2. THE BARYON BUDGET AT z 0

Stars in high surface density galaxies are the most prominent place for baryons. For our purpose it is reasonable to imagine the galaxies contain two distinct stellar populations, an old spheroidal component and a disk component consisting of generally younger stars, with a mix depending on galaxy type. Elliptical galaxies lack a significant disk component, and irregular (Im) galaxies are in the opposite extreme, having small or insignificant bulges. The formation of these two components seems to follow different histories, so it is appropriate to count baryons in stars divided into these two categories. The mass density for each component is obtained as

(2)

where is the mean luminosity density, f_B is the fraction of the luminosity density produced by the spheroid or disk component, and <M/L_B> is the mass-to-light ratio for each component including the stars and star remnants. The suffix B refers to luminosities measured in the B band, our choice for a standard wavelength band. Although the stellar population of irregular galaxies is similar to that of the disk component, we treat the former separately to emphasize its distinct role in the luminosity density at low redshift and a slightly smaller mass-to-light ratio because of its younger mean age.