1,087 research outputs found
Constraining local UV field geometry at reionization using Milky Way satellites
We present a new semi-analytical model of the population of satellite
galaxies of the Milky Way, aimed at estimating the effect of the geometry of
reionization at galaxy scale on the properties of the satellites. In this model
reionization can be either: (A) externally-driven and uniform, or (B)
internally-driven, by the most massive progenitor of the Milky Way. In the
latter scenario the propagation of the ionisation front and photon dilution
introduce a delay in the photo-evaporation of the outer satellites' gas with
respect to the inner satellites. As a consequence, outer satellites experience
a longer period of star formation than those in the inner halo. We use simple
models to account for star formation, the propagation of the ionisation front,
photo-evaporation and observational biases. Both scenarios yield a model
satellite population that matches the observed luminosity function and
mass-to-light ratios. However, the predicted population for scenario (B) is
significantly more extended spatially than for scenario (A), by about 0.3 dex
in distance, resulting in a much better match to the observations. The survival
of the signature left by the local UV field during reionization on the radial
distribution of satellites makes it a promising tool for studying the
reionization epoch at galaxy scale in the Milky Way and nearby galaxies
resolved in stars with forthcoming large surveys.Comment: Proceedings of the conference "Assembling the puzzle of the Milky
Way" held at Le Grand Bornand, 201
A study of simulated histories of reionization with merger trees of HII regions
We describe a new methodology to analyze the reionization process in
numerical simulations: the chronology and the geometry of reionization is
investigated by means of merger histories of individual HII regions. From the
merger tree of ionized patches, one can track the individual evolution of the
regions properties such as e.g. their size, or the intensity of the percolation
process by looking at the formation rate, the frequency of mergers and the
number of individual HII regions involved in the mergers. We apply the merger
tree technique to simulations of reionization with three different kinds of
ionizing source models and two resolutions. Two of them use star particles as
ionizing sources. In this case we confront two emissivity evolutions for the
sources in order to reach the reionization at z ~ 6. As an alternative we built
a semi-analytical model where the dark matter halos extracted from the density
fields are assumed as ionizing sources. We then show how this methodology is a
good candidate to quantify the impact of the adopted star formation on the
history of the observed reionization. The semi-analytical model shows a
homogeneous reionization history with 'local' hierarchical growth steps for
individual HII regions. On the other hand auto-consistent models for star
formation tend to present fewer regions with a dominant region in size which
governs the fusion process early in the reionization at the expense of the
'local' reionizations. The differences are attenuated when the resolution of
the simulation is increased.Comment: Accepted for publication in A&
Local reionizations histories with merger tree of HII regions
We constrain the initial stage of the reionization process around progenitors
of galaxies, such as the extent of the initial HII region before its fusion
with the UV background and the duration of its propagation. We use a set of
reionisation simulations with different resolutions and ionizing source
recipes. A catalog of the HII regions properties is built thanks to a merger
tree of HII regions. We draw local reionization histories as a function of time
and investigate variations according to the halo mass progenitors of the
regions. We then extrapolate the halo mass inside the region from high z to z=0
to make predictions about the reionization histories of z=0 galaxies. We found
that the later an HII region appears, the smaller will be its related lifetime
and volume before it sees the global UV background. Quantitatively the duration
and the extent of the initial growth of an HII region is strongly dependent on
the mass of the inner halo and can be as long as 50 % of the reionization
epoch. We found that the most massive is a halo today, the earlier it appears
and the larger are the extension and the duration of propagation of its HII
region. Quantitative predictions differ depending on the box size or the source
model: small simulated volumes are affected by proximity effects between HII
regions and halo-based source models predict smaller regions and slower I-front
expansion than in models using star particles as ionizing sources. Our results
suggests that Milky Way-type halos have a maximal extent of 1.1 Mpc/h for the
initial HII region that established itself in 150-200 Myrs. This is
consistent with prediction made using constrained Local Group simulation.
Considering halos with masses comparable to those of the Local Group (MW+M31),
our result suggests that statistically it has not been influenced by an
external front coming from a Virgo-like cluster.Comment: 16 pages, 9 figures, Accepted for publication in A&
The impact of binary-star yields on the spectra of galaxies
One of the complexities in modelling integrated spectra of stellar populations is the effect of interacting binary stars besides Type Ia supernovae (SNeIa). These include common envelope systems, cataclysmic variables, novae, and are usually ignored in models predicting the chemistry and spectral absorption line strengths in galaxies. In this paper, predictions of chemical yields from populations of single and binary stars are incorporated into a galactic chemical evolution model to explore the significance of the effects of these other binary yields. Effects on spectral line strengths from different progenitor channels of SNeIa are also explored. Small systematic effects are found when the yields from binaries, other than SNeIa, are included, for a given star formation history. These effects are, at present, within the observational uncertainties on the line strengths. More serious differences can arise in considering different types of SNIa models, their rates and contributions
Galaxy formation hydrodynamics: From cosmic flows to star-forming clouds
Major progress has been made over the last few years in understanding
hydrodynamical processes on cosmological scales, in particular how galaxies get
their baryons. There is increasing recognition that a large part of the baryons
accrete smoothly onto galaxies, and that internal evolution processes play a
major role in shaping galaxies - mergers are not necessarily the dominant
process. However, predictions from the various assembly mechanisms are still in
large disagreement with the observed properties of galaxies in the nearby
Universe. Small-scale processes have a major impact on the global evolution of
galaxies over a Hubble time and the usual sub-grid models account for them in a
far too uncertain way. Understanding when, where and at which rate galaxies
formed their stars becomes crucial to understand the formation of galaxy
populations. I discuss recent improvements and current limitations in
"resolved" modelling of star formation, aiming at explicitely capturing
star-forming instabilities, in cosmological and galaxy-sized simulations. Such
models need to develop three-dimensional turbulence in the ISM, which requires
parsec-scale resolution at redshift zero.Comment: To appear in the proceedings for IAU Symposium 270: Computational
Star Formation (eds. Alves, Elmegreen, Girart, Trimble
Fitting galaxy spectra with STECKMAP: a user guide
STECKMAP stands for STEllar Content and Kinematics via Maximum A Posteriori
likelihood. It is a tool for interpreting galaxy spectra in terms of their
stellar populations, through the derivation of their star formation history,
age-metallicity relation, kinematics and extinction. To do so, the observed
spectrum is projected onto a temporal sequence of models of single stellar
populations, so as to determine a linear combination of these models, that fit
the observed spectrum best. The weights of the various components of this
linear combination indicate the stellar content of the population. This
procedure is regularized using various penalizing functions. The principles of
the method are detailed in Ocvirk et al. 2006a,b. The STECKMAP software package
is public and freely available at http://astro.u-strasbg.fr/~ocvirk/. A number
of authors have already adopted it and use it in their daily research. This
user guide aims at accompanying the user through the setup and first runs of
STECKMAP. The last chapter will help the user to understand and improve his
results and experience with the code.Comment: 27 page
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