The Arecibo Galaxy Environment Survey IX: The Isolated Galaxy Sample

TL;DR

This study used the Arecibo telescope to survey three isolated galaxy regions, finding fewer dwarf companions than expected, which suggests environment influences the dwarf-to-giant galaxy ratio.

Contribution

It provides new observational data on dwarf galaxy companions around isolated galaxies and compares the findings with different HI mass functions.

Findings

Detected fewer dwarf companions than expected from field HI mass functions.

Number of dwarf companions aligns with a flat or declining HI mass function.

Supports the idea that environment affects dwarf galaxy abundance.

Abstract

We have used the Arecibo L-band Feed Array to map three regions, each of 5 square degrees, around the isolated galaxies NGC 1156, UGC 2082, and NGC 5523. In the vicinity of these galaxies we have detected two dwarf companions: one near UGC 2082, previously discovered by ALFALFA, and one near NGC 1156, discovered by this project and reported in an earlier paper. This is significantly fewer than the 15.4 $^{+1.7}_{-1.5}$ that would be expected from the field HI mass function from ALFALFA or the 8.9 $\pm$ 1.2 expected if the HI mass function from the Local Group applied in these regions. The number of dwarf companions detected is, however, consistent with a flat or declining HI mass function as seen by a previous, shallower, HI search for companions to isolated galaxies.We attribute this difference in Hi mass functions to the different environments in which they are measured. This agrees with the general observation that lower ratios of dwarf to giant galaxies are found in lower density environments.