The Molecular Clouds Fueling a 1/5 Solar Metallicity Starburst

TL;DR

This study uses high-resolution ALMA observations to analyze molecular gas in the low-metallicity galaxy II Zw 40, revealing unusual cloud properties, a high CO-to-H2 conversion factor, and intense star formation linked to galaxy merging processes.

Contribution

First detailed high-resolution molecular gas observations of II Zw 40, deriving a new CO-to-H2 conversion factor related to metallicity, and linking cloud properties to galaxy interactions.

Findings

CO emission is clumpy and mostly unassociated with star formation.

CO-to-H2 conversion factor is 4 to 35 times higher than Milky Way.

Star formation efficiency is at least 10 times higher than in normal spirals.

Abstract

Using the Atacama Large Millimeter/submillimeter Array, we have made the first high spatial and spectral resolution observations of the molecular gas and dust in the prototypical blue compact dwarf galaxy II Zw 40. The CO(2-1) and CO(3-2) emission is clumpy and distributed throughout the central star-forming region. Only one of eight molecular clouds has associated star formation. The continuum spectral energy distribution is dominated by free-free and synchrotron: at 870$\mu m$, only 50% of the emission is from dust. We derive a CO-to-H$_2$ conversion factor by several methods including a new method that uses simple photodissocation models and resolved CO line intensity measurements to derive a relationship that uniquely predicts $\alpha_{CO}$ for a given metallicity. We find that the CO-to-H$_2$ conversion factor is 4 to 35 times that of the Milky Way (18.1 to 150.5 M$_\odot$ / (K km/s pc$^2$)). The star formation efficiency of the molecular gas at least 10 times higher than that found in normal spiral galaxies, which is likely due to the burst-dominated star formation history of II Zw 40 rather than an intrinsically higher efficiency. The molecular clouds within II Zw 40 resemble those in other strongly interacting systems like the Antennae: overall they have high size-linewidth coefficients and molecular gas surface densities. These properties appear to be due to the high molecular gas surface densities produced in this merging system rather than to increased external pressure. Overall, these results paint a picture of II Zw 40 as a complex, rapidly evolving system whose molecular gas properties are dominated by the large-scale gas shocks from its on-going merger.