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