The Shapes, Orientation, and Alignment of Galactic Dark Matter Subhalos

Abstract

We present a study of the shapes, orientations, and alignments of Galactic dark matter subhalos in the Via Lactea simulation of a Milky Way-size ΛCDM host halo. Whereas isolated dark matter halos tend to be prolate, subhalos are predominantly triaxial. Overall subhalos are more spherical than the host halo, with minor-to-major and intermediate-to-major axis ratios of 0.68 and 0.83, respectively. Like isolated halos, subhalos tend to be less spherical in their central regions. The principal axis ratios are independent of subhalo mass when the shapes are measured within a physical scale such as rVmax, the radius of the peak of the circular velocity curve. Subhalos tend to be slightly more spherical closer to the host halo center. The spatial distribution of the subhalos traces the prolate shape of the host halo when they are selected by the largest Vmax they ever had, i.e., before they experienced strong tidal mass loss. The subhalos' orientation is not random: the major axis tends to align with the direction toward the host halo center. This alignment disappears for halos beyond 3r200 and is more pronounced when the shapes are measured in the outer regions of the subhalos. The radial alignment is preserved during a subhalo's orbit and they become elongated during pericenter passage, indicating that the alignment is likely caused by the host halo's tidal forces. These tidal interactions with the host halo act to make subhalos rounder over time.