2017ApJ...835..120M

We try to constrain the cosmic molecular gas mass density (CMGD) at z=1-1.5 and that in the local universe by combining stellar mass functions of star-forming galaxies and their average molecular gas mass fractions against the stellar mass. The average molecular gas mass fractions are taken from recent CO observations of star-forming galaxies at the redshifts. The CMGD is obtained to be ρ_H2_=(6.8-8.8)×10⁷ M_☉ Mpc^–3 at z=1-1.5 and 6.7×10⁶ M_☉ Mpc^–3 at z∼0 by integrating down to 0.03 M^* . Although the values have various uncertainties, the CMGD at z=1-1.5 is about 10 times larger than that in the local universe. The cosmic star formation rate density (CSFRD) at z∼1-2 is also about 10 times larger than that in the local universe. Our result suggests that the large CMGD at z=1-1.5 accounts for the large CSFRD at z∼1-2.