119 research outputs found
The infant Milky Way
We investigate the physical properties of the progenitors of today living
Milky Way-like galaxies that are visible as Damped Lya Absorption systems and
Lya Emitters at higher redshifts (z ~ 2.3,5.7). To this aim we use a
statistical merger-tree approach that follows the formation of the Galaxy and
its dwarf satellites in a cosmological context, tracing the chemical evolution
and stellar population history of the progenitor halos. The model accounts for
the properties of the most metal-poor stars and local dwarf galaxies, providing
insights on the early cosmic star-formation. Fruitful links between Galactic
Archaeology and more distant galaxies are presented.Comment: 6 pages, 6 figures; to appear in the proceedings of the Subaru
conference on Galactic Archaeology, Shuzenji, Japan (Nov. 1-4 2011);
Astronomical Society of the Pacific Conference Series 201
Cosmic stellar relics in the Galactic halo
We study the stellar population history and chemical evolution of the Milky
Way (MW) in a hierarchical LCDM model for structure formation. Using a Monte
Carlo method based on the semi-analytical EPS formalism, we reconstruct the
merger tree of our Galaxy and follow the evolution of gas and stars along the
hierarchy. Our approach allows us to compare the observational properties of
the MW with model results, exploring different properties of primordial stars,
such as their IMF and the critical metallicity for low-mass star formation,
Zcr. By matching our predictions to the Metallicity Distribution Function (MDF)
of metal-poor stars in the Galactic halo we find that: (i) supernova feedback
is required to reproduce the observed properties of the MW; (ii) stars with
[Fe/H]<-2.5 form in halos accreting Galactic Medium (GM) enriched by earlier
supernova explosions; (iii) the fiducial model (Zcr=10^-4Zsun,m_PopIII=200Msun)
provides an overall good fit to the MDF but cannot account for the two stars
with [Fe/H]<-5; the latter can be accommodated if Zcr<10^-6Zsun but such model
overpopulates the range -5.3<[Fe/H]<-4 in which no stars have been detected;
(iv) the current non-detection of metal-free stars robustly constrains either
Zcr>0 or the masses of the first stars m_PopIII>0.9Msun; (v) the statistical
impact of second generation stars, i.e stars forming out of gas polluted only
by metal-free stars, is negligible in current samples; (vi) independently of
Zcr, 60% of metals in the GM are ejected through winds by halos with masses
M<6x10^9 Msun, showing that low-mass halos are the dominant population
contributing to cosmic metal enrichment.Comment: 18 pages, 12 figures, submitted to MNRA
High-redshift quasars host galaxies: is there a stellar mass crisis?
We investigate the evolutionary properties of a sample of quasars at 5<z<6.4
using the semi-analytical hierarchical model GAMETE/QSOdust. We find that the
observed properties of these quasars are well reproduced by a common formation
scenario in which stars form according to a standard IMF, via quiescent star
formation and efficient merger-driven bursts, while the central BH grows via
gas accretion and BH-BH mergers. Eventually, a strong AGN driven wind starts to
clear up the ISM of dust and gas, damping the star formation and un-obscuring
the line of sight toward the QSO. In this scenario, all the QSOs hosts have
final stellar masses in the range , a factor 3-30
larger than the upper limits allowed by the observations. We discuss
alternative scenarios to alleviate this apparent tension: the most likely
explanation resides in the large uncertainties that still affect dynamical mass
measurements in these high-z galaxies. In addition, during the transition
between the starburst-dominated and the active QSO phase, we predict that about
40% of the progenitor galaxies can be classified as Sub Millimeter Galaxies,
although their number rapidly decreases with redshift.Comment: 14 pages, 9 figures, accepted for publication in MNRA
The faintest galaxies
We investigate the nature of Ultra Faint dwarf spheroidal galaxies (UF dSphs)
in a general cosmological context, simultaneously accounting for various
"classical" dSphs and Milky Way (MW) properties, including their Metallicity
Distribution Function (MDF). The model successfully reproduces both the
observed [Fe/H]-Luminosity relation and the mean MDF of UFs. According to our
results UFs are the living fossils of H2-cooling minihaloes formed at z>8.5,
i.e. before the end of reionization. They are the oldest and the most dark
matter-dominated (M/L > 100) dSphs in the MW system, with a total mass of M =
10^(7-8) Msun. The model allows to interpret the different shape of UFs and
classical dSphs MDF, along with the frequency of extremely metal-poor stars in
these objects. We discuss the "missing satellites problem" by comparing the UF
star formation efficiencies with those derived for minihaloes in the Via Lactea
simulation.Comment: To appear in the conference proceeding: "First Stars and Galaxies:
Challenges in the Next Decade" . Publisher: American Institute of Physics.
Editors: V. Bromm, D. Whalen, N. Yoshid
High redshift Lya emitters: clues on the Milky Way infancy
With the aim of determining if Milky Way (MW) progenitors could be identified
as high redshift Lyman Alpha Emitters (LAEs) we have derived the intrinsic
properties of z ~ 5.7 MW progenitors, which are then used to compute their
observed Lyman-alpha luminosity, L_alpha, and equivalent width, EW. MW
progenitors visible as LAEs are selected according to the canonical
observational criterion, L_alpha > 10^42 erg/s and EW > 20 A. Progenitors of
MW-like galaxies have L_alpha = 10^(39-43.25) erg/s, making some of them
visible as LAEs. In any single MW merger tree realization, typically only 1
(out of ~ 50) progenitor meets the LAE selection criterion, but the probability
to have at least one LAE is very high, P = 68%. The identified LAE stars have
ages, t_* ~ 150-400 Myr at z ~ 5.7 with the exception of five small progenitors
with t_* 10% of
the halo very metal-poor stars [Fe/H] < -2, thus establishing a potentially
fruitful link between high-z galaxies and the Local Universe.Comment: Accepted for publication in MNRAS lette
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