66 research outputs found
The Formation and Evolution of S0 Galaxies
This thesis studies the origin of local S0 galaxies and their possible links
to other morphological types. To address these issues, two different approaches
have been adopted: a detailed study of the stellar populations of S0s in the
Fornax Cluster and a study of the Tully-Fisher Relation (TFR) of local S0s in
different environments.Comment: Ph.D. Thesis submitted to The University of Nottingham, U.K. Please
download the complete version at
http://www.nottingham.ac.uk/~ppxapgg/phdthesis.pd
Central Stellar Populations of S0 Galaxies in The Fornax Cluster
Based on FORS2-VLT long-slit spectroscopy, the analysis of the central
absorption line indices of 9 S0 galaxies in the Fornax Cluster is presented.
Central indices correlate with central velocity dispersions as observed in
ellipticals. However, the stellar population properties of these S0s indicates
that the observed trends are produced by relative differences in age and
alpha-element abundances and not in metallicity ([Fe/H]) as previous studies
have found in elliptical galaxies. The observed scatter in the line indices vs.
velocity dispersion relations can be partially explained by the
rotationally-supported nature of many of these systems. The presence of tighter
line indices vs. maximum (circular) rotational velocity relations confirms this
statement. It was also confirmed that the dynamical mass is the driving
physical property of all these correlations and in our Fornax S0s it has to be
estimated assuming rotational support.Comment: To appear in the Proceedings of IAU Symposium 241: "Stellar
Populations as Building Blocks of Galaxies", 10-16 December, 2006 at La
Palma, Canary Islands, Spai
Stellar population gradients in Fornax Cluster S0 galaxies: connecting bulge and disk evolution
We present absorption-line index gradients for a sample of S0 galaxies in the
Fornax Cluster. The sample has been selected to span a wide range in galaxy
mass, and the deep VLT-FORS2 spectroscopy allows us to explore the stellar
populations all the way to the outer disk-dominated regions of these galaxies.
We find that globally, in both bulges and disks, star formation ceased earliest
in the most massive systems, as a further manifestation of downsizing. However,
within many galaxies, we find an age gradient which indicates that star
formation ended first in the outermost regions. Metallicity gradients, when
detected, are always negative such that the galaxy centres are more metal-rich.
This finding fits with a picture in which star formation continued in the
central regions, with enriched material, after it had stopped in the outskirts.
Age and metallicity gradients are correlated, suggesting that large differences
in star formation history between the inner and outer parts of S0 galaxies
yield large differences in their chemical enrichment. In agreement with
previous results, we conclude that the radial variations in the stellar
populations of S0 galaxies are compatible with the hypothesis that these
galaxies are the descendants of spiral galaxies whose star formation has
ceased. With the addition of radial gradient information, we are able to show
that this shutdown of star formation occurred from the outside inward, with the
later star formation in the central regions offering a plausible mechanism for
enhancing the bulge light in these systems, as the transformation to more
bulge-dominated S0 galaxies requires.Comment: 9 pages, 4 figures and Appendix, accepted for publication in MNRA
Spectroscopic bulge-disc decomposition: a new method to study the evolution of lenticular galaxies
A new method for spectroscopic bulge-disc decomposition is presented, in
which the spatial light profile in a two-dimensional spectrum is decomposed
wavelength-by-wavelength into bulge and disc components, allowing separate
one-dimensional spectra for each component to be constructed. This method has
been applied to observations of a sample of nine S0s in the Fornax Cluster in
order to obtain clean high-quality spectra of their individual bulge and disc
components. So far this decomposition has only been fully successful when
applied to galaxies with clean light profiles, consequently limiting the number
of galaxies that could be separated into bulge and disc components. Lick index
stellar population analysis of the component spectra reveals that in those
galaxies where the bulge and disc could be distinguished, the bulges have
systematically higher metallicities and younger stellar populations than the
discs. This correlation is consistent with a picture in which S0 formation
comprises the shutting down of star formation in the disc accompanied by a
final burst of star formation in the bulge. The variation in spatial-fit
parameters with wavelength also allows us to measure approximate colour
gradients in the individual components. Such gradients were detected separately
in both bulges and discs, in the sense that redder light is systematically more
centrally concentrated in all components. However, a search for radial
variations in the absorption line strengths determined for the individual
components revealed that they are absent from the vast majority of S0 discs and
bulges. The absence of gradients in line indices for most galaxies implies that
the colour gradient cannot be attributed to age or metallicity variations, and
is therefore most likely associated with varying degrees of obscuration by
dust.Comment: 10 pages, 10 figures, 1 table, accepted for publication in MNRA
Spatially Resolved [FeII] 1.64 \mu m Emission in NGC 5135. Clues for Understanding the Origin of the Hard X-rays in Luminous Infrared Galaxies
Spatially resolved near-IR and X-ray imaging of the central region of the
Luminous Infrared Galaxy NGC 5135 is presented. The kinematical signatures of
strong outflows are detected in the [FeII]1.64 \mu m emission line in a compact
region at 0.9 kpc from the nucleus. The derived mechanical energy release is
consistent with a supernova rate of 0.05-0.1 yr. The apex of the
outflowing gas spatially coincides with the strongest [FeII] emission peak and
with the dominant component of the extranuclear hard X-ray emission. All these
features provide evidence for a plausible direct physical link between
supernova-driven outflows and the hard X-ray emitting gas in a LIRG. This
result is consistent with model predictions of starbursts concentrated in small
volumes and with high thermalization efficiencies. A single high-mass X-ray
binary (HMXB) as the major source of the hard X-ray emission although not
favoured, cannot be ruled out. Outside the AGN, the hard X-ray emission in NGC
5135 appears to be dominated by the hot ISM produced by supernova explosions in
a compact star-forming region, and not by the emission due to HMXB. If this
scenario is common to U/LIRGs, the hard X-rays would only trace the most
compact (< 100 pc) regions with high supernova and star formation densities,
therefore a lower limit to their integrated star formation. The SFR derived in
NGC 5135 based on its hard X-ray luminosity is a factor of two and four lower
than the values obtained from the 24 \mu m and soft X-ray luminosities,
respectively.Comment: Accepted for Publication in ApJ, 18 pages, 2 figure
The link between the masses and central stellar populations of S0 galaxies
Using high signal-to-noise ratio VLT/FORS2 long-slit spectroscopy, we have
studied the properties of the central stellar populations and dynamics of a
sample of S0 galaxies in the Fornax Cluster. The central absorption-line
indices in these galaxies correlate well with the central velocity dispersions
(Sigma0) in accordance with what previous studies found for elliptical
galaxies. However, contrary to what it is usually assumed for cluster
ellipticals, the observed correlations seem to be driven by systematic age and
alpha-element abundance variations, and not changes in overall metallicity. We
also found that the observed scatter in the Index-Sigma0 relations can be
partially explained by the rotationally-supported nature of these systems.
Indeed, even tighter correlations exist between the line indices and the
maximum circular velocity of the galaxies. This study suggests that the
dynamical mass is the physical property driving these correlations, and for S0
galaxies such masses have to be estimated assuming a large degree of rotational
support. The observed trends imply that the most massive S0s have the shortest
star-formation timescales and the oldest stellar populations.Comment: 21 pages, 17 figures, 9 tables, accepted for publication in MNRA
The Tully-Fisher relation for S0 galaxies
We present a study of the local B and K-band Tully-Fisher Relation (TFR)
between absolute magnitude and maximum circular speed in S0 galaxies. To make
this study, we have combined kinematic data, including a new high-quality
spectral data set from the Fornax Cluster, with homogeneous photometry from the
RC3 and 2MASS catalogues, to construct the largest sample of S0 galaxies ever
used in a study of the TFR. Independent of environment, S0 galaxies are found
to lie systematically below the TFR for nearby spirals in both optical and
infrared bands. This offset can be crudely interpreted as arising from the
luminosity evolution of spiral galaxies that have faded since ceasing star
formation. However, we also find a large scatter in the TFR. We show that most
of this scatter is intrinsic, not due to the observational uncertainties. The
presence of such a large scatter means that the population of S0 galaxies
cannot have formed exclusively by the above simple fading mechanism after all
transforming at a single epoch. To better understand the complexity of the
transformation mechanism, we have searched for correlations between the offset
from the TFR and other properties of the galaxies such as their structural
properties, central velocity dispersions and ages (as estimated from line
indices). For the Fornax Cluster data, the offset from the TFR relates with the
estimated age of the stars in the individual galaxies, in the sense and of the
magnitude expected if S0 galaxies had passively faded since being converted
from spirals. This correlation implies that a significant part of the scatter
in the TFR arises from the different times at which galaxies began their
transformation.Comment: 17 pages, 11 figures, 3 tables, accepted for publication in MNRA
Measuring the fading of S0 galaxies using globular clusters
We test the hypothesis that S0 galaxies are the descendants of fading spirals
whose star formation has been shut down, by using the properties of their
globular cluster (GC) systems. We estimate the amount by which the GC specific
frequency (number of GCs per unit V-band luminosity) is enhanced in S0s
relative to spirals. If the transformation hypothesis is correct, and no GCs
are created or destroyed in the process, then this difference provides a
measure of the degree to which the S0's V-band luminosity has faded relative to
that of its spiral progenitor. We also explore whether the degree to which the
GC specific frequency is enhanced in S0s correlates with the colour of the
stellar population, as also predicted by this hypothesis in which galaxies
become redder as they fade. We find that, on average, the GC specific frequency
is a factor ~3 larger for S0s than for spirals, which can be interpreted as
meaning that passively-evolving S0s have faded by about a factor of 3 from
their spiral progenitors. This value fits remarkably well with the predictions
of stellar population synthesis calculations, and the offset between the S0 and
spiral Tully-Fisher relations. We also find that the global colours of S0
galaxies are strongly correlated with their GC specific frequencies: the redder
the stellar population of an S0, the larger its specific frequency, as we might
expect if we are catching different S0s at different stages of passively fading
and reddening. Comparison to the predictions of stellar population synthesis
models show that this explanation works quantitatively as well as
qualitatively. These tests strongly support the hypothesis that S0 galaxies
were once normal spirals, whose star formation was cut off, presumably due to a
change of environment.Comment: 7 pages, 2 figures, accepted for publication in A&
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