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 2×2 region at 14 (FWHM) resolution (corresponding to linear scales of ∼0.2-1.0 kpc). The LIR-Ldense 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, LIR/Ldense, shows systematic variations with LIR within individual spatially resolved galaxies, whereas the galaxy-integrated ratios vary little. A rising trend is also found between LIR/Ldense 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 (SFEdense), appear to be nearly independent of the dense gas fraction ( fdense) for our sample of galaxies. The SFE of the total molecular gas (SFEmol) is found to increase substantially with fdense when combining our data with those on local (ultra)luminous infrared galaxies and high-z quasars. The mean LHCN(4-3) LHCO+(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 LIR, nor with the warm-dust temperature, for the different populations of galaxies.
Bibliographical noteFunding Information:
We are very grateful to the EAO/JCMT staff for their help during the observations and data reduction, and we thank the anonymous referee for useful comments and suggestions that greatly improved this manuscript. The James Clerk Maxwell Telescope is operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan; Academia Sinica Institute of Astronomy and Astrophysics; the Korea Astronomy and Space Science Institute; the Operation, Maintenance and Upgrading Fund for Astronomical Telescopes and Facility Instruments, budgeted from the Ministry of Finance (MOF) of China and administrated by the Chinese Academy of Sciences (CAS), as well as the National Key R&D Program of China (no. 2017YFA0402700). Additional funding support is provided by the Science and Technology Facilities Council of the United Kingdom and participating universities in the United Kingdom and Canada. MALATANG is a JCMT Large Program with project code M16AL007. We are grateful to P. P. Papadopoulos for his warm help and support with the JCMT observations. We acknowledge the ORAC-DR, Starlink, and GILDAS software for the data reduction and analysis.
This work was supported by National Key R&D Program of China grant no. 2017YFA0402704, NSFC grant nos. 11420101002 and 11603075, and Chinese Academy of Sciences Key Research Program of Frontier Sciences grant no. QYZDJ-SSW-SLH008. This work has also been supported by National Research Foundation of Korea grant no. 2015R1D1A1A01060516 and the Center for Galaxy Evolution Research (no. 2010-0027910). C.D.W. acknowledges support from the Natural Sciences and Engineering Research Council of Canada. Z.-Y.Z. acknowledges support from ERC in the form of the Advanced Investigator Programme, 321302, COSMICISM. L.H. acknowledges the support of National Key R&D Program of China grant no. 2016YFA0400702 and NSFC grant no. 11721303. E.B. acknowledges support from the UK Science and Technology Facilities Council [grant no. ST/M001008/1]. M.J.M. acknowledges the support of the National Science Centre, Poland, through the POLONEZ grant 2015/19/P/ST9/04010. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 665778. S.M. acknowledges the support of the Ministry of Science and Technology (MoST) of Taiwan, MoST 103-2112-M-001-032-MY3 and MoST 106-2112-M-001-011. M.J.C. had support from STFC. J.-H.H. thanks the support from the National Natural Science Foundation of China (U1631237), Yunnan Province of China (no. 2017HC018) where the sum is over all grid cells and Nobsi and Nmodeli are the number of observed and model data points, respectively, in grid cell i.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science