154 research outputs found
Luminosity functions of cluster galaxies: The Near-ultraviolet luminosity function at
We derive NUV luminosity functions for 6471 NUV detected galaxies in 28 clusters and consider their dependence on cluster properties. We
consider optically red and blue galaxies and explore how their NUV LFs vary in
several cluster subsamples, selected to best show the influence of environment.
Our composite LF is well fit by the Schechter form with and in good agreement with values for the Coma
centre and the Shapley supercluster, but with a steeper slope and brighter
than in Virgo. The steep slope is due to the contribution of massive
quiescent galaxies that are faint in the NUV. There are significant differences
in the NUV LFs for clusters having low and high X-ray luminosities and for
sparse and dense clusters, though none are particularly well fitted by the
Schechter form, making a physical interpretation of the parameters difficult.
When splitting clusters into two subsamples by X-ray luminosity, the ratio of
low to high NUV luminosity galaxies is higher in the high X-ray luminosity
subsample (i.e the luminosity function is steeper across the sampled luminosity
range). In subsamples split by surface density, when characterised by Schechter
functions the dense clusters have an about a magnitude fainter than that
of the sparse clusters and is steeper ( vs.
respectively). The differences in the data appear to be driven by changes in
the LF of blue (star-forming) galaxies. This appears to be related to
interactions with the cluster gas [abridged]Comment: Accepted A&
Environmental Effects on the UV Upturn in Local Clusters of Galaxies
We explore the dependence of UV upturn colours in early type cluster galaxies
on the properties of their parent clusters (such as velocity dispersion and
X-ray luminosity) and on the positions and kinematics of galaxies within them.
We use a sample of 24 nearby clusters with highly complete spectroscopy and
optical/infrared data to select a suitable sample of red sequence galaxies,
whose FUV and NUV magnitudes we measure from archival GALEX data. Our results
show that the UV upturn colour has no dependence on cluster properties and has
the same range in all clusters. There is also no dependence on the projected
position within clusters or on line-of-sight velocity. Therefore, our
conclusion is that the UV upturn phenomenon is an intrinsic feature of cluster
early type galaxies, irrespective of their cluster environment.Comment: 8 pages. Accepted for publication MNRA
High-redshift galaxies and low-mass stars
The sensitivity available to near-infrared surveys has recently allowed us to probe the galaxy population at z ≈ 7 and beyond. The existing Hubble Wide Field Camera 3 (WFC3) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond 1 μm – a capability that will be further extended with the launch and commissioning of the James Webb Space Telescope (JWST). As new regions of parameter space in both colour and depth are probed, new challenges for distant galaxy surveys are identified. In this paper, we present an analysis of the colours of L- and T-dwarf stars in widely used photometric systems. We also consider the implications of the newly identified Y-dwarf population – stars that are still cooler and less massive than T-dwarfs for both the photometric selection and spectroscopic follow-up of faint and distant galaxies. We highlight the dangers of working in the low-signal-to-noise regime, and the potential contamination of existing and future samples. We find that Hubble/WFC3 and VISTA/VIRCam Y-drop selections targeting galaxies at z ∼ 7.5 are vulnerable to contamination from T- and Y-class stars. Future observations using JWST, targeting the z ∼ 7 galaxy population, are also likely to prove difficult without deep medium-band observations. We demonstrate that single emission line detections in typical low-signal-to-noise spectroscopic observations may also be suspect, due to the unusual spectral characteristics of the cool dwarf star population
On the evolution of young radio-loud AGN
This paper describes an investigation of the early evolution of extragalactic
radio sources using samples of faint and bright Gigahertz Peaked Spectrum (GPS)
and Compact Steep Spectrum (CSS) radio galaxies. Correlations found between
their peak frequency, peak flux density and angular size provide strong
evidence that synchrotron self absorption is the cause of the spectral
turnovers, and indicate that young radio sources evolve in a self-similar way.
In addition, the data seem to suggest that the sources are in equipartition
while they evolve. If GPS sources evolve to large size radio sources, their
redshift dependent birth-functions should be the same. Therefore, since the
lifetimes of radio sources are thought to be short compared to the Hubble time,
the observed difference in redshift distribution between GPS and large size
sources must be due to a difference in slope of their luminosity functions. We
argue that this slope is strongly affected by the luminosity evolution of the
individual sources. A scenario for the luminosity evolution is proposed in
which GPS sources increase in luminosity and large scale radio sources decrease
in luminosity with time. This evolution scenario is expected for a ram-pressure
confined radio source in a surrounding medium with a King profile density. In
the inner parts of the King profile, the density of the medium is constant and
the radio source builds up its luminosity, but after it grows large enough the
density of th e surrounding medium declines and the luminosity of the radio
source decreases. A comparison of the local luminosity function (LLF) of GPS
galaxies with that of extended sources is a good test for this evolution
scenario [abridged].Comment: LaTeX, 11 pages, 8 figures; Accepted by MNRAS. Related papers may be
found at http://www.ast.cam.ac.uk/~snellen . Valuable comments of referee
incorporated. More discussion on simulation
Morphological evolution of cluster red sequence galaxies in the past 9 Gyr
Galaxies arrive on the red sequences of clusters at high redshift (z>1) once their star formation is quenched and evolve passively thereafter. However, we have previously found that cluster red sequence galaxies (CRSGs) undergo significant morphological evolution subsequent to the cessation of star formation, at some point in the past 9-10~Gyr. Through a detailed study of a large sample of cluster red sequence galaxies spanning 0.2<z<1.4 we elucidate the details of this evolution. Below (in the last 5-6 Gyr) there is little or no morphological evolution in the population as a whole, unlike in the previous 4-5 Gyrs. Over this earlier time (i) disk-like systems with S{\'e}rsic n < 2 progressively disappear, as (ii) the range of their axial ratios similarly decreases, removing the most elongated systems (those consistent with thin disks seen at an appreciable inclination angle), and (iii) radial colour gradients (bluer outwards) decrease in an absolute sense from significant age-related gradients to a residual level consistent with the metallicity-induced gradients seen in low redshift cluster members. The distribution of their effective radii shows some evidence of evolution, consistent with growth of {\it at most} a factor <1.5 between and , significantly less than for comparable field galaxies, while the distribution of their central (<1kpc) bulge surface densities shows no evolution at least at z<1. A simple model involving the fading and thickening of a disk component after comparatively recent quenching (after ) around an otherwise passively evolving older spheroid component is consistent with all of these findings
Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
We determine the ensemble properties of z~5 Lyman break galaxies (LBGs)
selected as V-band dropouts to i(AB)<26.3 in the Chandra Deep Field South using
their rest-frame UV-to-visible SEDs. By matching the selection and performing
the same analysis that has been used for z~3 samples, we show clear differences
in the properties of two samples of LBGs which are separated by ~1Gyr in
lookback time. We find that z~5 LBGs are typically much younger (<100Myr) and
have lower stellar masses (10^9Msol) than their z~3 counterparts. The
difference in mass is significant even when considering the presence of an
older, underlying population in both samples. Such young and moderately massive
systems dominate the luminous z~5 LBG population (>70%), whereas they comprise
<30% of LBG samples at z~3. This result is robust under all reasonable
modelling assumptions. These intense starbursts appear to be experiencing their
first (few) generations of large-scale star formation and are accumulating
their first significant stellar mass. Their dominance in luminous LBG samples
suggests that z~5 witnesses a period of wide-spread, recent galaxy formation.
As such, z~5 LBGs are the likely progenitors of the spheroidal components of
present-day massive galaxies. This is supported by their high stellar mass
surface densities, their core phase-space densities, as well as the ages of
stars in the bulge of our Galaxy and other massive systems. Their high star
formation rates per unit area suggest that these systems host outflows or winds
that enrich the intra- and inter-galactic media with metals. Their estimated
young ages are consistent with inefficient metal-mixing on galaxy-wide scales.
Therefore these galaxies may contain a significant fraction of metal-free stars
as has been proposed for z~3 LBGs (Jimenez & Haiman 2006). [Abridged]Comment: Accepted for publication in MNRAS. 21 pages, 9 postscript figures.
For a PDF file with high resolution figures, see
http://www-astro.physics.ox.ac.uk/~averma
Discovery of a single faint AGN in a large sample of z>5 Lyman break galaxies
As part of a large spectroscopic survey of z>5 Lyman break galaxies (LBGs),
we have identified a single source which is clearly hosting an AGN. Out of a
sample of more than fifty spectroscopically-confirmed R-band dropout galaxies
at z~5 and above, only J104048.6-115550.2 at z=5.44 shows evidence for a high
ionisation-potential emission-line indicating the presence of a hard ionising
continuum from an AGN. Like most objects in our sample the rest-frame-UV
spectrum shows the UV continuum breaking across a Ly_alpha line. Uniquely
within this sample of LBGs, emission from NV is also detected, a clear
signature of AGN photo-ionisation. The object is spatially resolved in HST
imaging. This, and the comparatively high Ly_alpha/NV flux ratio indicates that
the majority of the Ly_alpha (and the UV continuum longward of it) originates
from stellar photo-ionisation, a product of the ongoing starburst in the Lyman
break galaxy. Even without the AGN emission, this object would have been
photometrically-selected and spectroscopically-confirmed as a Lyman break in
our survey. The measured optical flux (I(AB)=26.1) is therefore an upper limit
to that from the AGN and is of order 100 times fainter than the majority of
known quasars at these redshifts. The detection of a single object in our
survey volume is consistent with the best current models of high redshift AGN
luminosity function, providing a substantial fraction of such AGN are found
within luminous starbursting galaxies. We discuss the cosmological implications
of this discovery.Comment: 7 pages, 3 figures, MNRAS in pres
Large Scale Structure traced by Molecular Gas at High Redshift
We present observations of redshifted CO(1-0) and CO(2-1) in a field
containing an overdensity of Lyman break galaxies (LBGs) at z=5.12. Our
Australia Telescope Compact Array observations were centered between two
spectroscopically-confirmed z=5.12 galaxies. We place upper limits on the
molecular gas masses in these two galaxies of M(H_2) <1.7 x 10^10 M_sun and
<2.9 x 10^9 M_sun (2 sigma), comparable to their stellar masses. We detect an
optically-faint line emitter situated between the two LBGs which we identify as
warm molecular gas at z=5.1245 +/- 0.0001. This source, detected in the CO(2-1)
transition but undetected in CO(1-0), has an integrated line flux of 0.106 +/-
0.012 Jy km/s, yielding an inferred gas mass M(H_2)=(1.9 +/- 0.2) x 10^10
M_sun. Molecular line emitters without detectable counterparts at optical and
infrared wavelengths may be crucial tracers of structure and mass at high
redshift.Comment: 4 pages, accepted for publication in ApJ Letter
- …