| The cold gas (H1and H2) is ubiquitous in the Interstellar Medium (ISM), and it is the fundamental material of star formation and galaxy evolution. By observing cold gas in the Milky Way and galaxies, we can better understand dif-ferent astrophysical mechanism and astrochemical processes. Thanks to the great advances in observational instruments, especially radio/millimeter telescopes/in-terferometry, different molecular species at different transitions, are now being studied on different physical scales, and helping with many new discoveries about star formation and galaxy evolution. Astronomers have developed many theories and models for astrophysical interpretation and renewed our knowledge of how galaxies evolve. In the context of quick development of radio/mm astrophysics, I have been focusing on several observational projects studying molecular gas on different scales, and aim to reveal the role of molecular gas in the formation of stars and how it affect the evolution of galaxies.We performed Submillimeter Array (SMA) observations of C2H N=3-2tran-sition, HC3N J=30-29, and1.1mm continuum emission towards four OB cluster-forming region AFGL490, ON1, W33Main, and G10.6-0.4(on scale of～0.1pc), covering a broad range of bolometric luminosity (～103-106L⊙). We found that on large scales, the C2H line emission traces the dense molecular gas envelope. However, for all observed sources, the C2H emission peaks show several times104AU offset from the1.1mm continuum emission peaks, which are known to harbor the most luminous stars. The C2H hyperfine lines and the1.1mm continuum emission show that the C2H abundance around the continuum peaks is-10times lower than those in the ambient gas envelope. Our results support the theoreti-cal prediction for centrally embedded-103-106L⊙OB star-forming cores, while future higher resolution observations are required to examine the C2H deficient around the localized sites of high-mass star-formation.With respect to galaxy environment on1-10kpc scale, we present CO J=2-1observations towards32nearby gas-rich star-forming galaxies selected from the ALFALFA catalog, using the Sub-millimeter Telescope (SMT). Our sample is selected to be dominated by intermediate-M*galaxies. Combined with AMIGA sample we compare our results with the COLD GASS survey of massive galaxies. The scaling-relations between molecular gas, atomic gas and other galaxy prop-erties (stellar mass, NUV-γ color and infrared color W3-W2etc.) are examined and discussed. Our results show that1). in the intermediate-M*(M*≤1010M⊙) galaxies, Hi fraction (fHI=MHI/M*) is significantly higher than that of massive galaxies, while H2gas fraction (fH2=MH2/M*) keep unchanged.2).fHI corre-lates better with both M*and NUV-γ than fH2. We also adopt in the scaling relation a new parameter, W3-W2color, which is similar to NUV-γ in trac-ing star formation activity. We find that W3-W2has an anti-correlation with log fH2, which is tighter than the anti-correlation of (NUV-γ)-fHI,(NUV-γ)-fH2and (W3-W2)-fHI. This indicates that W3-W2is a good tracer of the H2fraction in galaxies. We also find, only in the intermediate-M*galaxies the log MH2/MHI appears to depend on log M*and NUV-γ. With the infrared data, we find a tight correlation between the molecular gas mass (MH2) and12μm lu-minosities (L12μm)i with a linear slope (1.03±0.06) for our SMT CO2-1sample. This correlation may reflect the connection between the molecular gas and dust in galaxies, that they are well mixed on global galactic scale, and implies that our sample share similar physical conditions especially the fraction of dense molecules, which is the gas phase with on-going star-formation. Moreover, using the all-sky12μm data available in catalog, we can use L12μm to predict molecular gas mass for most, if not all, star-forming galaxies.Other than the CO survey for general molecular gas on normal star forming galaxies, we also made3mm observations with the IRAM30-m telescope towards ten nearby gas-rich active galaxies (starburst and/or AGN) spanning three orders of magnitude in infrared luminosity. Emission lines of HCO+(1-0), HCN(1-0), and C2H(1-0) were simultaneously detected in most galaxies of our sample. We also tentatively detected the rare isotopic species, H13CN, HC15N and H13CO+in up to four galaxies (IC694, NGC3690, NGC4258and NGC6240). Our estimation shows that the optical depth of HCN is low to moderate in these galaxies (τ～1-5). After comparing the intensity ratios of different molecular emission lines (HCN/HCO+, C2H/HCN, and C2H/HCO+) with the infrared luminosity (LIR), we find that in the infrared luminous sample of five composite (AGN+Starburst) galaxies, these ratios vary with LIR:the HCN/HCO+ratio increases with LIR, which is consistent with previous studies, while there is a slight trend in the composite galaxies that both C2H/HCN and C2H/HCO+ratios decrease with increasing LIR. Although our sample is limited, this trend may possibly imply a relation between the overall gas density and the infrared luminosity of galaxies.During the several years of study, I have got much experience on radio/mm observational astrophysics via the afore-mentioned projects. We produce impor-tant scientific results, improved the understanding of the relationship between molecular gas and star-formation, as well as galaxy evolution. The cumulation of first-hand observational data combining with new chemical models and analytical tools will help advance my future research. |
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