1,004 research outputs found
The Spatial Distribution of the Galactic First Stars II: SPH Approach
We use cosmological, chemo-dynamical, smoothed particle hydrodynamical
simulations of Milky-Way-analogue galaxies to find the expected present-day
distributions of both metal-free stars that formed from primordial gas and the
oldest star populations. We find that metal-free stars continue to form until
z~4 in halos that are chemically isolated and located far away from the biggest
progenitor of the final system. As a result, if the Population III initial mass
function allows stars with low enough mass to survive until z=0 (< 0.8 Msol),
they would be distributed throughout the Galactic halo. On the other hand, the
oldest stars form in halos that collapsed close to the highest density peak of
the final system, and at z=0 they are located preferentially in the central
region of the Galaxy, i.e., in the bulge. According to our models, these trends
are not sensitive to the merger histories of the disk galaxies or the
implementation of supernova feedback. Furthermore, these full hydrodynamics
results are consistent with our N-body results in Paper I, and lend further
weight to the conclusion that surveys of low-metallicity stars in the Galactic
halo can be used to directly constrain the properties of primordial stars. In
particular, they suggest that the current lack of detections of metal-free
stars implies that their lifetimes were shorter than a Hubble time, placing
constraints on the metal-free initial mass function.Comment: Accepted by ApJ. Emulate ApJ styl
Passive Scalar Structures in Supersonic Turbulence
We conduct a systematic numerical study of passive scalar structures in
supersonic turbulent flows. We find that the degree of intermittency in the
scalar structures increases only slightly as the flow changes from transonic to
highly supersonic, while the velocity structures become significantly more
intermittent. This difference is due to the absence of shock-like
discontinuities in the scalar field. The structure functions of the scalar
field are well described by the intermittency model of She and L\'{e}v\^{e}que
[Phys. Rev. Lett. 72, 336 (1994)], and the most intense scalar structures are
found to be sheet-like at all Mach numbers.Comment: 4 pages, 3 figures, to appear in PR
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
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
Detecting the Gravitational Redshift of Cluster Gas
We examine the gravitational redshift of radiation emitted from within the
potential of a cluster. Spectral lines from the intracluster medium (ICM) are
redshifted in proportion to the emission-weighted mean potential along the line
of sight, amounting to approximately 50 km/s at a radius of 100 kpc/h, for a
cluster dispersion of 1200 km/s. We show that the relative redshifts of
different ionization states of metals in the ICM provide a unique probe of the
three-dimensional matter distribution. An examination of the reported peculiar
velocities of cD galaxies in well studied Abell clusters reveals they are
typically redshifted by an average of km/s. This can be achieved by
gravity with the addition of a steep central potential associated with the cD
galaxy. Note that in general gravitational redshifts cause a small overestimate
of the recessional velocities of clusters by an average of 20 km/s.Comment: 6 pages, 3 figures, accepted to the Astrophysical Journal Letter
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
AGN Feedback Causes Downsizing
We study the impact of outflows driven by active galactic nuclei (AGN) on
galaxy formation. Outflows move into the surrounding intergalactic medium (IGM)
and heat it sufficiently to prevent it from condensing onto galaxies. In the
dense, high-redshift IGM, such feedback requires highly energetic outflows,
driven by a large AGN. However, in the more tenuous low-redshift IGM,
equivalently strong feedback can be achieved by less energetic winds (and thus
smaller galaxies). Using a simple analytic model, we show that this leads to
the anti-hierarchical quenching of star-formation in large galaxies, consistent
with current observations. At redshifts prior to the formation of large AGN,
galaxy formation is hierarchical and follows the growth of dark-matter halos.
The transition between the two regimes lies at the z ~ 2 peak of AGN activity.Comment: 6 pages, 2 figures, ApJL in pres
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