Spatial and luminosity distributions of galactic satellites

TL;DR

This study compares the spatial and luminosity distributions of galactic satellites in simulations and SDSS data, revealing similarities in radial profiles but differences in satellite color and concentration.

Contribution

It provides a detailed comparison between GALFORM model predictions and SDSS observations, highlighting discrepancies in satellite color and distribution.

Findings

Model satellites have similar radial distributions to SDSS satellites.

Significant differences in satellite color distributions between model and SDSS.

Model blue satellites are less centrally concentrated than observed.

Abstract

We investigate the luminosity functions (LFs) and projected number density profiles of galactic satellites around isolated primaries of different luminosities. We measure these quantities for model satellites placed into the Millennium and Millennium II dark matter simulations by the GALFORM semi-analytic galaxy formation model for different bins of primary galaxy magnitude and we investigate their dependence on satellite luminosity. We compare our model predictions to the data of Guo et al. from the Sloan Digital Sky Survey Data Release 8 (SDSS DR8). First, we use a mock light-cone catalogue to verify that the method we used to count satellites in the SDSS DR8 is unbiased. We find that the radial distributions of model satellites are similar to those around comparable primary galaxies in the SDSS DR8, with only slight differences at low luminosities and small projected radii. However, when splitting the satellites by colour, the model and SDSS satellite systems no longer resemble one another, with many red model satellites, in contrast to the dominant blue fraction at similar luminosity in SDSS. The few model blue satellites are also significantly less centrally concentrated in the halo of their stacked primary than their SDSS counterparts. The implications of this result for the GALFORM model are discussed.