Molecular gas mass function of Herschel galaxies. Left: comparison of the 1/V_a estimate (green squares, based on Eq. (1)) with literature data (Keres et al. 2003) and models (Obreschkow & Rawlings 2009a; Lagos et al. 2011; Duffy et al. 2012). The red line and shaded area at z = 1.0−2.0 are obtained by scaling the Muzzin et al. (2013) stellar mass function using the molecular gas fractions reported by Tacconi et al. (2013). When needed, masses found in the literature were scaled by the factor 1.36 necessary to account for helium, and were matched to our set of cosmological parameters. Right: results of the parametric STY evaluation of a Schechter mass function (red lines) and its 3σ uncertainty (shaded areas).