The dense molecular gas mass, traced by HCN J = 1-0, linearly correlates with the far-infrared (FIR) luminosity (~ star formation rate, SFR) for essential all star-forming systems near and far. The spatially resolved FIR-HCN correlation in the disks of spiral galaxies, a local star formation law in terms of dense molecular gas across the disks, seems also to be linear and is essentially same as that established from the Galactic cloud cores and that of global galaxies. This linear FIR-HCN correlation appears to be valid for all other dense gas tracers, e.g., CS, HCO+ or high-J CO. Such tight linear SFR - dense molecular gas correlations suggest that the SFR depends linearly upon the mass of dense molecular gas. This is drastically different from the traditionally established Kennicutt-Schmidt (K-S) laws that relate the total gas and SFR in galaxies as there is no unique correlation in these K-S laws. Finally, we introduce the MALATANG large program on the JCMT to map the HCN and HCO+ J = 4 - 3 line emission in over 20 nearest IR-brightest galaxies. MALATANG bridges the gap, in terms of physical scale and luminosity, between extragalactic (i.e., galaxy-integrated) and Galactic (i.e., giant molecular clouds) observations, showing again such linear correlation in FIR vs. high-J HCN emission.
GAO Yu's research focus is mainly on the multi-wavelength observations (mostly mm/submm) of nearby galaxies (including Milky Way) and star-forming galaxies in the high redshift.