Properties of the Interstellar Medium in Star-Forming Galaxies at z~1.4
  revealed with ALMA
  We conducted observations of 12CO(J=5-4) and dust thermal continuum emission
toward twenty star-forming galaxies on the main sequence at z~1.4 using ALMA to
investigate the properties of the interstellar medium. The sample galaxies are
chosen to trace the distributions of star-forming galaxies in diagrams of
stellar mass-star formation rate and stellar mass-metallicity. We detected CO
emission lines from eleven galaxies. The molecular gas mass is derived by
adopting a metallicity-dependent CO-to-H2 conversion factor and assuming a
CO(5-4)/CO(1-0) luminosity ratio of 0.23. Molecular gas masses and its
fractions (molecular gas mass/(molecular gas mass + stellar mass)) for the
detected galaxies are in the ranges of (3.9-12) x 10^{10} Msun and 0.25-0.94,
respectively; these values are significantly larger than those in local spiral
galaxies. The molecular gas mass fraction decreases with increasing stellar
mass; the relation holds for four times lower stellar mass than that covered in
previous studies, and that the molecular gas mass fraction decreases with
increasing metallicity. Stacking analyses also show the same trends. The dust
thermal emissions were clearly detected from two galaxies and marginally
detected from five galaxies. Dust masses of the detected galaxies are (3.9-38)
x 10^{7} Msun. We derived gas-to-dust ratios and found they are 3-4 times
larger than those in local galaxies. The depletion times of molecular gas for
the detected galaxies are (1.4-36) x 10^{8} yr while the results of the
stacking analysis show ~3 x 10^{8} yr. The depletion time tends to decrease
with increasing stellar mass and metallicity though the trend is not so
significant, which contrasts with the trends in local galaxies.
