The MALATANG Survey: the L_gas-L_IR correlation on sub-kiloparsec scale
  in six nearby star-forming galaxies as traced by HCN J=4-3 and HCO^+ J=4-3
  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 arcmin $\times$ 2 arcmin region at 14 arcsec (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 \mu m/100
\mu m flux ratio. We find the luminosity ratios of IR/HCN(4-3) and
IR/HCO^+(4-3), which can be taken as a proxy for the efficiency of star
formation in the dense molecular gas (SFE_dense), appears 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 that 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 SFR as
traced by L_IR, nor with the warm-dust temperature, for the different
populations of galaxies.
