You are an expert at summarizing long articles. Proceed to summarize the following text: 
there has been enormous progress over the past decade in discovering galaxies and qsos at increasingly high redshifts . we are now probing far enough back in time that the universe at these early epochs was fundamentally different from its predominantly ionized state today . observations of @xmath24 qsos ( becker et al . 2001 , fan et al . 2002 ) show near - complete absorption of flux at wavelengths short - ward of lyman-@xmath3 ( gunn & peterson 1965 ) , indicating that the universe is optically thick to this line , and that the neutral fraction of hydrogen is much greater than at lower redshifts . initial cosmic microwave background results from the _ wmap _ satellite indicated that the universe was completely neutral at redshifts of @xmath23 ( kogut et al . there is an ongoing debate as to what reionized the universe at @xmath25 : is it agn or ionizing photons ( produced in hot , massive , short - lived stars ) escaping from star forming galaxies ? along with the escape fraction for these ionizing photons , it is crucial to know the global star formation rate at this epoch . here we detail our work in identifying star - forming galaxies within the first billion years of the big bang through deep imaging with the _ hubble space telescope ( hst)_. our selection method is based on the lyman break technique , and we have shown this is effective in isolating @xmath1 by obtaining spectroscopic confirmation with the 10-m keck telescopes . our analysis of the _ hubble ultra deep field _ ( the deepest images ever obtained ) enables us to explore the faint end of the luminosity function ( the bulk of galaxies with low star formation rates ) . our discovery of this @xmath0-drop galaxy population is used to infer the global star formation rate density at this epoch ( @xmath1 ) . we consider the implications for reionization of the uv flux from these galaxies . we also use infrared data from the _ spitzer space telescope _ to determine the spectral energy distributions ( seds ) of our @xmath0-drops , and constrain the previous star formation histories of these sources .    throughout we adopt the standard `` concordance '' cosmology of @xmath26 , @xmath27 , and use @xmath28 . all magnitudes are on the @xmath29 system . in order to select @xmath1 galaxies , we use the `` lyman break technique '' pioneered at @xmath30 using ground - based telescopes by steidel and co - workers ( steidel , pettini & hamilton 1995 ; steidel et al . 1996 ) and using _ hst _ by madau et al . ( 1996 ) . at @xmath31 , we can efficiently use only two filters , above ( @xmath2-band 9000 ) and below ( @xmath0-band 8000 ) the continuum break at lyman-@xmath3 line ( @xmath32 ) since the integrated optical depth of the lyman-@xmath3 forest is @xmath33 ( see figure  [ fig : filters ] ) and there is essentially no flux at shorter wavelengths . the key issue is to work at a sufficiently - high signal - to - noise ratio that lyman break `` drop - out '' galaxies can be safely identified through detection in a single redder band ( i.e. , @xmath2-band ) . this approach has been demonstrated to be effective by the sdss collaboration in the detection of @xmath1 qsos using the @xmath0- and @xmath2-bands alone ( fan et al . 2002 ) , using wide - area but shallow ground - based imaging ( sensitive to bright but rare quasars ) . our group has analysed deep imaging from the advanced camera for surveys ( acs ) on _ hst _ with the same sharp - sided sdss f775w ( @xmath0 ) and f850lp ( @xmath2 ) filters to locate the``@xmath6-drops '' which are candidate @xmath1 galaxies ( fainter but more numerous than the qsos ) . in figures  [ fig : tracks]&[fig : izzcolmag ] we illustrate how a colour cut of @xmath34 can be effective in selecting sources with @xmath35 . [ fig : tracks ]    we have analysed the acs images from the _ hst _ treasury `` great observatory origins deep survey '' ( goods ; giavalisco & dickinson 2003 ) to discover this @xmath0-drop population of @xmath1 galaxies . in stanway , bunker & mcmahon ( 2003 ) we searched 150arcmin@xmath36 of the goods - south field ( the _ chandra _ deep field - south ) to select @xmath37 objects , of which six were probable @xmath38 galaxies brighter than @xmath39 . subsequent papers by the goods team ( dickinson _ et al .  _ 2004 ; giavalisco _ et al . _ 2004 ) reproduced many of these @xmath0-drop candidates . to address potential cosmic variance issues , we performed a similar analysis in the goods - north field , which yielded a consistent estimate of the surface density of @xmath17 star forming sources ( stanway et al . .      the effectiveness of the lyman - break technique has been demonstrated at lower redshifts through spectroscopic confirmation of hundreds of @xmath13-band dropouts at @xmath12 and @xmath40-band drop - outs at @xmath41 . however , it is important to demonstrate that the @xmath0-drop selection is similarly isolating galaxies at @xmath1 , if we are to use the surface density of @xmath0-drops to draw global inferences about galaxies within the first billion years . a known possible contaminant is the extremely red object ( ero ) population of evolved galaxies at @xmath42 which can produce large @xmath43 colours : deep near - infrared imaging ( e.g. , from the _ hst_/nicmos survey of the udf ) should identify eros , and we consider this possible contamination stanway , mcmahon & bunker ( 2005 ) . low - mass galactic stars ( m / l / t dwarfs ) are another interloper population .    we have obtained deep spectroscopy with keck / deimos of several of the brightest @xmath0-drops identified by stanway , bunker & mcmahon ( 2003 ) in goods - south ; in the case of sbm03#3 ( @xmath44 ) and , an @xmath0-drop in the chandra deep field south photometrically - selected from hst / acs images to lie at @xmath1 .    for two of these objects , we discovered single emission lines ( fig . 3 ) at 8245  & 8305 , consistent with lyman-@xmath3 emission from galaxies at @xmath45 ( sbm03#3 , bunker et al .  2003 ) and @xmath46 ( sbm03#1 , stanway et al . the spectrally - resolved profiles of the emission lines are asymmetric ( as high-@xmath47 lyman-@xmath3 tends to be ) with a p - cygni - like profile and a sharp cut - off on the blue wing ( fig .  2 ) . the line fluxes are @xmath48 . the equivalent widths are @xmath49  using the @xmath2-band photometry from hst / acs , which is within the range seen in high-@xmath47 star - forming galaxies at @xmath50 . the velocity widths of the lyman-@xmath3 are comparatively narrow ( @xmath51 ) and we do not detect the high - ionization line n  v@xmath5212140 , which strongly support the view that this line emission is powered by star formation rather than an agn . our spectroscopic redshifts for these objects confirms the validity of the @xmath0-drop selection technique to select star - forming galaxies at @xmath1 . subsequently , a programme of spectroscopy with gmos `` nod&shuffle '' on gemini ( glare , stanway et al . 2004b ) has confirmed more galaxies in goods - south . slitless spectroscopy of the udf with _ hst_/acs ( grapes , malhotra et al . 2005 ) has also confirmed the spectral breaks of the @xmath0-drop galaxies , although the spectral resolution of @xmath53 is insufficient to see the lyman-@xmath3 emission lines . [ fig : madau_plot ] we base our measurement of the star formation rate on the intensity of the rest - frame uv continuum emission , redshifted into the @xmath2-band at @xmath1 ; this is dominated by the light from the most massive stars ( @xmath54 which are the hottest and bluest stars ) , which are the shortest lived , and hence a tracer of the instantaneous star formation rate . to go from the number of massive stars formed to the total star formation rate requires an assumption about the stellar initial mass function ( imf ) . in the absence of dust obscuration , the relation between the flux density in the rest - uv around @xmath55  and the star formation rate ( @xmath56 in @xmath57 ) is given by @xmath58 from madau , pozzetti & dickinson ( 1998 ) for a salpeter ( 1955 ) imf with @xmath59 . our relatively bright magnitude cut of @xmath4 for the goodsv0.5 data ( stanway , bunker & mcmahon 2003 ; stanway et al . 2004a ) corresponds to an unobscured star formation rate of @xmath60 at @xmath61 ( the luminosity - weighted average redshift ) , equivalent to @xmath62 for the @xmath13-band dropout population at @xmath12 . we recover the @xmath1 rest - frame uv luminosity function from the observed number counts of @xmath0-drops to faint magnitudes in the udf . although our colour cut selects galaxies with redshifts in the range @xmath63 , an increasing fraction of the @xmath2-band flux is attenuated by the redshifted lyman-@xmath3 forest . at higher redshifts we probe increasingly shortward of @xmath64  ( where the luminosity function is calculated ) so the @xmath65-corrections become significant beyond @xmath66 . we now compare the comoving star formation rate deduced for @xmath31 galaxies based on our candidate @xmath0-dropout source counts with predictions based on a range of rest - frame uv luminosity functions . for convenience we assume that there is no evolution over the sampled redshift range , @xmath67 , spanned by our @xmath0-drop selection ( equivalent to a range between @xmath68gyr after the big bang ) . we take as a starting point the luminosity function for the well - studied lyman - break @xmath13-dropout population , reported in steidel et al . ( 1999 ) , which has a characteristic rest - uv luminosity @xmath69 and a characteristic comoving number density at @xmath12 is @xmath70 in our cosmology . the faint end slope of the schechter function at @xmath12 is relatively steep ( @xmath71 ) compared with @xmath72 to @xmath73 for lower - redshift galaxy samples ( e.g. , blanton et al . 2003 ) . a tantalizing result from our work so far is that at @xmath74 there are far fewer uv - luminous star forming galaxies than would have been predicted if there was no evolution , based on a comparison to the well - studied @xmath75 lyman break population ( steidel et al.1999 ) . in fact , the volume averaged ( comoving ) star formation rate in galaxies with @xmath76 is @xmath77 _ less _ at @xmath1 than at @xmath12 . so it would appear that at this crucial epoch , where abundant sources of uv photons are needed to reionize the universe , there is in fact a deficit of luminous star forming galaxies . we have conclusively shown that the bright end of luminosity function has evolved greatly from @xmath74 to @xmath30 . other groups have claimed less dramatic evolution or even no evolution in the total volume - averaged star formation rate , based on the same fields ( giavalisco et al . 2004 ; dickinson et al . 2004 ) and similar _ hst_/acs data sets ( bouwens et al .  2003 ; yan , windhorst & cohen 2003 ) , but these groups work closer to the detection limit of the images and introduce large completeness corrections for the faint source counts . to do a complete inventory of the uv light from star formation we must address the contribution of low - luminosity star - forming galaxies to the ionizing flux . the public availability of the hubble ultra deep field ( udf ; beckwith , somerville & stiavelli ) can address this puzzle by pushing down the luminosity function at @xmath1 to well below the equivalent of @xmath78 for the @xmath12 population . the hubble ultra deep field ( udf ) was a cycle 12 stsci director s discretionary time programme executed over september 2003  january 200 , comprising 400 orbits in 4 broad - band filters ( including f775w @xmath0-band for 144 orbits ; f850lp @xmath2-band for 144 orbits ) . the udf lies within the chandra deep field south ( cdf - s ) . as the udf represents the deepest set of images yet taken , significantly deeper than the @xmath79-band exposures of the hubble deep fields ( williams et al . 1996 ) , and adds the longer - wavelength @xmath2-band , it is uniquely suited to the goals of our program . we performed the first analysis of @xmath0-drops in the udf ( bunker et al . 2004 ) , presenting details of 54 candidate star forming galaxies at @xmath1 in a preprint the day after the release of the udf data . our analysis was subsequently independently repeated by yan & windhorst ( 2004 ) , with agreement at the 98% level ( see bunker & stanway 2004 for a comparison ) . we take our magnitude limit as @xmath80 ( a @xmath81 cut).we measure a star formation density of @xmath82 at @xmath1 from galaxies in the udf with sfrs@xmath83 ( equivalent to @xmath11 at @xmath12 ) .    at the relatively bright cut of @xmath4 used in stanway , bunker & mcmahon ( 2003 ) from the goodsv0.5 survey , the udf data is 98% complete for sources as extended as @xmath84arcsec . interestingly , we detect no extended ( low surface brightness ) @xmath0-drops to this magnitude limit in addition to the targetted @xmath0-drop sbm03#1 in the deeper udf data . this supports our assertion that the @xmath0-drop population is predominantly compact and there can not be a large completeness correction arising from extended objects ( c.f . lanzetta et al . 2002 ) . the acs imaging is of course picking out hii star forming regions , and these uv - bright knots of star formation are typically compact ( @xmath85kpc , @xmath86arcsec at @xmath1 ) even within large galaxies at low redshift . it is interesting that the level of stellar contamination in the udf @xmath0-drops is only 2% , compared with about one in three at the bright end ( @xmath4 ; stanway , bunker & mcmahon 2003 ; stanway et al.2004 ) . this may be because we are seeing through the galactic disk at these faint limiting magnitudes to a regime where there are no stars ( see also pirzkal et al . we compare our @xmath0-drop luminosity function with the work of madau , haardt & rees ( 1999 ) for the density of star formation . we have updated their equation 27 for the more recent concordance cosmology estimate of the baryon density of spergel et al . ( 2003 ) , and for the predicted mean redshift of our sample ( @xmath87 ) : @xmath88    this relation is based on the same salpeter initial mass function as we have used in deriving our volume - averaged star formation rate . @xmath89 is the concentration factor of neutral hydrogen , @xmath90 . simulations suggest @xmath91 ( gnedin & ostriker 1997 ) . the escape fraction of ionizing photons ( @xmath92 ) for high - redshift galaxies is highly uncertain ( e.g. , steidel , pettini & adelberger 2001 ) , but even if we take @xmath93 ( no absorption by h  i ) this estimate of the star formation density required is a factor of @xmath94 higher than our measured star formation density of @xmath95 at @xmath1 from galaxies in the udf with sfrs@xmath83 . for faint end slopes of @xmath96 galaxies with @xmath97 account for @xmath98% of the total luminosity , so would fall short of the required density of lyman continuum photons required to reionize the universe . if the faint - end slope is as steep as @xmath99 then there would just be enough uv lyman continuum photons generated in star forming galaxies at @xmath1 ( assuming a salpeter imf ) , but the required escape fraction for complete reionization would still have to be implausibly high ( @xmath100 , whereas all high-@xmath47 measurements to date indicate @xmath101 : fernnadez - soto , lanzetta & chen 2003 ; steidel , adelberger & pettini 2001 ) . agn are also under - abundant at these epochs ( e.g. , dijkestra , haiman & loeb 2004 ) . if star forming galaxies at redshifts close to @xmath102 were responsible for the bulk of reionization , then a very different initial mass function would be required , or the calculations of the clumping factor of neutral gas would have to be significantly over - estimated ( see also stiavelli , fall & panagia 2004 ) . alternatively another low - luminosity population ( e.g. , forming globular clusters ; ricotti 2002 ) could be invoked to provide some of the shortfall in ionizing photons . it is also plausible that the bulk of reionization occurred at redshifts well beyond @xmath102 : the wmap polarization data indicate @xmath103 ( kogut et al .  2003 ) , and it is possible that the gunn - peterson troughs seen in @xmath104 qsos ( becker et al . 2001 ; fan et al . 2002 ) mark the very last period of a neutral igm . we have recently discovered evidence of an early epoch of star formation ( eyles et al . some of the spectroscopically - confirmed @xmath0-drops at @xmath1 show evidence of another spectral break in the near - infrared ( comparing _ spitzer _ images with irac at @xmath19 m with _ hst _ nicmos & acs images at @xmath105 m ) . this age - sensitive balmer/4000break indicates the ir light is dominated by stars older than 100myr ( and most probably 400myr old ) . this pushes the formation epoch for these galaxies to @xmath106 . our recent results also indicate that there were at least some galaxies already assembled with stellar masses @xmath21 ( equivalent to @xmath22 today ) within the first billion years . the early formation of such systems may have played a key role in reionizing the universe at @xmath23 .
we have determined the abundance of @xmath0-band drop - outs in the _ 
 hst_/acs goods surveys and the hubble ultra deep field ( udf ) . 
 the majority of these sources are likely to be @xmath1 galaxies whose flux decrement between the f775w @xmath0-band and f850lp @xmath2-band arises from lyman - alpha absorption . we have shown with keck / deimos and gemini / gmos spectroscopy that this technique does indeed select high redshift galaxies , and we discovered lyman-@xmath3 emission in the expected redshift range for about a third of the galaxies with @xmath4 in the 150arcmin@xmath5 of the goods - south field . 
 the @xmath6-drop number counts in the goods - north field are consistent , so cosmic variance is possibly not the dominant uncertainty . 
 the increased depth of udf enables us to reach a @xmath7 limiting magnitude of @xmath8 ( equivalent to @xmath9 at @xmath10 , or @xmath11 for the @xmath12 @xmath13-drop population ) . 
 the star formation rate at @xmath1 was approximately @xmath14 _ less _ than at @xmath12 . 
 this declining comoving star formation rate ( @xmath15 at @xmath1 at @xmath16 for a salpeter imf ) poses an interesting challenge for models which suggest that @xmath16 star forming galaxies at @xmath17 reionized the universe . 
 the short - fall in ionizing photons might be alleviated by galaxies fainter than our limit , or a radically different imf . 
 alternatively , the bulk of reionization might have occurred at @xmath18 . 
 we have recently discovered evidence of an early epoch of star formation in some of the @xmath0-drops at @xmath1 . _ 
 spitzer _ images with irac at @xmath19 m show evidence of the age - sensitive balmer/4000 , dominated by stars older than 100myr ( and most probably 400myr old ) . 
 this pushes the formation epoch for these galaxies to @xmath20 . 
 there are at least some galaxies already assembled with stellar masses @xmath21 ( equivalent to @xmath22 today ) within the first billion years . 
 the early formation of such systems may have played a key role in reionizing the universe at @xmath23 .