Joseph Silk

Abstract:

The chemical abundance patterns of the oldest stars in the Galaxy are expected to contain residual signatures of the first stars in the early universe. Numerous studies attempt to explain the intrinsic abundance scatter observed in some metal-poor populations in terms of chemical inhomogeneities dispersed throughout the early Galactic medium due to discrete enrichment events. Just how the complex data and models are to be interpreted with respect to "progenitor yields" remains an open question. Here we showthat stochastic chemical evolutionmodels to date have overlooked a crucial fact. Essentially, all stars today are born in highly homogeneous star clusters and it is likely that this was also true at early times. When this ingredient is included, the overall scatter in the abundance plane [Fe/H] versus [X/Fe] (C-space), where X is a nucleosynthetic element, can be much less than derived from earlier models. Moreover, for moderately flat clustermass functions (γ ≲ 2), and/or formass functions with a highmass cutoff (M max ≳ 105M⊙), stars exhibit a high degree of clumping in C-space that can be identified even in relatively small data samples. Since stellar abundances can be modified by mass transfer in close binaries, clustered signatures are essential for deriving the yields of the first supernovae.We present a statistical test to determine whether a given set of observations exhibit such behavior. Our initial work focuses on two dimensions in C-space, but we show that the clustering signal can be greatly enhanced by additional abundance axes. The proposed experiment will be challenging on existing 8-10 m telescopes, but relatively straightforward for a multi-object echelle spectrograph mounted on a 25-40 m telescope. © 2010. The American Astronomical Society. All rights reserved.

Abstract:

Early Release Science observations of the cluster NGC 3603 with the WFC3 on the refurbished Hubble Space Telescope allow us to study its recent star formation history. Our analysis focuses on stars with Hαexcess emission, a robust indicator of their pre-main sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Hαexcess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Abstract:

We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep HST/Advanced Camera for Surveys optical images from the Great Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts, which are Lyman break galaxy (LBG) candidates at z ≃ 1-3. These new HST/WFC3 observations were taken over 50 arcmin2 in the GOODS South field as a part of the Early Release Science program. The uniqueness of these new UV data is that they are observed in three UV/optical (WFC3 UVIS) channel filters (F225W, F275W, and F336W), which allows us to identify three different sets of UV-dropout samples. We apply Lyman break dropout selection criteria to identify F225W-, F275W-, and F336W-dropouts, which are z ≃ 1.7, 2.1, and 2.7 LBG candidates, respectively. We use multi-wavelength imaging combined with available spectroscopic and photometric redshifts to carefully access the validity of our UV-dropout candidates. Our results are as follows: (1) these WFC3 UVIS filters are very reliable in selecting LBGs with z ≃ 2.0, which helps to reduce the gap between the well-studied z ≳ 3 and z ∼ 0 regimes; (2) the combined number counts with average redshift z ≃ 2.2 agree very well with the observed change in the surface densities as a function of redshift when compared with the higher redshift LBG samples; and (3) the best-fit Schechter function parameters from the rest-frame UV luminosity functions at three different redshifts fit very well with the evolutionary trend of the characteristic absolute magnitude, M*, and the faint-end slope, a, as a function of redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS channel observations to select z ≃ 3 LBGs. The addition of the new WFC3 on the HST has made it possible to uniformly select LBGs from z ≃ 1 to z ≃ 9 and significantly enhance our understanding of these galaxies using HST sensitivity and resolution. © 2010. The American Astronomical Society.

Abstract:

We use the semi-analytic model GalICS to predict the Tully-Fisher relation in the B, I and for the first time, in the K band, and its evolution with redshift, up to z~1. We refined the determination of the disk galaxies rotation velocity, with a dynamical recipe for the rotation curve, rather than a simple conversion from the total mass to maximum velocity. The new recipe takes into account the disk shape factor, and the angular momentum transfer occurring during secular evolution leading to the formation of bulges. This produces model rotation velocities that are lower by ~20-25% for the majority of the spirals. We implemented stellar population models with a complete treatment of the TP-AGB, which leads to a revision of the mass-to-light ratio in the near-IR. I/K band luminosities increase by ~0.3/0.5 mags at redshift z=0 and by ~0.5/1 mags at z=3. With these two new recipes in place, the comparison between the predicted Tully-Fisher relation with a series of datasets in the optical and near-IR, at redshifts between 0 and 1, is used as a diagnostics of the assembly and evolution of spiral galaxies in the model. At 0.4

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