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the early pioneering sub - millimeter surveys in lensing clusters ( smail , ivison & blain 1997 ) and in blank fields ( hughes et al . 1998 , barger et al . 1998 ) demonstrated the feasibility of deep extragalactic surveys exploiting the favorable k - corrections in the sub - millimeter . deep @xmath11 m imaging has now resolved around half of the @xmath11 m extragalactic background ( hughes et al . 1998 , blain et al . 1999 , cowie , barger & kneib 2002 ) . these galaxies are often called scuba galaxies after the instrument with which they were first detected ( holland et al . the @xmath20 @xmath11 m scuba beam makes identifications at other wavelengths difficult ; nevertheless , @xmath21 of @xmath11 m sources are identifiable in @xmath22jy @xmath23ghz imaging ( ivison et al . 2002 ) . these radio identifications have led to optical identifications , morphologies and ultimately spectroscopic redshifts in multiwavelength follow - up campaigns ( e.g. chapman et al . furthermore , the non - detection of scuba galaxies in hard x - ray imaging ( e.g. alexander et al . 2003 ) suggests that the bulk of the population has far - infrared luminosities dominated by star formation . the morphologies , redshifts , clustering and molecular gas contents are so far consistent with at least some of the scuba population being the progenitors of giant elliptical galaxies ( e.g. dunlop 2002 ) , though other alternatives are still viable ( efstathiou & rowan - robinson 2003 ) and the scuba population is heterogeneous ( e.g. ivison et al . 1998 , 2000 ) . finally , the k - correction effects in the sub - millimeter make the sub - millimeter extragalactic background sensitive to contributions from the far - infrared luminous energy densities at all redshifts @xmath24 . the populations which contribute to the @xmath11 m extragalactic background are necessarily also significant contributors to the cosmic history of dust - shrouded star formation .    following the _ iras _ mid - infrared surveys of the local universe ( e.g. rush , malkan & spinoglio 1993 ) , the mid - infrared was first made accessible to deep extragalactic surveys by the infrared space observatory ( _ iso _ , kessler et al . 1996 ) which conducted a suite of surveys with a variety of depths and areal coverages ( e.g. genzel & cesarsky 2000 and refs . therein , rowan - robinson et al . 2004 ) . the rapid upturn in the @xmath25 m extragalactic source counts clearly demonstrated the existence of a strongly evolving population of obscured starbursts and active galaxies ( e.g. serjeant et al . 2000 , franceschini et al . 2001 , gruppioni et al . it has also been argued that the populations dominating the @xmath25 m extragalactic background light , which are resolved by _ iso _ , are also largely the same populations which dominate the unresolved @xmath26 m background ( elbaz et al . if correct , this is a significant breakthrough in determining the populations which dominated the far - infrared luminous energy density throughout the relatively recent history of the universe ( e.g. @xmath27 ) .    finding the population that supplies the luminous energy density at @xmath28 requires understanding the sub - millimeter background light . however , it has been difficult to find sub - millimeter source counterparts in the mid - infrared . very few sub - millimeter - selected sources have been detected by _ iso _ in the mid - infrared ( e.g. eales et al . 2000 , webb et al . 2003b , sato et al . the reverse procedure of looking for mid - infrared sources in the sub - millimeter via stacking analyses have not fared much better . serjeant et al . ( 2003a ) found no excess @xmath11 m flux at the locations of @xmath25 m sources in the hdf north . lyman break galaxies , in contrast , are detectable statistically ( e.g. peacock et al . 2000 , webb et al . if scuba galaxies are extreme star - forming galaxies in the most massive high - redshift halos , then their anomalously faint k - band identifications imply heavy obscuration in the observed - frame near - infrared ( serjeant et al . 2003b ) , suggesting that scuba galaxies may be detectable in @xmath22jy - level mid - infrared imaging .    the _ spitzer space telescope _ ( hereafter _ spitzer _ , werner et al . 2004 ) is an enormous advance over _ iso _ in terms of mapping speed , sensitivity , and wavelength coverage . it may now be possible to resolve the bulk of the extragalactic background light at @xmath3 m in exposures of the order @xmath29 minutes , an equivalent depth achieved at @xmath30 m in @xmath31 hours with _ iso _ by sato et al . 2003 . in this paper we present statistical sub - millimeter detections of galaxies selected in a new _ spitzer _ survey . the _ spitzer _ identifications of previously - published sub - millimeter sources are discussed by egami et al . identifications of mambo sources are discussed by ivison et al . the _ spitzer _ early release observations survey is one of the first extragalactic surveys conducted by _ spitzer_. besides the pragmatic goal of characterising the survey capabilities of the facility , the survey has the science goals of making the first constraints on the populations which dominate the extragalactic backgrounds in the shorter wavelength _ spitzer _ bands , and the links between these galaxies and other known populations . accordingly , the survey field was selected to lie in the lockman hole , an area with abundant multi - wavelength survey coverage , and in particular with @xmath32 galaxies from the @xmath11 m 8mjy survey @xmath33 catalog ( scott et al . 2002 , fox et al . 2002 , ivison et al . 2002 ; see below ) . the _ spitzer _ imaging is described by egami et al . ( 2004 ) and huang et al . in summary , irac ( fazio et al . 2004 ) imaged a @xmath34 field for @xmath35s per sky pixel at all four bands , resulting in median @xmath36 depths of @xmath37jy , @xmath38jy , @xmath39jy and @xmath40jy at @xmath14 m , @xmath15 m , @xmath6 m and @xmath7 m respectively , and sources were extracted to @xmath41 . mips ( rieke et al . 2004 ) observed the field in photometry mode at @xmath5 m for @xmath42s per sky pixel , resulting in a typical @xmath36 depth of @xmath43jy . source confusion and blending make the mips completeness and reliability more problematic than for scuba or irac . mips source catalogs were extracted to @xmath44jy ( @xmath45 complete , @xmath46 reliable ) and to @xmath47jy ( @xmath48 complete , @xmath48 reliable ) . we conservatively assume the catalogs are all extragalactic , as expected at these flux densities ; the deletion of any ( hypothetical ) contaminant population of galactic stars with negligible sub - millimeter fluxes would improve the confidence levels of our statistical detections .    the scuba 8mjy survey data ( which covers all our _ spitzer _ field ) is described in detail by scott et al . ( 2002 ) . we have reanalysed this data , modelling the intra - night scuba gain variations , improving the extinction corrections using improved fits to the @xmath49ghz skydips monitored at the caltech sub - millimeter observatory , and removing the cross - talking @xmath12 m bolometers a7 and a16 ( see mortier et al . additional sources are detectable in the zero - sum chopped / nodded sub - millimeter maps . after point source detection using noise - weighted psf convolution ( serjeant et al . 2003a ) including the effect of negative chop / nod positions , the median @xmath36 depth ( total flux ) in the _ spitzer _ early release observations field is @xmath50mjy at @xmath11 m , and @xmath51mjy at @xmath12 m . our reanalysis of the scuba 8mjy survey uncovered new candidate sub - millimeter sources ( mortier et al . figure [ fig : ids ] shows the _ spitzer _ identifications of the most significant of these , at @xmath52 @xmath53 @xmath54 , @xmath55 @xmath56 @xmath57 ( j2000 ) , detected with a @xmath58 @xmath11 m flux of @xmath59mjy . at @xmath12 m the source is not detected , though the @xmath12 m flux at its position is @xmath60mjy ( @xmath61 ) ; there is also a hint of positive flux at @xmath62 significance at @xmath63 m ( greve et al . 2004 ) at the position of this source . there are hints that the source is extended at @xmath11 m , suggesting a blend of more than one source , and indeed there are two candidate identifications in the _ spitzer _ imaging . the probabilities of a random association ( @xmath64 where @xmath65 is the surface density of catalogued objects brighter than the identification at distance @xmath66 ) are @xmath67 and @xmath68 for the brighter and fainter @xmath15 m flux respectively . interestingly , the low - significance contours of @xmath12 m emission are coincident with the brighter of the two candidates . the _ spitzer _ fluxes of this source are @xmath69 , @xmath70 , @xmath71 , @xmath72jy and @xmath73jy at @xmath14 m , @xmath15 m , @xmath6 m , @xmath7 m and @xmath5 m respectively . the fluxes of the second candidate identification are @xmath74jy and @xmath75jy at @xmath14 m , and @xmath15 m , but the source is not detected at longer wavelength _ spitzer _ bands . while deblending the fluxes from the two identifications is beyond the scope of this paper , it is interesting to note that either identification would have a sub - millimeter:_spitzer _ flux ratio which is redder than in the population as a whole , derived below . the @xmath33 sub - millimeter sources are already known to have _ identifications ( egami et al . 2004 ) , so we construct a mask to remove the sources at these positions from the stacking analysis . at @xmath11 m we masked a @xmath76 radius ( @xmath77 the scuba fwhm , i.e. the fwhm of point sources in our psf - convolved maps ) around the @xmath33 @xmath11 m point sources from scott et al . ( 2002 ) , as well as the new point sources from our reanalysis ( mortier et al . 2004 ) . at @xmath12 m the masking radius was @xmath78 , which we also applied to new @xmath12 m sources from our reanalysis . we also masked regions with high noise levels , arbitrarily selected as @xmath79mjy at @xmath11 m , or @xmath80mjy at @xmath12 m , and also masked all regions in the sub - millimeter maps without _ spitzer _ data . the unmasked area at @xmath11 m ( @xmath12 m ) is @xmath81 ( @xmath82 ) arcmin@xmath83 for the irac catalogues . for mips , the corresponding area is @xmath84 ( @xmath85 ) arcmin@xmath83 . the sub - millimeter maps give the best - fit point source flux at each location ( equation a4 of serjeant et al . 2003a ) , so we measure the values of the sub - millimeter images at the _ spitzer _ galaxy positions . figure [ fig : histograms ] shows the sub - millimeter signal - to - noise ratios ( equation a6 of serjeant et al . 2003a ) at the positions of the _ spitzer _ early release observations source catalogs at @xmath6 m and @xmath7 m , which lie in the unmasked regions . these figures also show histograms for the whole of the unmasked maps . note the clear positive skews in the sub - millimeter fluxes at the _ spitzer _ source positions , relative to the maps as a whole . we used kolmogorov - smirnov tests to determine the confidence level of these relative positive skews , listed in table  [ tab : coadd_results ] . there are significant detections at @xmath6 m and @xmath7 m , and marginal detections at @xmath5 m . no significant skew was found at @xmath14 m or @xmath15 m . we also calculated the mean fluxes in the sub - millimeter maps at the positions of the _ spitzer _ galaxies . these are almost all positive and are listed in table [ tab : coadd_results ] . this statistic is less efficient than the kolmogorov - smirnov test ( which does not just use the first moment ) , so the uncertainties on the mean fluxes are larger than suggested by the kolmogorov - smirnov confidence levels . the _ spitzer _ galaxies are individually undetected in the sub - millimeter , so one must be careful in interpreting the stacking analysis ; for example , we do not use the _ noise - weighted _ mean fluxes because they can give false positives , as noted by serjeant et al . also , many fluctuations in the sub - millimeter maps are blends ( e.g. scott et al . 2002 ) , so is there a risk of overestimating the flux of any given _ galaxy by also counting its neighbors ? the answer is no , as peacock et al . ( 2000 ) showed : for a poissonian sampling of the sub - millimeter map , the expected total sub - millimeter flux from all neighbors equals the mean flux of the map , which is exactly zero for the chopped , nodded scuba maps . ( the chopped / nodded psf is also zero - sum , so the psf - convolved images are still zero - sum . ) blending slightly degrades the uncertainty on the mean , but it does not affect the mean fluxes . another demonstration of this is to note that the total flux of any given source is exactly zero in these chopped , nodded maps . even confusing sources within the same scuba beam will have no _ net _ effect , provided that there are also similar sources uniformly distributed over the rest of the map . the histograms of the whole map in figure [ fig : histograms ] effectively act as the control sample . nevertheless , as a test of the stacking analysis methodology , we performed the same tests on simulated _ spitzer _ catalogs of the same size as the observed catalogs , with a poissonian distribution in the unmasked regions . no positive skews in the sub - millimeter flux distributions were detected relative to the maps as a whole . these simulations also verified that the mean sub - millimeter flux around randomly chosen positions is consistent with zero . although our _ spitzer _ @xmath6 m and @xmath7 m catalogues are small ( @xmath8 and @xmath9 respectively ) , we can use our statistical detections to obtain constraints on the contribution to the sub - millimeter extragalactic background light from the _ spitzer _ populations , by multiplying the mean sub - millimeter flux of a single _ spitzer _ galaxy ( table [ tab : coadd_results ] ) with the observed surface density of _ spitzer _ galaxies . at @xmath11 m , the integrated contribution from the @xmath6 m population is @xmath16 nw m@xmath17 sr@xmath18 , and from the ( overlapping ) @xmath7 m population @xmath86 nw m@xmath17 sr@xmath18 . this is already comparable to the total @xmath11 m extragalactic background reported by lagache et al . ( 2000 ) of @xmath87 nw m@xmath17 sr@xmath18 . at @xmath12 m the _ spitzer _ contributions can , within the uncertainties , account for all the observed @xmath88 nw m@xmath17 sr@xmath18 extragalactic background : @xmath19 nw m@xmath17 sr@xmath18 from the @xmath6m - selected sample , and @xmath89 nw m@xmath17 sr@xmath18 from the @xmath7 m galaxies . our sub - millimeter faint source density is consistent with those in the deepest @xmath11 m maps ( cowie et al . 2002 ) .    the sub - millimeter:_spitzer _ flux ratios in sub - millimeter point sources ( egami et al . 2004 , see above ) are much higher than average in the population ( table [ tab : coadd_results ] ) . despite the commonality between the scuba and _ spitzer _ galaxy populations , sub - millimeter galaxies are not at all representative of the _ spitzer _ population . the sub - millimeter background independently suggests the same result : if the @xmath5 m population had sub - millimeter:_spitzer _ flux ratios as high as scuba galaxies ( e.g. median @xmath90 for the egami et al . ( 2004 ) sample ) , the @xmath44jy @xmath5 m sources alone would over - predict the @xmath11 m background by @xmath91 . our weak sub - millimeter detection of the @xmath5 m population is less surprising in that context . one possible interpretation is that the scuba population has fewer ( or more heavily obscured ) active nuclei than the _ spitzer _ population as a whole . is our stacking analysis signal due to dusty galaxies , or to high-@xmath92 galaxies , or both ? our samples are too small to distinguish these possibilities , but stacking analyses of subsets show that either option is credible . almost all the @xmath6 m galaxies are detected at @xmath14 m , and as with the @xmath6 m population as whole ( table [ tab : coadd_results ] ) this subset ( @xmath93 galaxies ) is detected statistically . the @xmath94 remaining @xmath14 m galaxies are not detected in the stacking analyses ( e.g. , only @xmath95 confidence in the @xmath12 m signal : noise maps ) , so the redder galaxies are more likely to be sub - millimeter emitters . the lack of a stacking signal at @xmath14 m and @xmath15 m is at least in part due to dilution of the signal from the high density of bluer galaxies detected at these wavelengths . the red colors could be due to heavy obscuration , but the colors also become redder quickly with increasing redshift . simpson & eisenhardt ( 1999 ) show that @xmath96 or @xmath97 selects redshifts @xmath98 . there are @xmath99 galaxies satisfying either criteria , with detections in the relevant bands , and these galaxies are detected in the stacking analysis ( e.g. @xmath100 confidence at @xmath12 m ) ; the corresponding @xmath101 foreground @xmath102 galaxies are not detected ( e.g. only @xmath103 confidence at @xmath12 m ) . further work on larger samples is needed to discriminate between high - redshift and high - obscuration sub - populations . the sub - millimeter backgrounds are sensitive to contributions from the far - infrared luminous energy densities throughout the redshift range of favorable k - corrections . it follows that the _ spitzer _ @xmath6 m and @xmath7m - selected galaxies are necessarily significant contributors to the comoving volume averaged star formation density . many of the @xmath6 m and @xmath7 m galaxies are challenging targets for 8 - 10m - class optical / near - infrared spectroscopy , but redshift surveys of these populations , together with 2-d and 3-d clustering , would discriminate between competing semi - analytic descriptions of galaxy evolution , and determine whether ( or which ) scuba/_spitzer _ galaxies are the sites of the assembly of giant ellipticals . finally , it is worth stressing that the conclusions in this paper could only have been reached with co - ordinated multi - wavelength surveys . llllllllllll wave- & @xmath12 m & @xmath12 m & @xmath12 m & @xmath104 & @xmath105 & @xmath11 m & @xmath11 m & @xmath11 m & @xmath106 & @xmath107 + length & @xmath108 & k - s ( s : n ) & k - s ( flux ) & mjy & @xmath109 & @xmath108 & k - s ( s : n ) & k - s ( flux ) & mjy & @xmath109 + @xmath14 m & @xmath110 & @xmath111 & @xmath112 & @xmath113 & @xmath114 & @xmath115 & @xmath116 & @xmath117 & @xmath118 & @xmath119 + @xmath15 m & @xmath120 & @xmath121 & @xmath122 & @xmath123 & @xmath124 & @xmath125 & @xmath126 & @xmath127 & @xmath128 & @xmath129 + @xmath6 m & @xmath8 & @xmath130 & @xmath131 & @xmath132 & @xmath133 & @xmath134 & @xmath135 & @xmath136 & @xmath137 & @xmath138 + @xmath7 m & @xmath9 & @xmath131 & @xmath139 & @xmath140 & @xmath141 & @xmath141 & @xmath142 & @xmath143 & @xmath144 & @xmath145 + @xmath5m@xmath146 & @xmath147 & @xmath148 & @xmath149 & @xmath150 & @xmath151 & @xmath152 & @xmath153 & @xmath154 & @xmath155 & @xmath156 + @xmath5m@xmath157 & @xmath158 & @xmath159 & @xmath160 & @xmath161 & @xmath162 & @xmath114 & @xmath163 & @xmath164 & @xmath165 & @xmath166 +
we present sub - millimeter statistical detections of galaxies discovered in the @xmath0 spitzer early release observations ( to @xmath1jy @xmath2 at @xmath3 m , @xmath4jy at @xmath5 m ) through a stacking analysis of our reanalysed scuba 8mjy survey maps , and a spitzer identification of a new sub - millimeter point source in the 8mjy survey region . for sources detected at @xmath6 m or @xmath7 m ( @xmath8 and @xmath9 sources respectively ) , 
 we detect positive skews in the sub - millimeter flux distributions at @xmath10 confidence using kolmogorov - smirnov tests , at both @xmath11 m and @xmath12 m . we also marginally detect the _ spitzer _ @xmath5 m galaxies at @xmath11 m at @xmath13 confidence , and place limits on the mean sub - millimeter fluxes of the @xmath14 m and @xmath15 m sources . integrating the sub - millimeter fluxes of the _ spitzer _ populations , we find the @xmath6 m galaxies contribute @xmath16 nw m@xmath17 sr@xmath18 to the @xmath11 m background , and @xmath19 nw m@xmath17 sr@xmath18 to the @xmath12 m background ; similar contributions are made by the @xmath7m - selected sample . 
 we infer that the populations dominating the @xmath6 m and @xmath7 m extragalactic background light also contribute around a quarter of the @xmath11 m background and the majority of the @xmath12 m background .