2017ApJ...842..124G

We present Karl G. Jansky Very Large Array molecular line observations of the nearby starburst galaxy NGC 253, from SWAN, the 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 1 GHz bands from 21 to 36 GHz at 6'' ∼ 100 pc) spatial and 3.5 km s^–1 spectral resolution. In total we detect 19 transitions from 7 molecular and atomic species. We have targeted the metastable inversion transitions of ammonia (NH₃) from (1, 1) to (5, 5) and the (9, 9) line, the 22.2 GHz water (H₂O) (6₁₆–5₂₃) maser, and the 36.1 GHz methanol (CH₃OH) (4_–1–3₀) maser. Using NH₃ 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 130 K and a cooler 57 K 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 NH₃(3, 3) masers. Furthermore, we present velocities and luminosities of three water maser features related to the nuclear starburst. We partially resolve CH₃OH 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.