SMT CO (2-1) Observations of Nearby Star-Forming Galaxies

Abstract

We present CO $J$=2-1 observations towards 32 nearby gas-rich star-forming galaxies selected from the ALFALFA and WISE catalogs, using the Sub-millimeter Telescope. Our sample is selected to be dominated by intermediate-$M_{\ast }$ galaxies. The scaling-relations between molecular gas, atomic gas and galactic properties (stellar mass, NUV$-r$ and WISE color W3$-$W2) are examined and discussed. Our results show that (1). In the galaxies with stellar mass $M_{\ast }$ $⩽10^{10}$ $M_{\odot }$, HI fraction ($f_{\mathrm{HI}}$ $\equiv$ $M_{\mathrm{HI}}$/$M_{\ast }$) is significantly higher than that of more massive galaxies, while H${}_2$ gas fraction ($f_{{\mathrm{H}}_2}$ $\equiv$ $M_{{\mathrm{H}}_2}$/$M_{\ast }$) remain nearly unchanged. (2). Comparing with $f_{{\mathrm{H}}_2}$, $f_{\mathrm{HI}}$ correlates better with both $M_{\ast }$ and NUV$-r$. (3). A new parameter, WISE color W3$-$W2 (12\,$\mu$m$-$4.6\,$\mu$m) is introduced, which is similar to NUV$-r$ in tracing star formation activity, and we find that W3$-$W2 has a tighter anti-correlation with log $f_{{\mathrm{H}}_2}$ than the anti-correlation of (NUV$-r$) - $f_{\mathrm{HI}}$, (NUV$-r$) - $f_{{\mathrm{H}}_2}$ and (W3$-$W2) - $f_{\mathrm{HI}}$. This indicates that W3$-$W2 can trace the H${}_2$ fraction in galaxies. For gas ratio $M_{{\mathrm{H}}_2}$/$M_{\mathrm{HI}}$, only in the intermediate-$M_{\ast }$ galaxies it appears to depend on $M_{\ast }$ and NUV$-r$. We find a tight correlation between the molecular gas mass $M_{{\mathrm{H}}_2}$ and 12\,$\mu$m (W3) luminosities ($L_{12\mu \mathrm{m}}$), and the slope is close to unity (1.03 $\pm$ 0.06) for the SMT sample. This correlation may reflect that the cold gas and dust are well mixed on global galactic scale. Using the all-sky 12\,$\mu$m (W3) data available in WISE, this correlation can be used to estimate CO flux for molecular gas observations and can even predict H${}_2$ mass for star-forming galaxies.

15 pages, 7 figures, ApJ accepted

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