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Paper text:
the origin of the cosmic x - ray background ( xrb ) has been a puzzle for over 35 years , but now there is strong evidence that this will be explained by a large population of absorbed agn . if these are to explain the bump in the xrb spectrum at 30kev , however , the total energy output of this population must exceed that of broad - line agn by at least a factor of @xmath1 , with wide ranging implications @xcite .
this hidden population could also explain the apparent discrepancy between the black hole densities predicted by ordinary qsos and recent observations of local galaxy bulges @xcite , @xcite .    deep x - ray surveys with chandra and xmm will soon test this obscured agn hypothesis , although existing surveys with rosat , asca and bepposax have already revealed what could be the ` tip of the iceberg ' of this population .
several unambiguous cases of obscured qsos have been detected ( e.g. @xcite , @xcite , @xcite , @xcite , @xcite ) while at the faintest x - ray fluxes there is growing evidence for a large population of x - ray luminous emission - line galaxies , many of which show clear evidence for agn activity @xcite,@xcite,@xcite,@xcite .
arguably the most convincing evidence for the obscured agn model came from the ultra - deep survey of hasinger , schmidt et al ( @xcite , @xcite ) . using the keck telescope to identify sources from the deepest x - ray observation
ever taken they found that most of these x - ray galaxies could be classified as agn .
exciting new work has also been undertaken at harder energies with asca and beppo - sax ( @xcite , @xcite , @xcite ) resolving @xmath2 of the @xmath3kev xrb .
increasing numbers of these sources have been identified with absorbed agn .
deep sub - mm observations offer the potential to revolutionise our understanding of the high redshift universe . beyond 100@xmath4 m ,
both starburst galaxies and agn show a very steep decline in their continuum emission , which leads to a large negative k - correction as objects are observed with increasing redshift .
this effectively overcomes the ` inverse square law ' to pick out the most luminous objects in the universe to very high redshift @xcite .
since the commissioning of the scuba array at the james clerk maxwell telescope a number of groups have announced the results from deep sub - mm surveys , all of which find a high surface density of sources at @xmath5 m ( @xcite , @xcite , @xcite , @xcite , @xcite ) .
the implication is the existence of a large population of hitherto undetected dust enshrouded galaxies . in particular ,
the implied star - formation rate at high redshift ( @xmath6 ) is significantly higher than that deduced from uncorrected optical - uv observations , roughly a factor of two higher than even the dust - corrected version of the optically derived star - formation history @xcite .
the recent detections of the far - infrared / sub - mm background by the dirbe and firas experiments provide further constraints , representing the integrated far - infrared emission over the entire history of the universe ( @xcite , @xcite , @xcite ) .
since most of this background has now been resolved into discrete sources by scuba , the implication is that most high redshift star - forming activity occurred in rare , exceptionally luminous systems .    in other words , at high redshift ulirg - like starburst galaxies dominate the cosmic energy budget , in stark contrast to the situation today where ( to quote andy lawrence ) ` ulirgs are little more than a spectacular sideshow ' @xcite .
considerable excitement has been generated recently by the possibility that many of these scuba sources could be agn .
whether these agn are actually heating the dust is another matter ( see section 5 ) but there are now several independent lines of argument which suggest that agn are present in a significant fraction of these scuba sources . at the very least , the implication is that much of the star formation in the high redshift universe occurred in galaxies containing active quasars .
first we present the arguments predicting an agn contribution , followed by recent observational evidence .      * * the analogy with ulirgs : * in many ways a significant agn fraction would not be a surprise .
the scuba sources are exceptionally luminous systems , essentially the high redshift equivalents to local ulirgs . at the luminosities of the scuba sources ( @xmath7 ) we note that a least @xmath8 of local ulirgs show clear evidence for an agn @xcite . * * predictions based on agn luminosity functions : * one can estimate the agn contribution by transforming the x - ray luminosity function to the sub - mm waveband using a template agn sed .
we estimate that @xmath9 of the sources in recent scuba surveys could contain agn , perhaps higher if one allows for compton - thick objects @xcite .
major sources of uncertainty are in the extrapolation of local agn seds to high redshift , the dust temperature and in assuming the same underlying luminosity function for obscured agn .
note that an independent but very similar analysis by manners et al ( in preparation ) predicts a lower fraction ( @xmath10 ) .
* * re - radiating the absorbed energy : * another approach , also based on obscured agn models for the xrb , does not rely on uncertain seds but instead on thermally re - radiating the absorbed energy directly in the far - infrared @xcite .
this method also predicts a significant agn fraction among the scuba sources ( @xmath11 ) although the exact prediction is strongly dependent on the assumed dust temperature . *
* scuba observations of high - z quasars : * observations of the most luminous , very high redshift quasars ( @xmath12 ) suggest that many are exceptionally luminous in the far - infrared / sub - mm , with sub - mm luminosities comparable to arp 220 @xcite . whether this emission is due to dust heated by the quasar or associated with starburst activity
is unclear , but the lack of any correlation between the quasar power and the sub - mm luminosity would favour a starburst origin for the far - infrared emission . if one assumes that all high redshift quasars have similar sub - mm luminosities this would lead to a very large agn fraction in the deep scuba surveys ( @xmath13 ) . in reality
, however , some weak correlation between quasar power and the associated starburst is likely to exist , which would reduce this fraction significantly .
further sub - mm observations of high - z quasars are required to investigate these correlations .      * * the seds of detected scuba sources * : a recent analysis of the multi - wavelength spectral energy distributions ( seds ) of scuba sources has suggested that @xmath14 are likely to be agn @xcite . * * spectroscopic identification : * although the identification of many scuba deep survey sources remains elusive ( perhaps indicating their exceptionally high redshift ) there are growing indications that a significant fraction harbour agn .
the first clear - cut identification turned out to be an obscured qso @xcite and since then various surveys have been able to place limits on the agn fraction . from a study of @xmath15 sub - mm sources , barger and collaborators
have placed a lower limit of @xmath16 on the fraction showing evidence for agn activity @xcite .
of seven sub - mm sources studied in detail by ivison et al , at least @xmath17 show evidence for an agn @xcite .
these estimates are in good agreement with the predictions of the various models outlined above .
forthcoming deep x - ray surveys with chandra and xmm will push significantly fainter than ever before . in the soft x - ray band ,
we expect to reach at least an order of magnitude fainter than the deepest rosat surveys , while in the hard x - ray band the improvement is even more dramatic ( figure 1 )
. the puzzle of the xrb should therefore soon be solved , and we expect to detect large numbers of obscured agn and study their properties in detail .
in addition , we will be able to detect typical quasars to very high redshift ( @xmath18 ) and hence assess the importance of quasar activity during those early epochs .
these deep x - ray observations are potentially ideal for identifying agn in sub - mm surveys .
the source densities expected are very similar ( @xmath19 deg@xmath20 ) and hence with the resolution of chandra in particular it will be possible to pick out the agn directly from their x - ray flux , with very little confusion .
the @xmath21mjy survey of the uk scuba consortium is ideal for such a study ( figure 2 ) .
this is the only wide area scuba survey being conducted , covering @xmath22 square arcminutes in @xmath23 contiguous regions .
one of these regions is in the lockman hole ( which will be covered by chandra in pv time ) .
the other is concentrated on the n2 region of the elais survey , where we have deep chandra and xmm observations planned ( pi : almaini ) .
we will be able to detect the hard x - ray emission from the hidden agn _ and _ obtain a measurement of the absorbing column .
if the equivalent width is high enough , in some cases xmm may even allow a redshift determination from an iron line detection
several independent arguments now point to the conclusion that a significant fraction ( @xmath24 ) of the luminous sub - mm sources detected by scuba will contain agn .
this points to a very important link between x - ray astronomy and the newly emerging sub - mm field , both of which provide probes of the obscured , high redshift universe .
if a significant agn fraction is confirmed with forthcoming chandra / xmm surveys , considerable uncertainties will still remain . is the dust heated by the agn or by stellar processes ?
if the agn is responsible , and their contribution is large , the recent conclusions about star - formation at high redshift may require significant revision .
on the other hand the dust may be largely heated by stellar activity ( see @xcite ) but with the interesting implication that much of the star formation at high redshift occurred in galaxies containing active quasars .
it has recently been postulated that perhaps all quasars could go through an obscured phase during the growth of the black hole , a process which could be intimately linked with the formation of the galaxy bulge itself ( see @xcite ) .
future surveys combining x - ray and sub - mm observations will provide a powerful tool for disentangling these processes .
it is widely believed that most of the cosmic x - ray background ( xrb ) is produced by a vast , hitherto undetected population of obscured agn . 
 deep x - ray surveys with chandra and xmm will soon test this hypothesis . 
 similarly , recent sub - mm surveys with scuba have revealed an analogous population of exceptionally luminous , dust - enshrouded _ star - forming _ galaxies at high redshift . there is now growing evidence for an intimate link between these obscured populations . 
 there are currently large uncertainties in the models , but several independent arguments lead to the conclusion that a significant fraction of the scuba sources ( @xmath0 ) will contain quasars . 
 recent observational studies of scuba survey sources appear to confirm these predictions , although the relative roles of agn and star - forming activity in heating the dust are unclear . 
 forthcoming surveys combining x - ray and sub - mm observations will provide a very powerful tool for disentangling these processes .