191 research outputs found
The effect of gravitational tides on dwarf spheroidal galaxies
The effect of the local environment on the evolution of dwarf spheroidal
galaxies is poorly understood. We have undertaken a suite of simulations to
investigate the tidal impact of the Milky Way on the chemodynamical evolution
of dwarf spheroidals that resemble present day classical dwarfs using the SPH
code GEAR. After simulating the models through a large parameter space of
potential orbits the resulting properties are compared with observations from
both a dynamical point of view, but also from the, often neglected, chemical
point of view. In general, we find that tidal effects quench the star formation
even inside gas-endowed dwarfs. Such quenching, may produce the radial
distribution of dwarf spheroidals from the orbits seen within large
cosmological simulations. We also find that the metallicity gradient within a
dwarf is gradually erased through tidal interactions as stellar orbits move to
higher radii. The model dwarfs also shift to higher [Fe/H]/L
ratios, but only when losing of stellar mass.Comment: A&A accepte
The post-infall evolution of a satellite galaxy
As galaxy simulations increase in resolution more attention is being paid
towards the evolution of dwarf galaxies and how the simulations compare to
observations. Despite this increasing resolution we are however, far away from
resolving the interactions of satellite dwarf galaxies and the hot coronae
which surround host galaxies. We describe a new method which focuses only on
the local region surrounding an infalling dwarf in an effort to understand how
the hot baryonic halo will alter the chemodynamical evolution of dwarf
galaxies. Using this method we examine how a dwarf, similar to Sextans dwarf
spheroidal, evolves in the corona of a Milky Way like galaxy. We find that even
at high perigalacticons the synergistic interaction between ram pressure and
tidal forces transform a dwarf into a stream, suggesting that Sextans was much
more massive in the past in order survive its perigalacticon passage. In
addition the large confining pressure of the hot corona allows gas that was
originally at the outskirts to begin forming stars, initially forming stars of
low metallicity compared to the dwarf evolved in isolation. This increase in
star formation eventually allows a dwarf galaxy to form more metal rich stars
compared to one in isolation, but only if the dwarf retains gas for a
sufficiently long period of time. In addition, dwarfs which formed substantial
numbers of stars post-infall will have a slightly elevated [Mg/Fe] at high
metallicity ([Fe/H] -1.5).Comment: 29 pages, 26 figures, A&A accepte
Computational issues in chemo-dynamical modelling of the formation and evolution of galaxies
Chemo-dynamical N-body simulations are an essential tool for understanding
the formation and evolution of galaxies. As the number of observationally
determined stellar abundances continues to climb, these simulations are able to
provide new constraints on the early star formaton history and chemical
evolution inside both the Milky Way and Local Group dwarf galaxies. Here, we
aim to reproduce the low -element scatter observed in metal-poor stars.
We first demonstrate that as stellar particles inside simulations drop below a
mass threshold, increases in the resolution produce an unacceptably large
scatter as one particle is no longer a good approximation of an entire stellar
population. This threshold occurs at around , a mass limit
easily reached in current (and future) simulations. By simulating the Sextans
and Fornax dwarf spheroidal galaxies we show that this increase in scatter at
high resolutions arises from stochastic supernovae explosions. In order to
reduce this scatter down to the observed value, we show the necessity of
introducing a metal mixing scheme into particle-based simulations. The impact
of the method used to inject the metals into the surrounding gas is also
discussed. We finally summarise the best approach for accurately reproducing
the scatter in simulations of both Local Group dwarf galaxies and in the Milky
Way.Comment: 23 pages, 18 figures, accepted for publication in Astronomy and
Astrophysic
The Metallicity Distribution Function of Field Stars in M31's Bulge
We have used Hubble Space Telescope Wide Field Planetary Camera 2
observations to construct a color-magnitude diagram (CMD) for the bulge of M31
at a location ~1.6 kpc from the galaxy's center. Using scaled-solar abundance
theoretical red giant branches with a range of metallicities, we have
translated the observed colors of the stars in the CMD to abundances and
constructed a metallicity distribution function (MDF) for this region. The MDF
shows a peak at [M/H]~0 with a steep decline at higher metallicities and a more
gradual tail to lower metallicities. This is similar in shape to the MDF of the
Milky Way bulge but shifted to higher metallicities by ~0.1 dex. As is the case
with the Milky Way bulge MDF, a pure closed box model of chemical evolution,
even with significant pre-enrichment, appears to be inconsistent with the M31
bulge MDF. However, a scenario in which an initial infall of gas enriched the
bulge to an abundance of [M/H] ~ -1.6 with subsequent evolution proceeding as a
closed box provides a better fit to the observed MDF. The similarity between
the MDF of the M31 bulge and that of the Milky Way stands in stark contrast to
the significant differences in the MDFs of their halo populations. This
suggests that the bulk of the stars in the bulges of both galaxies were in
place before the accretion events that occurred in the halos could influence
them.Comment: 12 pages, 9 figures, accepted for publication in The Astronomical
Journal, October 200
The Local Cluster Survey II: Disk-Dominated Cluster Galaxies with Suppressed Star Formation
We investigate the role of dense environments in suppressing star formation
by studying star-forming galaxies in
nine clusters from the Local Cluster Survey () and a large
comparison field sample drawn from the Sloan Digital Sky Survey. We compare the
star-formation rate (SFR) versus stellar mass relation as a function of
environment and morphology. After carefully controlling for mass, we find that
in all environments, the degree of SFR suppression increases with increasing
bulge-to-total (B/T) ratio. In addition, the SFRs of cluster and infall
galaxies at a fixed mass are more suppressed than their field counterparts at
all values of B/T. These results suggest a quenching mechanism that is linked
to bulge growth that operates in all environments and an additional mechanism
that further reduces the SFRs of galaxies in dense environments. We limit the
sample to galaxies to control for the trends with morphology and
find that the excess population of cluster galaxies with suppressed SFRs
persists. We model the timescale associated with the decline of SFRs in dense
environments and find that the observed SFRs of the cluster core galaxies are
consistent with a range of models including: a mechanism that acts slowly and
continuously over a long (2-5 Gyr) timescale, and a more rapid ( Gyr)
quenching event that occurs after a delay period of 1-6 Gyr. Quenching may
therefore start immediately after galaxies enter clusters.Comment: 17 pages, 12 figure
The colour-magnitude relation of elliptical and lenticular galaxies in the ESO Distant Cluster Survey
In this paper we study the colour-magnitude relation (CMR) for a sample of 172 morphologically classified elliptical and S0 cluster galaxies from the ESO Distant Cluster Survey (EDisCS) at 0.4 âČzâČ 0.8. The intrinsic colour scatter about the CMR is very small (ăÏintă= 0.076) in rest-frame UâV. However, there is a small minority of faint early-type galaxies (7 per cent) that are significantly bluer than the CMR. We observe no significant dependence of Ïint with redshift or cluster velocity dispersion. Because our sample is strictly morphologically selected, this implies that by the time cluster elliptical and S0 galaxies achieve their morphology, the vast majority have already joined the red sequence. The only exception seems to be the very small fraction of faint blue early types. Assuming that the intrinsic colour scatter is due to differences in stellar population ages, we estimate the galaxy formation redshift zF of each cluster and find that zF does not depend on the cluster velocity dispersion. However, zF increases weakly with cluster redshift within the EDisCS sample. This trend becomes very clear when higher redshift clusters from the literature are included. This suggests that, at any given redshift, in order to have a population of fully formed ellipticals and S0s they needed to have formed most of their stars â2-4 Gyr prior to observation. That does not mean that all early-type galaxies in all clusters formed at these high redshifts. It means that the ones we see already having early-type morphologies also have reasonably old stellar populations. This is partly a manifestation of the âprogenitor bias', but also a consequence of the fact that the vast majority of the early-type galaxies in clusters (in particular the massive galaxies) were already red (i.e. already had old stellar populations) by the time they achieved their morphology. Elliptical and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. The scarcity of blue S0s indicates that, if they are the descendants of spirals whose star formation has ceased, the parent galaxies were already red when they became S0s. This suggests the red spirals found preferentially in dense environments could be the progenitors of these S0s. We also find that fainter early-type galaxies finished forming their stars later (i.e. have smaller zF), consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Combining the CMR scatter analysis with the observed evolution in the CMR zero-point we find that the early-type cluster galaxy population must have had their star formation truncated/stopped over an extended period Îtâł 1 Gy
The effect of the environment on the structure, morphology and star-formation history of intermediate-redshift galaxies
With the aim of understanding the effect of the environment on the star formation history and morphological transformation of galaxies, we present a detailed analysis of the colour, morphology and internal structure of cluster and field galaxies at 0.4â€zâ€0.8. We use {\em HST} data for over 500 galaxies from the ESO Distant Cluster Survey (EDisCS) to quantify how the galaxies' light distribution deviate from symmetric smooth profiles. We visually inspect the galaxies' images to identify the likely causes for such deviations. We find that the residual flux fraction (RFF), which measures the fractional contribution to the galaxy light of the residuals left after subtracting a symmetric and smooth model, is very sensitive to the degree of structural disturbance but not the causes of such disturbance. On the other hand, the asymmetry of these residuals (Ares) is more sensitive to the causes of the disturbance, with merging galaxies having the highest values of Ares. Using these quantitative parameters we find that, at a fixed morphology, cluster and field galaxies show statistically similar degrees of disturbance. However, there is a higher fraction of symmetric and passive spirals in the cluster than in the field. These galaxies have smoother light distributions than their star-forming counterparts. We also find that while almost all field and cluster S0s appear undisturbed, there is a relatively small population of star-forming S0s in clusters but not in the field. These findings are consistent with relatively gentle environmental processes acting on galaxies infalling onto clusters
HST-NICMOS Observations of M31's Metal Rich Globular Clusters and Their Surrounding Fields: II. Results
We have obtained HST-NICMOS observations of five of M31's most metal rich
globular clusters: G1, G170, G174, G177 & G280. For the two clusters farthest
from the nucleus we statistically subtract the field population and estimate
metallicities using K-(J-K) color-magnitude diagrams (CMDs). Based on the
slopes of their infrared giant branches we estimate [Fe/H]=-1.22+/-0.43 for G1
and -0.15+/-0.37 for G280. We combine our infrared observations of G1 with two
epochs of optical HST-WFPC2 V-band data and identify at least one LPV based on
color and variability. The location of G1's giant branch in the K-(V-K) CMD is
very similar to that of M107, indicating a higher metallicity than our purely
infrared CMD: [Fe/H]=-0.9+/-0.2.
For the field surrounding G280, we estimate the metallicity to be -1.3 with a
spread of 0.5 from the slope and width of the giant branch. Based on the
numbers and luminosities of the brightest giants, we conclude that only a small
fraction of the stars in this field could be as young as 2 Gyr, while the
majority have ages closer to 10 Gyr.
The K-band luminosity functions (LFs) of the upper few magnitudes of G1 and
G280, as well as for the fields surrounding all clusters, are indistinguishable
from the LF measured in the bulge of our Galaxy. This indicates that these
clusters are very similar to Galactic clusters, and at least in the surrounding
fields observed, there are no significant populations of young luminous stars.Comment: AAS LaTeX v5.0, 17 pages. Submitted to the A
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