Life in the last lane: star formation and chemical evolution in an extremely gas rich dwarf

Abstract

We present an analysis of H I, Hα and oxygen abundance data for NGC 3741. This galaxy has a very extended gas disc (^~8.8 times the Holmberg radius), and a dark-to-luminous (i.e. stellar) mass ratio of ^~149, which makes it one of the 'darkest' dwarf irregular galaxies known. However, its ratio of baryon (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution H I images of the galaxy show evidence for a large-scale (purely gaseous) spiral arm and central bar. From our H I data, a rotation curve can be derived out to ~37-44 disc scalelengths in the J and B bands, respectively. This is just slightly short of the radius at which one would expect a Navarro-Frenk-White type rotation curve to start falling. The galaxy has an integrated star formation rate (SFR) of ~0.0034 M_⊙ yr⁻¹, while the average SFR within the optical disc is ~0.0049 M_⊙yr⁻¹ kpc⁻². Despite the gaseous spiral feature and the ongoing star formation, we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion. This is consistent with the behaviour seen in other dwarf galaxies. We also find that the SFR is consistent with that expected from the observed correlations between H I mass and SFR and the global Kennicutt-Schmidt law, respectively. We measure the oxygen abundance to be 12 + log(O/H) = 7.66 ± 0.10, which is consistent with that expected from the metallicity-luminosity relation, despite its extreme gas mass ratio. We also examine the issue of chemical evolution of NGC 3741 in the context of the closed-box model of chemical evolution. The effective oxygen yield of NGC 3741 is consistent with recent model estimates of closed-box yields, provided one assumes that the gas has been efficiently mixed all the way to the edge of the H I disc (i.e. greater than eight times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric H I maps and chemical abundance measurements, we find that the effective yield is anticorrelated with the total dynamical mass, as expected in leaky box models.