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

TL;DR

This study analyzes the gas content, star formation, and chemical evolution of the extremely gas-rich dwarf galaxy NGC 3741, revealing its unique properties and implications for galaxy evolution models.

Contribution

It provides detailed HI, H-alpha, and oxygen abundance data for NGC 3741, highlighting its extended gas disk, spiral features, and insights into its chemical evolution in the context of closed-box and leaky box models.

Findings

NGC 3741 has an extended gas disk (~8.8 times the Holmberg radius).

Star formation rate aligns with the Kennicutt-Schmidt law.

Effective oxygen yield correlates inversely with dynamical mass.

Abstract

We present an analysis of HI, H-alpha, and oxygen abundance data for NGC 3741. This galaxy has a very extended gas disk (~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 baryons (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution HI images of the galaxy show evidence for a large scale (purely gaseous) spiral arm and central bar. Despite the gaseous spiral feature and the on-going star formation, we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion. We also find that the star formation rate is consistent with that expected from the observed correlations between HI mass and SFR and the global Kennicutt-Schmidt law respectively. We measure the oxygen abundance to be 12+ log(O/H)=7.66$\pm$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 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 edge of the HI disk (i.e. >8 times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric HI maps and chemical abundance measurements, we find that the effective yield is anti-correlated with the total dynamical mass, as expected in leaky box models (slightly abridged).