24 research outputs found
Is ram-pressure stripping an efficient mechanism to remove gas in galaxies?
We study how the gas in a sample of galaxies (M* > 10e9 Msun) in clusters,
obtained in a cosmological simulation, is affected by the interaction with the
intra-cluster medium (ICM). The dynamical state of each elemental parcel of gas
is studied using the total energy. At z ~ 2, the galaxies in the simulation are
evenly distributed within clusters, moving later on towards more central
locations. In this process, gas from the ICM is accreted and mixed with the gas
in the galactic halo. Simultaneously, the interaction with the environment
removes part of the gas. A characteristic stellar mass around M* ~ 10e10 Msun
appears as a threshold marking two differentiated behaviours. Below this mass,
galaxies are located at the external part of clusters and have eccentric
orbits. The effect of the interaction with the environment is marginal. Above,
galaxies are mainly located at the inner part of clusters with mostly radial
orbits with low velocities. In these massive systems, part of the gas, strongly
correlated with the stellar mass of the galaxy, is removed. The amount of
removed gas is sub-dominant compared with the quantity of retained gas which is
continuously influenced by the hot gas coming from the ICM. The analysis of
individual galaxies reveals the existence of a complex pattern of flows,
turbulence and a constant fuelling of gas to the hot corona from the ICM that
could make the global effect of the interaction of galaxies with their
environment to be substantially less dramatic than previously expected.Comment: 17 pages, 12 figures, accepted for publication in MNRA
Morphological Segregation in the Surroundings of Cosmic Voids
We explore the morphology of galaxies living in the proximity of cosmic
voids, using a sample of voids identified in the Sloan Digital Sky Survey Data
Release 7. At all stellar masses, void galaxies exhibit morphologies of a later
type than galaxies in a control sample, which represent galaxies in an average
density environment. We interpret this trend as a pure environmental effect,
independent of the mass bias, due to a slower galaxy build-up in the rarefied
regions of voids. We confirm previous findings about a clear segregation in
galaxy morphology, with galaxies of a later type being found at smaller
void-centric distances with respect to the early-type galaxies. We also show,
for the first time, that the radius of the void has an impact on the
evolutionary history of the galaxies that live within it or in its
surroundings. In fact, an enhanced fraction of late-type galaxies is found in
the proximity of voids larger than the median void radius. Likewise, an excess
of early-type galaxies is observed within or around voids of a smaller size. A
significant difference in galaxy properties in voids of different sizes is
observed up to 2 Rvoid, which we define as the region of influence of voids.
The significance of this difference is greater than 3sigma for all the
volume-complete samples considered here. The fraction of star-forming galaxies
shows the same behavior as the late-type galaxies, but no significant
difference in stellar mass is observed in the proximity of voids of different
sizes.Comment: Published in ApJ
The UV spectral window: new means to constrain the stellar populations of early-type galaxies
We discuss the possibilities of exploiting the UV spectral using the J-PAS survey to disentangle young contributions from apparently old galaxies. We show that the feasibility of determining such young stellar components in early-type galaxies at moderately high redshift
Systematic variation of the stellar Initial Mass Function with velocity dispersion in early-type galaxies
An essential component of galaxy formation theory is the stellar initial mass
function (IMF), that describes the parent distribution of stellar mass in star
forming regions. We present observational evidence in a sample of early-type
galaxies (ETGs) of a tight correlation between central velocity dispersion and
the strength of several absorption features sensitive to the presence of
low-mass stars. Our sample comprises ~40,000 ETGs from the SPIDER survey
(z<0.1). The data, extracted from the Sloan Digital Sky Survey, are combined,
rejecting both noisy data, and spectra with contamination from telluric lines,
resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N>
400 per A). A combined analysis of IMF-sensitive line strengths and spectral
fitting is performed with the latest state-of the art population synthesis
models (an extended version of the MILES models). A significant trend is found
between IMF slope and velocity dispersion, towards an excess of low-mass stars
in the most massive galaxies. Although we emphasize that accurate values of the
IMF slope will require a detailed analysis of chemical composition (such as
[a/Fe] or even individual element abundance ratios), the observed trends
suggest that low-mass ETGs are better fit by a Kroupa-like IMF, whereas massive
galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at velocity
dispersions above 200km/s.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
SHARDS: A global view of the star formation activity at z~0.84 and z~1.23
In this paper, we present a comprehensive analysis of star-forming galaxies
(SFGs) at intermediate redshifts (z~1). We combine the ultra-deep optical
spectro-photometric data from the Survey for High-z Absorption Red and Dead
Sources (SHARDS) with deep UV-to-FIR observations in the GOODS-N field.
Exploiting two of the 25 SHARDS medium-band filters, F687W17 and F823W17, we
select [OII] emission line galaxies at z~0.84 and z~1.23 and characterize their
physical properties. Their rest-frame equivalent widths
(EW([OII])), line fluxes, luminosities, star formation rates
(SFRs) and dust attenuation properties are investigated. The evolution of the
EW([OII]) closely follows the SFR density evolution of the
universe, with a trend of EW([OII])(1+z) up to
redshift z~1, followed by a possible flattening. The SF properties of the
galaxies selected on the basis of their [OII] emission are compared with
complementary samples of SFGs selected by their MIR and FIR emission, and also
with a general mass-selected sample of galaxies at the same redshifts. We
demonstrate observationally that the UVJ diagram (or, similarly, a cut in the
specific SFR) is only partially able to distinguish the quiescent galaxies from
the SFGs. The SFR-M relation is investigated for the different samples,
yelding a logarithmic slope ~1, in good agreement with previous results. The
dust attenuations derived from different SFR indicators (UV(1600), UV(2800),
[OII], IR) are compared and show clear trends with respect to both the stellar
mass and total SFR, with more massive and highly star-forming galaxies being
affected by stronger dust attenuation.Comment: Replaced to match the accepted version (24 pages, 1 table, 17
figures). Published in ApJ, 812, 155 (2015):
http://stacks.iop.org/0004-637X/812/15
The zCOSMOS 10k-Bright Spectroscopic Sample
We present spectroscopic redshifts of a large sample of galaxies with I_(AB) < 22.5 in the COSMOS field, measured from spectra of 10,644 objects that have been obtained in the first two years of observations in the zCOSMOS-bright redshift survey. These include a statistically complete subset of 10,109 objects. The average accuracy of individual redshifts is 110 km s^(–1), independent of redshift. The reliability of individual redshifts is described by a Confidence Class that has been empirically calibrated through repeat spectroscopic observations of over 600 galaxies. There is very good agreement between spectroscopic and photometric redshifts for the most secure Confidence Classes. For the less secure Confidence Classes, there is a good correspondence between the fraction of objects with a consistent photometric redshift and the spectroscopic repeatability, suggesting that the photometric redshifts can be used to indicate which of the less secure spectroscopic redshifts are likely right and which are probably wrong, and to give an indication of the nature of objects for which we failed to determine a redshift. Using this approach, we can construct a spectroscopic sample that is 99% reliable and which is 88% complete in the sample as a whole, and 95% complete in the redshift range 0.5 < z < 0.8. The luminosity and mass completeness levels of the zCOSMOS-bright sample of galaxies is also discussed
Mass and environment as drivers of galaxy evolution in SDSS and zCOSMOS and the origin of the Schechter function
We explore the inter-relationships between mass, star-formation rate and
environment in the SDSS, zCOSMOS and other surveys. The differential effects of
mass and environment are completely separable to z ~ 1, indicating that two
distinct processes are operating, "mass-quenching" and "environment-quenching".
Environment-quenching, at fixed over-density, evidently does not change with
epoch to z ~ 1, suggesting that it occurs as large-scale structure develops in
the Universe. The observed constancy of the mass-function shape for
star-forming galaxies, demands that the mass-quenching of galaxies around and
above M*, must be proportional to their star-formation rates at all z < 2. We
postulate that this simple mass-quenching law also holds over a much broader
range of stellar mass and epoch. These two simple quenching processes, plus
some additional quenching due to merging, then naturally produce (a) a
quasi-static Schechter mass function for star-forming galaxies with a value of
M* that is set by the proportionality between the star-formation and
mass-quenching rates, (b) a double Schechter function for passive galaxies with
two components: the dominant one is produced by mass-quenching and has exactly
the same M* as the star-forming galaxies but an alpha shallower by +1, while
the other is produced by environment effects and has the same M* and alpha as
the star-forming galaxies, and is larger in high density environments.
Subsequent merging of quenched galaxies modifies these predictions somewhat in
the denser environments, slightly increasing M* and making alpha more negative.
All of these detailed quantitative relationships between the Schechter
parameters are indeed seen in the SDSS, lending strong support to our simple
empirically-based model. The model naturally produces for passive galaxies the
"anti-hierarchical" run of mean ages and alpha-element abundances with mass.Comment: 66 pages, 19 figures, 1 movie, accepted for publication in ApJ. The
movie is also available at
http://www.exp-astro.phys.ethz.ch/zCOSMOS/MF_simulation_d1_d4.mo
Semi-analytical models of galaxy formation and comparison with observations
In this Thesis we attempted to answer to some of the fundamental questions concerning galaxy evolution.
In particular when and how galaxies got their
present-day stellar content and how this process depends on their mass. In order to address this key issues, we developed a new semi-analytic model of galaxy formation (GECO, Galaxy Evolution COde), that couples a Monte Carlo representation of the hierarchical clustering of dark matter haloes with analytic recipes for the baryonic physics, such as the cooling of the gas, the star formation, the feedback from SN and AGN. We set the model on observations in the local universe and then we predict and compare results for the high redshift galaxies