Anomalous Stellar Populations in LSB Galaxies

TL;DR

This study uses HST near-IR observations to analyze the stellar populations of two low surface brightness galaxies, revealing unique features like extremely low metallicity and evidence of multiple star formation episodes.

Contribution

It provides new near-IR CMD data for LSB galaxies, highlighting unusual stellar populations and star formation histories not previously documented.

Findings

F575-3 has the bluest RGB indicating very low metallicity.

F615-1 shows wide RGB and AGB sequences, suggesting multiple star formation episodes.

Both galaxies contain stars with abnormal near-IR colors, possibly analogous to peculiar local stars.

Abstract

We present new HST WFC3 near-IR observations of the CMD's in two LSB galaxies, F575-3 and F615-1, notable for having no current star formation based on a lack of H$\alpha$ emission. Key features of the near-IR CMD's are resolved, such as the red giant branch (RGB), the asymptotic giant branch (AGB) region and the top of the blue main sequence (bMS). F575-3 has the bluest RGB of any CMD in the literature, indicating an extremely low mean metallicity. F615-1 has unusually wide RGB and AGB sequences suggesting multiple episodes of star formation from metal-poor gas, possibly infalling material. Both galaxies have an unusual population of stars to the red of the RGB and lower in luminosity than typical AGB stars. These stars have normal optical colors but abnormal near-IR colors. We suggest that this population of stars might be analogous to local peculiar stars like Be stars with strong near-IR excesses owing to a surrounding disk of hot gas.