702 research outputs found
The Formation and Evolution of Virgo Cluster Galaxies - I. Broadband Optical & Infrared Colours
We use a combination of deep optical (gri) and near-infrared (H) photometry
to study the radially-resolved colours of a broad sample of 300 Virgo cluster
galaxies. For most galaxy types, we find that the median g-H colour gradient is
either flat (gas-poor giants and gas-rich dwarfs) or negative (i.e., colours
become bluer with increasing radius; gas-poor dwarfs, spirals, and gas-poor
peculiars). Later-type galaxies typically exhibit more negative gradients than
early-types. Given the lack of a correlation between the central colours and
axis ratios of Virgo spiral galaxies, we argue that dust likely plays a small
role, if at all, in setting those colour gradients. We search for possible
correlations between galaxy colour and photometric structure or environment and
find that the Virgo galaxy colours become redder with increasing concentration,
luminosity and surface brightness, while no dependence with cluster-centric
radius or local galaxy density is detected (over a range of ~2 Mpc and ~3-16
Mpc^-2, respectively). However, the colours of gas-rich Virgo galaxies do
correlate with neutral gas deficiency, such that these galaxies become redder
with higher deficiencies. Comparisons with stellar population models suggest
that these colour gradients arise principally from variations in stellar
metallicity within these galaxies, while age variations only make a significant
contribution to the colour gradients of Virgo irregulars. A detailed stellar
population analysis based on this material is presented in Roediger et al
(2011b; arXiv:1011.3511).Comment: 34 pages, 12 figures, 1 table, submitted to MNRAS; Paper II
(arXiv:1011.3511) has also been update
Evidence of a distinct stellar population in the counter-rotating core of NGC 1700
We find a distinct stellar population in the counter-rotating and
kinematically decoupled core of the isolated massive elliptical galaxy NGC
1700. Coinciding with the edge of this core we find a significant change in the
slope of the gradient of various representative absorption line indices. Our
age estimate for this core is markedly younger than the main body of the
galaxy. We find lower values for the age, metallicity and Mg/Fe abundance ratio
in the center of this galaxy when we compare them with other isolated
elliptical galaxies with similar velocity dispersion. We discuss the different
possible scenarios that might have lead to the formation of this younger
kinematically decoupled structure and conclude that, in light of our findings,
the ingestion of a small stellar companion on a retrograde orbit is the most
favoured.Comment: 13 pages, 5 figures, accepted for publication in ApJ
The Formation and Evolution of Virgo Cluster Galaxies - II. Stellar Populations
We use a combination of deep optical and near-infrared light profiles for a
morphologically diverse sample of Virgo cluster galaxies to study the
radially-resolved stellar populations of cluster galaxies over a wide range of
galaxy structure. We find that, in the median, the age gradients of Virgo
galaxies are either flat (lenticulars and Sa-Sb spirals) or positive
(ellipticals, Sbc+Sc spirals, gas-rich dwarfs, and irregulars), while all
galaxy types have a negative median metallicity gradient. Comparison of the
galaxy stellar population diagnostics (age, metallicity, and gradients thereof)
against structural and environmental parameters also reveals that the ages of
gas-rich systems depend mainly on their atomic gas deficiencies. Conversely,
the metallicities of Virgo gas-poor galaxies depend on their concentrations,
luminosities, and surface brightnesses. The stellar population gradients of all
Virgo galaxies exhibit no dependence on either their structure or environment.
We interpret these stellar population data for Virgo galaxies in the context of
popular formation and evolution scenarios, and suggest that gas-poor giants
grew hierarchically (through dissipative starbursts), gas-poor dwarfs have
descended from at least two different production channels (e.g., environmental
transformation and merging), while spirals formed inside-out, but with star
formation in the outskirts of a significant fraction of the population having
been quenched due to ram pressure stripping. (Abridged)Comment: 54 pages, 16 figures, 3 tables, re-submitted to MNRAS (edited to
reflect the referee's suggestions
Color and stellar population gradients in galaxies. Correlation with mass
We analyze the color gradients (CGs) of ~50000 nearby SDSS galaxies. From
synthetic spectral models based on a simplified star formation recipe, we
derive the mean spectral properties, and explain the observed radial trends of
the color as gradients of the stellar population age and metallicity (Z). The
most massive ETGs (M_* > 10^{11} Msun) have shallow CGs in correspondence of
shallow (negative) Z gradients. In the stellar mass range 10^(10.3-10.5) < M_*
< 10^(11) Msun, the Z gradients reach their minimum of ~ -0.5 dex^{-1}. At M_*
~ 10^{10.3-10.5} Msun, color and Z gradient slopes suddenly change. They turn
out to anti-correlate with the mass, becoming highly positive at the very low
masses. We have also found that age gradients anti-correlate with Z gradients,
as predicted by hierarchical cosmological simulations for ETGs. On the other
side, LTGs have gradients which systematically decrease with mass (and are
always more negative than in ETGs), consistently with the expectation from gas
infall and SN feedback scenarios. Z is found to be the main driver of the trend
of color gradients, especially for LTGs, but age gradients are not negligible
and seem to play a significant role too. We have been able to highlight that
older galaxies have systematically shallower age and Z gradients than younger
ones. Our results for high-mass galaxies are in perfect agreement with
predictions based on the merging scenario, while the evolution of LTGs and
younger and less massive ETGs seems to be mainly driven by infall and SN
feedback. (Abridged)Comment: 20 pages, 16 figures, accepted for publication on MNRAS. This version
includes revisions after the referee's report
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
Tidal streams in newly discovered M32 analogues: evidence for the stripping scenario
We present two newly-discovered compact elliptical (cE) galaxies, exhibiting
clear evidence of tidal steams, and found during a search of SDSS DR7 for cE
candidates. The structural parameters of the cEs are derived using GALFIT,
giving effective radii, Re, of 388 and 263 parsecs, and B-band mean surface
brightnesses within Re of 19.4 and 19.2 magnitudes per arcsec squared. We have
re-analysed the SDSS spectra, which indicate that they possess young to
intermediate-age stellar populations. These two cEs provide direct evidence, a
"smoking gun", for the process of tidal stripping that is believed to be the
origin of M32-type galaxies. Both are in small groups with a large spiral
fraction, suggesting that we may be seeing the formation of such cE galaxies in
dynamically young environments. The more compact of the galaxies is found in a
small group not unlike the Local Group, and thus provides an additional model
for understanding M32.Comment: 9 pages, 10 figures. Accepted to MNRA
Stellar populations of classical and pseudo-bulges for a sample of isolated spiral galaxies
In this paper we present the stellar population synthesis results for a
sample of 75 bulges in isolated spiral Sb-Sc galaxies, using the spectroscopic
data from the Sloan Digital Sky Survey and the STARLIGHT code. We find that
both pseudo-bulges and classical bulges in our sample are predominantly
composed of old stellar populations, with mean mass-weighted stellar age around
10 Gyr. While the stellar population of pseudo-bulges is, in general, younger
than that of classical bulges, the difference is not significant, which
indicates that it is hard to distinguish pseudo-bulges from classical bulges,
at least for these isolated galaxies, only based on their stellar populations.
Pseudo-bulges have star formation activities with relatively longer timescale
than classical bulges, indicating that secular evolution is more important in
this kind of systems. Our results also show that pseudo-bulges have a lower
stellar velocity dispersion than their classical counterparts, which suggests
that classical bulges are more dispersion-supported than pseudo-bulges.Comment: 10 pages, 8 figures. Accepted for publication in Astrophysics & Space
Scienc
Charged-Particle Thermonuclear Reaction Rates: III. Nuclear Physics Input
The nuclear physics input used to compute the Monte Carlo reaction rates and
probability density functions that are tabulated in the second paper of this
series (Paper II) is presented. Specifically, we publish the input files to the
Monte Carlo reaction rate code RatesMC, which is based on the formalism
presented in the first paper of this series (Paper I). This data base contains
overwhelmingly experimental nuclear physics information. The survey of
literature for this review was concluded in November 2009.Comment: 132 page
Mapping Dynamic Histone Acetylation Patterns to Gene Expression in Nanog-depleted Murine Embryonic Stem Cells
Embryonic stem cells (ESC) have the potential to self-renew indefinitely and
to differentiate into any of the three germ layers. The molecular mechanisms
for self-renewal, maintenance of pluripotency and lineage specification are
poorly understood, but recent results point to a key role for epigenetic
mechanisms. In this study, we focus on quantifying the impact of histone 3
acetylation (H3K9,14ac) on gene expression in murine embryonic stem cells. We
analyze genome-wide histone acetylation patterns and gene expression profiles
measured over the first five days of cell differentiation triggered by
silencing Nanog, a key transcription factor in ESC regulation. We explore the
temporal and spatial dynamics of histone acetylation data and its correlation
with gene expression using supervised and unsupervised statistical models. On a
genome-wide scale, changes in acetylation are significantly correlated to
changes in mRNA expression and, surprisingly, this coherence increases over
time. We quantify the predictive power of histone acetylation for gene
expression changes in a balanced cross-validation procedure. In an in-depth
study we focus on genes central to the regulatory network of Mouse ESC,
including those identified in a recent genome-wide RNAi screen and in the
PluriNet, a computationally derived stem cell signature. We find that compared
to the rest of the genome, ESC-specific genes show significantly more
acetylation signal and a much stronger decrease in acetylation over time, which
is often not reflected in an concordant expression change. These results shed
light on the complexity of the relationship between histone acetylation and
gene expression and are a step forward to dissect the multilayer regulatory
mechanisms that determine stem cell fate.Comment: accepted at PLoS Computational Biolog
VEGAS: A VST Early-type GAlaxy Survey. II. Photometric study of giant ellipticals and their stellar halos
Observations of diffuse starlight in the outskirts of galaxies are thought to
be a fundamental source of constraints on the cosmological context of galaxy
assembly in the CDM model. Such observations are not trivial because
of the extreme faintness of such regions. In this work, we investigate the
photometric properties of six massive early type galaxies (ETGs) in the VEGAS
sample (NGC 1399, NGC 3923, NGC 4365, NGC 4472, NGC 5044, and NGC 5846) out to
extremely low surface brightness levels, with the goal of characterizing the
global structure of their light profiles for comparison to state-of-the-art
galaxy formation models. We carry out deep and detailed photometric mapping of
our ETG sample taking advantage of deep imaging with VST/OmegaCAM in the g and
i bands. By fitting the light profiles, and comparing the results to
simulations of elliptical galaxy assembly, we identify signatures of a
transition between "relaxed" and "unrelaxed" accreted components and can
constrain the balance between in situ and accreted stars. The very good
agreement of our results with predictions from theoretical simulations
demonstrates that the full VEGAS sample of ETGs will allow us to use
the distribution of diffuse light as a robust statistical probe of the
hierarchical assembly of massive galaxies.Comment: Accepted for publication in Astronomy & Astrophysic
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