Primordial Dark Matter Halos and the Present Mass Distributions of Galaxies

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Dr. McGaugh will continue his work to determine the masses of the different components of disk galaxies, in particular the stellar disk, the gaseous disk and the dark halo. He will assemble velocity-resolved maps in the H-alpha line of ionized gas and in neutral hydrogen, to measure the density of gas in the galaxy disk, and its velocity field, for about 100 disk galaxies with a wide range in properties. He will use near-infrared K-band images to determine the distribution of stellar mass, fitting the rotation curves to obtain the mass contributions of the different components. In the low-surface-brightness disks where the dark halo dominates the gravitational forces, he will use the measured velocity field to search for evidence of the steep inner cusp predicted by simulations with standard Cold Dark Matter. The larger sample of galaxies will be used to derive the initial halo mass profiles by proper treatment of adiabatic compression. Finally, he will calibrate an empirical mass-discrepancy--acceleration relation. The mass discrepancy is defined locally as the difference between the observed circular speed of rotation and that predicted from the gas and stars alone. This discrepancy is related to the orbital speed of disk material; the present study aims to characterize that relation better as a step towards understanding its origin.Dr McGaugh will continue his program of outreach to elementary-school children. He will distribute the data products and models from this study to the astronomical community on a web page.