225 research outputs found
What Regulates Galaxy Evolution? Open Questions in Our Understanding of Galaxy Formation and Evolution
In April 2013, a workshop entitled "What Regulates Galaxy Evolution" was held
at the Lorentz Center. The aim of the workshop was to bring together the
observational and theoretical community working on galaxy evolution, and to
discuss in depth of the current problems in the subject, as well as to review
the most recent observational constraints. A total of 42 astrophysicists
attended the workshop. A significant fraction of the time was devoted to
identifying the most interesting "open questions" in the field, and to discuss
how progress can be made. This review discusses the four questions (one for
each day of the workshop) that, in our opinion, were the focus of the most
intense debate. We present each question in its context, and close with a
discussion of what future directions should be pursued in order to make
progress on these problems.Comment: 36 pages, 6 Figures, submitted to New Astronomy Review
The Mass Growth and Stellar Ages of Galaxies: Observations versus Simulations
Using observed stellar mass functions out to , we measure the main
progenitor stellar mass growth of descendant galaxies with masses of
at using an evolving
cumulative number density selection. From these mass growth histories, we are
able to measure the time at which half the total stellar mass of the descendant
galaxy was assembled, , which, in order of decreasing mass corresponds
to redshifts of and . We compare this to the
median light-weighted stellar age ( and
) of a sample of low redshift SDSS galaxies (from the literature) and
find the timescales are consistent with more massive galaxies forming a higher
fraction of their stars ex-situ compared to lower mass descendants. We find
that both and strongly correlate with mass which is in contrast
to what is found in the EAGLE hydrodynamical simulation which shows a flat
relationship between and . However, the semi-analytic model of
\citet{henriques2015} is consistent with the observations in both and
with , showing the most recent semi-analytic models are better
able to decouple the evolution of the baryons from the dark matter in
lower-mass galaxies.Comment: 6 pages, 3 figures, accepted for publication in ApJ
The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters
Recent observations have highlighted a significant population of faint but
large (r_eff>1.5 kpc) galaxies in the Coma cluster. The origin of these Ultra
Diffuse Galaxies (UDGs) remains puzzling, as the interpretation of the
observational results has been hindered by the subjective selection of UDGs,
and the limited study of only the Coma (and some examples in the Virgo-)
cluster. We extend the study of UDGs using 8 clusters in the redshift range
0.044<z<0.063 with deep g- and r-band imaging data taken with MegaCam at the
CFHT. We describe an automatic selection pipeline for quantitative
identification, tested for completeness using image simulations of these
galaxies. We find that the abundance of the UDGs we can detect increases with
cluster mass, reaching ~200 in typical haloes of M200~10^15 Msun. The cluster
UDGs have colours consistent with the cluster red sequence, and have a steep
size distribution that declines as n~r_eff^-3.4. Their radial distribution is
significantly steeper than NFW in the outskirts, and is significantly shallower
in the inner parts. They follow the same radial distribution as the more
massive quiescent galaxies in the clusters, except within the core region of
r<0.15XR200 (or <300 kpc). Within this region the number density of UDGs drops
and is consistent with zero. These diffuse galaxies can only resist tidal
forces down to this cluster-centric distance if they are highly centrally
dark-matter dominated. The observation that the radial distribution of more
compact dwarf galaxies (r_eff<1.0 kpc) with similar luminosities follows the
same distribution as the UDGs, but exist down to a smaller distance of 100kpc
from the cluster centres, indicates that they may have similarly massive
sub-haloes as the UDGs. Although several scenarios can give rise to the UDG
population, our results point to differences in the formation history as the
most plausible explanation.Comment: 12 pages, 11 figures. Accepted for publication in A&A after minor
revisio
Using Cumulative Number Densities to Compare Galaxies across Cosmic Time
Comparing galaxies across redshifts at fixed cumulative number density is a
popular way to estimate the evolution of specific galaxy populations. This
method ignores scatter in mass accretion histories and galaxy-galaxy mergers,
which can lead to errors when comparing galaxies over large redshift ranges
(Delta z > 1). We use abundance matching in the LCDM paradigm to estimate the
median change in number density with redshift and provide a simple fit (+0.16
dex per unit Delta z) for progenitors of z = 0 galaxies. We find that galaxy
descendants do not evolve in the same way as galaxy progenitors, largely due to
scatter in mass accretion histories. We also provide estimates for the 1-sigma
range of number densities corresponding to galaxy progenitors and descendants.
Finally, we discuss some limits on number density comparisons, which arise due
to difficulties measuring physical quantities (e.g., stellar mass) consistently
across redshifts. A public tool to calculate number density evolution for
galaxies, as well as approximate halo masses, is available online.Comment: 5 pages, minor revisions to match ApJL accepted version. Code
available at: http://code.google.com/p/nd-redshif
The Canadian Cluster Comparison Project: detailed study of systematics and updated weak lensing masses
Masses of clusters of galaxies from weak gravitational lensing analyses of
ever larger samples are increasingly used as the reference to which baryonic
scaling relations are compared. In this paper we revisit the analysis of a
sample of 50 clusters studied as part of the Canadian Cluster Comparison
Project. We examine the key sources of systematic error in cluster masses. We
quantify the robustness of our shape measurements and calibrate our algorithm
empirically using extensive image simulations. The source redshift distribution
is revised using the latest state-of-the-art photometric redshift catalogs that
include new deep near-infrared observations. Nonetheless we find that the
uncertainty in the determination of photometric redshifts is the largest source
of systematic error for our mass estimates. We use our updated masses to
determine b, the bias in the hydrostatic mass, for the clusters detected by
Planck. Our results suggest 1-b=0.76+-0.05(stat)}+-0.06(syst)}, which does not
resolve the tension with the measurements from the primary cosmic microwave
background.Comment: resubmitted to MNRAS after review by refere
The stellar mass function of galaxies in Planck-selected clusters at 0.5 < z < 0.7: new constraints on the timescale and location of satellite quenching
We study the abundance of star-forming and quiescent galaxies in a sample of
21 massive clusters at 0.5<z<0.7, detected with the Planck satellite. We
measure the cluster galaxy stellar mass function (SMF), which is a fundamental
observable to study and constrain the formation and evolution of galaxies. Our
measurements are based on homogeneous and deep multi-band photometry spanning
u- to the Ks-band for each cluster and are supported by spectroscopic data from
different programs. The galaxy population is separated between quiescent and
star-forming galaxies based on their rest-frame U-V and V-J colours. The SMF is
compared to that of field galaxies at the same redshifts, using data from the
COSMOS/UltraVISTA survey. We find that the shape of the SMF of star-forming
galaxies does not depend on environment, while the SMF of quiescent galaxies
has a significantly steeper low-mass slope in the clusters compared to the
field. We estimate the environmental quenching efficiency (f_EQ), i.e. the
probability for a galaxy that would normally be star forming in the field, to
be quenched due to its environment. The f_EQ shows no stellar-mass dependence
in any environment, but it increases from 40% in the cluster outskirts to ~90%
in the cluster centres. The radial signature of f_EQ provides constraints on
where the dominant quenching mechanism operates in these clusters and on what
timescale. Exploring these using a simple model based on galaxy orbits obtained
from an N-body simulation, we find a clear degeneracy between both parameters.
For example, the quenching process may either be triggered on a long (~3 Gyr)
time scale at large radii (r~8R_500), or happen well within 1 Gyr at r<R_500.
The radius where quenching is triggered is at least r_quench> 0.67R_500
(95%CL). The ICM density at this location suggests that ram-pressure stripping
of the cold gas is a likely cause of quenching. [Abridged]Comment: 16 pages, 12 figures, accepted for publication in A&
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