Which galaxies dominate the neutral gas content of the Universe?

We study the contribution of galaxies with different properties to the global densities of star formation rate (SFR), atomic (H i) and molecular hydrogen (H-2) as a function of redshift. We use the galform model of galaxy formation, which is set in the Lambda cold dark matter (Lambda CDM) framework. This model includes a self-consistent calculation of the SFR, which depends on the H-2 content of galaxies. The predicted SFR density and how much of this is contributed by galaxies with different stellar masses and infrared luminosities are in agreement with observations. The model predicts a modest evolution of the H i density at z < 3, which is also in agreement with the observations. The H i density is predicted to be always dominated by galaxies with SFR < 1 M-circle dot yr(-1). This contrasts with the H-2 density, which is predicted to be dominated by galaxies with SFR > 10 M-circle dot yr(-1) at z > 1. Current high-redshift galaxy surveys are limited to detect carbon monoxide in galaxies with SFR greater than or similar to 30 M-circle dot yr(-1), which in our model make up, at most, 20 per cent of the H-2 in the universe. In terms of stellar mass, the predicted H-2 density is dominated by massive galaxies, M-stellar > 10(10) M-circle dot, while the H i density is dominated by low-mass galaxies, M-stellar < 10(9) M-circle dot. In the context of upcoming neutral gas surveys, we suggest that the faint nature of the galaxies dominating the H i content of the Universe will hamper the identification of optical counterparts, while for H-2, we expect follow-up observations of molecular emission lines of already existing galaxy catalogues to be able to uncover the H-2 density of the Universe.