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