Forming Super Star Clusters in the Central Starburst of NGC 253
  NGC 253 hosts the nearest nuclear starburst. Previous observations show a
region rich in molecular gas, with dense clouds associated with recent star
formation. We used ALMA to image the 350 GHz dust continuum and molecular line
emission from this region at 2 pc resolution. Our observations reveal ~14
bright, compact (~2-3 pc FWHM) knots of dust emission. Most of these sources
are likely to be forming super star clusters (SSCs) based on their inferred
dynamical and gas masses, association with 36 GHz radio continuum emission, and
coincidence with line emission tracing dense, excited gas. One source coincides
with a known SSC, but the rest remain invisible in Hubble near-infrared (IR)
imaging. Our observations imply that gas still constitutes a large fraction of
the overall mass in these sources. Their high brightness temperature at 350 GHz
also implies a large optical depth near the peak of the IR spectral energy
distribution. As a result, these sources may have large IR photospheres and the
IR radiation force likely exceeds L/c. Still, their moderate observed velocity
dispersions suggest that feedback from radiation, winds, and supernovae are not
yet disrupting most sources. This mode of star formation appears to produce a
large fraction of stars in the burst. We argue for a scenario in which this
phase lasts ~1 Myr, after which the clusters shed their natal cocoons but
continue to produce ionizing photons. The strong feedback that drives the
observed cold gas and X-ray outflows likely occurs after the clusters emerge
from this early phase.
