Star formation in extremely faint dwarf galaxies

TL;DR

This study investigates the relationship between gas density and star formation in extremely faint dwarf galaxies, finding that star formation correlates with gas density without a clear threshold, often following a steeper power law than previously thought.

Contribution

It provides new insights into star formation laws at small scales in faint dwarf galaxies, challenging the notion of a universal threshold density for star formation.

Findings

Star formation rate correlates with gas density without a threshold.

Most galaxies follow a steeper power law than Kennicutt-Schmidt law.

Offsets between HI and UV peaks increase at smaller scales.

Abstract

We study the relationship between the gas column density (derived from GMRT 21 cm data) and the star formation rate surface density (derived from publicly available GALEX data) for a sample of 23 extremely faint dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS). Our sample galaxies have a median HI mass of 2.8e07 solar masses and a median blue magnitude -13.2. We find that gas column density averaged over the star forming region of the disk lies below most estimates of the "threshold density" for star formation, and that the average star formation rate surface density for most of the galaxies is also lower than would be expected from the "Kennicutt-Schmidt" law (Kennnicutt 1998}. We also use our data to look for small scale (400 pc and 200 pc) correlations. At 400 pc linear resolution, for 18 of our 23 galaxies, we find that star formation rate surface density can be parametrized as having a power law dependence on gas column density, which varies accross the sample and is in general steeper than "Kennicutt-Schmidt" law. The power law relation holds until one reaches the sensitivity limit of the GALEX data, i.e. we find no evidence for a "threshold density" below which star formation is completely cut off. For the 5 galaxies for which a power law does not provide a good parametrization, there are substantial offsets between the UV bright regions and the HI high column density maps. At 200 pc resolution, the offsets between the peaks in the HI and UV images are more pronounced, and a power law parametrization is possible for only 5 of 10 galaxies.