Characterizing the Star Formation of the Low-Mass SHIELD Galaxies from Hubble Space Telescope Imaging

TL;DR

This study investigates the star formation characteristics of low-mass, gas-rich galaxies from the SHIELD survey using Hubble imaging, revealing fluctuating, inefficient star formation not driven by external interactions.

Contribution

It provides detailed star-formation measurements in low-mass galaxies, highlighting their non-deterministic and internally regulated star-formation processes, with a focus on the properties of dwarf transition galaxies.

Findings

Recent SFRs are similar to lifetime SFRs with a mean birthrate of 1.4.

Dwarf transition galaxies have properties consistent with other low-mass galaxies.

No correlation between star formation activity and proximity to neighboring galaxies.

Abstract

The Survey of HI in Extremely Low-mass Dwarfs (SHIELD) is an on-going multi-wavelength program to characterize the gas, star formation, and evolution in gas-rich, very low-mass galaxies that populate the faint end of the galaxy luminosity function. The galaxies were selected from the first ~10% of the HI ALFALFA survey based on their low HI mass and low baryonic mass. Here, we measure the star-formation properties from optically resolved stellar populations for 12 galaxies using a color-magnitude diagram fitting technique. We derive lifetime average star-formation rates (SFRs), recent SFRs, stellar masses, and gas fractions. Overall, the recent SFRs are comparable to the lifetime SFRs with mean birthrate parameter of 1.4, with a surprisingly narrow standard deviation of 0.7. Two galaxies are classified as dwarf transition galaxies (dTrans). These dTrans systems have star-formation and gas properties consistent with the rest of the sample, in agreement with previous results that some dTrans galaxies may simply be low-luminosity dIrrs. We do not find a correlation between the recent star-formation activity and the distance to the nearest neighboring galaxy, suggesting that the star-formation process is not driven by gravitational interactions, but regulated internally. Further, we find a broadening in the star-formation and gas properties (i.e., specific SFRs, stellar masses, and gas fractions) compared to the generally tight correlation found in more massive galaxies. Overall, the star-formation and gas properties indicate these very low-mass galaxies host a fluctuating, non-deterministic, and inefficient star-formation process.