Sub-millimeter telescope CO (2-1) observations of nearby star-forming galaxies

Abstract

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 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 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 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 H2 mass for star-forming galaxies.