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:
black hole ( bh ) mass is believed to be one of the fundamental parameters that characterize quasar activity and much effort has been devoted to obtaining accurate bh masses for quasars and other agn @xcite . in recent years , much progress has been made in measuring bh masses in galactic nuclei , particularly with the remarkable discovery by @xcite and @xcite of a tight correlation between stellar velocity dispersion in galaxies and the mass of their central bh ( m@xmath3 ) .
the use of this correlation to derive bh masses in agn , however , is hampered by the difficulty in measuring velocity dispersions in host galaxies that tend to be overpowered by their very bright nuclei .
nevertheless , the correlation has been shown to be present at low redshift ( @xmath4 ) in low luminosity agn (  bl lac objects : @xcite ; or seyfert galaxies : @xcite ) .
seyfert galaxies at higher redshift ( @xmath5 and @xmath6 ) , however , appear to show an offset from the local relation ( * ? ? ?
* and references therein ) .
it is not yet known whether the m@xmath7 correlation holds for the highest luminosity agn .
a loose correlation has been found by using the width of [ oiii ] lines in active nuclei @xcite , but the width of these lines is dependent upon other parameters ( outflows , radio luminosity , etc . ) and therefore lead to a correlation with a large scatter .
bh masses derived from [ oiii ] emission line widths can only be accurate to within a factor of five at best @xcite .
more accurate determinations are necessary if we hope to use them to disentangle some of the other fundamental relationships among quasar parameters .
we are therefore carrying out a program to measure stellar velocity dispersions in quasar host galaxies .
we have selected a sample of @xmath8 @xmath9 quasars from 2mass .
red quasars are likely the dust obscured equivalent of the blue quasar population , and they have the advantage that the nucleus is highly extincted at optical wavelengths , so that the contrast between the stellar flux from the host galaxy and that of the nucleus is increased .
thus , the spectra of these objects show , at shorter wavelengths , stellar features that are useful to measure velocity dispersions and , at longer wavelengths , broad emission lines from which to obtain virial estimates of bh masses .
our sample of 11 objects is drawn from @xcite .
we obtained deep , medium resolution spectroscopic observations with the echelle spectrograph and imager ( esi ) on the keck ii telescope .
we placed the slit through the center of the host galaxies in order to measure velocity dispersions of the bulges of the hosts .
figure  1 shows the spectrum of one of the objects in the sample demonstrating that they suffer little contamination from the nucleus at wavelengths shorter than h@xmath2 .
we first estimated the size of the stellar bulges by inspecting archival @xmath10/wfpc2 images ( proposal i d 9057 ; pi d. hines ) and extracted spectra from these regions .
we measured velocity dispersions ( @xmath11 ) by fitting the spectra in the rest frame region between 5220 and 5550   for each of the targets .
we used templates formed from the combination of spectra of stars of different spectral types observed also with esi .
to these templates we added a small fraction of a continuum to simulate any potential contamination from the active nucleus .
we were able to obtain a reliable @xmath11 for eight of the targets , with typical errors at the 95% confidence level of @xmath12 @xmath13 20 km s@xmath14 .
we estimated virial masses for the bhs in the sample by first fitting the fwhm of the broad component of h@xmath2 , and then using the scaling relation given by @xcite . in this relation ,
the size of the broad line region is a function of the continuum luminosity at rest frame 5100  .
however , as mentioned before , the quasar continuum suffers from heavy extinction in this spectral region . to obtain the unobscured flux at 5100 
, we used the following procedure : ( 1 ) we measured the flux of the quasar at f814w in the hst images by fitting an empirical psf to the nucleus .
in this way , we also determined the relative flux contribution from the host galaxy and the quasar in the region covered by the slit .
( 2 ) we scaled the keck esi spectrum to match the flux obtained from the hst images . ( 3 ) we fitted a reddened version of the sdss composite quasar spectrum plus a reddened stellar population , using the relative contributions determined in ( 1 ) , and varying the amount of reddening , guided by the measured ratios of h@xmath2/h@xmath15 .
the e(b - v ) that we measured in the sample varied from 0.5 to 2 .
measuring the extinction accurately is currently our main source of uncertainty and , until we perform more detailed fitting of the spectra , our results are only tentative .
@xmath11 relation for red quasars .
the objects in our sample are plotted as solid circles , along with local objects ( open squares and triangles ) and seyfert galaxies at @xmath16 ( open stars ) .
the dotted line marks the empirical relation derived for local objects . ]
preliminary results for the eight objects we measured are plotted as solid circles in fig .  2 , along with results from local objects taken from @xcite and @xcite , and seyfert galaxies at @xmath16 taken from @xcite .
while half of the objects have positions consistent with the local relation , the other half seem to have an offset in the sense that @xmath0 has lower values for a given bh mass .
the positions of these objects in the plot are more consistent with those of seyfert galaxies at higher @xmath18 published by @xcite , who have found evidence for evolution in the relation from @xmath19 to the present @xcite .
the two objects that fall beneath the relation in fig .
2 are indeed the ones with the lowest @xmath18 in our sample . however , until we perform more careful modeling to determine the nuclear extinction , we can only speculate about this potential evidence pointing to evolution .
much progress has been made in measuring black hole ( bh ) masses in ( non - active ) galactic nuclei using the tight correlation between stellar velocity dispersions @xmath0 in galaxies and the mass of their central bh . 
 the use of this correlation in quasars , however , is hampered by the difficulty in measuring sigma in host galaxies that tend to be overpowered by their very bright nuclei . 
 we discuss results from a project that focuses on @xmath1 quasars suffering from heavy extinction at shorter wavelengths . 
 this makes it possible to obtain clean spectra of the hosts in the spectral regions of interest , while broad lines ( like h@xmath2 ) are still visible at longer wavelengths . 
 we compare bh masses obtained from velocity dispersions to those obtained from the blr and thus probe the evolution of this relation and bh growth with redshift and luminosity . 
 our preliminary results show an offset between the position of our objects and the local relation , in the sense that red quasars have , on average , lower velocity dispersions than local galaxies . 
 we discuss possible biases and systematic errors that may affect our results .     
 address = dept . of physics and astronomy and igpp , university of california , 
 riverside , ca 92521 , usa     address = institute of theoretical astrophysics , university of oslo , n-0315 oslo , norway     address = dept . of physics and astronomy and igpp , university of california , 
 riverside , ca 92521 , usa     address = spitzer science center , california institute of technology , pasadena , ca 91125 , usa