50 research outputs found
Properties of simulated Milky Way-mass galaxies in loose group and field environments
We test the validity of comparing simulated field disk galaxies with the
empirical properties of systems situated within environments more comparable to
loose groups, including the Milky Way's Local Group. Cosmological simulations
of Milky Way-mass galaxies have been realised in two different environment
samples: in the field and in environments with similar properties to the Local
Group. Apart from the environments of the galaxies, the samples are kept as
homogeneous as possible with equivalent ranges in last major merger time, halo
mass and halo spin. Comparison of these two samples allow for systematic
differences in the simulations to be identified. Metallicity gradients, disk
scale lengths, colours, magnitudes and age-velocity dispersion relations are
studied for each galaxy in the suite and the strength of the link between these
and environment of the galaxies is studied. The bulge-to-disk ratio of the
galaxies show that these galaxies are less spheroid dominated than many other
simulated galaxies in literature with the majority of both samples being disk
dominated. We find that secular evolution and mergers dominate the spread of
morphologies and metallicity gradients with no visible differences between the
two environment samples. In contrast with this consistency in the two samples
there is tentative evidence for a systematic difference in the velocity
dispersion-age relations of galaxies in the different environments. Loose group
galaxies appear to have more discrete steps in their velocity dispersion-age
relations. We conclude that at the current resolution of cosmological galaxy
simulations field environment galaxies are sufficiently similar to those in
loose groups to be acceptable proxies for comparison with the Milky Way
provided that a similar assembly history is considered.Comment: 16 pages, 11 figures, abstract abridged for arXiv. Accepted for
publication in Astronomy & Astrophysic
The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations
By means of high-resolution cosmological hydrodynamical simulations of Milky
Way-like disc galaxies, we conduct an analysis of the associated stellar
metallicity distribution functions (MDFs). After undertaking a kinematic
decomposition of each simulation into spheroid and disc sub-components, we
compare the predicted MDFs to those observed in the solar neighbourhood and the
Galactic bulge. The effects of the star formation density threshold are visible
in the star formation histories, which show a modulation in their behaviour
driven by the threshold. The derived MDFs show median metallicities lower by
0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic
bulge. Possible reasons for this apparent discrepancy include the use of low
stellar yields and/or centrally-concentrated star formation. The dispersions
are larger than the one of the observed MDF; this could be due to simulated
discs being kinematically hotter relative to the Milky Way. The fraction of low
metallicity stars is largely overestimated, visible from the more negatively
skewed MDF with respect to the observational sample. For our fiducial Milky Way
analog, we study the metallicity distribution of the stars born "in situ"
relative to those formed via accretion (from disrupted satellites), and
demonstrate that this low-metallicity tail to the MDF is populated primarily by
accreted stars. Enhanced supernova and stellar radiation energy feedback to the
surrounding interstellar media of these pre-disrupted satellites is suggested
as an important regulator of the MDF skewness.Comment: 20 pages, 14 figures, MNRAS, accepte
Anatomy of a post-starburst minor merger: a multi-wavelength WFC3 study of NGC 4150
(Abridged) We present a spatially-resolved near-UV/optical study of NGC 4150,
using the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope.
Previous studies of this early-type galaxy (ETG) indicate that it has a large
reservoir of molecular gas, exhibits a kinematically decoupled core (likely
indication of recent merging) and strong, central H_B absorption (indicative of
young stars). The core of NGC 4150 shows ubiquitous near-UV emission and
remarkable dusty substructure. Our analysis shows this galaxy to lie in the
near-UV green valley, and its pixel-by-pixel photometry exhibits a narrow range
of near-UV/optical colours that are similar to those of nearby E+A
(post-starburst) galaxies. We parametrise the properties of the recent star
formation (age, mass fraction, metallicity and internal dust content) in the
NGC 4150 pixels by comparing the observed near-UV/optical photometry to stellar
models. The typical age of the recent star formation (RSF) is around 0.9 Gyrs,
consistent with the similarity of the near-UV colours to post-starburst
systems, while the morphological structure of the young component supports the
proposed merger scenario. The RSF metallicity, representative of the
metallicity of the gas fuelling star formation, is around 0.3 - 0.5 Zsun.
Assuming that this galaxy is a merger and that the gas is sourced mainly from
the infalling companion, these metallicities plausibly indicate the gas-phase
metallicity (GPM) of the accreted satellite. Comparison to the local mass-GPM
relation suggests (crudely) that the mass of the accreted system is around
3x10^8 Msun, making NGC 4150 a 1:20 minor merger. A summation of the pixel RSF
mass fractions indicates that the RSF contributes about 2-3 percent of the
stellar mass. This work reaffirms our hypothesis that minor mergers play a
significant role in the evolution of ETGs at late epochs.Comment: 28 pages, 2 tables, accepted for publication in Ap
Signatures of minor mergers in the Milky Way disc I: The SEGUE stellar sample
It is now known that minor mergers are capable of creating structure in the
phase-space distribution of their host galaxy's disc. In order to search for
such imprints in the Milky Way, we analyse the SEGUE F/G-dwarf and the Schuster
et al. (2006) stellar samples. We find similar features in these two completely
independent stellar samples, consistent with the predictions of a Milky Way
minor-merger event. We next apply the same analyses to high-resolution,
idealised N-body simulations of the interaction between the Sagittarius dwarf
galaxy and the Milky Way. The energy distributions of stellar particle samples
in small spatial regions in the host disc reveal strong variations of structure
with position. We find good matches to the observations for models with a mass
of Sagittarius' dark matter halo progenitor M.
Thus, we show that this kind of analysis could be used to provide
unprecedentedly tight constraints on Sagittarius' orbital parameters, as well
as place a lower limit on its mass.Comment: 14 pages, 9 figures, 2 tables. Revised to reflect accepted versio
Gas Accretion and Galactic Chemical Evolution: Theory and Observations
This chapter reviews how galactic inflows influence galaxy metallicity. The
goal is to discuss predictions from theoretical models, but particular emphasis
is placed on the insights that result from using models to interpret
observations. Even as the classical G-dwarf problem endures in the latest round
of observational confirmation, a rich and tantalizing new phenomenology of
relationships between , , SFR, and gas fraction is emerging both in
observations and in theoretical models. A consensus interpretation is emerging
in which star-forming galaxies do most of their growing in a quiescent way that
balances gas inflows and gas processing, and metal dilution with enrichment.
Models that explicitly invoke this idea via equilibrium conditions can be used
to infer inflow rates from observations, while models that do not assume
equilibrium growth tend to recover it self-consistently. Mergers are an overall
subdominant mechanism for delivering fresh gas to galaxies, but they trigger
radial flows of previously-accreted gas that flatten radial gas-phase
metallicity gradients and temporarily suppress central metallicities. Radial
gradients are generically expected to be steep at early times and then
flattened by mergers and enriched inflows of recycled gas at late times.
However, further theoretical work is required in order to understand how to
interpret observations. Likewise, more observational work is needed in order to
understand how metallicity gradients evolve to high redshifts.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springer. 29 pages, 2 figure
Chemodynamics of a simulated disc galaxy: initial mass functions and Type Ia supernova progenitors
We trace the formation and advection of several elements within a cosmological adaptive mesh refinement simulation of an Lïżœ galaxy. We use nine realizations of the same initial conditions with different stellar initial mass functions (IMFs), mass limits for Type II and Type Ia supernovae (SNII, SNIa) and stellar lifetimes to constrain these subgrid phenomena. Our code includes self-gravity, hydrodynamics, star formation, radiative cooling and feedback from multiple sources within a cosmological framework. Under our assumptions of nucleosynthesis we find that SNII with progenitor masses of up to 100 Mïżœ are required to match low-metallicity gas oxygen abundances. Tardy SNIa are necessary to reproduce the classical chemical evolution âkneeâ in [O/Fe]â[Fe/H]: more prompt SNIa delayed time distributions do not reproduce this feature. Within our framework of hydrodynamical mixing of metals and galaxy mergers we find that chemical evolution is sensitive to the shape of the IMF and that there exists a degeneracy with the mass range of SNII. We look at the abundance plane and present the properties of different regions of the plot, noting the distinct chemical properties of satellites and a series of nested discs that have greater velocity dispersions are more α-rich and metal poor with age
Evolution of the specific Star Formation Rate Function at z<1.4 - Dissecting the mass-SFR plane in COSMOS and GOODS
The relation between the stellar mass and the star formation rate
characterizes how the instantaneous star formation is determined by the galaxy
past star formation history and by the growth of the dark matter structures. We
deconstruct the M-SFR plane by measuring the specific SFR functions in several
stellar mass bins from z=0.2 out to z=1.4. Our analysis is primary based on a
MIPS 24 selected catalogue combining the COSMOS and GOODS surveys. We
estimate the SFR by combining mid- and far-infrared data for 20500 galaxies.
The sSFR functions are derived in four stellar mass bins within the range
9.5<log(M/Msun)<11.5. First, we demonstrate the importance of taking into
account selection effects when studying the M-SFR relation. Secondly, we find a
mass-dependent evolution of the median sSFR with redshift varying as , with increasing from to between
and , respectively. At low masses, this
evolution is consistent with the cosmological accretion rate and predictions
from semi-analytical models (SAM). This agreement breaks down for more massive
galaxies showing the need for a more comprehensive description of the
star-formation history in massive galaxies. Third, we obtain that the shape of
the sSFR function is invariant with time at z<1.4 but depends on the mass. We
observe a broadening of the sSFR function ranging from 0.28 dex at
to 0.46 dex at . Such increase in the
scatter of the M-SFR relation suggests an increasing diversity of SFHs as the
stellar mass increases. Finally, we find a gradual decline of the sSFR with
mass as . We discuss the numerous physical processes,
as gas exhaustion in hot gas halos or secular evolution, which can gradually
reduce the sSFR and increase the SFH diversity.Comment: 24 pages, 16 figures, 3 tables, published version in A&
Metallicity gradients in disks: Do galaxies form inside-out?
Publisher's Version/PDFAims. We examine radial and vertical metallicity gradients using a suite of disk galaxy hydrodynamical simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient slope and reconcile the differences existing between extant models and observations within the canonical “inside-out” disk growth paradigm. Methods. A suite of 25 cosmological disks is used to examine the evolution of metallicity gradients; this consists of 19 galaxies selected from the RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code RAMSES, including eight drawn from the “field” and six from “loose group” environments. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code GASOLINE. Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. Results. We first show that generically flatter gradients are observed at redshift zero when comparing older stars with those forming today, consistent with expectations of kinematically hot simulations, but counter to that observed in the Milky Way. The vertical abundance gradients at ∼1–3 disk scalelengths are comparable to those observed in the thick disk of the Milky Way, but significantly shallower than those seen in the thin disk. Most importantly, we find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. Conclusions. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical “inside-out” growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z ∼ 1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required. 
The effects of the interaction on the kinematics, stellar population and metallicity of AM2322-821 with Gemini/GMOS
We present an observational study about the impacts of the interactions in
the kinematics, stellar populations, and oxygen abundances of the components of
the galaxy pair AM\,2322-821. A fairly symmetric rotation curve for the
companion (AM\,2322B) galaxy with a deprojected velocity amplitude of 110 km
s was obtained, and a dynamical mass of 1.1 - 1.3 \times 10^{10}
M_{\sun} within a radius of 4 kpc was estimated using this deprojected
velocity. Asymmetries in the radial velocity field were detected for the
companion, very likely due the interaction between the galaxies. The
interaction between the main and companion galaxies was modelled using
numerical N-body/hydrodynamical simulations, with the result indicating that
the current stage of the system would be about 90 Myr after perigalacticum. The
spatial variation in the distribution of the stellar-population components in
both galaxies was analysed using the stellar population synthesis code {\sc
STARLIGHT}. The companion galaxy is dominated by a very young (t yr) population, with the fraction of this population to the
total flux at , increasing outwards in the galaxy disc.
On the other hand, the stellar population of AM\,2322A is heterogeneous along
the slit positions observed. Spatial profiles of oxygen abundance in the
gaseous phase were obtained using the diagnostic diagram R23 vs. [OIII]/[OII],
where we compared the observed values with the ones obtained from
photoionization models.
Such gradients of oxygen abundance are significantly flatter for this pair of
galaxies than in typical isolated spiral galaxies. This metallicity
distribution is interpreted as the gradients having been destroyed by
interaction-induced gas flows from the outer parts to the centre of the galaxyComment: accepted by MNRA
The distribution of metals in cosmological hydrodynamical simulations of dwarf disc galaxies
Publisher's Version/PDFWe examine the chemical properties of five cosmological hydrodynamical simulations of an M33-like disc galaxy which have been shown previously to be consistent with the morphological characteristics and bulk scaling relations expected of late-type spirals. These simulations are part of the Making Galaxies in a Cosmological Context Project, in which stellar feedback is tuned to match the stellar mass–halo mass relationship. Each realization employed identical initial conditions and assembly histories, but differed from one another in their underlying baryonic physics prescriptions, including (a) the efficiency with which each supernova energy couples to the surrounding interstellar medium, (b) the impact of feedback associated with massive star radiation pressure, (c) the role of the minimum shut-off time for radiative cooling of Type II supernovae remnants, (d) the treatment of metal diffusion and (e) varying the initial mass function. Our analysis focusses on the resulting stellar metallicity distribution functions (MDFs) in each simulated (analogous) ‘solar neighbourhood’ (2–3 disc scalelengths from the galactic centre) and central ‘bulge’ region. We compare and contrast the simulated MDFs’ skewness, kurtosis and dispersion (inter-quartile, inter-decile, inter-centile and inter-tenth-percentile regions) with that of the empirical solar neighbourhood MDF and Local Group dwarf galxies.We find that the MDFs of the simulated discs are more negatively skewed, with higher kurtosis, than those observed locally in the Milky Way and Local Group dwarfs. We can trace this difference to the simulations' very tight and correlated age–metallicity relations (compared with that of the MilkyWay's solar neighbourhood), suggesting that these relations within ‘dwarf’ discs might be steeper than in Lâ
discs (consistent with the simulations' star formation histories and extant empirical data), and/or the degree of stellar orbital redistribution and migration inferred locally has not been captured in their entirety, at the resolution of our simulations. The important role of metal diffusion in ameliorating the overproduction of extremely metal-poor stars is highlighted