2014A&A...564A..66B

We study the properties of the cold gas component of the interstellar medium of the Herschel Reference Survey, a complete volume-limited (15≤D≤25Mpc), K-band-selected sample of galaxies spanning a wide range in morphological type (from ellipticals to dwarf irregulars) and stellar mass (10⁹≤M_star≤10¹¹M_☉). The multifrequency data in our hands are used to trace the molecular gas mass distribution and the main scaling relations of the sample, which put strong constraints on galaxy formation simulations. We extend the main scaling relations concerning the total and the molecular gas component determined for massive galaxies (M_star>10¹⁰M_☉) from the COLD GASS survey down to stellar masses M_star≃10⁹M_☉. As scaling variables we use the total stellar mass M_star, the stellar surface density µ_star, the specific star formation rate SSFR, and the metallicity of the target galaxies. By comparing molecular gas masses determined using a constant or a luminosity dependent X_CO conversion factor, we estimate the robustness of these scaling relations on the very uncertain assumptions used to transform CO line intensities into molecular gas masses. The molecular gas distribution of a K-band-selected sample is significantly different from that of a far-infrared-selected sample since it includes a significantly smaller number of objects with M(H₂)≤6x10⁹M_☉. In spiral galaxies the molecular gas phase is only 25-30% of the atomic gas. The analysis also indicates that the slope of the main scaling relations depends on the adopted conversion factor. Among the sampled relations, all those concerning M(gas)/M_star are statistically significant and show little variation with X_CO. We observe a significant correlation between M(H₂)/M_star and SSFR, M(H₂)/M(HI) and µ_star, M(H₂)/M(HI) and 12+log(O/H), regardless of the adopted X_CO. The total and molecular gas consumption timescales are anticorrelated with the specific star formation rate. The comparison of HRS and COLD GASS data indicates that some of the observed scaling relations are nonlinear.