125 research outputs found
Stellar Populations in Spiral Galaxies
We report preliminary results of the characterization of bulge and inner disk
stellar populations for 8 nearby spiral galaxies using Gemini/GMOS. The
long-slit spectra extend out to 1-2 disk scale lengths with S/N/Ang > 50. Two
different model fitting techniques, absorption-line indices and full spectral
synthesis, are found to weigh age, metallicity, and abundance ratios
differently, but with careful attention to the data/model matching (resolution
and flux calibration), we are able constrain real signatures of age and
metallicity gradients in star-forming galaxies.Comment: 4 pages, 3 figures. To appear in the proceedings for IAUS 241
"Stellar Populations as Building Blocks of Galaxies", Eds. R.F. Peletier and
A. Vazdeki
Integrated Stellar Populations: Confronting Photometry with Spectroscopy
We investigate the ability of spectroscopic techniques to yield realistic
star formation histories (SFHs) for the bulges of spiral galaxies based on a
comparison with their observed broadband colors. Full spectrum fitting to
optical spectra indicates that recent (within ~1 Gyr) star formation activity
can contribute significantly to the V-band flux, whilst accounting for only a
minor fraction of the stellar mass budget which is made up primarily of old
stars. Furthermore, recent implementations of stellar population (SP) models
reveal that the inclusion of a more complete treatment of the thermally
pulsating asymptotic giant branch (TP-AGB) phase to SP models greatly increases
the NIR flux for SPs of ages 0.2-2 Gyr. Comparing the optical--NIR colors
predicted from population synthesis fitting, using models which do not include
all stages of the TP-AGB phase, to the observed colors reveals that observed
optical--NIR colors are too red compared to the model predictions. However,
when a 1 Gyr SP from models including a full treatment the TP-AGB phase is
used, the observed and predicted colors are in good agreement. This has strong
implications for the interpretation of stellar populations, dust content, and
SFHs derived from colors alone.Comment: 6 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Environmental Effects in the Evolution of Galactic Bulges
We investigate possible environmental trends in the evolution of galactic
bulges over the redshift range 0<z<0.6. For this purpose, we construct the
Fundamental Plane (FP) for cluster and field samples at redshifts =0.4 and
=0.54 using surface photometry based on HST imaging and velocity dispersions
based on Keck spectroscopy. As a reference point for our study we include data
for pure ellipticals, which we model as single-component Sersic profiles;
whereas for multi-component galaxies we undertake decompositions using Sersic
and exponential models for the bulge and disk respectively. Although the FP for
both distant cluster and field samples are offset from the local relation,
consistent with evolutionary trends found in earlier studies, we detect
significant differences in the zero point of ~=0.2 dex between the field and
cluster samples at a given redshift. For both clusters, the
environmentally-dependent offset is in the sense expected for an accelerated
evolution of bulges in dense environments. By matching the mass range of our
samples, we confirm that this difference does not arise as a result of the
mass-dependent downsizing effects seen in larger field samples. Our result is
also consistent with the hypothesis that - at fixed mass and environment - the
star formation histories of galactic bulges and pure spheroids are
indistinguishable, and difficult to reconcile with the picture whereby the
majority of large bulges form primarily via secular processes within spiral
galaxies.Comment: 5 pages, 3 figures, accepted for publication in ApJ Letter
The Dynamical Distinction between Elliptical and Lenticular Galaxies in Distant Clusters: Further Evidence for the Recent Origin of S0 Galaxies
We examine resolved spectroscopic data obtained with the Keck II telescope
for 44 spheroidal galaxies in the fields of two rich clusters, Cl0024+16
(z=0.40) and MS0451-03 (z=0.54), and contrast this with similar data for 23
galaxies within the redshift interval 0.3<z<0.65 in the GOODS northern field.
For each galaxy we examine the case for systemic rotation, derive central
stellar velocity dispersions sigma and photometric ellipticities, epsilon.
Using morphological classifications obtained via Hubble Space Telescope imaging
as the basis, we explore the utility of our kinematic quantities in
distinguishing between pressure-supported ellipticals and
rotationally-supported lenticulars (S0s). We demonstrate the reliability of
using the v/(1-epsilon) vs sigma and v/sigma vs epsilon distributions as
discriminators, finding that the two criteria correctly identify 63%+-3% and
80%+-2% of S0s at z~0.5, respectively, along with 76%+8-3% and 79%+-2% of
ellipticals. We test these diagnostics using equivalent local data in the Coma
cluster, and find that the diagnostics are similarly accurate at z=0. Our
measured accuracies are comparable to the accuracy of visual classification of
morphologies, but avoid the band-shifting and surface brightness effects that
hinder visual classification at high redshifts. As an example application of
our kinematic discriminators, we then examine the morphology-density relation
for elliptical and S0 galaxies separately at z~0.5. We confirm, from kinematic
data alone, the recent growth of rotationally-supported spheroidals. We discuss
the feasibility of extending the method to a more comprehensive study of
cluster and field galaxies to z~1, in order to verify in detail the recent
density-dependent growth of S0 galaxies.Comment: 7 pages, 4 figures, updated with version accepted to Ap
Faint dwarf galaxies in the Next Generation Virgo cluster Survey
The Next Generation Virgo Cluster Survey (NGVS) is a CFHT Large Program that
is using the wide field of view capabilities of the MegaCam camera to map the
entire Virgo Cluster from its core to virial radius. The observing strategy has
been optimized to detect very low surface brightness structures in the cluster,
including intracluster stellar streams and faint dwarf spheroidal galaxies. We
present here the current status of this ongoing survey, with an emphasis on the
detection and analysis of the very low-mass galaxies in the cluster that have
been revealed by the NGVS.Comment: 6 pages, 2 figures, Conference Proceedings: "A Universe of Dwarf
Galaxies", 14-18 June 2010, Lyon, Franc
A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globular
clusters, obtained with the Blanco 4-m telescope and the R-C spectrograph at
the Cerro Tololo Interamerican Observatory. The spectra cover the range ~ 3350
-- 6430 A with ~ 3.1 A (FWHM) resolution. The spectroscopic observations and
data reduction were designed to integrate the full projected area within the
cluster core radii in order to properly sample the light from stars in all
relevant evolutionary stages. The S/N values of the flux-calibrated spectra
range from 50 to 240/A at 4000 A and from 125 to 500/A at 5000 A. The selected
targets span a wide range of cluster parameters, including metallicity,
horizontal-branch morphology, Galactic coordinates, Galactocentric distance,
and concentration. The total sample is thus fairly representative of the entire
Galactic globular cluster population and should be valuable for comparison with
similar integrated spectra of unresolved stellar populations in remote systems.
For most of the library clusters, our spectra can be coupled with deep
color-magnitude diagrams and reliable metal abundances from the literature to
enable the calibration of stellar population synthesis models. In this paper we
present a detailed account of the observations and data reduction. The spectral
library is publicly available in electronic format from the National Optical
Astronomical Observatory website.Comment: 39 Pages, including 2 tables and 15 Figures. To appear in the
Astrophysical Journal, Supplement Serie
The Tully-Fisher Relation of Barred Galaxies
We present new data exploring the scaling relations, such as the Tully-Fisher
relation (TFR), of bright barred and unbarred galaxies. A primary motivation
for this study is to establish whether barredness correlates with, and is a
consequence of, virial properties of galaxies. Various lines of evidence
suggest that dark matter is dominant in disks of bright unbarred galaxies at
2.2 disk scale lengths, the point of peak rotation for a pure exponential disk.
We test the hypothesis that the TF plane of barred high surface brightness
galaxies is offset from the mean TFR of unbarred galaxies, as might be expected
if barred galaxies are ``maximal'' in their inner parts. We use existing and
new TF data to search for basic structural differences between barred and
unbarred galaxies. Our new data consist of 2-dimensional Halpha velocity fields
derived from SparsePak integral field spectroscopy (IFS) and V,I-band CCD
images collected at the WIYN Observatory for 14 strongly barred galaxies. We
use WIYN/SparsePak (2-D) velocity fields to show that long-slit (1-D) spectra
yield reliable circular speed measurements at or beyond 2.2 disk scale lengths,
far from any influence of the bar. This enables us to consider line width
measurements from extensive TF surveys which include barred and nonbarred disks
and derive detailed scaling relation comparisons. We find that for a given
luminosity, barred and unbarred galaxies have comparable structural and
dynamical parameters, such as peak velocities, scale lengths, or colors. In
particular, the location of a galaxy in the TF plane is independent of
barredness. In a global dynamical sense, barred and unbarred galaxies behave
similarly and are likely to have, on average, comparable fractions of luminous
and dark matter at a given radius. (abridged)Comment: Accepted for publication in the ApJ (September 1, 2003 issue, v594).
Appendix figures with I-band image and superimposed 2-D velocity field plus
rotation curves must be downloaded separately (due to size constraints) from
http://www.astro.ubc.ca/people/courteau/public/courteau03_TFbars.ps.g
M31's Heavy Element Distribution and Outer Disk
Hubble Space Telescope imaging of 11 fields in M31 were reduced to
color-magnitude diagrams. The fields were chosen to sample all galactocentric
radii to 50 kpc. Assuming that the bulk of the sampled stellar populations are
older than a few Gyr, the colors of the red giants map to an abundance
distribution with errors of order 0.1 dex in abundance. The radially sampled
abundance distributions are all about the same width, but show a mild abundance
gradient that flattens outside ~20 kpc. The various distributions were weighted
and summed with the aid of new surface brightness profile fits to obtain an
abundance distribution representative of the entirety of M31. M31 is a system
near chemical maturity. This ``observed closed box'' is compared to analytical
closed box models. M31 suffers from a lack of metal-poor stars and metal-rich
stars relative to the simplest closed-box model in the same way as the solar
neighborhood.Comparing to several simple chemical evolution models, neither
complete mixing of gas at all times nor zero mixing, inhomogeneous models give
the most convincing match to the data. As noted elsewhere, the outer disk of
M31 is a factor of ten more metal-rich than the Milky Way halo, ten times more
metal-rich than the dwarf spheroidals cospatial with it, and more metal-rich
than most of the globular clusters at the same galactocentric radius.
Difficulties of interpretation are greatly eased if we posit that the M31 disk
dominates over the halo at all radii out to 50 kpc. In fact, scaling from
current density models of the Milky Way, one should not expect to see halo
stars dominating over disk stars until beyond our 50 kpc limit. A corollary
conclusion is that most published studies of the M31 "halo" are actually
studies of its disk.Comment: 28 pages, 11 black-and-white figures, in press, Astrophysical Journa
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