2015ApJ...799...92J

We present CO J = 2-1 observations toward 32 nearby gas-rich star-forming galaxies selected from the ALFALFA and Wide-field Infrared Survey Explorer (WISE) catalogs, using the Sub-millimeter Telescope (SMT). 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 the following. (1) In the galaxies with stellar mass M_* ≤ 10¹⁰ M_☉, the H I fraction (f_ H I_≡ M_H I_/M*_) is significantly higher than that of more massive galaxies, while the H₂gas fraction (f_H2 ≡ M_H2/M_*) remains nearly unchanged. (2) Compared to f_H2, f_H I_correlates better with both M_*and NUV - r. (3) A new parameter, WISE color W3 - W2 (12-4.6 µm), 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_H2 than the anti-correlation of (NUV - r)-f_ H I_, (NUV - r)-f_H2, and (W3 - W2)-f_H I_. This indicates that W3 - W2 can trace the H₂fraction in galaxies. For the gas ratio M_H2/M_H I_, 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 M_H2 and 12 µm (W3) luminosities (L_12 µm_), 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 a global galactic scale. Using the all-sky 12 µm (W3) data available in WISE, this correlation can be used to estimate CO flux for molecular gas observations and can even predict H₂mass for star-forming galaxies.