Star formation histories of the LEGUS dwarf galaxies (III): the non-bursty nature of 23 star forming dwarf galaxies

TL;DR

This study analyzes the recent star formation histories of 23 dwarf galaxies, revealing mostly steady activity over the last 100 million years and exploring spatial and temporal patterns of star formation.

Contribution

It provides detailed star formation histories using CMD fitting with two stellar models, confirming the non-bursty nature of these dwarf galaxies and comparing different SFR indicators.

Findings

Most galaxies show no recent burst activity in last 100 Myr.

CMD-based SFRs are generally higher than FUV-based SFRs.

About half exhibit spatial progression of star formation in last 60 Myr.

Abstract

We derive the recent star formation histories of 23 active dwarf galaxies using HST observations from the Legacy Extragalactic UV Survey (LEGUS). We apply a color-magnitude diagram fitting technique using two independent sets of stellar models, PARSEC-COLIBRI and MIST. Despite the non-negligible recent activity, none of the 23 star forming dwarfs show enhancements in the last 100 Myr larger than three times the 100-Myr-average. The unweighted mean of the individual SFHs in the last 100 Myr is also consistent with a rather constant activity, irrespective of the atomic gas fraction. We confirm previous results that for dwarf galaxies the CMD-based average star formation rates (SFRs) are generally higher than the FUV-based SFR. For half of the sample, the 60-Myr-average CMD-based SFR is more than two times the FUV SFR. In contrast, we find remarkable agreement between the 10-Myr-average CMD-based SFR and the H${\alpha}$-based SFR. Finally, using core helium burning stars of intermediate mass we study the pattern of star formation spatial progression over the past 60 Myr, and speculate on the possible triggers and connections of the star formation activity with the environment in which these galaxies live. Approximately half of our galaxies show spatial progression of star formation in the last 60 Myr, and/or very recent diffuse and off-center activity compared to RGB stars.