The Baryonic Mass Function of Spiral Galaxies. Clues to Galaxy Formation and to the Nature of Damped Lyman-α Clouds

We compute the baryonic mass function, ψS (Mb), of disk galaxies using the first luminosity functions and baryonic mass-to-light ratios reliable for this goal. On scales from 108M⊙ to 10 11M⊙ such function is featureless, ψ S ∝ M −1/2 b . Outside this mass range ψ S is a strong inverse function of Mb. The contributions to the baryon density ΩSb from objects of different mass indicate that spirals have a characteristic mass scale at M b ≃ 2 × 10 11M⊙, around which more than 50% of the total baryonic mass is concentrated. The integral value, ΩSb = (1.2 ± 0.2) × 10 , confirms, to a higher accuracy, the previous evidence (Persic & Salucci 1992) that the fraction of BBN baryons locked in disk galaxies is negligible and matches that of high-z Damped Lyα systems (DLAs). We have investigated the scenario in which DLAs are progenitors of present-day spirals. An essential condition for this is that their cross-sections should be larger than the areas of present-day stellar disks. Moreover, we find that: (i) the baryon surface densities of spirals, translated into pre-star-formation HI column densities NHI , are in good agreement with the DLAs’ column densities; (ii) at z ∼ 3, the number of DLA absorbers per unit absorbing interval, f(NHI , z), agrees with the projected number of present-day spirals; (iii) the baryon mass functions of DLAs and spirals match closely.