Galaxy Rotation Curves in the Context of LambdaCDM Cosmology

Abstract

The motion of stars and gas in spiral galaxies provide a means of measuring the density profile of such galaxies. In the late 1960s, Vera Rubin started to take observations of the rotation velocities of disk galaxies (otherwise known as rotation curves). One of the most famous rotation curves she obtained was that of M31 (Rubin & Ford 1970). A key result of this study is that the rotation velocity of M31 remains high at very large radii (r > 20 kpc; see Fig. 1). The conclusion from these rotation curve data is that galaxies must contain much more mass than the visible light would otherwise indicate. These so-called flat rotation curves were confirmed in a series of papers in the late-1970s (Rubin et al., 1977, 1978a, b, 1979; Peterson et al., 1978) and early 1980s (Rubin et al., 1980, 1982, 1985). These rotation curves have now been extended to much larger radii with sensitive HI measurements (e.g., Carignan et al., 2006) demonstrating that the rotation velocities remain high far beyond the optical disk in galaxies. This suggests that the visible light (and thus stellar mass) accounts for only a small fraction (∼ 15%) of the mass in spiral galaxies. This still remains one of the best pieces of evidence in favor of dark matter cosmology. The outline of this chapter is as follows: Section 2 describes the growth of structure in the Universe and how halos of cold dark matter assemble over cosmological times. Section 3 describes one of the fundamental problemswith cold darkmatter cosmology, that simulations predict cuspy central densities in dark matter halos, yet observations (particularly of dwarf galaxies) prefer constant density cores. Section 4 describes the Tully-Fisher zeropoint problem, the fact that cosmological models cannot reproduce the luminosity-rotation velocity relation for galaxies (Tully & Fisher, 1977) without overproducing the number density of galaxies at fixed luminosity. Finally, in Section 5 we make some conclusions about the present state of cosmological simulations on galaxy-sized scales.