703 research outputs found
Modeling the High-z Universe: Probing Galaxy Formation
We discuss how the conditions at high redshift differ from those at low
redshift, and what the impact is on the galaxy population. We focus in
particular on the role of gaseous dissipation and its impact on sustaining high
star formation rates as well as on driving star-bursts in mergers. Gas
accretion onto galaxies at high redshifts occurs on a halo dynamical time
allowing for very efficiently sustained star formation. In addition cold
accretion flows are able to drive turbulence in high redshift disks at the
level observed if at least 20% of the accretion energy is converted into random
motion in the gaseous disk. In general we find that the fraction of gas
involved in galaxy mergers is a strong function of time and increases with
redshift. A model combining the role of dissipation during mergers and
continued infall of satellite galaxies allows to reproduce the observed
size-evolution of early-type galaxies with redshift. Furthermore we investigate
how the evolution of the faint-end of the luminosity function can be explained
in terms of the evolution of the underlying dark matter evolution.Comment: To appear in "Reviews in Modern Astronomy", volume 21, Proceedings of
JENAM 2008, Vienn
The imprint of cosmological non-Gaussianities on primordial structure formation
We study via numerical N-body/SPH chemistry simulations the effects of
primordial non-Gaussianities on the formation of the first stars and galaxies,
and investigate the impact of supernova feedback in cosmologies with different
fnl. Density distributions are biased to higher values, so star formation and
the consequent feedback processes take place earlier in high-fnl models and
later in low-fnl ones. Mechanical feedback is responsible for shocking and
evacuating the gas from star forming sites earlier in the highly non-Gaussian
cases, because of the larger bias at high densities. Chemical feedback
translates into high-redshift metal filling factors that are larger by some
orders of magnitude for larger fnl, but that converge within one Gyr, for both
population III and population II-I stellar regimes. The efficient enrichment
process, though, leads to metallicities > 0.01 Zsun by redshift ~9, almost
independently from fnl. The impact of non-Gaussianities on the formation of
dark-matter haloes at high redshift is directly reflected in the properties of
the gas in these haloes, as models with larger fnl show more concentrated gas
profiles at early times. Non-Gaussian signatures in the gas behaviour are lost
after the first feedback takes place and introduces a significant degree of
turbulence and chaotic motions.Comment: 10 pages, 9 figures - accepted for publication in MNRA
Revised rate coefficients for H and H destruction by realistic stellar spectra
Understanding the processes that can destroy H and H species is
quintessential in governing the formation of the first stars, black holes and
galaxies. In this study we compute the reaction rate coefficients for H
photo--dissociation by Lyman--Werner photons ( eV), and H
photo--detachment by 0.76 eV photons emanating from self-consistent stellar
populations that we model using publicly available stellar synthesis codes. So
far studies that include chemical networks for the formation of molecular
hydrogen take these processes into account by assuming that the source spectra
can be approximated by a power-law dependency or a black-body spectrum at
10 or K. We show that using spectra generated from realistic stellar
population models can alter the reaction rates for photo-dissociation, , and photo-detachment, , significantly. In
particular, can be up to orders of magnitude lower
in the case of realistic stellar spectra suggesting that previous calculations
have over-estimated the impact that radiation has on lowering H abundances.
In contrast to burst modes of star formation, we find that models with
continuous star formation predict increasing and , which makes it necessary to include the star formation history of
sources to derive self-consistent reaction rates, and that it is not enough to
just calculate J for the background. For models with constant star
formation rate the change in shape of the spectral energy distribution leads to
a non-negligible late-time contribution to and , and we present self-consistently derived cosmological reaction
rates based on star formation rates consistent with observations of the high
redshift Universe.Comment: Submitted to MNRAS, 9 pages, 7 figures. Comments and communication
welcom
Suppression of accretion onto low-mass Population III stars
Motivated by recent theoretical work suggesting that a substantial fraction
of Population (Pop) III stars may have had masses low enough for them to
survive to the present day, we consider the role that the accretion of
metal-enriched gas may have had in altering their surface composition, thereby
disguising them as Pop II stars. We demonstrate that if weak, Solar-like winds
are launched from low-mass Pop III stars formed in the progenitors of the dark
matter halo of the Galaxy, then such stars are likely to avoid significant
enrichment via accretion of material from the interstellar medium. We find that
at early times accretion is easily prevented if the stars are ejected from the
central regions of the haloes in which they form, either by dynamical
interactions with more massive Pop III stars, or by violent relaxation during
halo mergers. While accretion may still take place during passage through
sufficiently dense molecular clouds at later times, we find that the
probability of such a passage is generally low (< 0.1), assuming that stars
have velocities of order the maximum circular velocity of their host haloes and
accounting for the orbital decay of merging haloes. In turn, due to the higher
gas density required for accretion onto stars with higher velocities, we find
an even lower probability of accretion (~ 0.01) for the subset of Pop III stars
formed at z > 10, which are more quickly incorporated into massive haloes than
stars formed at lower redshift. While there is no a priori reason to assume
that low-mass Pop III stars do not have Solar-like winds, without them surface
enrichment via accretion is likely to be inevitable. We briefly discuss the
implications that our results hold for stellar archaeology.Comment: 9 pages; 2 figures; MNRAS accepte
The specific star formation rate of high redshift galaxies: the case for two modes of star formation
We study the specific star formation rate (SSFR) and its evolution at
z\gtsim 4, in models of galaxy formation, where the star formation is driven
by cold accretion flows. We show that constant star formation and feedback
efficiencies cannot reproduce the observed trend of SSFR with stellar mass and
its observed lack of evolution at . Model galaxies with \log(M_*) \ltsim
9.5 M show systematically lower specific star formation rates by
orders of magnitudes, while massive galaxies with M_* \gtsim 5 \times 10^{10}
M have up to an order of magnitude larger SSFRs, compared to recent
observations by Stark et al.. To recover these observations we apply an
empirical star formation efficiency in galaxies that scales with the host halo
velocity dispersion as during galaxy mergers. We find that
this modification needs to be of stochastic nature to reproduce the
observations, i.e. only applied during mergers and not during accretion driven
star formation phases. Our choice of star formation efficiency during mergers
allows us to capture both, the boost in star formation at low masses and the
quenching at high masses, and at the same time produce a constant SSFR-stellar
mass relation at z\gtsim 4 under the assumption that most of the observed
galaxies are in a merger triggered star formation phase. Our results suggest
that observed high-z low mass galaxies with high SSFRs are likely to be
frequently interacting systems, which experienced bursts in their star
formation rate and efficiency (mode 1), in contrast to low redshift z \ltsim
3 galaxies which are cold accretion-regulated star forming systems with lower
star formation efficiencies (mode 2).Comment: 5 pages, accepted to MNRAS, replaced by version with including
referees comment
Gravity at Work: How the Build-Up of Environments Shape Galaxy Properties
We present results on the heating of the inter-cluster medium (ICM) by
gravitational potential energy from in-falling satellites. We calculate the
available excess energy of baryons once they are stripped from their satellite
and added to the ICM of the hosting environment. this excess energy is a strong
function of environment and we find that it can exceed the contribution from
AGNs or supernovae (SN) by up to two orders of magnitude in the densest
environments/haloes. Cooling by radiative losses is in general fully
compensated by gravitational heating in massive groups and clusters with hot
gas temperature > 1 keV. The reason for the strong environment dependence is
the continued infall of substructure onto dense environments during their
formation in contrast to field-like environments. We show that gravitational
heating is able to reduce the number of too luminous galaxies in models and to
produce model luminosity functions in agreement with observations.Comment: 8 pages, 3 figures. To be published in Proceedings of JENAM 2010,
Symposium 2: "Environment and the formation of galaxies: 30 years later
The First Billion Years project - IV: Proto-galaxies reionising the Universe
The contribution of stars in galaxies to cosmic reionisation depends on the
star formation history in the Universe, the abundance of galaxies during
reionisation, the escape fraction of ionising photons and the clumping factor
of the inter-galactic medium (IGM). We compute the star formation rate and
clumping factor during reionisation in a cosmological volume using a
high-resolution hydrodynamical simulation. We post-process the output with
detailed radiative transfer simulations to compute the escape fraction of
ionising photons. Together, this gives us the opportunity to assess the
contribution of galaxies to reionisation self-consistently. The strong mass and
redshift dependence of the escape fraction indicates that reionisation occurred
between z=15 and z=10 and was mainly driven by proto-galaxies forming in
dark-matter haloes with masses between 1e7 and 1e8 solar mass. More massive
galaxies that are rare at these redshifts and have significantly lower escape
fractions contribute less photons to the reionisation process than the
more-abundant low-mass galaxies. Star formation in the low-mass haloes is
suppressed by radiative feedback from reionisation, therefore these
proto-galaxies only contribute when the part of the Universe they live in is
still neutral. After z~10, massive galaxies become more abundant and provide
most of the ionising photons. In addition, we find that Population (Pop) III
stars are too short-lived and not frequent enough to have a major contribution
to reionisation. Although the stellar component of the proto-galaxies that
produce the bulk of ionising photons during reionisation is too faint to be
detected by the James Webb Space Telescope (JWST), these sources are brightest
in the H-alpha and Ly-alpha recombination lines, which will likely be detected
by JWST in deep surveys.Comment: 5 pages, 4 figures, accepted for publication in MNRAS letter
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