Hubble Space Telescope Imaging of Isolated Local Volume Dwarfs GALFA-Dw3 and Dw4

TL;DR

This study uses Hubble Space Telescope imaging to analyze two isolated dwarf galaxies, revealing their stellar populations, distances, and star formation histories, and highlighting the effectiveness of compact HI sources in identifying such galaxies.

Contribution

The paper presents detailed observations of two isolated dwarf galaxies, demonstrating their properties and star formation activities, and emphasizes the utility of compact HI sources in discovering similar galaxies.

Findings

GALFA Dw3 is 7.61 Mpc away with recent star formation activity.

GALFA Dw4 is 3.10 Mpc away, unusually compact for its luminosity.

Both galaxies are extremely isolated, with no nearby objects within 1.5 Mpc.

Abstract

We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). These galaxies were initially discovered as optical counterparts to compact HI clouds in the GALFA survey. Both objects resolve into stellar populations which display an old red giant branch, younger helium burning, and massive main sequence stars. We use the tip of the red giant branch method to determine the distance to each galaxy, finding distances of 7.61$_{-0.29}^{+0.28}$ Mpc and 3.10$_{-0.17}^{+0.16}$ Mpc, respectively. With these distances we show that both galaxies are extremely isolated, with no other confirmed objects within ~1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain HII regions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and H$\alpha$ observations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows some evidence for current star formation in spatially limited HII regions. Compact HI sources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 & Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution.