315 research outputs found
Linking stellar mass and star formation in Spitzer/MIPS 24 micron galaxies
We present deep Ks<21.5 (Vega) identifications, redshifts and stellar masses
for most of the sources composing the bulk of the 24 micron background in the
GOODS/CDFS. Our identified sample consists of 747 Spitzer/MIPS 24 micron
objects, and includes ~94% of all the 24 micron sources in the GOODS-South
field which have fluxes Snu(24)>83 microJy (the 80% completeness limit of the
Spitzer/GTO 24 micron catalog). 36% of our galaxies have spectroscopic
redshifts (mostly at z<1.5) and the remaining ones have photometric redshifts
of very good quality, with a median of |dz|=|zspec-zphot|/(1+zspec)=0.02. We
find that MIPS 24 micron galaxies span the redshift range z~0-4, and that a
substantial fraction (28%) lie at high redshifts z>1.5. We determine the
existence of a bump in the redshift distribution at z~1.9, indicating the
presence of a significant population of galaxies with PAH emission at these
redshifts. Massive (M>10^11 Msun) star-forming galaxies at redshifts 2<z<3 are
characterized by very high star-formation rates (SFR>500 Msun/yr), and some of
them are able to construct a mass of 10^10-10^11 Msun in a single burst
lifetime (~0.01-0.1 Gyr). At lower redshifts z<2, massive star-forming galaxies
are also present, but appear to be building their stars on long timescales,
either quiescently or in multiple modest burst-like episodes. At redshifts
z~1-2, the ability of the burst-like mode to produce entire galaxies in a
single event is limited to some lower (M<7x10^10 Msun) mass systems, and it is
basically negligible at z<1. Our results support a scenario where
star-formation activity is differential with assembled stellar mass and
redshift, and where the relative importance of the burst-like mode proceeds in
a down-sizing way from high to low redshifts. (abridged)Comment: Accepted for publication in the ApJ. 19 pages, 10 figures. Uses
emulateap
Number counts and clustering properties of bright Distant Red Galaxies in the UKIDSS Ultra Deep Survey Early Data Release
We describe the number counts and spatial distribution of 239 Distant Red
Galaxies (DRGs), selected from the Early Data Release of the UKIDSS Ultra Deep
Survey. The DRGs are identified by their very red infrared colours with
(J-K)AB>1.3, selected over 0.62 sq degree to a 90% completeness limit of
KAB~20.7. This is the first time a large sample of bright DRGs has been studied
within a contiguous area, and we provide the first measurements of their number
counts and clustering. The population shows strong angular clustering,
intermediate between those of K-selected field galaxies and
optical/infrared-selected Extremely Red Galaxies. Adopting the redshift
distributions determined from other recent studies, we infer a high correlation
length of r0~11 h-1 Mpc. Such strong clustering could imply that our galaxies
are hosted by very massive dark matter halos, consistent with the progenitors
of present-day L>L* elliptical galaxies.Comment: 5 pages, 4 figures, revised version accepted to MNRAS.
Higher-resolution figures available from the authors on reques
Spectroscopy of the near-nuclear regions of Cygnus A: estimating the mass of the supermassive black hole
We use a combination of high spatial resolution optical and near-IR
spectroscopic data to make a detailed study of the kinematics of the NLR gas in
the near-nuclear regions of the powerful, FRII radio galaxy Cygnus A
(z=0.0560), with the overall goal of placing limits on the mass of any
supermassive black hole in the core. Our K-band infrared observations (0.75
arcsec seeing) -- taken with NIRSPEC on the Keck II telescope -- show a smooth
rotation pattern across the nucleus in the Paschen alpha and H_2 emission lines
along a slit position (PA180) close to perpendicular to the radio axis,
however, there is no evidence for such rotation along the radio axis (PA105).
Higher spatial resolution observations of the [OIII]5007 emission line -- taken
with STIS on the Hubble Space Telescope (HST) -- confirm the general rotation
pattern of the gas in the direction perpendicular to the radio axis, and
provide evidence for steep velocity gradients within a radius of 0.1 arcsec of
the core. The circular velocities measured from both the Keck and HST data lead
to an estimate of the mass of the supermassive black hole of 2.5+/-0.7x10^9
solar masses. For the host galaxy properties of Cygnus A, this mass is
consistent with the global correlations between black hole mass and host galaxy
properties deduced for non-active galaxies. Therefore, despite the extreme
power of its radio source and the quasar-like luminosity of its AGN, the black
hole in Cygnus A is not unusually massive considering theluminosity of its host
galaxy. Indeed, the estimated mass of the black hole in Cygnus A is similar to
that inferred for the supermassive black hole in the FRI radio galaxy M87,
despite the fact that the AGN and radio jets of Cygnus A are 2 -- 3 orders of
magnitude more powerful.Comment: 17 pages, 12 figure
On the evolution of clustering of 24um-selected galaxies
This paper investigates the clustering properties of a complete sample of
1041 24um-selected sources brighter than F[24um]=400 uJy in the overlapping
region between the SWIRE and UKIDSS UDS surveys. We have concentrated on the
two (photometric) interval ranges z=[0.6-1.2] (low-z sample) and z>1.6 (high-z
sample) as it is in these regions were we expect the mid-IR population to be
dominated by intense dust-enshrouded activity such as star formation and black
hole accretion. Investigations of the angular correlation function produce a
correlation length are r0~15.9 Mpc for the high-z sample and r0~8.5 Mpc for the
low-z one. Comparisons with physical models reveal that the high-z sources are
exclusively associated with very massive (M>~10^{13} M_sun)haloes, comparable
to those which locally host groups-to-clusters of galaxies, and are very common
within such (rare) structures. Conversely, lower-z galaxies are found to reside
in smaller halos (M_min~10^{12} M_sun) and to be very rare in such systems.
While recent studies have determined a strong evolution of the 24um luminosity
function between z~2 and z~0, they cannot provide information on the physical
nature of such an evolution. Our clustering results instead indicate that this
is due to the presence of different populations of objects inhabiting different
structures, as active systems at z<~1.5 are found to be exclusively associated
with low-mass galaxies, while very massive sources appear to have concluded
their active phase before this epoch. Finally, we note that the small-scale
clustering data seem to require steep profiles for the distribution of galaxies
within their halos. This is suggestive of close encounters and/or mergers which
could strongly favour both AGN and star-formation activity.Comment: 13 pages, 8 figures, to appear in MNRA
The role of the LIRG and ULIRG phases in the evolution of Ks-selected galaxies
We investigate the role of the luminous infrared galaxy (LIRG) and
ultra-luminous infrared galaxy (ULIRG) phases in the evolution of Ks-selected
galaxies and, in particular, Extremely Red Galaxies (ERGs). With this aim, we
compare the properties of a sample of 2905 Ks<21.5 (Vega mag) galaxies in the
GOODS/CDFS with the sub-sample of those 696 sources which are detected at 24
microns. We find that LIRGs constitute 30% of the galaxies with stellar mass
M>1x10^{11} Msun assembled at redshift z=0.5. A minimum of 65% of the galaxies
with M>2.5x10^{11} Msun at z~2-3 are ULIRGs at those redshifts. 60% of the
ULIRGs in our sample have the characteristic colours of ERGs. Conversely, 40%
of the ERGs with stellar mass M>1.3x10^{11} Msun at 1.5<z<2.0 and a minimum of
52% of those with the same mass cut at 2.0<z<3.0 are ULIRGs. The average
optical/near-IR properties of the massive ERGs at similar redshifts that are
identified with ULIRGs and that are not have basically no difference,
suggesting that both populations contain the same kind of objects in different
phases of their lives.
LIRGs and ULIRGs have an important role in galaxy evolution and mass
assembly, and, although they are only able to trace a fraction of the massive
(M>1x10^{11} Msun) galaxies present in the Universe at a given time, this
fraction becomes very significant (>50%) at redshifts z>~2.Comment: Accepted for publication in A&A. 9 pages, 6 figure
A comparative HST imaging study of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies: Paper I
We present the first results from a major HST WFPC2 imaging study aimed at
providing the first statistically meaningful comparison of the morphologies,
luminosities, scalelengths and colours of the host galaxies of radio-quiet
quasars, radio-loud quasars, and radio galaxies. We describe the design of this
study and present the images which have been obtained for the first half of our
33-source sample. We find that the hosts of all three classes of luminous AGN
are massive elliptical galaxies, with scalelengths ~=10 kpc, and R-K colours
consistent with mature stellar populations. Most importantly this is the the
first unambiguous evidence that, just like radio-loud quasars, essentially all
radio-quiet quasars brighter than M_R = -24 reside in massive ellipticals. This
result removes the possibility that radio `loudness' is directly linked to host
galaxy morphology, but is however in excellent accord with the
black-hole/spheroid mass correlation recently highlighted by Magorrian et al.
(1998). We apply the relations given by Magorrian et al. to infer the expected
Eddington luminosity of the putative black hole at the centre of each of the
spheroidal host galaxies we have uncovered. Comparison with the actual nuclear
R-band luminosities suggests that the black holes in most of these galaxies are
radiating at a few percent of the Eddington luminosity; the brightest host
galaxies in our low-z sample are capable of hosting quasars with M_R = -28,
comparable to the most luminous quasars at z = 3. Finally we discuss our
host-derived black-hole masses in the context of the
radio-luminosity:black-hole mass correlation recently uncovered for nearby
galaxies by Franceschini et al. (1998), and the resulting implications for the
physical origin of radio loudness.Comment: Submitted for publication in the Astrophysical Journal, 55 pages of
latex, plus 12 postscript figures (Figures 1a-1s (greyscales of images and
model fits, and Figures 2a-2g (luminosity profiles and model fits) can be
downloaded from http://www.roe.ac.uk/astronomy/html/rjm1.shtml
Quasar induced galaxy formation: a new paradigm ?
We discuss observational evidence that quasars play a key role in the
formation of galaxies starting from the detailed study of the quasar
HE0450-2958 and extending the discussion to a series of converging evidence
that radio jets may trigger galaxy formation. The direct detection with VISIR
at the ESO-VLT of the 7 kpc distant companion galaxy of HE0450-2958 allows us
to spatially separate the sites of quasar and star formation activity in this
composite system made of two ultra-luminous infrared galaxies (ULIRGs). No host
galaxy has yet been detected for this quasar, but the companion galaxy stellar
mass would bring HE0450-2958 in the local M(BH)-M(stellar bulge) relation if it
were to merge with the QSO. This is bound to happen because of their close
distance (7 kpc) and small relative velocity (~60-200 km/s). We conclude that
we may be witnessing the building of the M(BH)-M(stellar bulge) relation, or at
least of a major event in that process. The star formation rate (~340 Msun/yr),
age (40-200 Myr) and stellar mass ([5-6]x10^10 Msun) are consistent with
jet-induced formation of the companion galaxy. We suggest that HE0450-2958 may
be fueled in fresh material by cold gas accretion from intergalactic filaments.
We map the projected galaxy density surrounding the QSO as a potential tracer
of intergalactic filaments and discuss a putative detection. Comparison to
other systems suggests that inside-out formation of quasar host galaxies and
jet-induced galaxy formation may be a common process. Two tests are proposed
for this new paradigm: (1) the detection of offset molecular gas or dust
emission with respect to the position of distant QSOs, (2) the delayed
formation of host galaxies as a result of QSO activity, hence the two step
building of the M(BH)/M(stellar bulge) ratio.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics (with minor corrections
The sizes, masses and specific star formation rates of massive galaxies at 1.3 < z < 1.5: strong evidence in favour of evolution via minor mergers
We report the results of a comprehensive study of the relationship between
galaxy size, stellar mass and specific star-formation rate (sSFR) at redshifts
1.3= 6x10^10 Msun), spectroscopic
sample from the UKIDSS Ultra-deep Survey (UDS), with accurate stellar-mass
measurements derived from spectro photometric fitting, we find that at z~1.4
the location of massive galaxies on the size-mass plane is determined primarily
by their sSFR. At this epoch we find that massive galaxies which are passive
(sSFR <= 0.1 Gyr^-1) follow a tight size-mass relation, with half-light radii a
factor f=2.4+/-0.2 smaller than their local counterparts. Moreover, amongst the
passive sub-sample we find no evidence that the off-set from the local
size-mass relation is a function of stellar population age. Based on a
sub-sample with dynamical mass estimates we also derive an independent estimate
of f=2.3+/-0.3 for the typical growth in half-light radius between z~1.4 and
the present day. Focusing on the passive sub-sample, we conclude that to
produce the necessary evolution predominantly via major mergers would require
an unfeasible number of merger events and over populate the high-mass end of
the local stellar mass function. In contrast, we find that a scenario in which
mass accretion is dominated by minor mergers can produce the necessary
evolution, whereby an increase in stellar mass by a factor of ~2, accompanied
by an increase in size by a factor of ~3.5, is sufficient to reconcile the
size-mass relation at z~1.4 with that observed locally. Finally, we note that a
significant fraction (44+/-12%) of the passive galaxies in our sample have a
disk-like morphology, providing additional evidence that separate physical
processes are responsible for the quenching of star-formation and the
morphological transformation of massive galaxies (abridged).Comment: 21 pages, 11 figures, accepted for publication in MNRAS. Replaced to
match accepted versio
Galaxies in Southern Bright Star Fields I. Near-infrared imaging
As a prerequisite for cosmological studies using adaptive optics techniques,
we have begun to identify and characterize faint sources in the vicinity of
bright stars at high Galactic latitudes. The initial phase of this work has
been a program of K_s imaging conducted with SOFI at the ESO NTT. From
observations of 42 southern fields evenly divided between the spring and autumn
skies, we have identified 391 additional stars and 1589 galaxies lying at
separations 60" from candidate guide stars in the magnitude range 9.0 R 12.4.
When analyzed as a "discrete deep field" with 131 arcmin^2 area, our dataset
gives galaxy number counts that agree with those derived previously over the
range 16 K_s 20.5. This consistency indicates that in the aggregate, our fields
should be suitable for future statistical studies. We provide our source
catalogue as a resource for users of large telescopes in the southern
hemisphere.Comment: 10 pages, 7 figures, accepted by A&A; Table 3 is available at
http://www.rzg.mpg.de/~ajb/data.html pending upload to CD
- …