257 research outputs found
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
Double barred galaxies at intermediate redshifts: A feasibility study
Despite the increasing number of studies of barred galaxies at intermediate
and high redshifts, double-barred (S2B) systems have only been identified in
the nearby (z<0.04) universe thus far. In this feasibility study we demonstrate
that the detection and analysis of S2Bs is possible at intermediate redshifts
(0.1 < z < 0.5) with the exquisite resolution of the Hubble Space Telescope
Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the
HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a
novel method. The radial profile of the Gini coefficient -- a model-independent
structure parameter -- is able to detect bars in early-type galaxies that are
large enough that they might host an inner bar of sufficient angular size.
Using this method and subsequent examination with unsharp masks and ellipse
fits we identified the two most distant S2Bs currently known (at redshifts
z=0.103 and z=0.148). We investigate the underlying stellar populations of
these two galaxies through a detailed colour analysis, in order to demonstrate
the analysis that could be performed on a future sample of
intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates
in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of
distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to
push the limit for S2B detection and analysis out by a factor of ten in
redshift and lookback time (z=0.5, t=5Gyr) compared to the previously known
S2Bs. This in turn would provide new insight into the formation of these
objects.Comment: 9 pages + 10 figures. Accepted for publication in MNRAS. Main change
from version 1 is an extension of the introduction/motivation and discussion
section. A full resolution version including colour figures is available at
http://www.astro.unibas.ch/~tlisker/papers/lisker2006_s2b.pd
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
Identifying the progenitor set of present-day early-type galaxies: a view from the standard model
We present a comprehensive theoretical study, using a semi-analytical model
within the standard LCDM framework, of the photometric properties of the
progenitors of present-day early-type galaxies in the redshift range 0<z<1. We
explore progenitors of all morphologies and study their characteristics as a
function of the luminosity and local environment of the early-type remnant at
z=0. In agreement with previous studies, we find that, while larger early-types
are generally assembled later, their luminosity-weighted stellar ages are
typically older. In dense cluster-like environments, 70% of early-type systems
are `in place' by z=1 and evolve without interactions thereafter, while in the
field the corresponding value is 30%. Averaging across all environments at z~1,
less than 50% of the stellar mass which ends up in early-types today is
actually in early-type progenitors at this redshift, in agreement with recent
observational work. We develop probabilistic prescriptions which provide a
means of including spiral (i.e. non early-type) progenitors at intermediate and
high redshifts, based on their luminosity and optical colours. For example, we
find that, at intermediate redshifts (z~0.5), large (M_V0.7)
spirals have 75-95% chance of being an early-type progenitor, while the
corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is
50-75%. The prescriptions developed here can be used to address, from the
perspective of the standard model, the issue of `progenitor bias', whereby the
exclusion of late-type progenitors in observational studies can lead to
inaccurate conclusions regarding the evolution of the early-type population
over cosmic time. (abridged)Comment: Published in A&A, 2009, 503, 445. The article can be downloaded at:
http://adsabs.harvard.edu/abs/2009A%26A...503..445
Spectral dating of high-redshift stellar populations
Age derivation techniques for unresolved stellar populations at high redshifts are explored using the near-ultraviolet (NUV) spectrum of LBDS 53W091 (z = 1.55) and LBDS 53W069 (z = 1.43). The photometry and morphology of these galaxies - which are weak radio sources - suggest they are early-type systems, a feature that makes them ideal testbeds for the analysis of their ages and metallicities with the use of population-synthesis models. In the analysis that is based on simple stellar population models, we find a significant degeneracy between the derived ages and metallicities both in optical+near-infrared (NIR) photometric and NUV spectroscopic analyses. This degeneracy is not so strong for LBDS 53W069. However, even in this case the stellar age cannot be constrained better than to a range roughly encompassing one-third of the age of the Universe at z = 1.43 (90 per cent confidence level). We have explored several independent population-synthesis models and consistently found similar results. Broad-band photometry straddling the rest-frame 4000-Å break is also subject to a strong age-metallicity degeneracy. The use of realistic chemical enrichment assumptions significantly helps in disentangling the degeneracy. Based on this method, we derive the average stellar age for both galaxies around 〈t⋆〉∼ 3.6-3.8 Gyr with better constraints on the youngest possible ages (∼3 Gyr at the 90 per cent confidence level). The comparison with simple stellar population models suggest subsolar metallicities (log Z/Z⊙ =−0.2). A composite model using chemical enrichment gives slightly higher metallicities in both galaxies (log Z/Z⊙ =− 0.1). Given that the stellar component in galaxies forms over times which are larger than a typical chemical enrichment time-scale, we conclude that composite stellar populations must be used in all photospectroscopic analyses of galaxies. From the observational point of view, the most efficient (and feasible) way to set limits on unresolved stellar populations comprises a combination of Balmer absorption lines along with either low signal-to-noise ratio (SNR) rest-frame NUV spectroscopy or accurate optical and NIR photometr
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
How young are early-type cluster galaxies ? Quantifying the young stellar component in a rich cluster at z=0.41
We present a new method of quantifying the mass fraction of young stars in
galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our
attention on early-type cluster galaxies, whose star formation history is at
present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of
cluster Abell 851 (z=0.41) using HST/WFPC2 allow us to determine a NUV-optical
color-magnitude relation, whose slope is incompatible with a monolithic
scenario for star formation at high redshift. A degeneracy between a young
stellar component and its fractional mass contribution to the galaxy is found,
and a photometric analysis comparing the data with the predictions for a simple
two-stage star formation history is presented. The analysis shows that some of
the early-type galaxies may have fractions higher than 10% of the total mass
content in stars formed at z~0.5. An increased scatter is found in the
color-magnitude relation at the faint end, resulting in a significant fraction
of faint blue early-type systems. This would imply that less massive galaxies
undergo more recent episodes of star formation, and this can be explained in
terms of a positive correlation between star formation efficiency and
luminosity.Comment: Accepted for publication in ApJ Letters. Uses emulateapj.sty. 5 pages
with 3 embedded EPS figure
Diagnostics of baryonic cooling in lensing galaxies
Theoretical studies of structure formation find an inverse proportionality between the concentration of dark matter haloes and virial mass. This trend has been recently confirmed for Mvir≳ 6 × 1012 M⊙ by the observation of the X-ray emission from the hot halo gas. We present an alternative approach to this problem, exploring the concentration of dark matter haloes over galaxy scales on a sample of 18 early-type systems. Our c -Mvir relation is consistent with the X-ray analysis, extending towards lower virial masses, covering the range from 4 × 1011 up to 5 × 1012 M⊙. A combination of the lensing analysis along with photometric data allows us to constrain the baryon fraction within a few effective radii, which is compared with prescriptions for adiabatic contraction (AC) of the dark matter haloes. We find that the standard methods for AC are strongly disfavoured, requiring additional mechanisms - such as mass loss during the contraction process - to play a role during the phases following the collapse of the haloe
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