344 research outputs found
Constraints on feedback processes during the formation of early-type galaxies
Galaxies are found to obey scaling relations between a number of observables.
These relations follow different trends at the low- and the high-mass ends. The
processes driving the curvature of scaling relations remain uncertain. In this
letter, we focus on the specific family of early-type galaxies, deriving the
star formation histories of a complete sample of visually classified galaxies
from SDSS-DR7 over the redshift range 0.01<z<0.025, covering a stellar mass
interval from 10^9 to 3 x 10^11 Msun. Our sample features the characteristic
"knee" in the surface brightness vs. mass distribution at Mstar~3 x 10^10 Msun.
We find a clear difference between the age and metallicity distributions of the
stellar populations above and beyond this knee, which suggests a sudden
transition from a constant, highly efficient mode of star formation in
high-mass galaxies, gradually decreasing towards the low-mass end of the
sample. At fixed mass, our early-type sample is more efficient in building up
the stellar content at early times in comparison to the general population of
galaxies, with half of the stars already in place by redshift z~2 for all
masses. The metallicity-age trend in low-mass galaxies is not compatible with
infall of metal-poor gas, suggesting instead an outflow-driven relation.Comment: 12 pages,3 figures, accepted for publication in ApJ
The Influence of Galaxy Environment on the Stellar Initial Mass Function of Early-Type Galaxies
In this paper we investigate whether the stellar initial mass function of
early-type galaxies depends on their host environment. To this purpose, we have
selected a sample of early-type galaxies from the SPIDER catalogue,
characterized their environment through the group catalogue of Wang et al. and
used their optical SDSS spectra to constrain the IMF slope, through the
analysis of IMF-sensitive spectral indices. To reach a high enough
signal-to-noise ratio, we have stacked spectra in velocity dispersion
() bins, on top of separating the sample by galaxy hierarchy and host
halo mass, as proxies for galaxy environment. In order to constrain the IMF, we
have compared observed line strengths to predictions of MIUSCAT/EMILES
synthetic stellar population models, with varying age, metallicity, and
"bimodal" (low-mass tapered) IMF slope (). Consistent with
previous studies, we find that increases with ,
becoming bottom-heavy (i.e. an excess of low-mass stars with respect to the
Milky-Way-like IMF) at high . We find that this result is robust
against the set of isochrones used in the stellar population models, as well as
the way the effect of elemental abundance ratios is taken into account. We thus
conclude that it is possible to use currently state-of-the-art stellar
population models and intermediate resolution spectra to consistently probe IMF
variations. For the first time, we show that there is no dependence of
on environment or galaxy hierarchy, as measured within the SDSS
fibre, thus leaving the IMF as an intrinsic galaxy property, possibly set
already at high redshift
Radial variations in the stellar initial mass function of early-type galaxies
The hypothesis of a universal initial mass function (IMF) -- motivated by
observations in nearby stellar systems -- has been recently challenged by the
discovery of a systematic variation of the IMF with the central velocity
dispersion, {\sigma}, of early-type galaxies (ETGs), towards an excess of
low-mass stars in high-{\sigma} galaxies. This trend has been derived so far
from integrated spectra, and remains unexplained at present. To test whether
such trend depends on the local properties within a galaxy, we have obtained
new, extremely deep, spectroscopic data, for three nearby ETGs, two galaxies
with high {\sigma} (~300 km/s), and one lower mass system, with {\sigma} ~ 100
km/s. From the analysis of IMF-sensitive spectral features, we find that the
IMF depends significantly on galactocentric distance in the massive ETGs, with
the enhanced fraction of low-mass stars f mostly confined to their central
regions. In contrast, the low-{\sigma} galaxy does not show any significant
radial gradient in the IMF, well described by a shallower distribution,
relative to the innermost regions of massive galaxies, at all radii. Such a
result indicates that the IMF should be regarded as a local (rather than
global) property, and suggests a significant difference between the formation
process of the core and the outer regions of massive ETGs.Comment: 17 pages, 13 figures. Accepted for publication in MNRA
Strong Gravitational Lensing and the Stellar IMF of Early-type Galaxies
Systematic variations of the IMF in early-type galaxies, and their connection
with possible drivers such as velocity dispersion or metallicity, have been
much debated in recent years. Strong lensing over galaxy scales combined with
photometric and spectroscopic data provides a powerful method to constrain the
stellar mass-to-light ratio and hence the functional form of the IMF. We
combine photometric and spectroscopic constraints from the latest set of
population synthesis models of Charlot & Bruzual, including a varying IMF, with
a non-parametric analysis of the lens masses of 18 ETGs from the SLACS survey,
with velocity dispersions in the range 200-300 km/s. We find that very
bottom-heavy IMFs are excluded. However, the upper limit to the bimodal IMF
slope (, accounting for a dark matter fraction of 20-30%,
where corresponds to a Kroupa-like IMF) is compatible at the
level with constraints imposed by gravity-sensitive line strengths. A
two-segment power law parameterisation of the IMF (Salpeter-like for high
masses) is more constrained (, where is the power
index at low masses) but requires a dark matter contribution of
to reconcile the results with a Salpeter IMF. For a standard Milky Way-like IMF
to be applicable, a significant dark matter contribution is required within
. Our results reveal a large range of allowed IMF slopes, which, when
interpreted as intrinsic scatter in the IMF properties of ETGs, could explain
the recent results of Smith et al., who find Milky Way-like IMF normalisations
in a few massive lensing ETGs.Comment: Accepted for publication in MNRAS, 18 pages, 12 figures, 4 table
SPIDER X - Environmental effects in central and satellite early-type galaxies through the stellar fossil record
A detailed analysis of how environment affects the star formation history of
early-type galaxies (ETGs) is undertaken via high signal to noise ratio stacked
spectra obtained from a sample of 20,977 ETGs (morphologically selected) from
the SDSS-based SPIDER survey. Two major parameters are considered for the
study: the central velocity dispersion (sigma), which relates to local drivers
of star formation, and the mass of the host halo, which relates to
environment-related effects. In addition, we separate the sample between
centrals (the most massive galaxy in a halo) and satellites. We derive trends
of age, metallicity, and [alpha/Fe] enhancement, with sigma. We confirm that
the major driver of stellar population properties in ETGs is velocity
dispersion, with a second-order effect associated to the central/satellite
nature of the galaxy. No environmental dependence is detected for satellite
ETGs, except at low sigma - where satellites in groups or in the outskirts of
clusters tend to be younger than those in the central regions of clusters. In
contrast, the trends for centrals show a significant dependence on halo mass.
Central ETGs in groups (i.e. with a halo mass >10^12.5 M_Sun) have younger
ages, lower [alpha/Fe], and higher internal reddening, than "isolated" systems
(i.e. centrals residing in low-mass, <10^12.5 M_Sun, halos). Our findings imply
that central ETGs in groups formed their stellar component over longer time
scales than "isolated" centrals, mainly because of gas-rich interactions with
their companion galaxies.Comment: 22 pages, 19 figures, accepted for publication in MNRA
The Origin of Color Gradients in Early-Type Systems and Their Compactness at High-z
In this Letter, we present mean optical+NIR color gradient estimates for 5080
early-type galaxies (ETGs) in the grizYJHK wavebands of the Sloan Digital Sky
Survey (SDSS) plus UKIRT Infrared Deep Sky Survey (UKIDSS). The color gradient
is estimated as the logarithmic slope of the radial color profile in ETGs. With
such a large sample size, we study the variation of the mean color gradient as
a function of waveband with unprecedented accuracy. We find that (i) color
gradients are mainly due, on average, to a metallicity variation of about
-0.4dex per decade in galaxy radius; and (ii) a small, but significant,
positive age gradient is present, on average, in ETGs, with the inner stellar
population being slightly younger, by ~0.1dex per radial decade, than the outer
one. Also, we show that the presence of a positive mean age gradient in ETGs,
as found in the present study, implies their effective radius to be smaller at
high z, consistent with observations.Comment: 4 pages, 2 color figures, accepted for publication in the
Astrophysical Journal Letter
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