Organisateur : Simona Mei (GEPI)

In the last few years, much progress has been done to understand stellar populations in the
Milky Way, close galaxies and galaxies at high redshifts.
The pioneer works by Maraston et al. (2005) have shown that stellar population models are
not yet adequate to describe real galaxy colors. A different treatment of the thermally pulsing
asymptotic giant branch (TP-AGB) in the models, leads to a higher prediction for the infrared
flux in galaxy with ages around 1 Gyr, when this phase dominates the flux from galaxy stellar
populations. This is key not only because predicted and observed colors at a given redshift
would be different for these young populations, but also because estimates of the galaxy age
and mass are based on the rest-frame K-band flux, directly or by the fitting of the galaxy
spectral energy distribution. In standard models, galaxies with ages around 1Gyr with these
high fluxes due to the TP-AGB phase, would be predicted to have older ages and higher
masses.
In local galaxies, this differences can lead to very different estimates of the stellar mass for
young and often faint galaxies (Maraston et al. 2005), while at high redshifts the bias will
affect a larger number of bright galaxies still star-forming (Raichoor et al. 2011).
Observations of both local and high redshift galaxies have not brought yet to discriminate
these different models.
In the Milky Way the stellar populations can be observed with great detail. Both kinematic
and chemical information can be assembled for large samples of stars, taking advantage of the
high multiplex spectrographs (Giraffe@VLT, SDSS, 6dF, etc..). This also implies that we are
able to find subtle differences among populations that were once believed to be homogeneous.
The obvious example are the multiple populations in Galactic Globular clusters (Gratton et al.
2001) or the “low-alpha” and “high-alpha” populations in the Galactic Halo (Nissen &
Schuster 2010, Bonifacio et al. 2011).
From the dynamic point of view many “stellar streams” in the Galactic Halo have been
discovered (Belokurov et al. 2006) , probably associated to disrupted or disrupting galaxy
satellites. It is interesting that the chemical composition of these streams indicates that they
represent a sub-population of the parent galaxy (Monaco et al. 2007, Chou et al. 2007).
Galaxies in the local universe, and in particular in the Local Group, also offer the ideal
laboratory to study stellar populations in detail, in particular the process of star formation in a
variety of environments (galaxy type, gas content, dynamical state), over a wide range of
metallicities. Such galaxies are close enough that resolved studies of their stellar constituents
are possible with HST (e.g., PHAT for M31 (Dalcanton et al. 2012). A HST treasury program
with PI Luciana Bianchi [see e.g., Bianchi et al. 2012a, b, 2011] covers the most conspicuous
star-forming regions in six dwarfs and the two nearest spirals, M31 and M33. The ANGST
survey among many others), and spectroscopy of their brightest stars and clusters is possible
with 8m-class ground-based telescopes.
These detailed studies of local star-forming regions, combined with wide-field imaging in the
UV (GALEX), IR (e.g., Spitzer, Herschel), H ! (ground-based), provide critical tests to galaxy
evolution models, and shed light on the co-evolution of stars and dust.
On such models rests our interpretation of integrated measurements of distant galaxies
samples.
In this workshop, we would like to discuss recent results from local and high redshift surveys
and compare them with predictions from standard and new stellar population models. The
workshop will put together scientists working in both modeling and observations, in the local
and high redshift Universe to present new results and discuss of new ways to address the
study of stellar populations in the local and high redshift Universe.