Survey of Water and Ammonia in Nearby galaxies (SWAN): Resolved Ammonia
  Thermometry, Water and Methanol Masers in the Nuclear Starburst of NGC 253
  We present Karl G Jansky Very Large Array molecular line observations of the
nearby starburst galaxy NGC 253, from SWAN: "Survey of Water and Ammonia in
Nearby galaxies". SWAN is a molecular line survey at centimeter wavelengths
designed to reveal the physical conditions of star forming gas over a range of
star forming galaxies. NGC 253 has been observed in four 1GHz bands from 21 to
36 GHz at 6" ($\sim100$pc) spatial and 3.5 km s$^{-1}$ spectral resolution. In
total we detect 19 transitions from seven molecular and atomic species. We have
targeted the metastable inversion transitions of ammonia (NH$_{3}$) from (1,1)
to (5,5) and the (9,9) line, the 22.2 GHz water (H$_2$O) ($6_{16}-5_{23}$)
maser, and the 36.1 GHz methanol (CH$_3$OH) ($4_{-1}-3_{0}$) maser. Utilizing
NH$_{3}$ as a thermometer, we present evidence for uniform heating over the
central kpc of NGC 253. The molecular gas is best described by a two kinetic
temperature model with a warm 130K and a cooler 57K component. A comparison of
these observations with previous ALMA results suggests that the molecular gas
is not heated in photon dominated regions or shocks. It is possible that the
gas is heated by turbulence or cosmic rays. In the galaxy center we find
evidence for NH$_{3}$(3,3) masers. Furthermore we present velocities and
luminosities of three water maser features related to the nuclear starburst. We
partially resolve CH$_3$OH masers seen at the edges of the bright molecular
emission, which coincides with expanding molecular superbubbles. This suggests
that the masers are pumped by weak shocks in the bubble surfaces.
