New emerging results on molecular gas, stars, and dust at z ∼ 2, as revealed by low star formation rate and low stellar mass star-forming galaxies

Abstract

Recent CO surveys of star-forming galaxies (SFGs) at z ∼ 2 have revolutionized our picture of massive galaxies. It is time to expand these studies toward the more common z ∼ 2 SFGs with SFR < 40 M O yr-1 and M ∗ < 2.5 × 1010 MO. We have derived molecular gas, stars, and dust in 8 such lensed SFGs. They extend the L IR-L'CO(1-0) distribution of massive z>1 SFGs and increase the spread of the SFG star formation efficiency (SFE). A single star formation relation is found when combining all existing CO-detected galaxies. Our low-M ∗ SFGs also reveal a SFE decrease with M ∗ as found locally. A rise of the molecular gas fraction (f gas) with redshift is observed up to z ∼ 1.6, but it severely flattens toward higher redshifts. We provide the first insight into the f gas upturn at the low-M ∗ end 109.4 < 1010 reaching f gas ∼ 0.7, it is followed by a f gas decrease toward higher M. Finally, we find a non-universal dust-to-gas ratio among local and high-redshift SFGs and starbursts with near-solar metallicities.