7 research outputs found
Feedback and the Structure of Simulated Galaxies at redshift z=2
We study the properties of simulated high-redshift galaxies using
cosmological N-body/gasdynamical runs from the OverWhelmingly Large Simulations
(OWLS) project. The runs contrast several feedback implementations of varying
effectiveness: from no-feedback, to supernova-driven winds to powerful
AGN-driven outflows. These different feedback models result in large variations
in the abundance and structural properties of bright galaxies at z=2. We find
that feedback affects the baryonic mass of a galaxy much more severely than its
spin, which is on average roughly half that of its surrounding dark matter halo
in our runs. Feedback induces strong correlations between angular momentum
content and galaxy mass that leave their imprint on galaxy scaling relations
and morphologies. Encouragingly, we find that galaxy disks are common in
moderate-feedback runs, making up typically ~50% of all galaxies at the centers
of haloes with virial mass exceeding 1e11 M_sun. The size, stellar masses, and
circular speeds of simulated galaxies formed in such runs have properties that
straddle those of large star-forming disks and of compact early-type galaxies
at z=2. Once the detailed abundance and structural properties of these rare
objects are well established it may be possible to use them to gauge the
overall efficacy of feedback in the formation of high redshift galaxies.Comment: 16 pages, 12 figures. Accepted for publication in MNRAS. Minor
changes to match published versio
Cosmic Reionisation by Stellar Sources: Population III Stars
We combine fast radiative transfer calculations with high resolution
hydrodynamical simulations to study an epoch of early hydrogen reionisation by
primordial stellar sources at redshifts 15<z<30. With relatively conservative
assumptions, population III star formation proceeds in a self-regulated manner
both locally and globally and, for a conventional LCDM cosmology, can
significantly reionise the intergalactic medium between 15<z<20 as long as a
large fraction of ionising photons can escape from these earliest galaxies. We
then combine these results with our earlier work focusing on the role of
population II stars in galaxies with virial temperatures >10^4K at redshifts
5<z< 20. Hence, we construct a complete reionisation history of the Universe
which matches the Thomson optical depths as measured by the WMAP satellite as
well as the evolution of the Gunn Peterson optical depth as seen in the
asborption spectra of the higest redshift quasars. We find that even with
conservative estimates for the impact of negative feedback mechanisms,
primordial stellar sources contribute significantly to early reionisation.
Future observations of a Thomson optical depth of tau_e>~0.13 would bolster the
claim for the existence of population III stars similar to the ones studied
here.Comment: 40 pages, 9 figure
From dwarf spheroidals to cDs: Simulating the galaxy population in a LCDM cosmology
We apply updated semi-analytic galaxy formation models simultaneously to the
stored halo/subhalo merger trees of the Millennium and Millennium-II
simulations. These differ by a factor of 125 in mass resolution, allowing
explicit testing of resolution effects on predicted galaxy properties. We have
revised the treatments of the transition between the rapid infall and cooling
flow regimes of gas accretion, of the sizes of bulges and of gaseous and
stellar disks, of supernova feedback, of the transition between central and
satellite status as galaxies fall into larger systems, and of gas and star
stripping once they become satellites. Plausible values of efficiency and
scaling parameters yield an excellent fit not only to the observed abundance of
low-redshift galaxies over 5 orders of magnitude in stellar mass and 9
magnitudes in luminosity, but also to the observed abundance of Milky Way
satellites. This suggests that reionisation effects may not be needed to solve
the "missing satellite" problem except, perhaps, for the faintest objects. The
same model matches the observed large-scale clustering of galaxies as a
function of stellar mass and colour. The fit remains excellent down to ~30kpc
for massive galaxies. For M* < 6 x 10^10Msun, however, the model overpredicts
clustering at scales below 1 Mpc, suggesting that the sigma_8 adopted in the
simulations (0.9) is too high. Galaxy distributions within rich clusters agree
between the simulations and match those observed, but only if galaxies without
dark matter subhalos (so-called orphans) are included. Our model predicts a
larger passive fraction among low-mass galaxies than is observed, as well as an
overabundance of ~10^10Msun galaxies beyond z~0.6, reflecting deficiencies in
the way star-formation rates are modelled.Comment: Accepted for publication in MNRAS. SQL databases containing the full
galaxy data at all redshifts and for both the Millennium and Millennium-II
simulations are publicly released at
http://www.mpa-garching.mpg.de/millenniu
Simulations of Cosmic Chemical Enrichment
Using a new numerical model for cosmic chemical evolution, we study the
influence of hypernova feedback on the star formation and metal enrichment
history of the universe. For assumptions which produce plausible results in
idealized collapse models of individual galaxies, our cosmological simulations
of the standard Lambda-CDM cosmology show a peak of the cosmic star formation
rate at z~4, with ~10% of the baryons turning into stars. We find that the
majority of stars in present-day massive galaxies formed in much smaller
galaxies at high redshifts, giving them a mean stellar age as old as 10 Gyr,
despite their late assembly times. The hypernova feedback drives galactic
outflows efficiently in low mass galaxies, and these winds eject heavy elements
into the intergalactic medium. The ejected baryon fraction is larger for less
massive galaxies, correlates well with stellar metallicity, and amounts to ~20%
of all baryons in total. The resulting enrichment history is broadly consistent
with the observed abundances of Lyman break galaxies, of damped Lyman alpha
systems, and of the intergalactic medium. The metallicity of the cold gas in
galaxies increases with galaxy mass, which is comparable to observations with a
significant scatter. The stellar mass-metallicity relation of the observed
galaxy population is well reproduced by the simulation model as a result of
mass-dependent galactic winds. However, star formation does not terminate in
massive galaxies at late times in our model, and too few dwarf galaxies are
still forming stars. These problems may be due to a lack of resolution, to
inappropriate modelling of supernova feedback, or to a neglect of other
feedback processes such as active galactic nuclei.Comment: 16 pages, 20 figures. Accepted for publication in MNRA