CO Luminosity Function and the First Estimate for ΩHI+H ₂

Abstract

We derive a non-parametric CO luminosity function using a 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 Surveys (BGS; IRAS 60 micron flux density ≥ 5.24 Jy). Although our CO sample is not complete, the normalization using the BGS reproduces the IRAS 60 micron luminosity function in excellent agreement with those found in the literature. Similarly, a B- band selected sample defined using the Revised Shapley-Ames (RSA) catalog is used to derive a CO luminosity function for a comparison. A Schechter function describes both CO luminosity functions reasonably well. Adopting the standard CO-to-H₂ conversion factor, we derive a molecular gas density of ρH ₂ = (3.1±1.2) x 10⁷/hM_⊙ Mpc^-3 for the local volume. Combining with the measurements of the local HI 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) x 10^-4 h ^-1, which is about 20% of the total stellar mass density Ω_*.