Joseph Silk

Abstract:

The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multiband images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near-ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function (LF) for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL ∝ L^α, with α =-2.04 ± 0.08, down to MV ≈-5.5. We test the sensitivity of the LF to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI, Hα colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power law, dN/dτ ∝ τ^γ, with γ =-0.9 ± 0.2, for M ≳ few × 103 M⊙ and τ ≲ 4 × 10⁸ yr. This indicates that clusters are disrupted quickly, with ≈80%-90% disrupted each decade in age over this time. The mass function of clusters over the same M-τ range is a power law, dN/dM ∝ M^β, with β =-1.94 ± 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, MC, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e., mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters. © 2010. The American Astronomical Society. All rights reserved.

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.