298 research outputs found
Disk galaxies at Z=0 and at High Redshift: an explanation of the observed evolution of damped Ly-alpha systems
The analysis of disk formation is based on the White & Rees (1978) picture,
in which disk galaxies form by continuous cooling and accretion of gas within a
merging hierarchy of dark matter halos. A simple Kennicutt law of star
formation for disks, based on a single- fluid gravitational instability model
is introduced. Since the gas supply in the disk is regulated by infall from the
surrounding halo, the gas is always maintained at a critical threshold surface
density. Chemical enrichment of the disks occurs when the surrounding hot halo
gas is enriched with heavy elements ejected during surpernova explosions. This
gas then cools onto the disk producing a new generation of metal-rich stars.
I first show that models of this type can reproduce many of the observed
properties of a typical spiral galaxy like the Milky Way, including its gas and
stellar surface density profiles and the observed relationship between the ages
and metallicities of solar neighbourhood stars. I then use the models to make
inferences about the properties of disk galaxies at high redshift. The total
neutral hydrogen density Omega(HI) increases at higher z. The predicted
increase is mild, but is roughly consistent with the latest derivation of
Omega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The models
are also able to account for some of the other trends other trends seen in the
high-redshift data, including the increase in the number of high column-density
systems at high redshift, as well as the metallicity distribution of damped
Lyman-alpha systems at redshifts 2-3.Comment: 14 pages with 8 figs included, uuencoded postscript file, to appear
in the Proceedings of the Workshop on Coldgas at High Redshift, Kluwe
Physical origin of the large-scale conformity in the specific star formation rates of galaxies
Two explanations have been put forward to explain the observed conformity
between the colours and specific star formation rates (SFR/) of galaxies
on large scales: 1) the formation times of their surrounding dark matter halos
are correlated (commonly referred to as "assembly bias"), 2) gas is heated over
large scales at early times, leading to coherent modulation of cooling and star
formation between well-separated galaxies (commonly referred to as
"pre-heating") . To distinguish between the pre-heating and assembly bias
scenarios, we search for relics of energetic feedback events in the
neighbourhood of central galaxies with different specific star formation rates.
We find a significant excess of very high mass () galaxies out
to a distance of 2.5 Mpc around low SFR/ central galaxies compared to
control samples of higher SFR/ central galaxies with the same stellar mass
and redshift. We also find that very massive galaxies in the neighbourhood of
low SFR/ galaxies have much higher probability of hosting radio loud
active galactic nuclei. The radio-loud AGN fraction in neighbours with is four times higher around passive, non star-forming centrals at
projected distances of 1 Mpc and two times higher at projected distances of 4
Mpc. Finally, we carry out an investigation of conformity effects in the
recently publicly-released Illustris cosmological hydrodynamical simulation,
which includes energetic input both from quasars and from radio mode accretion
onto black holes. We do not find conformity effects of comparable amplitude on
large scales in the simulations and we propose that gas needs to be pushed out
of dark matter halos more efficiently at high redshifts.Comment: 8 pages, 8 figures, submitted to MNRA
Properties of AGN selected by their mid-IR colours: evidence for a physically distinct mode of black hole growth
We study the narrow emission line properties and stellar populations of a
sample of 1385 AGN selected to have strong excess emission at mid-infrared
wavelengths based on comparing Wide-field Infrared Survey Explorer W1-W2 band
colours with optical stellar absorption line indicators. Our goal is to
understand whether the physical conditions in the interstellar medium of these
objects differ from those of local AGN selected by their optical emission line
ratios. To enable this comparison, we construct a control sample of 50,000
optically-selected AGN with the same redshifts that do not have strong mid-IR
excess emission. The mid-IR excess and control samples differ strongly in
[OIII] line luminosity, ionized gas excitation mechanism, ionization state and
electron density. We show that the radio-detected, mid-IR excess AGN constitute
the most luminous and highly ionized AGN in the local Universe and they
contribute primarily to the growth of black holes in the most massive galaxies.
At least half of this black hole growth is occurring in galaxies with recent
starbursts. The morphologies of these systems indicate that the starbursts have
probably been triggered by galaxy-galaxy mergers and interactions. The most
luminous AGN in our mid-IR excess sample have properties that are similar to
the Type II quasars studied at higher redshifts. In contrast, the control
sample constitute a class of lower ionization, less luminous AGN in more
quiescent galaxies that contribute primarily to the growth of low mass black
holes.Comment: 15 pages, 17 figures, accepted for publication in MNRA
Hierarchical Clustering and the Butcher-Oemler Effect
We show that the rapid evolution in the fraction of blue, star-forming
galaxies seen in clusters as a function of redshift (the Butcher-Oemler effect)
can be explained very simply if structure formation in the universe proceeeds
hierarchically. We show that a rich cluster observed at high redshift has had a
significantly different evolutionary history to a cluster of the same richness
observed today. High redshift clusters take longer to assemble and thus under-
go more merging at small lookback times. We have investigated two models of
star formation in cluster galaxies: 1) a model in which star formation is
induced by galaxy-galaxy mergers and interactions and 2) a model in which star
formation is regulated by the infall of galaxies onto larger systems such as
groups and clusters. Both models produce trends consistent with the Butcher-
Oemler effect. Our models of cluster formation and evolution allow us to make
predictions about trends in the observed properties of clusters with redshift.
We find that there should be a correlation between the mass of the cluster or
group and the strength of the observed Butcher-Oemler effect, with more massive
systems exhibiting more evolution than less massive systems. We also predict
that both the blue galaxy fraction and the incidence of interacting galaxies in
rich clusters should rise continuously with redshift. Finally, we have explored
the influence of cosmological parameters on our predictions for cluster
evolution. We find that models in which structure formation occurs at very
early epochs, such as low models, predict rather little recent star
formation and merging activity in clusters at redshifts of around 0.4.Comment: Latex file, 12 pages, postscript figures on reques
The Physical Properties of Galaxies with Unusually Red Mid-Infrared Colours
The goal of this paper is to investigate the physical nature of galaxies in
the redshift range that have strong excess emission at mid-IR
wavelengths and to determine whether they host a population of accreting black
holes that cannot be identified using optical emission lines. We show that at
fixed stellar mass and , the distribution of [3.4]-[4.6]
m (WISE W1-W2 band) colours is sharply peaked, with a long tail to much
redder W1-W2 colours. We introduce a procedure to pull out the red outlier
population based on a combination of three stellar population diagnostics. When
compared with optically-selected AGN, red outliers are more likely to be found
in massive galaxies, and they tend to have lower stellar mass densities,
younger stellar ages and higher dust content than optically-selected AGN hosts.
They are twice as likely to be detected at radio wavelengths. We examine W1-W2
colour profiles for a subset of the nearest, reddest outliers and find that
most are not centrally peaked, indicating that the hot dust emission is spread
throughout the galaxy. We find that radio luminosity is the quantity that is
most predictive of a redder central W1-W2 colour. Radio-loud galaxies with
centrally concentrated hot dust emission are almost always morphologically
disturbed, with compact, unresolved emission at 1.4 Ghz. Eighty percent of such
systems are identifiable as AGN using optical emission line diagnostics.Comment: 12 pages, 15 figures, accepted in MNRA
Large scale correlations in gas traced by MgII absorbers around low mass galaxies
The physical origin of the large-scale conformity in the colours and specific
star formation rates of isolated low mass central galaxies and their neighbours
on scales in excess of 1 Mpc is still under debate. One possible scenario is
that gas is heated over large scales by feedback from active galactic nuclei
(AGN), leading to coherent modulation of cooling and star formation between
well-separated galaxies. In this Letter, the metal line absorption catalogue of
Zhu & Menard (2013) is used to probe gas out to large projected radii around a
sample of a million galaxies with stellar masses ~10^{10} M_{sun} and
photometric redshifts in the range 0.4<z<0.8 selected from Sloan Digital Sky
Survey imaging data. This galaxy sample covers an effective volume of 2.2
Gpc^3. A statistically significant excess of MgII absorbers is present around
the red low mass galaxies compared to their blue counterparts out to projected
radii of 10 Mpc. In addition, the equivalent width distribution function of
MgII absorbers around low mass galaxies is shown to be strongly affected by the
presence of a nearby (R_p<2 Mpc) radio-loud AGN out to projected radii of 5
Mpc.Comment: 5 pages, accepted in MNRAS Letter
The Outer Stellar Populations and Environments of Unusually HI-rich Galaxies
We investigate the nature of HI-rich galaxies from the ALFALFA and GASS
surveys, which are defined as galaxies in the top 10th percentile in atomic gas
fraction at a given stellar mass. We analyze outer (R>1.5 Re) stellar
populations for a subset of face-on systems using optical g-r versus r-z
colour/colour diagrams. The results are compared with those from control
samples that are defined without regard to atomic gas content, but are matched
in redshift, stellar mass and structural parameters. HI-rich early-type (C>2.6)
and late-type (C<2.6) galaxies are studied separately. When compared to the
control sample, the outer stellar populations of the majority of HI-rich
early-type galaxies are shifted in the colour/colour plane along a locus
consistent with younger stellar ages, but similar metallicities. The outer
colours of HI-rich late-type galaxies are much bluer in r-z than the HI-rich
early types, and we infer that they have outer disks which are both younger and
more metal-poor. We then proceed to analyze the galaxy environments of HI-rich
galaxies on scales of 500 kpc. HI-rich early-type galaxies with low (log M* <
10.5) stellar masses differ significantly from the control sample in that they
are more likely to be central rather than satellite systems. Their satellites
are also less massive and have younger stellar populations. Similar, but weaker
effects are found for low mass HI-rich late-type galaxies. In addition, we find
that the satellites of HI-rich late-types exhibit a greater tendency to align
along the major axis of the primary. No environmental differences are found for
massive (log M* > 10.5) HI-rich galaxies, regardless of type.Comment: 12 pages, 14 figures, submitted to MNRA
The Observed Properties of High-Redshift Cluster Galaxies
We use the semi-analytic models of galaxy formation developed by Kauffmann,
White \& Guiderdoni to generate predictions for the observed properties of
cluster and group galaxies at redshifts between 0 and 0.6. We examine four sets
of cosmological initial conditions: low-density CDM models with and without
cosmological constant, a flat CDM model and a mixed dark matter model. These
models were selected because they span a wide range in cluster formation epoch.
The semi-analytic models that we employ are able to follow both the evolution
of the dark matter component of clusters and the formation and evolution of the
stellar populations of the cluster galaxies. We are thus able to generate model
predictions that can be compared directly with the observational data. In the
low-density CDM models, clusters form at high red- shift and accrete very
little mass at recent times. Our models predict that essentially no evolution
in the observed properties of clusters will have occurred by a redshift of 0.6,
in direct contradiction with the data. In contrast, in the MDM model, both
galaxies and clusters form extremely late. This model predicts evolution which
appears to be too extreme to be in agreement with the observations. The flat
CDM model, which is intermediate in structure formation epoch, is most
successful. This model is able to account for the evolution of the blue
fraction of rich clusters with redshift, the relationship between blue fraction
and cluster richness at different epochs, and the changes in the distribution
of the morphologies of cluster galaxies by a redshift of 0.4.Comment: Latex file, 12 pages, postscript figures on request, 99
The K-band luminosity function at z=1: a powerful constraint on galaxy formation theory
There are two major approaches to modelling galaxy evolution. The traditional
view is that the most massive galaxies were assembled early and have evolved
with steeply declining star formation rates since a redshift of 2 or higher.
According to hierarchical theories, massive galaxies were assembled much more
recently from mergers of smaller subunits. Here we present a simple
observational test designed to differentiate between the two. The observed
K-band flux from a galaxy is a good measure of its stellar mass even at high
redshift. It is probably only weakly affected by dust extinction. We compute
the evolution of the observed K-band luminosity function for traditional, pure
luminosity evolution (PLE) models and for hierarchical models. At z=0, both
models can fit the observed local K-band luminosity function. By redshift 1,
they differ greatly in the predicted abundance of bright galaxies. We calculate
the redshift distributions of K-band selected galaxies and compare these with
available data. We show that the number of K<19 galaxies with redshifts greater
than 1 is well below the numbers predicted by the PLE models. In the Songaila
et al (1994) redshift sample of 118 galaxies with 16<K<18, 33 galaxies are
predicted to lie at z>1. Only 2 are observed. In the Cowie et al. (1996)
redshift sample of 52 galaxies with 18<K<19, 28 galaxies are predicted to lie
at at z>1. Only 5 are observed. Both these samples are more than 90% complete.
We conclude that there is already strong evidence that the abundance of massive
galaxies at z=1 is well below the local value. This is inconsistent with the
traditional model, but similar to the expectations of hierarchical models.Comment: 13 pages, Latex, 4 figures included in text, submitted to MNRAS pink
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