1,810 research outputs found
Mass dependent Evolution of Field Early-Type Galaxies Since z=1
We present the Fundamental Plane (FP) of field early-type galaxies at
0.5<z<1.0. Our project is a continuation of our efforts to understand the
formation and evolution of early-type galaxies in different environments. The
target galaxies were selected from the comprehensive and homogeneous data set
of the Gemini/HST Galaxy Cluster Project. The distant field early-type galaxies
follow a steeper FP relation compared to the local FP. The change in the slope
of the FP can be interpreted as a mass-dependent evolution. Similar results
have been found for cluster early-type galaxies in high redshift galaxy
clusters at 0.8<z<1. Therefore, the slope change of the FP appears to be
independent of the environment of the galaxies.Comment: 2 pages, 1 figure, to appear in the proceedings of the IAU Symposium
no. 262, "Stellar Populations - Planning for the Next Decade", eds. G. R.
Bruzual and S. Charlo
Star Clusters in M 31. IV. A Comparative Analysis of Absorption Line Indices in Old M 31 and Milky Way Clusters
We present absorption line indices measured in the integrated spectra of
globular clusters both from the Galaxy and from M 31. Our samples include 41
Galactic globular clusters, and more than 300 clusters in M 31. The conversion
of instrumental equivalent widths into the Lick system is described, and
zero-point uncertainties are provided. Comparison of line indices of old M 31
clusters and Galactic globular clusters suggests an absence of important
differences in chemical composition between the two cluster systems. In
particular, CN indices in the spectra of M 31 and Galactic clusters are
essentially consistent with each other, in disagreement with several previous
works. We reanalyze some of the previous data, and conclude that reported CN
differences between M 31 and Galactic clusters were mostly due to data
calibration uncertainties. Our data support the conclusion that the chemical
compositions of Milky Way and M 31 globular clusters are not substantially
different, and that there is no need to resort to enhanced nitrogen abundances
to account for the optical spectra of M 31 globular clusters.Comment: 72 pages, including 15 figures and 14 tables. Published by the
Astronomical Journa
Effective temperatures of red giants in the APOKASC catalogue and the mixing length calibration in stellar models
Red giants in the updated APOGEE-Kepler catalogue, with estimates of mass,
chemical composition, surface gravity and effective temperature, have recently
challenged stellar models computed under the standard assumption of solar
calibrated mixing length. In this work, we critically reanalyse this sample of
red giants, adopting our own stellar model calculations. Contrary to previous
results, we find that the disagreement between the effective temperature scale
of red giants and models with solar calibrated mixing length disappears when
considering our models and the APOGEE-Kepler stars with scaled solar metal
distribution. However, a discrepancy shows up when alpha-enhanced stars are
included in the sample. We have found that assuming mass, chemical composition
and effective temperature scale of the APOGEE-Kepler catalogue, stellar models
generally underpredict the change of temperature of red giants caused by
alpha-element enhancements at fixed [Fe/H]. A second important conclusion is
that the choice of the outer boundary conditions employed in model calculations
is critical. Effective temperature differences (metallicity dependent) between
models with solar calibrated mixing length and observations appear for some
choices of the boundary conditions, but this is not a general resultComment: 8 pages, 10 figures, Astronomy & Astrophysics, in pres
UV observations of the globular cluster M10 from HST and GALEX. The BSS population
We present a combination of high-resolution Hubble Space Telescope and
wide-field ground-based and Galaxy Evolution Explorer data of the Galactic
Globular Cluster M10 (NGC6254). By using this large data-set we determined the
center of gravity of the cluster and we built its density profile from star
counts over its entire radial extension. We find that the density profile is
well reproduced by a single-mass King model with structural parameters c=1.41
and r_c=41". We also studied the Blue Straggler Star population and its radial
distribution. We count a total number of 120 BSS within the tidal radius. Their
radial distribution is bimodal: highly peaked in the cluster center, decreasing
at intermediate distances and rising again outwards. We discuss these results
in the context of the dynamical clock scheme presented by Ferraro et al. (2012)
and of recent results about the radial distribution of binary systems in this
cluster.Comment: Accepted for publication by ApJ; 26 pages, 11 figures, 1 tabl
Fake star formation bursts: blue horizontal branch stars masquerade as young stars in optical integrated light spectroscopy
Model color magnitude diagrams of low-metallicity globular clusters usually
show a deficit of hot evolved stars with respect to observations. We
investigate quantitatively the impact of such modelling inaccuracies on the
significance of star formation history reconstructions obtained from optical
integrated spectra. To do so, we analyse the sample of spectra of galactic
globular clusters of Schiavon et al. with STECKMAP (Ocvirk et al.) and the
stellar population models Vazdekis et al. and Bruzual & Charlot, and focus on
the reconstructed stellar age distributions. Firstly, we show that
background/foreground contamination correlates with E(B-V), which allows us to
define a clean subsample of uncontaminated GCs, on the basis of a E(B-V)
filtering.
We then identify a "confusion zone" where fake young bursts of star formation
pop up in the star formation history although the observed population is
genuinely old. These artifacts appear for 70-100% of cases depending on the
population model used, and contribute up to 12% of the light in the optical.
Their correlation with the horizontal branch ratio indicates that the confusion
is driven by HB morphology: red horizontal branch clusters are well fitted by
old stellar population models while those with a blue HB require an additional
hot component. The confusion zone extends over [Fe/H]=[-2,-1.2], although we
lack the data to probe extreme high and low metallicity regimes. As a
consequence, any young starburst superimposed on an old stellar population in
this metallicity range could be regarded as a modeling artifact, if it weighs
less than 12% of the optical light, and if no emission lines typical of an HII
region are present.Comment: 14 pages, 6 figures, accepted for publication in Ap
RXJ0848.6+4453: The Evolution of Galaxy Sizes and Stellar Populations in a z=1.27 Cluster
RXJ0848.6+4453 (Lynx W) at redshift 1.27 is part of the Lynx Supercluster of
galaxies. Our analysis of stellar populations and star formation history in the
cluster covers 24 members and is based on deep optical spectroscopy from Gemini
North and imaging data from HST. Focusing on the 13 bulge-dominated galaxies
for which we can determine central velocity dispersions, we find that these
show a smaller evolution of sizes and velocity dispersions than reported for
field galaxies and galaxies in poorer clusters. The galaxies in RXJ0848.6+4453
populate the Fundamental Plane similar to that found for lower redshift
clusters with a zero point offset corresponding to an epoch of last star
formation at z_form= 1.95+-0.2. The spectra of the galaxies in RXJ0848.6+4453
are dominated by young stellar populations at all galaxy masses and in many
cases show emission indicating low level on-going star formation. The average
age of the young stellar populations (estimated from H-zeta) is consistent with
a major star formation episode 1-2 Gyr prior, which in turn agrees with
z_form=1.95. Galaxies dominated by young stellar populations are distributed
throughout the cluster. We speculate that low level star formation has not yet
been fully quenched in the center of this cluster may be because the cluster is
significantly poorer than other clusters previously studied at similar
redshifts, which appear to have very little on-going star formation in their
centers.Comment: Accepted for publication in Astronomical Journal. High-resolution
figures available from the first author by reques
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