The Various Kinematics of Dwarf Irregular Galaxies in Nearby Groups and Their Dark Matter Distributions

Abstract

Eight dwarf irregular galaxies, in the two nearby groups of galaxies Sculptor and Centaurus A (at 2.5 Mpc and 3.5 Mpc), have been imaged in neutral hydrogen (H I) with the Australia Telescope and the Very Large Array. These galaxies have absolute magnitudes ranging from MB = -15.7 to -11.3. Yet they are mostly rotationally supported, with maximum velocities going from 19 to 67 km s-1. Multicomponent mass models have been fitted to the rotation curves to investigate the properties of their dark matter halos and the scaling laws of dark matter halo parameters. Dwarf galaxies have, on average, a higher dark to luminous mass ratio, as well as higher dark halo central densities than spiral galaxies. They have a larger dispersion of their dark matter properties both in terms of their total dark matter amount and of their dark halo parameters, compared to spiral galaxies. It is therefore very difficult to predict a dwarf galaxy rotation curve shape based only on its optical properties. Dwarfs are not well fitted by cold dark matter (CDM) halos of the type proposed by Navarro, Frenk, & White, even for ΛCDM models with Ω0 as low as 0.3. For two of our dwarfs we also have Hα rotation curves confirming the H I velocities, so the discrepancy with the CDM models cannot be attributed to beam-smearing effects.