Shape and Spin of Minihaloes: From Large Scales to the Centres

Abstract

The spin and shape of galaxies at the present day have been well-studied both observationally and theoretically. At high redshifts, however, we have to rely on numerical simulations. In this study, we investigate the shape and spin of minihaloes with masses of $M \sim 10^5$--$10^7 \, {\rm M_{\odot}}$ which are of particular interest as they are the sites where the first stars in the Universe form. We analyse a large sample of these minihaloes, selected from a high resolution cosmological simulation. The first minihaloes form at $z \simeq 24$ and by the end of the simulation at $z \simeq 14$ our sample includes $\sim 9000$ minihaloes. We find that the spin parameter of the minihaloes follows a log-normal distribution with minimal dependence on redshift. Most minihaloes are prolate, but those formed at the highest redshifts are more prolate than those formed at lower redshifts. On the scale of the virial radius, there is a good correlation between the shape and spin of the gas and that of the dark matter. However, this correlation breaks down in gas which is cooling and undergoing gravitational collapse. We show, contrary to previous assumptions, that although the direction of the spin of the central dense gas correlates well with that of the halo, the magnitude of the spin of the dense gas is uncorrelated with that of the halo. Therefore, measurements of the spin of minihaloes on large scales tell us little about the angular momentum of the gas responsible for forming the first stars.