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-$M_{ \rm *}$ 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 $M_{ \rm *}$ $\leqslant 10^{ 10}$ $M_{ \odot}$, HI fraction ($f_{ \rm HI}$ $\equiv$ $M_{ \rm HI}$/$M_{ \rm *}$) is significantly higher than that of more massive galaxies, while H$_2$ gas fraction ($f_{ \rm H_2}$ $\equiv$ $M_{ \rm H_2}$/$M_{ \rm *}$) remain nearly unchanged. (2). Comparing with $f_{ \rm H_2}$, $f_{ \rm HI}$ correlates better with both $M_{ \rm *}$ 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 $f_{ \rm H_2}$ than the anti-correlation of (NUV$- r$) - $f_{ \rm HI}$, (NUV$- r$) - $f_{ \rm H_2}$ and (W3$-$W2) - $f_{ \rm HI}$. This indicates that W3$-$W2 can trace the H$_2$ fraction in galaxies. For gas ratio $M_{ \rm H_2}$/$M_{ \rm HI}$, only in the intermediate-$M_{ \rm *}$ galaxies it appears to depend on $M_{ \rm *}$ and NUV$- r$. We find a tight correlation between the molecular gas mass $M_{ \rm H_2}$ and 12\,$\mu$m (W3) luminosities ($L_{ \rm 12\,\mu m}$), 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.
Categories: astro-ph.GA