Title: Gas Fraction in 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∗ 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∗ ⩽10^10 M⊙, HI fraction (fHI ≡ MHI/M∗) is significantly higher than that 
of more massive galaxies, while H2 gas fraction (fH2 ≡ MH2/M∗) remain nearly 
unchanged. (2). Comparing with fH2, fHI correlates better with both M∗ and NUV−r. (3). A 
new parameter, WISE color W3−W2 (12μm−4.6μm) 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 fH2 than the anti-correlation of (NUV−r) - fHI, (NUV−r) - fH2 and (W3−W2) - fHI. 
This indicates that W3−W2 can trace the H2 fraction in galaxies. For gas ratio MH2/MHI, only 
in the intermediate-M∗ galaxies it appears to depend on M∗ and NUV−r. We find a tight 
correlation between the molecular gas mass MH2 and 12\,μm (W3) luminosities (L_12μm), 
and the slope is close to unity (1.03 ± 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 12μm 
(W3) data available in WISE, this correlation can be used to estimate CO flux for molecular gas 
observations and can even predict H2 mass for star-forming galaxies.