Joseph Silk

Abstract:

We calculate solar models including dark matter (DM) weakly interacting massive particles (WIMPs) of mass 5-50 GeV and test these models against helioseismic constraints on sound speed, convection-zone depth, convection-zone helium abundance, and small separations of low-degree p-modes. Our main conclusion is that both direct detection experiments and particle accelerators may be complemented by using the Sun as a probe for WIMP DM particles in the 5-50 GeV mass range. The DM most sensitive to this probe has suppressed annihilations and a large spin-dependent elastic scattering cross section. For the WIMP cross section parameters explored here, the lightest WIMP masses <10 GeV are ruled out by constraints on core sound speed and low-degree frequency spacings. For WIMP masses 30-50 GeV, the changes to the solar structure are confined to the inner 4% of the solar radius and so do not significantly affect the solar p-modes. Future helioseismology observations, most notably involving g-modes, and future solar neutrino experiments may be able to constrain the allowable DM parameter space in a mass range that is of current interest for direct detection. © 2010 The American Physical Society.

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.