We present Atacama Large Millimeter/submillimeter Array observations of 99.02 GHz freefree and H40α 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 ± 0.2) × 10 53 s −1and a star formation rate of 1.73 ± 0.12 M ⊙ yr −1 within the central 20×10 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.

We present observations of the 85.69 GHz continuum emission and H42α 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 H42α and 85.69 GHz free-free emission (and using a distance of 3.8 Mpc) is 4.35±0.25 M ⊙ yr −1 . 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 µm data and Wide-field Infrared Survey Explorer 22 µm data yield SFRs ∼10× 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.

We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of H30α (231.90 GHz) emission from the low metallicity dwarf galaxy NGC 5253 to measure the star formation rate (SFR) within the galaxy and to test the reliability of SFRs derived from other commonly-used metrics. The H30α emission, which originates mainly from the central starburst, yields a photoionizing photon production rate of (1.9±0.3)×10 52 s −1 and an SFR of 0.087±0.013 M ⊙ yr −1 based on conversions that account for the low metallicity of the galaxy and for stellar rotation. Among the other star formation metrics we examined, the SFR calculated from the total infrared flux was statistically equivalent to the values from the H30α data. The SFR based on previously-published versions of the Hα flux that were extinction corrected using Paα and Paβ lines were lower than but also statistically similar to the H30α value. The mid-infrared (22 µm) flux density and the composite star formation tracer based on Hα and mid-infrared emission give SFRs that were significantly higher because the dust emission appears unusually hot compared to typical spiral galaxies. Conversely, the 70 and 160 µm flux densities yielded SFR lower than the H30α value, although the SFRs from the 70 µm and H30α data were within 1-2σ of each other. While further analysis on a broader range of galaxies are needed, these results are instructive of the best and worst methods to use when measuring SFR in low metallicity dwarf galaxies like NGC 5253.

We describe the design and ground testing of a prototype S-band cryogenic phased array feed. The feed uses 6-mm thick solid aluminium Vivaldi antennas, which were designed using a commercial electromagnetic simulation software. The antennas are designed for easy removal and 40 of them have been integrated into an array. The return loss, inter element coupling and beam patterns have been tested at room temperature and found to agree with simulations. For cryogenic testing the array has been combined with an existing analogue receiver chain and integrated into an cryostat. When tested at 30 K in aperture array mode the system was found to possess a minimum receiver temperature of 12 K. Increasing the number of elements and combining the array with a beamformer would make it a suitable phased array feed (PAF) for radio-astronomy applications.