Locating the Youngest HII Regions in M82 with 7 mm Continuum Maps

TL;DR

This study uses high-resolution 7mm VLA imaging to identify and analyze the youngest HII regions in M82, revealing their properties, distribution, and relation to molecular gas and star formation activity.

Contribution

First detailed 7mm continuum maps of M82's HII regions, linking radio emission features with star formation and molecular gas distribution.

Findings

Identified 14 compact HII regions, including 9 with high ionizing photon rates.

Found strong correlation between dense gas and star formation efficiency.

Detected extended high-brightness radio structures indicating complex gas configurations.

Abstract

We present 7mm Very Large Array continuum images of the starburst galaxy M82. On arcsecond scales, two-thirds of the 7mm continuum consists of free-free emission from HII regions. In the subarcsecond resolution map, we identify 14 compact sources, including 9 bright HII regions with N_Lyc > 10^51 sec^-1. Four of the HII regions have rising spectra, implying emission measures > 10^8 cm^-6 pc. Except for one compact source with peculiar features, all other compact radio sources are found in dust lanes and do not have optical or near-infrared continuum counterparts. Four regions of extended, high brightness (EM > 10^7 cm-6 pc) radio emission are found in our high resolution map, including some as large as ~2", or 30 pc, representing either associations of small HII regions, or sheetlike structures of denser gas. The good correlation between 7 mm emission and Spitzer IRAC 8 micron continuum-removed PAH feature suggests that PAH emission may track the recently formed OB stars. We find an excellent correlation between molecular gas and star formation, particularly dense gas traced by HCN, down to the ~ 45 pc scale in M82. We also find star formation efficiencies (SFEs) of 1-10% on the same scale, based on CO maps. The highest SFE are found in regions with the highest dense gas fractions.