Two Local Volume Dwarf Galaxies Discovered in 21 cm Emission: Pisces A and B

TL;DR

This paper reports the discovery of two faint dwarf galaxies, Pisces A and B, identified via 21 cm HI emission, which are among the faintest star-forming galaxies and may represent a large, previously undetected population of similar dwarfs.

Contribution

First detection of dwarf galaxies Pisces A and B through blind 21 cm HI survey, highlighting their potential as typical faint, HI-rich dwarfs in the Local Volume.

Findings

Galaxies are consistent with being in the Local Volume (<10 Mpc).

One galaxy may be on the outer edge of the Local Group (~1 Mpc from M31).

Results suggest these galaxies are among the faintest star-forming galaxies known.

Abstract

We report the discovery of two dwarf galaxies, Pisces A and B, from a blind 21 cm HI search. These were the only two galaxies found via optical imaging and spectroscopy of 22 HI clouds identified in the GALFA-HI survey as dwarf galaxy candidates. They have properties consistent with being in the Local Volume ($<10$ Mpc), and one has resolved stellar populations such that it may be on the outer edge of the Local Group ($\sim 1 \, {\rm Mpc}$ from M31). While the distance uncertainty makes interpretation ambiguous, these may be among the faintest starforming galaxies known. Additionally, rough estimates comparing these galaxies to $\Lambda$CDM dark matter simulations suggest consistency in number density, implying that dark matter halos likely to host these galaxies are primarily HI-rich. The galaxies may thus be indicative of a large population of dwarfs at the limit of detectability that are comparable to the faint satellites of the Local Group. Because they are outside the influence of a large dark matter halo to alter their evolution, these galaxies can provide critical anchors to dwarf galaxy formation models.