2018ApJ...860..165T

We present HCN J=4→3 and HCO⁺ J=4→3 maps of six nearby star-forming galaxies, NGC 253, NGC 1068, IC 342, M82, M83, and NGC 6946, obtained with the James Clerk Maxwell Telescope as part of the MALATANG survey. All galaxies were mapped in the central 2' x 2' region at 14'' (FWHM) resolution (corresponding to linear scales of ∼0.2-1.0 kpc). The L_IR-L'_dense relation, where the dense gas is traced by the HCN J=4→3 and the HCO⁺ J=4→3 emission, measured in our sample of spatially resolved galaxies is found to follow the linear correlation established globally in galaxies within the scatter. We find that the luminosity ratio, L_IR/L'_dense, shows systematic variations with L_IR within individual spatially resolved galaxies, whereas the galaxy-integrated ratios vary little. A rising trend is also found between L_IR/L'_dense ratio and the warm-dust temperature gauged by the 70 µm/100 µm flux ratio. We find that the luminosity ratios of IR/HCN (4-3) and IR/HCO⁺ (4-3), which can be taken as a proxy for the star formation efficiency (SFE) in the dense molecular gas (SFE_dense), appear to be nearly independent of the dense gas fraction (f_dense) for our sample of galaxies. The SFE of the total molecular gas (SFE_mol) is found to increase substantially with f_dense when combining our data with those on local (ultra)luminous infrared galaxies and high-z quasars. The mean L′_HCN(4–3)/L′_HCO^+(4–3) line ratio measured for the six targeted galaxies is 0.9 ± 0.6. No significant correlation is found for the L′_HCN(4–3)/L′_HCO^+(4–3) ratio with the star formation rate as traced by L_IR, nor with the warm-dust temperature, for the different populations of galaxies.