Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

The peculiar motions of galaxies can be used to infer the distribution of matter in the Universe. It has recently been shown that measurements of the peculiar velocity field indicates an anomalously high bulk flow of galaxies in our local volume. In this paper we find the implications of the high bulk flow for the power spectrum of density fluctuations. We find that analyzing only the dipole moment of the velocity field yields an average power spectrum amplitude which is indeed much higher than the LCDM value. However, by also including shear and octupole moments of the velocity field, and marginalizing over possible values for the growth rate, an average power spectrum amplitude which is consistent with LCDM is recovered. We attempt to infer the shape of the matter power spectrum from moments of the velocity field, and find a slight excess of power on scales ~ h-1 Gpc.

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 ∝ Lα, 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 ∝ τγ, 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 ∝ Mβ, 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.