A simple quantitative model is presented for the history of galaxies to explain galaxy number counts, redshift distributions, and some other related observations. We first infer that irregular galaxies and the disks of spiral galaxies are young, probably formed at z ≈ 0.5-2, from a simultaneous consideration of colors and gas content under a moderate assumption on the star formation history. Assuming that elliptical galaxies and bulges of spiral galaxies, both called spheroids in the discussion, had formed early in the universe, the resulting scenario is that spiral galaxies formed as intergalactic gas accreting onto preexisting bulges mostly at z ≈ 1-2; irregular galaxies as seen today formed by aggregation of clouds at z ≈ 0.5-1.5. Taking the formation epochs thus estimated into account, we construct a model for the history of galaxies employing a stellar population synthesis model. We assume that the number of galaxies does not change, except that some of them (irregular galaxies) were newly born, and use a morphology-dependent local luminosity function to constrain the number of galaxies. We represent the galaxies by E/S0, Sab, Sc, and Irr; low-luminosity dwarfs or any objects unobservable today do not play a role in our considerations. In our model, spheroids follow passive evolution and the luminosity of spiral galaxies evolves only very slowly for a wide redshift interval due to a counterbalance between fading stars and new star formation from the gas replenished from intergalactic space. Irregular galaxies evolve moderately fast for z 1. The predictions of the model are compared with the observation of galaxy number counts and redshift distributions for the B, I, and K color bands. We show that K-band observations are largely controlled by spheroids, which make them particularly suitable to study cosmology. We argue that Ω = 1 models are disfavored, unless the basic assumptions of the present model are abandoned. The K-band observations reach quite high redshift: for instance, observations at K = 23 mag may explore the formation epoch, which could be as high as z 5. On the other hand, galaxies observed in the B band are dominated by disks and irregular galaxies, spheroids making a very small contribution. It is shown that young irregular galaxies cause the steep slope of the counts. The fraction of irregular galaxies increases with decreasing brightness: at B = 24 mag, they contribute as much as spiral galaxies. Thus, "the faint blue galaxy problem" is solved by invoking young galaxies. This interpretation is corroborated by a comparison of our prediction with the morphologically classified galaxy counts in the I band. We do not invoke sporadic star bursting: star formation takes place steadily as does today, but galaxies (especially irregular galaxies) are gaseous at higher redshift, and hence star formation is much more active than today. Consistency is also shown with the constraint on the luminosity evolution from a Mg II quasar absorption-line-selected sample. We estimate that two-thirds of the baryons in stars are stored in spheroids and one-third in disks, only less than 10% being in irregular galaxies. The amount of baryons in disk stars is increasing, since they form to Ωb ~ 0.001, which just offsets the decrease of neutral gas toward the present epoch, as inferred from quasar absorption-line surveys.
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