2018PASP..130h4101L

We report the discovery of hydrogen radio recombination line (RRL) emission from two galaxies with star formation rates (SFRs) similar to that of the Milky Way: M51 and NGC 628. We use the Green Bank Telescope (GBT) to measure ∼15 Hnα recombination transitions simultaneously and average these data to improve our spectral signal-to-noise ratio. We show that our data can be used to estimate the total ionizing photon flux of these two sources, and we derive their SFRs within the GBT beam: Ψ_OB = 3.46 M_☉ yr^–1 for M51 and Ψ_OB = 0.56 M_☉ yr^–1 for NGC 628. Here, we demonstrate that it is possible to detect RRLs from normal galaxies that are not undergoing a starburst with current instrumentation and reasonable integration times (∼12 hr for each source). We also show that we can characterize the overall star-forming properties of M51 and NGC 628, although the GBT beam cannot resolve individual H II region complexes. Our results suggest that future instruments, such as the Square Kilometre Array and the Next Generation Very Large Array, will be able to detect RRL emission from a multitude of Milky Way-like galaxies, making it possible to determine SFRs of normal galaxies unaffected by extinction and to measure global star formation properties in the local universe.