The MALATANG survey: dense gas and star formation from high-transition HCN and HCO⁺ maps of NGC 253

To study the high-transition dense-gas tracers and their relationships to the star formation of the inner ∼2 kpc circumnuclear region of NGC 253, we present HCN J = 4−3 and HCO ⁺ J = 4−3 maps obtained with the James Clerk Maxwell Telescope. Using the spatially resolved data, we compute the concentration indices r 90 /r 50 for the different tracers. HCN and HCO ⁺ 4–3 emission features tend to be centrally concentrated, which is in contrast to the shallower distribution of CO 1–0 and the stellar component. The dense-gas fraction (f dense , traced by the velocity-integrated-intensity ratios of HCN/CO and HCO ⁺ /CO) and the ratio R 31 (CO 3–2/1–0) decline towards larger galactocentric distances, but increase with higher star formation rate surface density. The radial variation and the large scatter of f dense and R 31 imply distinct physical conditions in different regions of the galactic disc. The relationships of f dense versus Σ stellar , and SFE dense versus Σ stellar are explored. SFE dense increases with higher Σ stellar in this galaxy, which is inconsistent with previous work that used HCN 1–0 data. This implies that existing stellar components might have different effects on the high-J HCN and HCO ⁺ than their low-J emission. We also find that SFE dense seems to be decreasing with higher f dense  which is consistent with previous works, and it suggests that the ability of the dense gas to form stars diminishes when the average density of the gas increases. This is expected in a scenario where only the regions with high-density contrast collapse and form stars.