Sub-millimeter Telescope CO (2-1) Observations of Nearby Star-forming Galaxies

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 H2?gas fraction ( ? /M *) remains nearly unchanged. (2) Compared to , f H I correlates better with both M * and NUV?? r. (3) A new parameter, WISE color W3???W2 (12-4.6 ?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 than the anti-correlation of (NUV?? r)-f H I , (NUV?? r)-, and (W3???W2)-f H I . This indicates that W3???W2 can trace the H2?fraction in galaxies. For the gas ratio /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 ?and 12 ?m (W3) luminosities (L 12 ?m), 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 ?m (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.