Hydrogen Radio Recombination Line Emission from M51 and NGC628
  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 NGC628. We use the Green Bank Telescope (GBT) to measure $\sim$15
Hn$\alpha$ 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: $\Psi_{\rm OB} = 3.46$ M$_{\odot}$ yr$^{-1}$
for M51 and $\Psi_{\rm OB} = 0.56$ M$_{\odot}$ yr$^{-1}$ for NGC628. 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 ($\sim$12 hr for each source). We also show that we can
characterize the overall star-forming properties of M51 and NGC628, although
the GBT beam cannot resolve individual HII 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.
