# Survey of Water and Ammonia in Nearby galaxies (SWAN): Resolved Ammonia Thermometry, Water and Methanol Masers in the Nuclear Starburst of NGC 253

## TL;DR

This paper presents a detailed molecular line survey of NGC 253, revealing the physical conditions of star-forming gas, including temperature, maser activity, and possible heating mechanisms, using VLA observations across multiple molecular transitions.

## Contribution

It provides the first comprehensive analysis of ammonia, water, and methanol masers in NGC 253, offering insights into the heating processes and gas dynamics in a starburst galaxy.

## Key findings

- Molecular gas exhibits two kinetic temperature components at 130K and 57K.
- Evidence suggests turbulence or cosmic rays, not photon-dominated regions or shocks, heat the gas.
- Detection of ammonia and water masers indicates active star formation and complex gas motions.

## Abstract

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.