Star formation in the centre of NGC 1808 as observed by ALMA

TL;DR

This study uses ALMA observations to accurately measure star formation rates in NGC 1808's center, demonstrating the effectiveness of free-free emission as a dust-unaffected tracer and comparing it with other metrics.

Contribution

It provides a detailed analysis of star formation indicators in a dusty starburst galaxy, highlighting the reliability of ALMA free-free emission measurements over traditional methods.

Findings

ALMA free-free emission accounts for 60-90% of the continuum in the 85-100 GHz range.

Star formation rate from ALMA data is approximately 3.1 solar masses per year.

Radio and UV-based SFR estimates are lower or less reliable due to diffusion and dust extinction.

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 85.69 and 99.02 GHz continuum emission and H42$\alpha$ and H40$\alpha$ lines emission from the central 1~kpc of NGC 1808. These forms of emission are tracers of photoionizing stars but unaffected by dust obscuration that we use to test the applicability of other commonly star formation metrics. An analysis of the spectral energy distributions shows that free-free emission contributes about 60 to 90 per cent of the continuum emission in the 85-100 GHz frequency range, dependent on the region. The star formation rate (SFR) derived from the ALMA free-free emission is $3.1\pm0.3$~M$_\odot$~yr$^{-1}$. This is comparable to the SFRs measured from the infrared emission, mainly because most of the bolometric energy from the heavily obscured region is emitted as infrared emission. The radio 1.5~GHz emission yields a SFR 25 per cent lower than the ALMA value, probably because of the diffusion of the electrons producing the synchrotron emission beyond the star-forming regions. The SFRs measured from the extinction-corrected H$\alpha$ line emission are about 40 to 65 per cent of the SFR derived from the ALMA data, likely because this metric was not calibrated for high extinction regions. Some SFRs based on extinction-corrected ultraviolet emission are similar to those from ALMA and infrared data, but given that the ultraviolet terms in the extinction correction equations are very small, these metrics seem inappropriate to apply to this dusty starburst.