Companions to Isolated Elliptical Galaxies: Revisiting the Bothun-Sullivan Sample

TL;DR

This study revisits the companion galaxy count around isolated elliptical galaxies using NED data, finding a higher number of companions than previous visual searches, and confirms some are physically associated through radial velocities.

Contribution

It provides an updated, data-driven analysis of satellite galaxies around ellipticals, improving upon earlier visual surveys with quantitative NED data and radial velocity confirmation.

Findings

Average of 1.0 companions per galaxy within 75 kpc

Significantly more companions than previous estimates

Radial velocities confirm some companions are physically associated

Abstract

(Abridged) We investigate the number of physical companion galaxies for a sample of relatively isolated elliptical galaxies. NED has been used to reinvestigate the incidence of satellite galaxies for a sample of 34 elliptical galaxies, first investigated by Bothun & Sullivan in a visual search out to a projected search radius of 75 kpc. We have repeated their original investigation using data cataloged in NED. Nine of these elliptical galaxies appear to be members of galaxy clusters; the remaining sample of 25 galaxies reveals an average of +1.0+/-0.5 apparent companions per galaxy within a projected search radius of 75 kpc, in excess of two equal-area comparison regions displaced by 150-300 kpc. This is significantly larger than the +0.12+/-0.42 companions/galaxy found by Bothun & Sullivan for the identical sample. Making use of published radial velocities identifies physical companions and gives a somewhat lower estimate of +0.4 companions per elliptical galaxy. An expansion of the search radius out to 300 kpc.