The Spatial Distribution of Satellite Galaxies

We use the first Millennium Run simulation (MRS) to investigate the spatial locations of the satellites of relatively isolated host galaxies. The stellar masses of the MRS hosts span a range of 10.3 6 log10 [M∗/M⊙] 6 11.5, and host-satellite systems were selected via typical redshift space proximity criteria. The 3-D locations of the MRS satellites are well-fitted by a combination of a Navarro, Frenk and White (NFW) density profile and a power law distribution. At fixed host stellar mass, the NFW scale parameter, rs, for the satellites of red MRS host galaxies exceeds that for the satellites of blue MRS host galaxies. In both cases the dependence of rs on host stellar mass is well-fitted by a power law. For the satellites of red MRS hosts, r s ∝ (M∗/M⊙)0.71±0.05, while for the satellites of blue MRS hosts, r s ∝ (M∗/M⊙)0.48±0.07. We perform model fits to the 2-D (i.e., projected) locations of the MRS satellites from which we find that, with the exception of the satellites of the most massive red MRS hosts, the 2-D analysis accurately recovers the values of rs that were found using the full 3-D analysis. Finally, we investigate the locations of the satellites of relatively isolated host galaxies in the seventh data release of the Sloan Digital Sky Survey (SDSS). The SDSS hosts have stellar masses in the range 10.6 6 log10 [M∗/M⊙] 6 11.2, and the host-satellite systems were selected using the same criteria that were used for the MRS. The locations of the SDSS satellites are well-fitted by the NFW and power law model, but due to the small size of the SDSS sample and the fact that few SDSS satellites are found within projected radii Rp 6 75 kpc of their hosts, the error bounds on rs are large. There is, however, a weak indication that the values of rs for the satellites of the red SDSS hosts exceeds that of the satellites of the blue SDSS hosts: r s = 122± 40 kpc and r s = 38± 29 kpc. 2 Ingólfur Ágústsson and Tereasa G. Brainerd