101 research outputs found
The host galaxies and black-hole:galaxy mass ratios of luminous quasars at z~4
We present and analyse the deepest, high-quality Ks-band images ever obtained
of luminous quasars at z~4, in an attempt to determine the basic properties of
their host galaxies less than 1 Gyr after the first recorded appearance of
black holes with Mbh > 10^9 Msol. To maximise the robustness of our results we
have carefully selected two SDSS quasars at z~4. These quasars are
representative of the most luminous quasars known at this epoch but they also,
crucially, lie within 40 arcsec of comparably-bright foreground stars (required
for accurate PSF definition), and have redshifts which ensure line-free Ks-band
imaging. The data were obtained in excellent seeing (<0.4-arcsec) at the ESO
VLT with integration times of ~5.5 hours per source. Via carefully-controlled
separation of host-galaxy and nuclear light, we estimate the luminosities and
stellar masses of the host galaxies, and set constraints on their half-light
radii. The quasar host galaxies have K-band luminosities similar to radio
galaxies at comparable redshifts, suggesting that these quasar hosts are also
among the most massive galaxies in existence at this epoch. However, the quasar
hosts are a factor ~5 smaller than the host galaxies of luminous low-redshift
quasars. We estimate the stellar masses of the z~4 host galaxies to lie in the
range 2-10x10^11 Msol, and use the CIV emission line in the Sloan spectra to
estimate the masses of their black holes. The results imply a
black-hole:host-galaxy mass ratio Mbh:Mgal~0.01-0.05. This is an order of
magnitude higher than typically seen in the low-redshift Universe, and is
consistent with existing evidence for a systematic growth in this mass ratio
with increasing redshift, at least for objects selected as powerful AGN.Comment: 10 pages, 6 figure
The host galaxies of luminous quasars
We present results of a deep HST/WFPC2 imaging study of 17 quasars at z~0.4,
designed to determine the properties of their host galaxies. The sample
consists of quasars with absolute magnitudes in the range -24>M_V>-28, allowing
us to investigate host galaxy properties across a decade in quasar luminosity,
but at a single redshift. We find that the hosts of all the RLQs, and all the
RQQs with nuclear luminosities M_V<-24, are massive bulge-dominated galaxies,
confirming and extending the trends deduced from our previous studies. From the
best-fitting model host galaxies we have estimated spheroid and black-hole
masses, and the efficiency (with respect to Eddington luminosity) with which
each quasar is radiating. The largest inferred black-hole mass in our sample is
\~3.10^9 M_sun, comparable to those at the centres of M87 and Cygnus A. We find
no evidence for super-Eddington accretion in even the most luminous objects. We
investigate the role of scatter in the black-hole:spheroid mass relation in
determining the ratio of quasar to host-galaxy luminosity, by generating
simulated populations of quasars lying in hosts with a Schechter mass function.
Within the subsample of the highest luminosity quasars, the observed variation
in nuclear-host luminosity ratio is consistent with being the result of the
scatter in the black-hole:spheroid relation. Quasars with high nuclear-host
ratios can be explained by sub-Eddington accretion onto black holes in the
high-mass tail of the black-hole:spheroid relation. Our results imply that,
owing to the Schechter cutoff, host mass should not continue to increase
linearly with quasar luminosity, at the very highest luminosities. Any quasars
more luminous than M_V=-27 should be found in massive elliptical hosts which at
the present day would have M_V ~ -24.5.Comment: Accepted for publication in MNRAS. 18 pages; 7 figures and 17
greyscale images are reproduced here at low quality due to space limitations.
High-resolution figures are available from
ftp://ftp.roe.ac.uk/pub/djef/preprints/floyd2004
A near-infrared morphological comparison of high-redshift submm and radio galaxies: massive star-forming discs vs relaxed spheroids
We present deep, high-quality K-band images of complete subsamples of
powerful radio and sub-mm galaxies at z=2. The data were obtained in the best
available seeing at UKIRT and Gemini North, with integration times scaled to
ensure that comparable rest-frame surface brightness levels are reached for all
galaxies. We fit two-dimensional axi-symmetric galaxy models to determine
galaxy morphologies at rest-frame optical wavelengths > 4000A, varying
luminosity, axial ratio, half-light radius, and Sersic index. We find that,
while some images show evidence of galaxy interactions, >95% of the rest-frame
optical light in all galaxies is well-described by these simple models. We also
find a clear difference in morphology between these two classes of galaxy; fits
to the individual images and image stacks reveal that the radio galaxies are
moderately large (=8.4+-1.1kpc; median r{1/2}=7.8), de Vaucouleurs
spheroids ( = 4.07+-0.27; median n=3.87), while the sub-mm galaxies appear
to be moderately compact (=3.4+-0.3kpc; median r{1/2}=3.1kpc)
exponential discs (=1.44+-0.16; median n=1.08). We show that the z=2 radio
galaxies display a well-defined Kormendy relation but that, while larger than
other recently-studied high-z massive galaxy populations, they are still ~1.5
times smaller than their local counterparts. The scalelengths of the starlight
in the sub-mm galaxies are comparable to those reported for the molecular gas.
Their sizes are also similar to those of comparably massive quiescent galaxies
at z>1.5. In terms of stellar mass surface density, the majority of the radio
galaxies lie within the locus defined by local ellipticals. In contrast, while
best modelled as discs, most of the sub-mm galaxies have higher stellar mass
densities than local galaxies, and appear destined to evolve into present-day
massive ellipticals.Comment: 24 pages, 9 figure
The cosmological evolution of quasar black-hole masses
Virial black-hole mass estimates are presented for 12698 quasars in the
redshift interval 0.1<z<2.1, based on modelling of spectra from the Sloan
Digital Sky Survey (SDSS) first data release. The black-hole masses of the SDSS
quasars are found to lie between \simeq10^{7}\Msun and an upper limit of
\simeq 3\times 10^{9}\Msun, entirely consistent with the largest black-hole
masses found to date in the local Universe. The estimated Eddington ratios of
the broad-line quasars (FWHM\geq 2000 kms^{-1}) show a clear upper boundary at
, suggesting that the Eddington luminosity is still a
relevant physical limit to the accretion rate of luminous broad-line quasars at
. By combining the black-hole mass distribution of the SDSS quasars
with the 2dF quasar luminosity function, the number density of active black
holes at is estimated as a function of mass. In addition, we
independently estimate the local black-hole mass function for early-types using
the and correlations. Based on the SDSS
velocity dispersion function and the 2MASS band luminosity function, both
estimates are found to be consistent at the high-mass end (M_{bh}\geq
10^{8}\Msun). By comparing the estimated number density of active black holes
at with the local mass density of dormant black holes, we set lower
limits on the quasar lifetimes and find that the majority of black holes with
mass \geq 10^{8.5}\Msun are in place by .Comment: 15 pages, 10 figures, revised version, accepted for publication by
MNRA
A search for the first massive galaxy clusters
We have obtained deep, multi-band imaging observations around three of the
most distant known quasars at redshifts z>6. Standard accretion theory predicts
that the supermassive black holes present in these quasars were formed at a
very early epoch. If a correlation between black hole mass and dark matter halo
mass is present at these early times, then these rare supermassive black holes
will be located inside the most massive dark matter halos. These are therefore
ideal locations to search for the first clusters of galaxies. We use the
Lyman-break technique to identify star-forming galaxies at high redshifts. Our
observations show no overdensity of star-forming galaxies in the fields of
these quasars. The lack of (dust-free) luminous starburst companions indicates
that the quasars may be the only massive galaxies in their vicinity undergoing
a period of intense activity.Comment: 6 pages, 2 figures, contributed paper to Proceedings of the
Conference "Growing Black Holes" held in Garching, Germany, June 21-25, 2004,
edited by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of
"ESO Astrophysics Symposia
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