ALMA observations of 99 GHz free-free and H40$\alpha$ line emission from star formation in the centre of NGC 253

TL;DR

This study uses ALMA observations at 99 GHz to measure free-free and H40α emissions in NGC 253's center, providing precise star formation rates and revealing higher dust opacities than previous estimates.

Contribution

First detailed millimeter-wave measurements of ionized gas and star formation in NGC 253's core, improving constraints and highlighting dust opacity differences.

Findings

Electron temperatures of 3700-4500 K in the central gas

Star formation rate of 1.73 solar masses per year

Dust opacities are ~3 dex higher than near-infrared estimates

Abstract

We present Atacama Large Millimeter/submillimeter Array observations of 99.02 GHz free-free and H40$\alpha$ emission from the centre of the nearby starburst galaxy NGC 253. We calculate electron temperatures of 3700-4500 K for the photoionized gas, which agrees with previous measurements. We measure a photoionizing photon production rate of $(3.2\pm0.2)\times10^{53}$ s$^{-1}$ and a star formation rate of $1.73\pm0.12$ M$_\odot$ yr$^{-1}$ within the central 20$\times$10 arcsec, which fall within the broad range of measurements from previous millimetre and radio observations but which are better constrained. We also demonstrate that the dust opacities are ~3 dex higher than inferred from previous near-infrared data, which illustrates the benefits of using millimetre star formation tracers in very dusty sources.