Free-free and H42alpha emission from the dusty starburst within NGC 4945 as observed by ALMA

TL;DR

This study uses ALMA observations to analyze free-free and H42alpha emissions in NGC 4945, revealing detailed properties of its starburst region and providing a more accurate star formation rate estimate than mid-infrared data.

Contribution

The paper presents high-resolution ALMA measurements of free-free and recombination line emissions in NGC 4945, offering new insights into the starburst's physical conditions and star formation rate estimation methods.

Findings

84% of 85.69 GHz emission is free-free emission

Electron temperature of ~5400 K in the starburst region

Star formation rate of ~4.35 M/yr from ALMA data

Abstract

We present observations of the 85.69 GHz continuum emission and H42alpha line emission from the central 30 arcsec within NGC 4945. Both sources of emission originate from nearly identical structures that can be modelled as exponential discs with a scale length of ~2.1 arcsec (or ~40 pc). An analysis of the spectral energy distribution based on combining these data with archival data imply that 84% +/- 10% of the 85.69 GHz continuum emission originates from free-free emission. The electron temperature is 5400 +/- 600 K, which is comparable to what has been measured near the centre of the Milky Way Galaxy. The star formation rate (SFR) based on the H42alpha and 85.69 GHz free-free emission (and using a distance of 3.8 Mpc) is 4.35 +/- 0.25 M/yr. This is consistent with the SFR from the total infrared flux and with previous measurements based on recombination line emission, and it is within a factor of ~2 of SFRs derived from radio data. The Spitzer Space Telescope 24 micron data and Wide-field Infrared Survey Explorer 22 micron data yield SFRs ~10x lower than the ALMA measurements, most likely because the mid-infrared data are strongly affected by dust attenuation equivalent to A_V=150. These results indicate that SFRs based on mid-infrared emission may be highly inaccurate for dusty, compact circumnuclear starbursts.