Low Metallicity ISM: excess submillimetre emission and CO-free H2 gas

TL;DR

This paper investigates the peculiar submillimetre emission and molecular gas properties in low metallicity dwarf galaxies, revealing excess emission, inconsistent dust-to-gas ratios, and a large reservoir of CO-free H2 gas.

Contribution

It provides new insights into the dust and molecular gas content in low metallicity galaxies, highlighting the presence of CO-free H2 gas and the breakdown of standard dust-metallicity relations.

Findings

Excess submm emission begins at or beyond 500 μm.

Low metallicity galaxies show high [CII]/CO ratios indicating CO-free H2 gas.

Dust-to-gas ratios deviate from expected linear relations at metallicities around 12+log(O/H) ~ 8.0-8.2.

Abstract

The low metallicity interstellar medium of dwarf galaxies gives a different picture in the far infrared(FIR)/submillimetre(submm)wavelengths than the more metal-rich galaxies. Excess emission is often found in the submm beginning at or beyond 500 mu. Even without taking this excess emission into account as a possible dust component, higher dust-to-gas mass ratios (DGR) are often observed compared to that expected from their metallicity for moderately metal-poor galaxies. The SEDs of the lowest metallicity galaxies, however, give very low dust masses and excessively low values of DGR, inconsistent with the amount of metals expected to be captured into dust if we presume the usual linear relationship holding for all metallicities, including the more metal-rich galaxies. This transition seems to appear near metalllicities of 12 + log(O/H) ~ 8.0 - 8.2. These results rely on accurately quantifying the total molecular gas reservoir, which is uncertain in low metallicity galaxies due to the difficulty in detecting CO(1-0) emission. Dwarf galaxies show an exceptionally high [CII] 158 mu/CO (1-0) ratio which may be indicative of a significant reservoir of 'CO-free' molecular gas residing in the photodissociated envelope, and not traced by the small CO cores.