71 research outputs found
More than just halo mass: Modelling how the red galaxy fraction depends on multiscale density in a HOD framework
The fraction of galaxies with red colours depends sensitively on environment,
and on the way in which environment is measured. To distinguish competing
theories for the quenching of star formation, a robust and complete description
of environment is required, to be applied to a large sample of galaxies. The
environment of galaxies can be described using the density field of neighbours
on multiple scales - the multiscale density field. We are using the Millennium
simulation and a simple HOD prescription which describes the multiscale density
field of Sloan Digital Sky Survey DR7 galaxies to investigate the dependence of
the fraction of red galaxies on the environment. Using a volume limited sample
where we have sufficient galaxies in narrow density bins, we have more dynamic
range in halo mass and density for satellite galaxies than for central
galaxies. Therefore we model the red fraction of central galaxies as a constant
while we use a functional form to describe the red fraction of satellites as a
function of halo mass which allows us to distinguish a sharp from a gradual
transition. While it is clear that the data can only be explained by a gradual
transition, an analysis of the multiscale density field on different scales
suggests that colour segregation within the haloes is needed to explain the
results. We also rule out a sharp transition for central galaxies, within the
halo mass range sampled.Comment: 24 pages, 21 figures, accepted for publication by MNRA
The Hierarchical Origins of Observed Galaxy Morphology
Galaxies grow primarily via accretion-driven star formation in discs and
merger-driven growth of bulges. These processes are implicit in semi-analytical
models of galaxy formation, with bulge growth in particular relating directly
to the hierarchical build-up of halos and their galaxies. In this paper, we
consider several implementations of two semi-analytical models. Focusing on
implementations in which bulges are formed during mergers only, we examine the
fractions of elliptical galaxies and both passive and star-forming disk
galaxies as functions of stellar and halo mass, for central and satellite
systems. This is compared to an observational cross-matched SDSS+RC3 z ~ 0
sample of galaxies with accurate visual morphological classifications and
M_{stellar} > 10^10.5 M_{sol}. The models qualitatively reproduce the observed
increase of elliptical fraction with stellar mass, and with halo mass for
central galaxies, supporting the idea that observed ellipticals form during
major mergers. However, the overall elliptical fraction produced by the models
is much too high compared with the z ~ 0 data. Since the "passive" -- i.e.
non-star-forming -- fractions are approximately reproduced, and since the
fraction which are star-forming disc galaxies is also reproduced, the problem
is that the models overproduce ellipticals at the expense of passive S0 and
spiral galaxies. Bulge-growth implementations (tuned to reproduce simulations)
which allow the survival of residual discs in major mergers still destroy too
much of the disc. Increasing the lifetime of satellites, or allowing
significant disc regrowth around merger remnants, merely increases the fraction
of star-forming disc galaxies. Instead, it seems necessary to reduce the mass
ratios of merging galaxies, so that most mergers produce modest bulge growth in
disc-galaxy remnants instead of ellipticals. [Abridged]Comment: latex, 20 pages, 13 figures. Accepted by Monthly Notices. Source
package includes full version of Table 1 from paper (file
sdssrc3_table_for_paper.tab
On the influence of environment on star forming galaxies
We use our state-of-the-art semi analytic model for GAlaxy Evolution and
Assembly (GAEA), and observational measurements of nearby galaxies to study the
influence of the environment on the gas content and gaseous/stellar disc sizes
of star-forming galaxies. We analyse the origin of differences between physical
properties of satellites and those of their central counterparts, identified by
matching the Vmax of their host haloes at the accretion time of the satellites.
Our model reproduces nicely the differences between centrals and satellites
measured for the HI mass, size of the star-forming region, and stellar radii.
In contrast, our model predicts larger differences with respect to data for the
molecular gas mass and star formation rate. By analysing the progenitors of
central and satellite model galaxies, we find that differences in the gas
content arise after accretion, and can be entirely ascribed to the
instantaneous stripping of the hot gas reservoir. The suppression of cold gas
replenishment via cooling and star formation leads to a reduction of the cold
gas and of its density. Therefore, more molecular gas is lost than lower
density HI gas, and model satellites have less molecular gas and lower star
formation rates than observed satellites. We argue that these disagreements
could be largely resolved with the inclusion of a proper treatment for
ram-pressure stripping of cold gas and a more gradual stripping of the hot gas
reservoir. A more sophisticated treatment of angular momentum exchanges,
accounting for the multi-phase nature of the gaseous disc is also required.Comment: 15 pages, 9 figures, accepted for publication in MNRA
The GEEC2 spectroscopic survey of Galaxy Groups at
We present the data release of the Gemini-South GMOS spectroscopy in the
fields of 11 galaxy groups at , within the COSMOS field. This forms
the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project
to study galaxy evolution in haloes with across cosmic
time. The final sample includes spectroscopically--confirmed members with
per cent complete for galaxies within the virial
radius, and with stellar mass . Including
galaxies with photometric redshifts we have an effective sample size of galaxies within the virial radii of these groups. We present group
velocity dispersions, dynamical and stellar masses. Combining with the GCLASS
sample of more massive clusters at the same redshift we find the total stellar
mass is strongly correlated with the dynamical mass, with
. This stellar
fraction of per cent is lower than predicted by some halo occupation
distribution models, though the weak dependence on halo mass is in good
agreement. Most groups have an easily identifiable most massive galaxy (MMG)
near the centre of the galaxy distribution, and we present the spectroscopic
properties and surface brightness fits to these galaxies. The total stellar
mass distribution in the groups, excluding the MMG, compares well with an NFW
profile with concentration , for galaxies beyond . This is
more concentrated than the number density distribution, demonstrating that
there is some mass segregation.Comment: Accepted for publication in MNRAS. The appendix is omitted due to
large figures. The full version will be available from the MNRAS website and
from http://quixote.uwaterloo.ca/~mbalogh/papers/GEEC2_data.pdf. Long data
tables are available from MNRAS or by contacting the first autho
Evolution in the Disks and Bulges of Group Galaxies since z=0.4
We present quantitative morphology measurements of a sample of optically
selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope
(HST) Advanced Camera for Surveys (ACS) and the GIM2D surface
brightness--fitting software package. The group sample is derived from the
Canadian Network for Observational Cosmology Field Redshift survey (CNOC2) and
follow-up Magellan spectroscopy. We compare these measurements to a similarly
selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z <
0.12. We find that, at both epochs, the group and field fractional bulge
luminosity (B/T) distributions differ significantly, with the dominant
difference being a deficit of disk--dominated (B/T < 0.2) galaxies in the group
samples. At fixed luminosity, z=0.4 groups have ~ 5.5 +/- 2 % fewer
disk--dominated galaxies than the field, while by z=0.1 this difference has
increased to ~ 19 +/- 6 %. Despite the morphological evolution we see no
evidence that the group environment is actively perturbing or otherwise
affecting the entire existing disk population. At both redshifts, the disks of
group galaxies have similar scaling relations and show similar median
asymmetries as the disks of field galaxies. We do find evidence that the
fraction of highly asymmetric, bulge--dominated galaxies is 6 +/- 3 % higher in
groups than in the field, suggesting there may be enhanced merging in group
environments. We replicate our group samples at z=0.4 and z=0 using the
semi-analytic galaxy catalogues of Bower et al (2006). This model accurately
reproduces the B/T distributions of the group and field at z=0.1. However, the
model does not reproduce our finding that the deficit of disks in groups has
increased significantly since z=0.4.Comment: Accepted for publication in MNRAS. 20 pages, 17 figure
The definition of environment and its relation to the quenching of galaxies at z=1-2 in a hierarchical Universe
A well calibrated method to describe the environment of galaxies at all
redshifts is essential for the study of structure formation. Such a calibration
should include well understood correlations with halo mass, and the possibility
to identify galaxies which dominate their potential well (centrals), and their
satellites. Focusing on z = 1 and 2 we propose a method of environmental
calibration which can be applied to the next generation of low to medium
resolution spectroscopic surveys. Using an up-to-date semi-analytic model of
galaxy formation, we measure the local density of galaxies in fixed apertures
on different scales. There is a clear correlation of density with halo mass for
satellite galaxies, while a significant population of low mass centrals is
found at high densities in the neighbourhood of massive haloes. In this case
the density simply traces the mass of the most massive halo within the
aperture. To identify central and satellite galaxies, we apply an
observationally motivated stellar mass rank method which is both highly pure
and complete, especially in the more massive haloes where such a division is
most meaningful. Finally we examine a test case for the recovery of
environmental trends: the passive fraction of galaxies and its dependence on
stellar and halo mass for centrals and satellites. With careful calibration,
observationally defined quantities do a good job of recovering known trends in
the model. This result stands even with reduced redshift accuracy, provided the
sample is deep enough to preserve a wide dynamic range of density.Comment: 19 pages, 12 figures, accepted for publication in MNRA
Impact of Redshift Information on Cosmological Applications with Next-Generation Radio Surveys
In this paper, we explore how the forthcoming generation of large-scale radio
continuum surveys, with the inclusion of some degree of redshift information,
can constrain cosmological parameters. By cross-matching these radio surveys
with shallow optical to near-infrared surveys, we can essentially separate the
source distribution into a low- and a high-redshift sample, thus providing a
constraint on the evolution of cosmological parameters such as those related to
dark energy. We examine two radio surveys, the Evolutionary Map of the Universe
(EMU) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN).
A crucial advantage is their combined potential to provide a deep, full-sky
survey. The surveys used for the cross-identifications are SkyMapper and SDSS,
for the southern and northern skies, respectively. We concentrate on the galaxy
clustering angular power spectrum as our benchmark observable, and find that
the possibility of including such low redshift information yields major
improvements in the determination of cosmological parameters. With this
approach, and provided a good knowledge of the galaxy bias evolution, we are
able to put strict constraints on the dark energy parameters, i.e.
w_0=-0.9+/-0.041 and w_a=-0.24+/-0.13, with type Ia supernovae and CMB priors
(with a one-parameter bias in this case); this corresponds to a Figure of Merit
(FoM) > 600, which is twice better than what is obtained by using only the
cross-identified sources and greater than four time better than the case
without any redshift information at all.Comment: 12 pages, 6 figures, 6 tables; accepted for publication in MNRA
First results from the VIRIAL survey: the stellar content of -selected quiescent galaxies at from KMOS
We investigate the stellar populations of 25 massive, galaxies
() at using data obtained with
the K-band Multi-Object Spectrograph (KMOS) on the ESO VLT. Targets were
selected to be quiescent based on their broadband colors and redshifts using
data from the 3D-HST grism survey. The mean redshift of our sample is , where KMOS YJ-band data probe age- and metallicity-sensitive
absorption features in the rest-frame optical, including the band, Fe I,
and high-order Balmer lines. Fitting simple stellar population models to a
stack of our KMOS spectra, we derive a mean age of Gyr.
We confirm previous results suggesting a correlation between color and age for
quiescent galaxies, finding mean ages of Gyr and
Gyr for the reddest and bluest galaxies in our sample.
Combining our KMOS measurements with those obtained from previous studies at
we find evidence for a Gyr spread in the formation epoch of
massive galaxies. At the measured stellar ages are consistent with
passive evolution, while at they appear to saturate at
1 Gyr, which likely reflects changing demographics of the (mean)
progenitor population. By comparing to star-formation histories inferred for
"normal" star-forming galaxies, we show that the timescales required to form
massive galaxies at are consistent with the enhanced
-element abundances found in massive local early-type galaxies.Comment: 6 pages, 5 figures, accepted for publication in ApJ
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