Unveiling Extragalactic Star Formation Using Radio Recombination Lines:
  An EVLA Pilot Study with NGC 253
  Radio recombination lines (RRLs) are powerful, extinction-free diagnostics of
the ionized gas in young, star-forming regions. Unfortunately, these lines are
difficult to detect in external galaxies. We present the results of EVLA
observations of the RRL and radio continuum emission at 33 GHz from NGC 253, a
nearby nuclear starburst galaxy. We detect the previously unobserved H58a and
H59a RRLs and make simultaneous sensitive measurements of the continuum. We
measure integrated line fluxes of $44.3 \pm 0.7$ W m$^{-2}$ and $39.9 \pm 0.8$
W m$^{-2}$ for the H58a and H59a lines, respectively. The thermal gas in NGC
253 is kinematically complex with multiple velocity components. We constrain
the density of the thermal gas to $1.4 - 4 \times 10^4$ cm$^{-3}$ and estimate
an ionizing photon flux of $1 \times 10^{53}$ s$^{-1}$. We use the RRL
kinematics and the derived ionizing photon flux to show that the nuclear region
of NGC 253 is not gravitationally bound, which is consistent with the outflow
of gas inferred from the X-ray and Halpha measurements. The line profiles,
fluxes, and kinematics of the H58a and H59a lines agree with those of RRLs at
different frequencies confirming the accuracy of the previous, more difficult,
high frequency observations. We find that the EVLA is an order of magnitude
more efficient for extragalactic RRL observations than the VLA. These
observations demonstrate both the power of the EVLA and the future potential of
extragalactic RRL studies with the EVLA.
