Fumiya MAEDA
(Department of Astronomy, Kyoto Univ.)
We try to constrain the cosmic molecular gas mass density 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 cosmic molecular gas mass density is obtained to be ρ(H_2) = (6.8−8.8) ×10^7 M⊙ Mpc^(−3) at z = 1−1.5 and 6.7×10^6 M⊙ Mpc^(−3) at z ∼ 0 by integrating down to 0.03 M∗. Although the values have various uncertainties, the cosmic molecular gas mass density at z = 1 − 1.5 is about ten times larger than that in the local universe. The cosmic star formation rate density at z ∼ 1 − 2 is also about ten times larger than that in the local universe. Our result suggests that the large cosmic molecular gas mass density at z = 1 − 1.5 accounts for the large cosmic star formation rate density at z ∼ 1 − 2.