MALT-45: A 7mm survey of the southern Galaxy - II. ATCA follow-up observations of 44GHz class I methanol masers

TL;DR

This study uses ATCA follow-up observations to analyze 44GHz class I methanol masers detected in the Galactic plane, revealing their spatial, velocity, and evolutionary properties in star-forming regions.

Contribution

It provides high-resolution positions and comparisons with other maser species, identifying quasi-thermal emission and characterizing the physical environments of class I methanol masers.

Findings

Most masers have small spatial and velocity ranges.

Masers are associated with clumps of 1000-3000 solar masses.

Evolutionary stage correlates with presence of OH masers and radio lines.

Abstract

We detail interferometric observations of 44GHz class I methanol masers detected by MALT-45 (a 7mm unbiased auto-correlated spectral-line Galactic-plane survey) using the Australia Telescope Compact Array. We detect 238 maser spots across 77 maser sites. Using high-resolution positions, we compare the class I CH$_3$OH masers to other star formation maser species, including CS (1-0), SiO $v=0$ and the H53$\alpha$ radio-recombination line. Comparison between the cross- and auto-correlated data has allowed us to also identify quasi-thermal emission in the 44GHz class I methanol maser line. We find that the majority of class I methanol masers have small spatial and velocity ranges ($<$0.5pc and $<$5 km s$^{-1}$), and closely trace the systemic velocities of associated clouds. Using 870$\mu$m dust continuum emission from the ATLASGAL survey, we determine clump masses associated with class I masers, and find they are generally associated with clumps between 1000 and 3000 $M_\odot$. For each class I methanol maser site, we use the presence of OH masers and radio recombination lines to identify relatively evolved regions of high-mass star formation; we find that maser sites without these associations have lower luminosities and preferentially appear toward dark infrared regions.