674 research outputs found
Stellar haloes in Milky-Way mass galaxies: From the inner to the outer haloes
We present a comprehensive study of the chemical properties of the stellar
haloes of Milky-Way mass galaxies, analysing the transition between the inner
to the outer haloes. We find the transition radius between the relative
dominance of the inner-halo and outer-halo stellar populations to be ~15-20 kpc
for most of our haloes, similar to that inferred for the Milky Way from recent
observations. While the number density of stars in the simulated inner-halo
populations decreases rapidly with distance, the outer-halo populations
contribute about 20-40 per cent in the fiducial solar neighborhood, in
particular at the lowest metallicities. We have determined [Fe/H] profiles for
our simulated haloes; they exhibit flat or mild gradients, in the range
[-0.002, -0.01 ] dex/kpc. The metallicity distribution functions exhibit
different features, reflecting the different assembly history of the individual
stellar haloes. We find that stellar haloes formed with larger contributions
from massive subgalactic systems have steeper metallicity gradients. Very
metal-poor stars are mainly contributed to the halo systems by lower-mass
satellites. There is a clear trend among the predicted metallicity distribution
functions that a higher fraction of low-metallicity stars are found with
increasing radius. These properties are consistent with the range of behaviours
observed for stellar haloes of nearby galaxies.Comment: 11 pages, 6 figures. Accepted MNRAS. Revised version after referee's
comment
Effects of Supernova Feedback on the Formation of Galaxies
We study the effects of Supernova (SN) feedback on the formation of galaxies
using hydrodynamical simulations in a Lambda-CDM cosmology. We use an extended
version of the code GADGET-2 which includes chemical enrichment and energy
feedback by Type II and Type Ia SN, metal-dependent cooling and a multiphase
model for the gas component. We focus on the effects of SN feedback on the star
formation process, galaxy morphology, evolution of the specific angular
momentum and chemical properties. We find that SN feedback plays a fundamental
role in galaxy evolution, producing a self-regulated cycle for star formation,
preventing the early consumption of gas and allowing disks to form at late
times. The SN feedback model is able to reproduce the expected dependence on
virial mass, with less massive systems being more strongly affected.Comment: To appear in "The Galaxy Disk in Cosmological Context"; Proceedings
of IAU254; 9-13 June 2008; Copenhagen; v2: typo corrected; uses iaus.cl
How the Universe got its Spots
The universe displays a three-dimensional pattern of hot and cold spots in
the radiation remnant from the big bang. The global geometry of the universe
can be revealed in the spatial distribution of these spots. In a topologically
compact universe, distinctive patterns are especially prominent in spatial
correlations of the radiation temperature. Whereas these patterns are usually
washed out in statistical averages, we propose a scheme which uses the
universe's spots to observe global geometry in a manner analogous to the use of
multiple images of a gravitationally lensed quasar to study the geometry of the
lens. To demonstrate how the geometry of space forms patterns in observations
of the microwave sky, we develop a simple real-space approximation to estimate
temperature correlations for any set of cosmological parameters and any global
geometry. We present correlated spheres which clearly show geometric pattern
formation for compact flat universes as well as for the compact negatively
curved space introduced by Weeks and another discovered by Best. These examples
illustrate how future satellite-based observations of the microwave background
can determine the full geometry of the universe.Comment: 16 pages, 26 figure
Dark matter response to galaxy formation
We have resimulated the six galaxy-sized haloes of the Aquarius Project
including metal-dependent cooling, star formation and supernova feedback. This
allows us to study not only how dark matter haloes respond to galaxy formation,
but also how this response is affected by details of halo assembly history. In
agreement with previous work, we find baryon condensation to lead to increased
dark matter concentration. Dark matter density profiles differ substantially in
shape from halo to halo when baryons are included, but in all cases the
velocity dispersion decreases monotonically with radius. Some haloes show an
approximately constant dark matter velocity anisotropy with , while others retain the anisotropy structure of their baryon-free
versions. Most of our haloes become approximately oblate in their inner
regions, although a few retain the shape of their dissipationless counterparts.
Pseudo-phase-space densities are described by a power law in radius of altered
slope when baryons are included. The shape and concentration of the dark matter
density profiles are not well reproduced by published adiabatic contraction
models. The significant spread we find in the density and kinematic structure
of our haloes appears related to differences in their formation histories. Such
differences already affect the final structure in baryon-free simulations, but
they are reinforced by the inclusion of baryons, and new features are produced.
The details of galaxy formation need to be better understood before the inner
dark matter structure of galaxies can be used to constrain cosmological models
or the nature of dark matter.Comment: 14 pages, 9 figures. Accepted MNRAS. Revised version includes
discussion on resolution effects and minor changes
The role of tidal interactions in driving galaxy evolution
We carry out a statistical analysis of galaxy pairs selected from chemical
hydrodynamical simulations with the aim at assessing the capability of
hierarchical scenarios to reproduce recent observational results for galaxies
in pairs. Particularly, we analyse the effects of mergers and interactions on
the star formation (SF) activity, the global mean chemical properties and the
colour distribution of interacting galaxies. We also assess the effects of
spurious pairs.Comment: to appear in "Groups of galaxies in the nearby Universe" ESO
Workshop, (Dec 2005) Santiago, Chil
The Formation and Survival of Discs in a Lambda-CDM Universe
We study the formation of galaxies in a Lambda-CDM Universe using high
resolution hydrodynamical simulations with a multiphase treatment of gas,
cooling and feedback, focusing on the formation of discs. Our simulations
follow eight haloes similar in mass to the Milky Way and extracted from a large
cosmological simulation without restriction on spin parameter or merger
history. This allows us to investigate how the final properties of the
simulated galaxies correlate with the formation histories of their haloes. We
find that, at z = 0, none of our galaxies contain a disc with more than 20 per
cent of its total stellar mass. Four of the eight galaxies nevertheless have
well-formed disc components, three have dominant spheroids and very small
discs, and one is a spheroidal galaxy with no disc at all. The z = 0 spheroids
are made of old stars, while discs are younger and formed from the inside-out.
Neither the existence of a disc at z = 0 nor the final disc-to-total mass ratio
seems to depend on the spin parameter of the halo. Discs are formed in haloes
with spin parameters as low as 0.01 and as high as 0.05; galaxies with little
or no disc component span the same range in spin parameter. Except for one of
the simulated galaxies, all have significant discs at z > ~2, regardless of
their z = 0 morphologies. Major mergers and instabilities which arise when
accreting cold gas is misaligned with the stellar disc trigger a transfer of
mass from the discs to the spheroids. In some cases, discs are destroyed, while
in others, they survive or reform. This suggests that the survival probability
of discs depends on the particular formation history of each galaxy. A
realistic Lambda-CDM model will clearly require weaker star formation at high
redshift and later disc assembly than occurs in our models.Comment: 14 pages, 10 figures, mn2e.cls. MNRAS in press, updated to match
published versio
The host galaxies of long-duration GRBs in a cosmological hierarchical scenario
We developed a Monte Carlo code to generate long-duration gamma ray burst
(LGRB) events within cosmological hydrodynamical simulations consistent with
the concordance model. As structure is assembled, LGRBs are generated in the
substructure that formed galaxies today. We adopted the collapsar model so that
LGRBs are produced by single, massive stars at the final stage of their
evolution. We found that the observed properties of the LGRB host galaxies
(HGs) are reproduced if LGRBs are also required to be generated by low
metallicity stars. The low metallicity condition imposed on the progenitor
stars of LGRBs selects a sample of HGs with mean gas abundances of 12 + log O/H
\~ 8.6. For z<1 the simulated HGs of low metallicity LGRB progenitors tend to
be faint, slow rotators with high star formation efficiency, compared with the
general galaxy population, in agreement with observations. At higher redshift,
our results suggest that larger systems with high star formation activity could
also contribute to the generation of LGRBs from low metallicity progenitors
since the fraction of low metallicity gas available for star formation
increases for all systems with look-back time. Under the hypothesis of our LGRB
model, our results support the claim that LGRBs could be unbiased tracers of
star formation at high redshifts.Comment: Final revised version with minor changes. 9 pages, 9 figures,
mn2e.cls. To appear in MNRA
Non-equilibrium Ionization States Within Galactic Outflows: Explaining Their O VI and N V Column Densities
We present a suite of one-dimensional spherically-symmetric hydrodynamic
simulations that study the atomic ionization structure of galactic outflows. We
track the ionization state of the outflowing gas with a non-equilibrium atomic
chemistry network that includes photoionization, photo-heating, and ion-by-ion
cooling. Each simulation describes a steady-state outflow that is defined by
its mass and energy input rates, sonic radius, metallicity, and UV flux from
both the host galaxy and meta-galactic background. We find that for a large
range of parameter choices, the ionization state of the material departs
strongly from what it would be in photo-ionization equilibrium, in conflict
with what is commonly assumed in the analysis of observations. In addition,
nearly all the models reproduce the low N V to O VI column density ratios and
the relatively high O VI column densities that are observed.Comment: 16 pages, 11 figures, accepted to ApJ. Comments welcom
Stellar haloes of simulated Milky Way-like galaxies: Chemical and kinematic properties
We investigate the chemical and kinematic properties of the diffuse stellar
haloes of six simulated Milky Way-like galaxies from the Aquarius Project.
Binding energy criteria are adopted to defined two dynamically distinct stellar
populations: the diffuse inner and outer haloes, which comprise different
stellar sub-populations with particular chemical and kinematic characteristics.
Our simulated inner- and outer-halo stellar populations have received
contributions from debris stars (formed in sub-galactic systems while they were
outside the virial radius of the main progenitor galaxies) and endo-debris
stars (those formed in gas-rich sub-galactic systems inside the dark matter
haloes). The inner haloes possess an additional contribution from disc-heated
stars in the range , with a mean of . Disc-heated
stars might exhibit signatures of kinematical support, in particular among the
youngest ones. Endo-debris plus disc-heated stars define the so-called \insitu
stellar populations. In both the inner- and outer-halo stellar populations, we
detect contributions from stars with moderate to low [/Fe] ratios,
mainly associated with the endo-debris or disc-heated sub-populations. The
observed abundance gradients in the inner-halo regions are influenced by both
the level of chemical enrichment and the relative contributions from each
stellar sub-population. Steeper abundance gradients in the inner-halo regions
are related to contributions from the disc-heated and endo-debris stars, which
tend to be found at lower binding energies than debris stars. (Abridged).Comment: 12 pages, 6 figures. Accepted version. To appear in MNRA
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