The relation between atomic gas and star formation rate densities in faint dwarf irregular galaxies

Abstract

We use data for faint (MB > -14.5) dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT Survey to study the correlation between the surface densities of atomic gas (Σgas,atomic) and star formation rate (ΣSFR) in the galaxies. The estimated gas-phase metallicity of our sample galaxies is Z ~ 0.1 Z⊙. Understanding star formation in such molecule-poor gas is of particular importance since it is likely to be of direct relevance to simulations of early galaxy formation. For about 20 per cent (9/43) of our sample galaxies, we find that the HI distribution is significantly disturbed, with little correspondence between the optical and HI distributions. We exclude these galaxies from the comparison. We also exclude galaxies with very low star formation rates, for which stochastic effects make it difficult to estimate the true star formation rates. For the remaining galaxies, we compute the Σgas,atomic and ΣSFR averaged over the entire star-forming disc of the galaxy. For these galaxies, we find a nearly linear relation between the star formation rate and the atomic gas density, namely log ΣSFR = 0.91-0.25+0.23 log Σgas,atomic - 3.84-0.19+0.15. The corresponding gas consumption timescale is ~10 Gyr, i.e. significantly smaller than the ~100 Gyr estimated for the outer regions of spiral galaxies. We also estimate the gas consumption time-scale computed using the global gas content and the global star formation rate for all galaxies with a reliable measurement of the star formation rate, regardless of whether the HI distribution is disturbed or not. The mean gas consumption time-scale computed using this entire gas reservoir is ~18 Gyr, i.e. still significantly smaller than that estimated for the outer parts of spirals. The gas consumption time-scale for dwarfs is intermediate between the values of ~100 and ~2Gyr estimated for the outer molecule-poor and inner molecule-rich regions of spiral discs.