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:
the seemingly disjoint topics of strong gravitational lensing and of the host galaxies that surround the luminous qsos at high redshift , enjoy a symbiotic relationship .
_ hosts constrain lensing :  _ gravitationally lensed radio lobes can provide - through their spatially extended nature - more constraints on the lensing potential than the two or four point source images that are found in optically selected or radio - quiet qsos . as the majority of high - z qsos are radio - quiet , the extended stellar light of the host galaxies that surround qsos , can provide a welcome subsitute for extended radio emission ; the host galaxies form  optical einstein rings  or small arcs when lensed .
recent examples include pg  1115 + 080 ( impey 1998 ) , mg  1131 + 0456 ( kochanek 1999 ) , q  0957 + 561 ( keeton 1999 , _ in prep .
_ ) , and b  1938 + 666 ( king 1998 ) .
because gravitational lensing preserves the surface brightness of the source , different images of the same host position in the source plane should only differ in surface brightness through differential dust extinction along the various image paths . in this way
, optical images of the lensed hosts could provide a more sensitive probe of the ism in distant galaxies than the point sources alone ( falco 1999 ) .
_ lensing helps to see host galaxies :  _ gravitational lensing magnifies both the nuclear qso emission and the surrounding host galaxy . for the unresolved nucleus this magnification simply results in a flux increase without observable changes in the source morphology ; in contrast , the extended host emission is `` stretched away '' from the nucleus at a constant surface brightness . if the psf falls off with radius @xmath2 , as for hst , gravitational lensing changes the local contrast in favor of the extended light proportional to the square of the magnification .
0.3truein    in the data of the castles ( see _ http://cfa-www.harvard.edu/castles/_ ) project several factors come together to provide unprecedented sensitivity to detecting qso host galaxies : ( 1 ) hst provides a relatively stable psf with a fwhm of @xmath3 ; ( 2 ) for a qso at @xmath4 , the host galaxy is an order of magnitude more prominent at @xmath5 compared to the agn than at optical wavelengths ; ( 3 ) the low near - ir background from space improves the sensitivity to low - surface brightness features in the outer parts of the host galaxy ; ( 4 ) gravitational lensing further improves the host 
agn contrast as discussed above .    for these reasons
, we initiated a study of the host galaxies around gravitationally lensed quasars .
as the lens magnification is physically unrelated to the source structure , our sample of lensed qsos should not be significantly biased compared to other samples , save some possible luminosity bias . in this paper
, we summarize briefly the state of other host galaxy studies (  2 ) , describe the results from the castles study (  3 ) and compare them to recent theoretical models of central black hole and host galaxy growth at high redshifts (  4 ) .
explicit empirical demonstrations that qsos are indeed active nuclei surrounded by host galaxies have been pursued for 25 years , but have been hampered , especially for distant sources , by contrast problems with the much brighter nucleus , particularly because @xmath6 .
nonetheless , for qsos at @xmath7 , host galaxy detections have now become routine both at visible and near - ir wavelengths ( hutchings and neff 1992 ; mcleod and rieke 1995 ; bahcall 1995 ) , and a fairly coherent picture is emerging : radio - quiet qsos ( rqq ) typically live in early - type disk galaxies ( s0-sb ) whose luminosity range is centered around @xmath8 ; radio - loud qsos live in slightly more luminous hosts ( @xmath9 ) . while tidal interactions may be conducive to qso activity they are not necessary .
mcleod ( 1999 ) showed that in all nearby qsos ( @xmath10 ) there is a maximum nuclear luminosity for any given h - band host luminosity ( figure 1 ) . converting the host luminosity into a stellar mass , @xmath11 , and applying to these qsos the locally inferred relation @xmath12 ( magorrian 1998 ) between @xmath11 and the nuclear black hole ( bh )
mass , mcleod ( 1999 ) deduce that these qsos can shine at most with @xmath13 .    at higher redshifts ( @xmath14 )
the host galaxy picture becomes observationally less clear .
the hosts of radio - loud qsos ( rlq ) have been detected in sizeable samples with reasonable completeness ( @xmath15 ) , mostly through near - ir imaging from the ground ( kotilainen 1998 , carballo 1998 ) .
these have become possible in part because rlqs at these epochs seem to live in extremely luminous galaxies .
the observed rlq hosts do not seem to have comparably bright present - day counter parts , but it is still not entirely clear whether most of their ( rest ) optical emission really is stellar . in most ground - based searches for rqq hosts beyond @xmath16 the detection completeness has been very low : findings are still published on individual objects ( aretxaga 1998 ) .
the relatively few existing detections ( hutchings 1985 ) , and by implication the non - detections , suggest that the rqq hosts are of modest luminosity .
the existing observations leave open a number of important questions about the hosts of distant qsos , which we can address through the lensing studies :    * are all qsos embedded in substantial host galaxies , or are there `` naked '' qsos ? * do the most luminous qsos ( mostly rqqs ) at their heyday ( @xmath4 ) , live in the most luminous galaxies at their epoch ? *
do the hosts of rqqs and rlqs differ increasingly with growing redshift ? * is there the same maximum nuclear luminosity at a given host luminosity in distant qsos than at @xmath10 ? as qsos were more luminous in the past , and most galaxies were in lower mass fragments , one might expect an evolution of such a relation .
to study gravitationally lensed qsos systematically , the castles project has obtained deep f160w ( h - band ) images ( typically for one orbit ) of most known lens systems .
we have begun to derive de - magnified h - band magnitudes for the host galaxies wherever they are detected . in the four image systems we should expect that at least some portion of the host is highly magnified and becomes a detectable arc .
indeed , in most highly magnified cases there is such a qualitative detection , indicating that `` naked quasars '' are rare , if they exist at all .
however , the demagnification of such systems ( pg  1115 + 080 , fig .
2 ) can be quite complex ; certainly , the de - magnification of two - image lenses ( q0957 + 561 , fig .
2 ) is more straightforward .    in figure 3 , we have summarized the magnitudes ( and upper limit ) for an initial set of six objects ( rix 1999 ) . clearly , these host galaxies of rqqs are much fainter than bright radio galaxies ( @xmath17 ) at similar redshifts .
for comparison , figure 3 also shows the apparent brightness of two galaxies with two different star formation histories ( sfh ) that both result in @xmath18 galaxies at the present epoch .
if the hosts had formed all their stars at high redshift ( an @xmath18 s worth ) , they would appear much brighter .
however , if the hosts formed stars continuously similar to the milky way , the predicted magnitudes match the observations .
this is consistent with qso living in unexceptional host galaxies with typical sfhs that lead to @xmath18 now .
it is instructive to compare the relation between nuclear and host luminosity for our sources at @xmath0 to the low redshift results of mcleod 1999 .
the large filled circles in figure 1 represent the initial results from castles and show that at higher redshift the hosts are comparatively much fainter than at later epochs .
this discrepancy would increase considerably if we compared the minimum stellar mass ( rather than h - band luminosity ) at high and low redshift . indeed , after accounting for about one magnitude of h - band m / l evolution , we would be lead to conclude that these luminous qsos at high redshift have super - eddington luminosities , _ if the same @xmath19 @xmath20 relation were to hold .
_    instead , it appears more plausible to infer that the central bhs grew faster at early epochs than the surrounding hosts , so that @xmath21 in this case , a less massive and less luminous host galaxy could sustain a more luminous nucleus .
recently , kauffmann and hhnelt ( 1999 ) have explored the connection between galaxy formation and central bh growth through semi - analytic models , trying to explain evolution of the galaxy population and of the qso luminosity function simultaneously .
their models predict a quite steep evolution in the @xmath22 vs @xmath23 relation ( see figure 4 ) , but one that exactly matches our findings .
support for the castles project was provided by nasa through grant numbers go-7495 and go-7887 from the space telescope science institute , which is operated by the association of universities for research in astronomy , inc .
aretxaga , i. , le mignant , d. , melnick , j. , terlevich , r.j . , & boyle , b.j .
1998 , , 296 , 643 bahcall , j.n . , kirhakos , s. , & schneider , d. 1995 , , 450 , 486 carballo , r. , snchez , s.f .
, gonlez - serrano , j.i . , benn , c.r . , & vigotti , m. 1998 , , 115 , 1234 falco e.e .
1999 , astro - ph/9901037 hutchings , j.b . , &
neff , s.g .
1992 , , 104 , 1 hutchings , j.b .
1995 , , 110 , 994 impey , c.d .
1998 , , 509 , 551 kauffmann , g. , & hhnelt , m. 1999 , astro - ph/9906493 king , l.j .
1998 , 295l , 41k kochanek , c.s .
1998 , astro - ph/9809371 kotilainen , j.k . ,
falomo , r. , & scarpa , r. 1998 , , 332 , 503 magorrian , j. 1998 , , 115 , 2285 mcleod , b.a . , rieke , g.h . , & storrie - lombardi , l.j .
1999 , , 511 , l67 mcleod , k.k . , & rieke , g.h .
1995 , , 454 , 77 rix , h .- w .
, 1999 , _ in prep .
we present h - band observations of gravitationally lensed qso host galaxies obtained with nicmos on hst as part of the cfa - arizona - gravitational - lens - survey ( castles ) . 
 the detections are greatly facilitated by the lensing magnification in these systems ; we find that most hosts of radio - quiet qsos ( rqq ) at @xmath0 are of modest luminosity ( @xmath1 ) . 
 they are 2 - 5 times fainter than the hosts of radio - loud qsos at the same epoch .    compared to low redshifts , 
 rqq hosts at @xmath0 also support higher nuclear luminosities at given stellar host mass . 
 this suggests that the supermassive black holes at their centers grew faster at early epochs than the stellar body of their surrounding host galaxies .