Galaxy Luminosity Functions, M/L Ratios, and Closure of the Universe

Extract

Data on the luminosity function (LF) of galaxies are reviewed and compared on a common magnitude system and with common assumptions. The result of Kirshner et al. (Astron. J. 88, 1285, 1983) is chosen as a best guess. Departures from this “standard LF” for specific galaxy types and environments (clusters, groups, field) are discussed briefly. If the Galactic absorption is A_B = 0.2 csc|b| and the solar absolute magnitude is M_B⊙ = 5.48, this LF leads to a mean luminosity density on the B_T system, or about 1.4 × 10⁻² h “Galaxies” Mpc⁻³. The mean M/L ratio needed to give critical cosmological density (Ω₀ = 1) is then 920h in solar units on the B_T⁰ (face-on) system. [The latter number can be larger by a factor ∼2 if a different LF is used, and larger still on different systems and with different assumptions (see Table).] The “weighed” M/L on this system is in clusters, but it is smaller in binaries and small groups. Estimates of the weighed (clumped) Ω₀ vary, but it is definitely < 1. Comparison with constraints imposed by inflation and nucleosynthesis suggests that we distinguish at least two “dark-matter” problems: (1) What is the weighed mass, and how is it distributed? It contributes an Ω₀ ∼ 0.1–0.5 and could be all baryonic if at the lower end of this range. (2) Is there additional matter, more smoothly distributed and probably nonbaryonic, which brings Ω₀ up to unity? For details see Comments on Astrophys., in press.