  Studying molecular gas in nearby galaxies using hydrogen cyanide (HCN) as a
tracer for higher densities than CO emission still poses a significant
challenge. Even though several galaxies have HCN maps on a few kpc scales,
higher-resolution maps are still required. Our goal is to examine the contrast
in intensity between two tracers that probe different density regimes -
HCN(1-0)/CO(2-1) ratio - and their kinematics across NGC 253. By utilizing the
advanced capabilities of the Atacama Large Millimeter/submillimeter Array
(ALMA), we can map these features at high resolution across a large field of
view and uncover the nature of such dense gas in extragalactic systems. We
present new ALMA Atacama Compact Array and Total Power (ACA+TP) observations of
the HCN emission across NGC 253, covering the inner 8.6' of the galaxy disk at
300 pc scales. We analyze the integrated intensity and mean velocity of HCN and
CO along each line of sight and use SCOUSE software to perform spectral
decomposition, which considers each velocity component separately. Molecular
gas traced by HCN piles up in a ring-like structure at a radius of 2 kpc. The
HCN emission is enhanced by 2 orders of magnitude in the central 2 kpc 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. 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 indicates the star formation efficiency of dense gas, is
anti-correlated with the molecular gas surface density up to approximately 200
Msul/pc^2. In contrast, beyond this point, the ratio starts to increase.
