230 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
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
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
Validation of stellar population and kinematical analysis of galaxies
3D spectroscopy produces hundreds of spectra from which maps of the
characteristics of stellar populations (age-metallicity) and internal
kinematics of galaxies can be derived. We carried on simulations to assess the
reliability of inversion methods and to define the requirements for future
observations. We quantify the biases and show that to minimize the errors on
the kinematics, age and metallicity (in a given observing time) the size of the
spatial elements and the spectral dispersion should be chosen to obtain an
instrumental velocity dispersion comparable to the physical dispersion.Comment: 5 pages, 3 figures, extended version of a poster proceeding to appear
in "Science Perspectives for 3D Spectroscopy", eds. M. Kissler-Patig, M. M.
Roth and J. R. Walsh, ESO Astrophysics Symposia. (The two last pages with
figures are not in the conference proceedings.
Fake star formation bursts: blue horizontal branch stars masquerade as young stars in optical integrated light spectroscopy
Model color magnitude diagrams of low-metallicity globular clusters usually
show a deficit of hot evolved stars with respect to observations. We
investigate quantitatively the impact of such modelling inaccuracies on the
significance of star formation history reconstructions obtained from optical
integrated spectra. To do so, we analyse the sample of spectra of galactic
globular clusters of Schiavon et al. with STECKMAP (Ocvirk et al.) and the
stellar population models Vazdekis et al. and Bruzual & Charlot, and focus on
the reconstructed stellar age distributions. Firstly, we show that
background/foreground contamination correlates with E(B-V), which allows us to
define a clean subsample of uncontaminated GCs, on the basis of a E(B-V)
filtering.
We then identify a "confusion zone" where fake young bursts of star formation
pop up in the star formation history although the observed population is
genuinely old. These artifacts appear for 70-100% of cases depending on the
population model used, and contribute up to 12% of the light in the optical.
Their correlation with the horizontal branch ratio indicates that the confusion
is driven by HB morphology: red horizontal branch clusters are well fitted by
old stellar population models while those with a blue HB require an additional
hot component. The confusion zone extends over [Fe/H]=[-2,-1.2], although we
lack the data to probe extreme high and low metallicity regimes. As a
consequence, any young starburst superimposed on an old stellar population in
this metallicity range could be regarded as a modeling artifact, if it weighs
less than 12% of the optical light, and if no emission lines typical of an HII
region are present.Comment: 14 pages, 6 figures, accepted for publication in Ap
Vast planes of satellites in a high resolution simulation of the Local Group: comparison to Andromeda
We search for vast planes of satellites (VPoS) in a high resolution
simulation of the Local Group performed by the CLUES project, which improves
significantly the resolution of former similar studies. We use a simple method
for detecting planar configurations of satellites, and validate it on the known
plane of M31. We implement a range of prescriptions for modelling the satellite
populations, roughly reproducing the variety of recipes used in the literature,
and investigate the occurence and properties of planar structures in these
populations. The structure of the simulated satellite systems is strongly
non-random and contains planes of satellites, predominantly co-rotating, with,
in some cases, sizes comparable to the plane observed in M31 by Ibata et al..
However the latter is slightly richer in satellites, slightly thinner and has
stronger co-rotation, which makes it stand out as overall more exceptional than
the simulated planes, when compared to a random population. Although the
simulated planes we find are generally dominated by one real structure, forming
its backbone, they are also partly fortuitous and are thus not kinematically
coherent structures as a whole. Provided that the simulated and observed planes
of satellites are indeed of the same nature, our results suggest that the VPoS
of M31 is not a coherent disc and that one third to one half of its satellites
must have large proper motions perpendicular to the plane
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
Comparison of different spectral population models
We have compared simple stellar populations (SSPs) generated with different population synthesis tools: BC03, Vazdekis and Pegase.HR and different stellar libraries: ELODIE3.1, SteLib and MILES. We find that BC03/SteLib SSPs are biased toward solar metallicity, however Pegase.HR/ELODIE3.1 and Vazdekis/MILES are extremely consistent. The extensive coverage of the space of atmospheric parameters in the large stellar libraries allows precise synthesis for a large range of ages (0.1 .. 10 Gyr) and metallicities (-2 .. +0.4 dex) limited by the quality of the determination of stellar parameters (like temperature scale of the giants)
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