You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
over the last five years , remarkable observational progress has been made in constructing large samples of local and distant galaxy clusters with the aim of quantifying the evolution of their space density and providing the basis for follow - up studies of their physical properties .
the rosat satellite is largely responsible for this progress , both with all - sky survey data and pointed observations , which have been a gold mine for serendipitous discoveries .
about a thousand clusters have now been selected from the rosat all - sky survey and several statistical complete subsamples have been used to obtain a firm measurement of the local abundance of clusters @xcite and their spatial distribution ( cf .
bhringer this volume ) .
serendipitous searches for distant clusters , selected as extended x - ray sources in deep pspc pointings @xcite , have boosted the number of known clusters at @xmath2 by an order of magnitude , being just a few before the rosat era . as we will show below , this recent work has complemented the original einstein medium sensitivity survey ( emss ) @xcite , and has corroborated its findings .    in this paper
, we provide a brief update on our current knowledge of the redshift dependent cluster x - ray luminosity function ( xlf ) @xcite from results published over the last year .
the reader is referred to the contributions of h.  ebeling , i.  gioia , l.  jones , a.  vikhlinin in this volume for additional details and recent findings on specific surveys .
we also report the most recent results from the rosat deep cluster survey ( rdcs ) which has allowed these studies to be pushed beyond @xmath3 for the first time .
unless otherwise stated , we assume @xmath5 km s@xmath6 mpc@xmath6 , @xmath7 .
rosat distant cluster surveys @xcite , besides employing different x - ray selection methods , have adopted different strategies in terms of survey depth and solid angle . in fig.[fig : vmax ] , we show the sky coverage of three surveys which span a wide region of the solid angle  limiting flux plane , from the large , shallow emss survey @xcite , to the moderately deep 160 deg@xmath9 cfa survey @xcite , and the deep , small area ( 50 deg@xmath9 ) rdcs @xcite ( the other rosat surveys generally fill the space in between ) .
this complementary coverage of the @xmath10 plane has the advantage of providing a better sampling of the xlf at different redshifts when results from various surveys are combined . as an example , we also show in fig.[fig : vmax ] the corresponding survey volume which is covered at @xmath11 for an @xmath12 cluster .
this illustrates the good sensitivity of the rdcs for detecting very distant  common " clusters , whereas a similar plot would show that the emss explores a larger volume for the most luminous rare systems ( @xmath13 ) .    in fig.[fig :
xlf_all ] we show several measurements of the cluster xlf that have been published to date .
sample sizes and median redshifts of each sample are also indicated .
based on these data , several groups have argued that no significant evolution is observed in the space density of distant clusters with @xmath14\ { \raise -2.truept\hbox{\rlap{\hbox{$\sim$}}\raise5.truept \hbox{$<$}\ } } 3\times 10^{44 } { \,{\rm erg\,s^{-1}}}$ ] @xcite . as demonstrated by the rdcs , this trend persists out to @xmath15 .
measurements of the distant xlf at @xmath16 are difficult with current samples , due to low number statistics . as a result
, the evolution of the high end of the xlf has remained a hotly debated issue , ever since it was first reported in the emss @xcite .
more recently , vikhlinin et al .
@xcite have confirmed the emss findings by comparing the observed number of very luminous systems with the no evolution prediction .
this result seems to be also in agreement with a preliminary analysis of the bright sharc sample @xcite ( fig .
[ fig : xlf_all ] ) .
the binned representation of the xlf in fig.[fig : xlf_all ] does not provide a full picture of the space density evolution observed in a given sample . for example , it fails to provide the statistical significance of a possible departure from no evolution models @xcite .
the information contained in the rdcs can be more readily recovered by analyzing the unbinned @xmath17 distribution with a maximum - likelihood ( ml ) approach , which compares the observed cluster distribution on the @xmath17 plane with that expected from a given xlf model .
we characterize the cluster xlf as an evolving schechter function , + @xmath18 , with @xmath19 ; where @xmath20 and @xmath21 are two evolutionary parameters .
different surveys find consistent values for the faint end slope @xmath22 , which is not observed to vary as a function of redshift ( fig .
[ fig : xlf_all ] ) .
for the local xlf , we use here the measurement of the bcs sample @xcite , i.e. @xmath23 , @xmath24 , @xmath25 .    for this analysis
, we use a complete flux limited sample ( @xmath26 erg @xmath27 s@xmath6 ) of 81 spectroscopically confirmed rdcs clusters drawn from 33 deg@xmath9 ( @xmath28 ) .
observed flux errors are included in the likelihood computation .
the resulting @xmath29 , @xmath30 and @xmath31 c.l . contours in the a - b plane are shown in fig .
[ fig : like ] , for two different cosmologies .
best fit values for the @xmath32 case are @xmath33 and @xmath34 ( @xmath30 errors ) . the no evolution model ( @xmath35 ) is excluded at more than a @xmath31 confidence level , even when the uncertainties of the local xlf are taken into account .
the departure of our best fit model from the no - evolution scenario is due to the small number of observed clusters in the rdcs at @xmath36 with luminosities @xmath37 compared to the no - evolution prediction .
interestingly , this effect is barely significant with a slightly shallower sample ( @xmath38 , 70 clusters ) .
this evolutionary trend is similar to that observed in the emss @xcite .    by excluding the most luminous clusters from our ml analysis
, we find that there is _ no evidence of evolution _
( with @xmath30 confidence level ) at luminosities @xmath39 , confirming previous results obtained with smaller samples . a redshift dependent inspection of the likelihood also shows that little can be said on the evolution of the high end of the xlf at @xmath40 with the current rdcs sample .
these findings lead to a consistent picture in which the comoving space density of the bulk of the cluster population is approximately constant out to @xmath41 , but the most luminous ( @xmath37 ) , presumably most massive clusters were indeed rarer at high redshifts .
constraints on cosmological models based on this same rdcs sample are discussed elsewhere ( borgani et al .
this volume ; @xcite ) .
an inspection of fig .
[ fig : vmax ] indicates that the rdcs probes an appreciable volume at high redshifts . the maximum sensitivity for clusters at @xmath42
is reached at fluxes below @xmath43 and for luminosities @xmath44 .
the discovery of the first x - ray selected cluster ( rxj0848.9 + 4452 , fig .
[ fig : c0848 ] ) at @xmath45 in the rdcs @xcite has confirmed these expectations . deep near - ir imagery and optical spectroscopy with keck / lris were required to secure this identification .
this system has @xmath46 ( in rest frame [ 0.5 - 2 kev ] band ) and is found to lie only @xmath47 away ( @xmath48 comoving mpc ) from an ir selected cluster previously discovered by stanford et al .
@xcite at @xmath49 , also known to be x - ray luminous with half the @xmath50 of rxj0848.9 .
this is , most likely , the first example of a high - redshift supercluster consisting of two separate systems in an advanced stage of collapse . scheduled chandra and xmm observations of this field should provide important information on the temperature and metal enrichment of their intra - cluster media .
recently , two additional faint rdcs candidates have been spectroscopically confirmed , using keck / lris and vlt / fors , as clusters at @xmath51 and @xmath52 .
it should be stressed that these clusters , with @xmath53 , are low surface brightness `` fluctuations '' in pspc images , and therefore the fraction of spurious candidates can be significant at these low fluxes . moreover , the spectroscopic follow - up work of such distant clusters is particularly time consuming , even with 8 - 10 meter class telescopes .
although the optical identification is not yet complete at these faint flux levels , the high-@xmath54 tail of the rdcs can be used to set an interesting lower limit to the space density of clusters at @xmath55 .
this is shown in fig .
[ fig : xlf_new ] , where an extended rdcs sample has been used to obtain the best estimate of the xlf of distant clusters out to @xmath56 .
such a sample contains 107 clusters drawn from a 47 deg@xmath9 area , with 8 clusters at @xmath57 .
we also plot in fig .
[ fig : xlf_new ] the best fit xlf model described above , at @xmath58 and @xmath59 .    to better understand the constraints that these newly - identified high redshift clusters set on the xlf evolution , we have plotted in fig .
[ fig : like ] ( right ) the loci of the a - b plane for which the corresponding xlf , @xmath60}(l , z)$ ] , predicts @xmath61 clusters at @xmath62 , for the entire rdcs sample .
about 20 clusters would have to be identified in the no - evolution scenario .
this seems very unlikely , unless the sample is severely incomplete at faint fluxes .
given that 4 clusters have already been discovered , the portion of the a - b plane which is allowed by this preliminary analysis suggests that the evolution is still rather mild at @xmath42 , at luminosities just above @xmath63 .
the next obvious step in the effort to understand cluster formation and evolution is to push the cluster ( or proto - cluster ) search out to even higher redshifts , namely out to @xmath64 where the signature of large scale stucture has already been unveiled @xcite .
finding clusters around high-@xmath54 agn is a viable method ( e.g.@xcite ) , although not suitable for assessing the cluster abundance .
serendipitous searches with chandra and xmm will of course be actively pursued , but it will take several years to build large enough survey areas , and furthermore , the spectroscopic follow - up of cluster candidates at @xmath65 may turn out to be too difficult with existing telescopes .
while the short - term prospects for exploring the era at @xmath66 may appear somewhat bleak , it should be kept in mind that earlier this decade many theorists and observers were convinced that clusters at @xmath62 were either out of reach , or did not exist .
borgani , s. , rosati , p. , tozzi , p. , & norman , c. 1999 , apj , 517 , 40 burke , d.j .
1997 , apj , 488 , l83 carilli , c.l .
1998 , apj , 494 , l143 crawford , c.s . &
fabian , a.c .
1996 , mnras , 282 , 1483 de grandi et al . 1999 , apj , 514 , 148 dickinson m. et al . 1999 ,
apj , submitted ebeling , h. et al .
1997 , apj , 479 , l101 henry , j.p .
1992 , apj , 386 , 408 gioia , i.m .
1990 , apj , 356 , l35 jones , l.r .
1998 , apj , 495 , 100 nichol , r.c .
et al . 1999 , apj , 521 , l21 page , m.j . , carrera , f.j . 1999 , mnras , in press rosati , p. 1998 , in wide field surveys in cosmology , 14th iap meeting ( paris , publ .
: editions frontieres ) p.219 rosati , p. , et al .
1998 , apj , 492 , l21 rosati , p. et al .
1999 , aj , 118 , 76 stanford , s.a .
1997 , aj , 114 , 2232 steidel c.c .
et al .  1999 ,
apj , 519 , 1 vikhlinin a. et al .
1998 , apj , 498 , l21
we briefly review our current knowledge of the space density of distant x - ray clusters as measured by several rosat serendipitous surveys . 
 we compare old and new determinations of the cluster x - ray luminosity function ( xlf ) at increasing redshifts , addressing the controversial issue of the evolution of its high end . 
 we use complete subsamples , drawn from the rosat deep cluster survey ( rdcs ) , to quantify the statistical significance of the xlf evolution out to @xmath0 . 
 a consistent observational picture emerges in which the bulk of the cluster population shows no significant evolution out to @xmath0 , whereas the most luminous systems ( @xmath1}\simeq 5\times 10^{44}{\,{\rm erg\,s^{-1}}}$ ] ) were indeed rarer , at least at @xmath2 , in keeping with the original findings of the emss . 
 we also report on the recent spectroscopic identification of four clusters in the rdcs lying beyond @xmath3 , the most distant x - ray clusters known to date , which set an interesting lower limit on the space density of clusters at @xmath4 .