2003ApJ...582..659K

We derive a nonparametric CO luminosity function using an FIR- and an optical B-band-selected sample of the galaxies included in the FCRAO Extragalactic CO Survey. The FIR-selected sample is defined using the IRAS bright galaxy samples (BGS; IRAS 60 µm flux density ≥5.24 Jy). Although our CO sample is not complete, the normalization using the BGS reproduces the IRAS 60 µm luminosity function in excellent agreement with those found in the literature. Similarly, a B-band-selected sample defined using the Revised Shapley-Ames catalog is used to derive a CO luminosity function for a comparison. A Schechter function describes both the derived CO luminosity functions reasonably well. Adopting the standard CO-to-H₂ conversion factor, we derive a molecular gas density of ρ_H2=(3.1±1.2)x10⁷hM_☉.Mpc^–3 for the local volume. Combining with the measurements of the local H I mass density and the helium contribution, we estimate that the total mass density of cold neutral gas in the local universe is Ω_gas=(4.3±1.1)x10^–4h^–1, which is about 20% of the total stellar mass density Ω_*.