Dense Molecular Gas And Star Formation In Nearby Galaxy M51

Stars form in the dense cores of giant molecular clouds(GMCs).The dense cores are traced by high-dipole-moment molecules such as HCN and HCO+.The star formation rate(traced by LIR)and dense molecular gas mass(traced by LHCN1-0)show linear correlation in the global galaxies.It is a front topic in astrophysics that how does the dense gas form stars in galaxies.This thesis focused on the relation between the dense molecular gas and star formation in the disk of M51.In the introduction,we introduced the process of star formation,the mea-surements of star formation rate and(dense)molecular gas mass in galaxies,the relation between(dense)molecular gas mass and star formation rate in galaxies and the advantage of researching M51.In this thesis,we observed the dense molecular gas in the disk of M51 with IRAM 30,NOEMA and JCMT telescope,and we find three major results as following:1.We mapped the HCN J=1-0 emission in the disk of M51 using IRAM 30m telescope with spatially resolution of 1 kpc scale,and first present the HCN J=1-0 integrated intensity distribution in the disk of M51.Combined with IR data(tracing star formation rate),we find that the SFEdense(LIR/LHCN1-0)is lower in the center than in the outer disk of M51.This could be explained by:(1)the turbulence is stronger in the center of M51 than in the outer disk,and the strong turbulence prevent the formation of stars in the center,(2)the metallicity is lower in the outer disk than in the center,the low metallicity leads to weaker HCN J=1-0 emission per dense gas mass in the outer disk,and then the observed LIR/LHCN1-0 should be higher in the outer disk than in the center.2.We detected dense gas(traced by HCN J=1-0 and HCO+J=1-0)us-ing NOEMA interferometer array with spatially resolution of 100 pc in 6 giant molecular associations(GMAs)on the outer spiral arm of M51.Comparing with multiple scale targets(global galaxies?1 kpc scale regions in the disk of nearby galaxies?giant molecular associations in the nearby galaxies and dense molecular cloud cores in the Milky Way),we find that the linear relation between LIR and LHCN1-0 extended to the spatial scale of 100 pc GMAs.We also find that SFEdense(LIR/LHCN1-0 and LIR/LHCO+1-0)is larger for the giant molecular associations with massive stars.3.We detected extremely dense molecular gas(traced by HCN J =4-3 and HCO+J =4-3)with JCMT telescope in the inner disk of M51.We find that the relation between LIR and LHCN4-3 in the center disk of M51 follows the relation of global galaxies.While the relation between LIR and LHCN1-0 in the center disk of M51 does not follow the relation of global galaxies,LHCN1-0 is stronger than which estimate from LIR with the linear relation of global galaxies.At the same time,we studied the reason for the suddenly stop of starburst in the IR bright post starburst galaxies,SDSS J144816.04+173305.9.We ob-served 12CO 1-0,13CO 1-0 and 12CO 2-1 with IRAM 30m and find that 12CO 1-0 and 13CO 1-0 luminosity ratio(17.2)is two times larger than that of normal galactic disk(6),and is similar with the ratio of starburst galaxies.The optical thin 13CO 1-0 could trace the molecular gas mass better than the optical thick 12CO 1-0.So we should adopt the CO-to-H2 conversion factor of starburst galax-ies for the post star burst galaxy in calculating the molecular gas mass.When we adopt the CO-to-H2 conversion factor of normal galactic disk for the post star-burst galaxy,we will over estimate the molecular gas mass.Then we find that the mass of molecular gas and star formation rate in the post-starburst galaxy follow the Kennicutt-Schmidt Law.