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 r₉₀/r₅₀ 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 R₃₁ (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 R₃₁ 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.