z-GAL. III. Physical properties

In seven sources the para-H_2_O (2_11_-2_02_) transition is detected and follows the IR versus H_2_O luminosity relation of sub-millimetre galaxies. The molecular gas mass of the z-GAL sources is derived from their ^12^CO, [CI], and sub-millimetre dust continuum emission. The three tracers lead to consistent results, with the dust continuum showing the largest scatter when compared to ^12^CO.

The gas-to-dust mass ratio of these sources was computed by combining the information derived from ^12^CO and the dust continuum and has a median value of 107, similar to star-forming galaxies of near-solar metallicity. The same combined analysis leads to depletion timescales in the range between 0.1 and 1.0Gyr, which place the z-GAL sources between the 'main sequence' of star formation and the locus of starbursts. Finally, we derived a first estimate of stellar masses - modulo possible gravitational magnification - by inverting known gas scaling relations: the z-GAL sample is confirmed to be mostly composed by starbursts, whereas ~25% of its members lie on the main sequence of star-forming galaxies (within +/-0.5dex).