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

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 _⊙ yr^-1 and M _* < 2.5 × 10¹⁰ M_⊙. 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 10^9.4 < M _*/M_⊙ < 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.