12CO and HCN datacubes of NGC 253

We present new ALMA Atacama Compact Array and Total Power (ACA+TP) observations of the HCN emission across NGC 253. The observations cover the inner 8.6' of the galaxy disk at a spatial resolution of 300pc. Our study examines the distribution and kinematics of the HCN- traced gas and its relationship with the bulk molecular gas traced by CO(2-1).

We analyze the integrated intensity and mean velocity of HCN and CO along each line of sight. We also used the SCOUSE software to perform spectral decomposition, which considers each velocity component separately. We find that the denser molecular gas traced by HCN piles up in a ring-like structure at a radius of 2kpc.

The HCN emission is enhanced by two orders of magnitude in the central 2kpc regions, beyond which its intensity decreases with increasing galactocentric distance. The number of components in the HCN spectra shows a robust environmental dependence, with multiple velocity features across the center and bar. The HCN spectra exhibit multiple velocity features across the center and bar, which shows a robust environmental dependence.

We have identified an increase in the HCN/CO ratio in these regions, corresponding to a velocity component likely associated with a molecular outflow. We have also discovered that the ratio between the total infrared luminosity and dense gas mass, which is an indicator of the star formation efficiency of dense gas, is anticorrelated with the molecular gas surface density up to approximately 200M_{sun}_/pc^2^. However, beyond this point, the ratio starts to increase.

We argue that using information about spectroscopic features of molecular emission is an important aspect of understanding molecular properties in galaxies.