153 research outputs found
The host galaxies of active galactic nuclei
Hubble Space Telescope A-band images of a sample of 10 radio galaxies (RG), 10
radio-loud quasars (RLQ) and 13 radio-quiet quasars (RQQ) at redshift z â 0.2 are
presented. The sources comprising the radio-loud sub-samples have been selected to
be indistinguishable in the Pâ
Gââ ~ z plane. The two quasar sub-samples have been
similarly selected to have indistinguishable distributions in the V â z plane. All three
sub-samples have been analysed with two-dimensional modelling software, designed to
accurately determine the host-galaxy parameters.The modelling results show all of the RG, and all of the quasars with Mᔄ < -23,
to have bulge-dominated host galaxies, excluding the hypothesis that host morphology
is responsible for the radio-loudness dichotomy. Furthermore, the host galaxies are
found to be essentially identical to a de Vaucouleurs (Ă = 0.25) law, with 30/33 objects
having Ă parameters in the range 0.2 < Ă < 0.3. The hosts of all of the objects studied
are found to be extremely luminous (L â„ L*), with 25/33 objects having L â„ 2L*, the
hosts of the RQQs are found to be typically 0.4 magnitudes fainter than their radio-loud
counterparts. The host galaxies of all three sub-samples are found to be larger than
average ellipticals (= 10.5 kpc), with a trend found for the RQQ hosts to be
some 20% smaller than those of the RLQs. For the first time it is demonstrated that
the hosts of quasars, as well as RG, display a Kormendy relation basically identical to
that of inactive ellipticals.In combination with the previous K-band modelling of this sample (Taylor et
al. 1996) the first set of reliable optical-infrared colours for a significant sample of
AGN host galaxies has been determined. The R - K colours of the hosts in all three
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sub-samples are shown to be consistent with each other, with the close agreement of the
median colour of R - K=2.5 with that expected from a ~ 14 Gyr-old stellar population
implying that the host galaxies of both RGs and quasars are consistent with having
formed at high redshift (z â„ 3).The nuclear colours of the RGs are shown to be perfectly consistent with those of
dust reddened RLQ nuclei. The combined luminosity, scalelength, Kormendy relation
and R - K colour results for the RG and RLQ sub-samples clearly demonstrate that
they are drawn from the same parent population, in strong support of the orientationbased
unification scheme of Barthel (1989). The corresponding results for the full 33
object sample show that, in terms of their global structural parameters, the hosts of
powerful AGN are no different to normal massive ellipticals.The host galaxy-black-hole mass correlations of Magorrian et al. (1998) and Franceschini
et al. (1998) are combined with the AGN modelling results to investigate the
quasar radio-loudness dichotomy. All methods of black-hole mass estimation lead to
the conclusion that RLQs harbour black-holes of mass â„ 10Âčâ°Mâš. The RQQ blackhole
masses estimated via the Magorrian relation are typically smaller than those of the
RLQs (~ 5 x 10âčMâš), but show sufficient overlap to require the influence of another
physical parameter, possibly black-hole spin, to explain the radio-loudness dichotomy.
In contrast, the use of the Franceschini black-hole estimator predicts RQQ black-hole
masses a factor of two smaller than those of the RLQs, suggesting that black-hole mass
alone may be the crucial factor.The application of the modelling technique to an existing HST survey of 3CR radio
galaxies at z ~ 1 shows them to have a scalelength distribution and Kormendy relation
indistinguishable from their z ~ 0.2 counterparts. The implied luminosity evolution
between z ~ 0.2 and z ~ 0.8 is shown to be consistent with that expected from passive
evolution alone in a low density Universe, with a high galaxy formation redshift (z â„ 3).
The lack of evidence for significant merger activity in the redshift range 0.2 < z < 0.8
leads to the conclusion that the traditional passive evolution interpretation of the radio
galaxy K - z relation remains acceptable
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
Deep GMOS Spectroscopy of Extremely Red Galaxies in GOODS-South: Ellipticals, Mergers and Red Spirals at 1<z<2
We have performed a deep spectroscopic survey of extremely red galaxies on
the GOODS-South field, using GMOS on Gemini South. We present here spectra and
redshifts for 16 ERGs at 0.87<z<2.02, to a limit K=20.2. The ERHs are a mixture
of spheroidals, mergers and spirals, with one AGN. For at least 10 of these
galaxies we observe [OII] emission lines. We perform an age-dating analysis by
fitting the spectra and 9-band photometry of the ERGs with models of passively
evolving stellar populations combined with a younger star-forming component.
The best-fitting ages for the old stellar components range from 0.6 to 4.5 Gyr,
with a mean 2.1 Gyr. Masses range from 3 to 20 times 10^10 solar masses. The
star-forming component typically forms a few per cent of the total mass, with
dust reddening averaging E(B-V)=0.35. Its timescale tends to be short for
mergers (<50 Myr) and longer (200-800 Myr) for spiral ERGs.Comment: 19 pages, 9 figure
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
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
An optimal ALMA image of the Hubble Ultra Deep Field in the era of JWST: obscured star formation and the cosmic far-infrared background
We combine archival ALMA data targeting the Hubble Ultra Deep Field (HUDF) to
produce the deepest currently attainable 1-mm maps of this key, extragalactic
survey field. Combining all existing data in Band 6, our deepest map covers
4.2arcmin^2, with a beamsize of 1.49"x1.07" at an effective frequency of 243GHz
(1.23mm). It reaches an rms of 4.6uJy/beam, with 1.5arcmin^2 below 9.0uJy/beam,
an improvement of >5% over the best previously published map and 50%
improvement in some regions. We also make a wider, but shallower map, covering
25.4arcmin^2. We detect 45 galaxies in the deep map down to 3.6sigma, including
10 more 1-mm sources than previously detected. 38 of these galaxies have a JWST
ID from the JADES NIRCam imaging and the new sources are typically faint and
red. A stacking analysis on the positions of ALMA-undetected JADES galaxies
yields detections for z<4 and stellar masses from 10^(8.4) to 10^(10.4)Msun,
extracting 10% of additional stacked signal from our map compared to previous
analyses. Detected sources and stacking contribute (10.0+/-0.5)Jy/deg^2 of the
cosmic infrared background (CIB) at 1.23mm. Although this is short of the
(uncertain) background level of about 20Jy/deg^2, after taking into account
intrinsic fluctuations in the CIB, our measurement is consistent with the
background if the HUDF is a mild (~2sigma) negative fluctuation. This suggests
that within the HUDF, JWST may have detected essentially all of the galaxies
that contribute to the CIB. Our stacking analysis predicts that the field
contains around 60 additional galaxies with 1.23mm flux densities averaging
around 15uJy, and over 300 galaxies at the few uJy level. However, the
contribution of these fainter more modestly-obscured objects to the background
is small, and converging, as anticipated from the now well-established strong
correlation between galaxy stellar mass and obscured star formation.Comment: Submitted to MNRA
Accounting for Cosmic Variance in Studies of Gravitationally-Lensed High-Redshift Galaxies in the Hubble Frontier Field Clusters
Strong gravitational lensing provides a powerful means for studying faint
galaxies in the distant universe. By magnifying the apparent brightness of
background sources, massive clusters enable the detection of galaxies fainter
than the usual sensitivity limit for blank fields. However, this gain in
effective sensitivity comes at the cost of a reduced survey volume and, in this
{\it Letter}, we demonstrate there is an associated increase in the cosmic
variance uncertainty. As an example, we show that the cosmic variance
uncertainty of the high redshift population viewed through the Hubble Space
Telescope Frontier Field cluster Abell 2744 increases from ~35% at redshift z~7
to >~65% at z~10. Previous studies of high redshift galaxies identified in the
Frontier Fields have underestimated the cosmic variance uncertainty that will
affect the ultimate constraints on both the faint end slope of the
high-redshift luminosity function and the cosmic star formation rate density,
key goals of the Frontier Field program.Comment: 5 pages, 3 figures. Version accepted by ApJ
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