2016MNRAS.455.3430M

We have used the Arecibo L-band Feed Array (ALFA) to map three regions, each of 5 deg2, 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 the Arecibo Legacy Fast ALFA (ALFALFA) survey, 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±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.