ALMA 3 mm spectroscopic mapping across the disk of the closest starburst galaxy NGC 253

Star formation correlates mostly directly with dense gas (traced by, e.g. HCN), but spectroscopic tracers of dense gas are incredibly faint with low surface brightness. As a result, we still know relatively little about the resolved structure of dense gas in galaxy disks and its relation to other parts of the galaxy, particularly in extreme systems such as starburst galaxies. I present new ALMA ACA observations of 90 and 100 GHz molecular lines across the disk of the closest starburst galaxy NGC 253 at spatial scales of 300 pc. This yields continuous, uniform measurements of the dense gas abundance and efficiency spanning from the starburst nucleus across the disk environments. We find that all detected lines show enhanced emission towards the centre of this galaxy, then steeply decreases along the bar and remains continuous in the rest of the disk. Dense molecular gas gets piled up in a ring, after which it inflows to the nuclear region, and the presence of an outflow in NGC 253 impacts the dense molecular gas at these scales. In assessing the dynamics of the dense gas by using the spectral decomposition algorithm (SCOUSE), we find a strong environmental dependence on the observed spectral features. In particular, the bar and central sightlines contain complex spectra indicating the presence of multiple gas flows. Similarly, we observed environmental dependence on the observed HCN/CO (the proxy for the dense gas fraction) and the IR/HCN, which traces star formation efficiency. Dense gas fraction strongly decreases towards the outer parts of the disk of NGC 253, whereas SFE shows different behaviour for gas surface densities below and above 200 Msol/pc2.

Our work illustrates the importance of using ALMA to push such "mm-spectroscopy" towards small spatial scales and high sensitivity in understanding dense molecular gas content across disks of extreme environments that are examples of high-redshift galaxies.