Clustered Star Formation in the center of NGC 253 Contributes to Driving
  the Ionized Nuclear Wind
  We present new 3 mm observations of the ionized gas toward the nuclear
starburst in the nearby (D ~ 3.5 Mpc) galaxy NGC 253. With ALMA, we detect
emission from the H40-alpha and He40-alpha lines in the central 200 pc of this
galaxy on spatial scales of ~4 pc. The recombination line emission primarily
originates from a population of approximately a dozen embedded super star
clusters in the early stages of formation. We find that emission from these
clusters is characterized by electron temperatures ranging from 7000-10000 K
and measure an average singly-ionized helium abundance <Y+> = 0.25 +/- 0.06,
both of which are consistent with values measured for HII regions in the center
of the Milky Way. We also report the discovery of unusually broad-linewidth
recombination line emission originating from seven of the embedded clusters. We
suggest that these clusters contribute to the launching of the large-scale hot
wind observed to emanate from the central starburst. Finally, we use the
measured recombination line fluxes to improve the characterization of overall
embedded cluster properties, including the distribution of cluster masses and
the fractional contribution of the clustered star formation to the total
starburst, which we estimate is at least 50%.
