Satellites in the field and lens galaxies: SDSS/COSMOS vs. SLACS/CLASS

TL;DR

This study compares satellite galaxy incidences around elliptical galaxies in different surveys to test predictions of Cold Dark Matter models and understand gravitational lensing anomalies.

Contribution

It provides a large-scale observational analysis of satellite galaxy frequencies around ellipticals, contrasting lens and non-lens systems to evaluate CDM predictions.

Findings

Luminous satellites within 20 kpc are rare, about a few tenths of a percent.

SDSS survey satellites match typical incidence rates, while CLASS lenses show excess.

SLACS lenses are consistent with general elliptical galaxy environments.

Abstract

The incidence of sub-galactic level substructures is an important quantity, as it is a generic prediction of high-resolution Cold Dark Matter (CDM) models which is susceptible to observational test. Confrontation of theory with observations is currently in an uncertain state. In particular, gravitational lens systems appear to show evidence for flux ratio anomalies, which are expected from CDM substructures although not necessarily in the same range of radius as observed. However, the current small samples of lenses suggest that the lens galaxies in these systems are unusually often accompanied by luminous galaxies. Here we investigate a large sample of unlensed elliptical galaxies from the COSMOS survey, and determine the fraction of objects with satellites, in excess of background counts, as a function of satellite brightness and separation from the primary object. We find that the incidence of luminous satellites within 20 kpc is typically a few tenths of one percent for satellites of a few tenths of the primary flux, comparable to what is observed for the wider but shallower SDSS survey. Although the environments of lenses in the SLACS survey are compatible with this observation, the CLASS radio survey lenses are significantly in excess of this.