Serendipitous discovery of a faint dwarf galaxy near a Local Volume dwarf

TL;DR

A faint dwarf irregular galaxy was serendipitously discovered near a known dwarf galaxy, with detailed distance and star formation analysis indicating it is a physical satellite, marking the first such discovery beyond 4 Mpc.

Contribution

This paper reports the first discovery of a very faint dwarf galaxy beyond 4 Mpc, including detailed distance measurements and star formation evidence.

Findings

The faint dwarf galaxy is a likely satellite of LV J1157+5638.

Both galaxies show recent or ongoing star formation.

The discovery extends the known range of faint dwarf galaxies.

Abstract

A faint dwarf irregular galaxy has been discovered in the HST/ACS field of LV J1157+5638. The galaxy is resolved into individual stars, including the brightest magnitude of the red giant branch. The dwarf is very likely a physical satellite of LV J1157+5638.The distance modulus of LV J1157+5638 using the tip of the red giant branch (TRGB) distance indicator is 29.82+-0.09 mag (D = 9.22+-0.38 Mpc). The TRGB distance modulus of LV J1157+5638 sat is 29.76+-0.11 mag (D = 8.95+-0.42 Mpc). The distances to the two galaxies are consistent within the uncertainties. The projected separation between them is only 3.9 kpc. LVJ1157+5638 has a total absolute V-magnitude of -13.26+-0.10 and linear Holmberg diameter of 1.36 kpc, whereas its faint satellite LV J1157+5638 sat has M_V = -9.38+-0.13 mag and Holmberg diameter of 0.37 kpc. Such a faint dwarf was discovered for the first time beyond the nearest 4 Mpc from us. The presence of main sequence stars in both galaxies unambiguously indicates the classification of the objects as dwarf irregulars (dIrrs) with recent or ongoing star formation events in both galaxies.