes of van den bergh et  al . 
   ( 2001 ) for galaxies from @xmath36 in the hdf - n and the  flanking fields . to simplify the morphological filtering 
   , we have cast objects in the van den bergh et  al . 
   ( 2001 ) catalog into the following four classes : ( 1 )  e / s0 " and  e " , ( 2 )  merger " , ( 3 )  sa"sc " , including proto - spirals and spiral / irregulars , and ( 4 )  irr " ,  peculiar " and/or  tadpole " . 
   we then filtered the catalog to keep only classes ( 2 ) and ( 3 ) .    filtering the photometric sample is more difficult due to the faintness of many of the sources and problems due to morphological evolution with redshift . 
   we have used the spectral energy distribution ( sed ) classifications of fernndez - soto et al . 
   ( 2001 ) to exclude all galaxies of type  e " . 
   comparison of the source lists reveals that , within the area covered by both , @xmath37  70% of galaxies identified through the two methods are in common . 
   since the evolution of x - ray properties with redshift is of interest , we have made an effort to study objects with comparable optical luminosities at different redshifts . 
   this is particularly important due to the non - linear relationship between x - ray luminosity and @xmath2-band luminosity for some types of spiral galaxies ( @xmath38 ; e.g. fabbiano & shapley 2001 ) . using the value of @xmath39 in the @xmath40-band as determined by blanton et  al . 
   ( 2001 ) for a large sample of galaxies in the sloan digital sky survey , we determined the value of @xmath39 in the @xmath2-band . 
   the sloan filter @xmath40 is best matched to @xmath2 ; the resulting value of @xmath39 in the @xmath2 band is @xmath41 .    to ensure that our results are not sensitively dependent upon the galaxy sed used to determine the optical properties , we have used both the sa and sc galaxy seds of poggianti et al . 
   ( 1997 ) to calculate @xmath33 and @xmath34 vs. @xmath42 for an @xmath39 galaxy using the synthetic photometry package synphot in iraf . 
   these calculations are shown in figure  [ sample_definition ] . 
   note the close similarity between the sa and sc tracks ; this is because the @xmath33 band corresponds to rest - frame @xmath2 in the middle of our redshift range . 
   also plotted in figure 1 are the 151 galaxies in the spectroscopic sample with spiral or merger morphology having @xmath43 and the 651 galaxies in the photometric sample with sed class other than  e " having @xmath44 . 
   these galaxies were filtered by optical flux to lie within 1.5 mags of the @xmath39 galaxy tracks discussed above ; the galaxy samples constructed assuming sa and sc seds were identical ( or nearly so ) for all redshifts up to @xmath45 . 
   galaxies meeting the optical magnitude filter were then divided by redshift into several bins ; these bins were constructed to ensure that there were @xmath46 galaxies per bin . 
   the number of galaxies , median redshift , median look - back time and median optical magnitude for each bin are listed in table  1 . in figure  1 
   , we mark all the objects in the sc sed sample with colors indicating the different redshift bins . 
   table  1 also includes the number of galaxies rejected from each redshift bin due to the presence of an x - ray detection within 40 ; this exclusion radius ensures that our results will not be adversely affected by the wings of the psf of very bright x - ray sources . 
   these galaxies satisfied both the optical magnitude and morphology filtering constraints and were rejected only due to x - ray detection . 
   this exclusion criterion is very conservative , however , considering that our astrometry is accurate to @xmath476 in the area under consideration ( see paper  v ) . to allow for the off - nuclear nature of some of the x - ray sources found in normal galaxies ( e.g. , paper iv ) , we consider galaxies to be highly confident x - ray detections if the x - ray source is within 15 of the galaxy s center 
   this matching radius is also well matched to the chandra psf . 
   off - axis for 0.52 kev and the 83% encircled - energy radius for 0.58 kev . 
   ] we therefore also give the number of galaxies having an x - ray detection within 15 in table  1 . 
   the x - ray imaging data at each position were stacked in the same manner as in paper  vii , keeping the 30 pixels whose centers fall within an aperture of radius 15 . 
   the detection significance in each band was assessed by performing 100,000 monte - carlo stacking simulations using local background regions as in paper  vii . 
   a source is considered to be significantly detected if the number of counts over background exceeds that of 99.99% of the simulations . 
   no single source in the stacking sample appeared to dominate the distribution , demonstrating the effectiveness of our selection criteria . 
   stacking of the galaxies in the redshift bins described in table  [ sample_table ] and figure  [ sample_definition ] resulted in significant detections in the soft band for all of the redshift bins up to @xmath48 ( see table  2 ) . 
   we also stacked galaxies in the redshift bin @xmath49 , but there was not a significant detection . 
   the results for the two different spiral galaxy sed samples are nearly or exactly identical except for the detection in the highest redshift bin ( @xmath50 ) . 
   we adopt a @xmath51 power law for the calculation of x - ray fluxes and luminosities , assuming that these galaxies are similar to spiral galaxies in the local universe and have their x - ray emission dominated by x - ray binaries ( e.g. , kim , fabbiano , & trinchieri 1992 ) . 
   while there were several cases of significant detections in the full band , there were no highly significant detections in the hard band . 
   given the variation of effective area and background rate with energy , the signal - to - noise ratio for sources with the assumed spectrum is highest in the soft band and lowest in the hard band , so this behavior is expected . the flux level of the soft - band detections for the spectroscopic sample is ( 56)@xmath52 erg  @xmath6  s@xmath7 . the corresponding rest - frame 0.52  kev luminosities for the average galaxy are @xmath53 erg  s@xmath7 for the lowest redshift bin and @xmath54 erg  s@xmath7 for the highest redshift bin . 
   for the photometric sample , the soft - band flux level of the detections is ( 35)@xmath52 erg  @xmath6  s@xmath7 . 
   we also give fluxes for the less significant detections in the full band for those redshift bins having highly significant soft - band detections . 
   we have investigated the properties of the sources which were rejected from the stacking samples due to an x - ray detection at or near the position of the galaxy ( the numbers of such sources are given in the last column of table  1 ) . 
   there are only 15 distinct galaxies with x - ray detections within 15 . of these 15 sources , 
   three are broad - line agn , which are clearly not normal galaxies . 
   one object has a photometric redshift which differs significantly from its spectroscopic redshift . since the optical properties of this object at its spectroscopic redshift place it outside our sample boundaries 
   , we have rejected it . 
   one object is very near another x - ray source which has been positively identified with a narrow - line agn . 
   thus , there are a total of 10  normal " galaxies positively identified with x - ray sources within this sample , consituting a small minority of the galaxies under study here . 
   the worst case is in the lowest redshift bin where 15% of the galaxies had x - ray detections . 
   figure  [ sbhistogram ] shows a histogram of @xmath55 values calculated for both the stacking samples and the individually x - ray detected galaxies . 
   the individually x - ray detected galaxy set does possibly contain some lower - luminosity agn , including the agn candidate ( see papers ii and iv ) . with the exception of the objects in the redshift bin with median @xmath56 , 
   figure  [ sbhistogram ] shows that typically the x - ray luminosities of the individually detected objects are on average much higher than those of the stacked galaxies ; they are sufficiently more luminous as to appear atypical of the normal galaxy population . for the lowest redshift bin , 
   it is plausible that our results are moderately biased by the exclusion of the x - ray detected sources . in  4 , we therefore also give results which include the individually x - ray detected objects in the sample average for this lowest redshift bin .    for additional comparison 
   , we have considered the radio properties of the individually detected galaxies and the stacked galaxies using the catalogs of richards et  al . 
   ( 1998 ) and richards ( 2000 ) . 
   the percentage of radio detections among the individually x - ray detected galaxies is higher than that among the stacked galaxies ( @xmath57% vs. @xmath585% ) . 
   this possible difference between the two populations is significant at the 93% level as determined using the fisher exact probability test for two independent samples ( see siegel & castellan 1988 ) . due to the x - ray luminosity difference and possible difference in radio properties , and to the fact that these galaxies constitute a small minority of those under study 
   , we are confident we have not biased our determination of the properties of the typical galaxy by omitting these x - ray detected objects from further consideration . 
   in figure  [ lxlb]a , we plot the x - ray - to - optical luminosity ratio @xmath59 for each stacked detection , where @xmath13 is calculated for the rest - frame 0.52  kev band . 
   we have also plotted our approximate sensitivity limit in figure  [ lxlb]a , which is simply the corresponding @xmath60 x - ray luminosity detection limit achieved for a 30  ms stacking analysis divided by @xmath61 . 
   we do not expect to detect galaxies having less x - ray emission per unit @xmath2-band luminosity than this value . 
   for comparison with the local universe , we also plot the mean @xmath59 for spiral galaxies of comparable @xmath62 from the sample of shapley et  al . 
   this sample includes 234 spiral galaxies observed with _ 
   einstein _ and excludes agn where the x - ray emission is clearly dominated by the nucleus . 
   the galaxies in the shapley et al . 
   ( 2001 ) sample all have @xmath63 ; median redshift is @xmath64 .    in figure  [ lxlb]b , 
   we plot @xmath59 versus @xmath65 ; the values up to @xmath66 are consistent with what is observed in the shapley et  al . 
   sample for objects with comparable @xmath65 , although they are toward the high end of what is observed . 
   this is consistent with figure [ lxlb]a , which shows the average @xmath59 derived from stacking being somewhat higher than that for the shapley et  al . 
   galaxies of comparable optical luminosity . 
   there is a slight increase ( factor of 1.6 ) in the average @xmath67 from the local universe to @xmath68 . for the highest redshift bin ( @xmath50 ) , 
   the results become somewhat sensitive to the galaxy sed one assumes for determining the optical properties . 
   we suspect that an sc galaxy sed is more appropriate at this epoch due to the higher prevalence of star formation . 
   adopting this sed , we find that @xmath69 has increased somewhat more substantially ( @xmath70 times ) at @xmath50 . 
   one may also constrain star - formation models using only the x - ray properties of these galaxies . 
   the average x - ray luminosity of the spiral galaxies in the shapley et  al . 
   ( 2001 ) sample having the same range of @xmath65 as used in this study is @xmath71  erg  s@xmath7 ( converted to 0.52 kev ) . 
   the average galaxy in our stacking sample has a luminosity @xmath72 times higher at @xmath68 . 
   this increases to @xmath73 times higher at @xmath74 . 
   the @xmath68 value is most likely affected by some bias due to the exclusion of legitimate normal galaxies in the lowest redshift bin ( see  3 ) . 
   if we include these individually x - ray detected objects , then the average galaxy in our stacking sample has a luminosity @xmath75 times higher at @xmath68 , consistent with the predictions of ghosh & white ( 2001 ) that the x - ray luminosity of the typical sa - sbc spiral galaxy should be @xmath75 times higher at @xmath76 . including the x - ray detected galaxies does not significantly affect our results for the interval @xmath77 ( the difference is @xmath78% ) . 
   however , since the x - ray luminosities of the x - ray detected objects with @xmath74 are substantially higher ( by an order of magnitude ) than the x - ray stacking averages ( see figure  2 ) , it is not appropriate to include these objects in the calculation of the average x - ray luminosity . 
   we thus find a smaller increase in the average x - ray luminosity of galaxies at @xmath79 than the increase by a factor of @xmath80 predicted by ghosh & white ( 2001 ) . 
   we find that the average x - ray luminosities of galaxies have not evolved upward by more than a factor of @xmath73 by @xmath74 , regardless of exclusion or inclusion of x - ray detected objects . 
   the range of average 0.52  kev fluxes for the spiral galaxies studied here is ( @xmath46@xmath5 erg  @xmath6  s@xmath7 . 
   these x - ray fluxes are consistent with independent predictions made by ptak et al . 
   ( 2001 ) that galaxies of this type will be detected at 0.52 kev flux levels of @xmath81 erg  @xmath6  s@xmath7 ( converted from their 210 kev prediction assuming a @xmath82 power law ) .    assuming a 0.52  kev xrb flux density of @xmath83  erg  @xmath6  s@xmath7  deg@xmath84 ( garmire et al . 
   2001 ) , we have identified @xmath85% of the soft xrb as arising from spiral galaxies not yet individually detected in deep  surveys . 
   many of these objects should be sufficently bright to be detected with  acis exposures of @xmath86  ms , which should be achievable in the next several years of the mission . 
   we thank alice shapley for useful discussions and sharing data . 
   we gratefully acknowledge the financial support of nasa grant nas  8 - 38252 ( gpg , pi ) , nasa gsrp grant ngt5 - 50247 ( aeh ) , nsf career award ast-9983783 ( wnb , dma , feb ) , and nsf grant ast99 - 00703  ( dps ) . 
   this work would not have been possible without the enormous efforts of the entire  team . 
   ccccccc + 0.400.75 & sc & 0.635 & 6.34 & 21.73 & 29 & 12/6 + 0.750.90 & sc & 0.821 & 7.39 & 21.83 & 38 & 2/2 + 0.901.10 & sc & 0.960 & 8.05 & 22.29 & 30 & 6/4 +   +   + 0.501.00 & sc & 0.920 & 7.87 & 22.99 & 37 & 9/4 + 1.001.50 & sc & 1.200 & 8.97 & 24.28 & 64 & 6/2 + 1.001.50 & sa & 1.240 & 9.09 & 24.69 & 80 & 8/2 +    rrrrrrrrrcrrc + 0.400.75 & sc & 49.6 & 31.7 & 99.99 & @xmath87 & 25.41 & 25.44 & 1.52 & 0.65 & 2.94 & 1.26 & 0.67 + 0.750.90 & sc & 58.0 & 36.4 & @xmath87 & @xmath87 & 33.05 & 33.09 & 1.36 & 0.57 & 4.92 & 2.07 & 1.33 + 0.901.10 & sc & 54.4 & 25.9 & @xmath87 & @xmath87 & 26.54 & 26.56 & 1.60 & 0.51 & 8.44 & 2.68 & 1.40 +   +   + 0.501.00 & sc & 42.0 & 33.5 & 99.77 & @xmath87 & 34.51 & 34.54 & 0.95 & 0.51 & 4.51 & 2.40 & 1.12 + 1.001.50 & sc & 44.8 & 38.5 & 98.94 & @xmath87 & 59.67 & 59.73 & 0.58 & 0.34 & 5.34 & 3.06 & 0.79 + 1.001.50 & sa & 45.6 & 34.2 & 98.29 & 99.95 & 74.61 & 74.68 & 0.48 & 0.24 & 4.70 & 2.36 & 0.81 +    . 
   the blue curves give @xmath33 vs. @xmath42 for @xmath39 sa and sc galaxies ( the lower curve at higher redshift is for the sa galaxy ) . galaxies without a colored circle were not included in the stacking sample because they either were not within the range of optical luminosity specified or because an x - ray detection was found within 40 . 
   ( b ) the black filled circles are the photometric redshift sample of fernndez - soto et  al . 
   ( 1999 ) , excluding the  e "- type galaxies . 
   the blue curves give @xmath34 vs. @xmath42 for @xmath39 sa and sc galaxies ( the lower curve at higher redshift is for the sa galaxy ) . 
   [ sample_definition ] ]       as a function of redshift for the stacking samples . 
   the redshift error bars indicate the full extent of the redshift bin ; the data points are at the median redshift value for that bin . 
   the solid line indicates the 2@xmath88 x - ray sensitivity limit normalized by @xmath89 . 
   the dashed lines above and below the solid line indicate the effect of decreasing and increasing the optical luminosity by one magnitude , respectively . 
   objects which have less x - ray luminosity per unit @xmath2-band luminosity than this are not expected to be detected in the current data . 
   the error bar on the shapley et al . 
   ( 2001 ) data point indicates the dispersion of values in this sample . 
   ( b ) @xmath59 vs. @xmath65 for both the shapley et al . 
   ( 2001 ) sample ( open circles ) and the stacked detections presented here . 
   the error bars on @xmath59 in both figures were calculated following the numerical method described in  1.7.3 . of lyons ( 1991 ) . 
   the solid line in ( b ) indicates @xmath89 ; again the dashed lines correspond to one magnitude fainter and brighter than @xmath89 .
summary: we present a statistical x - ray study of spiral galaxies in the hubble deep field - north and its flanking fields using the chandra deep field  north 1  ms dataset . 
   we find that @xmath0 galaxies with @xmath1 have ratios of x - ray to @xmath2-band luminosity similar to those in the local universe , although the data indicate a likely increase in this ratio by a factor of @xmath33 . 
   we have also determined that typical spiral galaxies at @xmath1 should be detected in the 0.52 kev band in the flux range ( @xmath46@xmath5 erg  @xmath6  s@xmath7 . 
   1_heao-1 _