THE HISTORY OF GALAXIES AND GALAXY NUMBER COUNTS

A simple quantitative model is presented for the history of galaxies t o 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 approximate t o 0.5-2, from a simultaneous consideration of colors and gas content u nder a moderate assumption on the star formation history. Assuming tha t elliptical galaxies and bulges of spiral galaxies, both called spher oids in the discussion, had formed early in the universe, the resultin g scenario is that spiral galaxies formed as intergalactic gas accreti ng onto preexisting bulges mostly at z approximate to 1-2; irregular g alaxies as seen today formed by aggregation of clouds at z approximate to 0.5-1.5. Taking the formation epochs thus estimated into account, we construct a model for the history of galaxies employing a stellar p opulation synthesis model. We assume that the number of galaxies does not change, except that some of them (irregular galaxies) were newly b orn, and use a morphology-dependent local luminosity function to const rain the number of galaxies. We represent the galaxies by E/SO, Sab, S c, 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 betwe en fading stars and new star formation from the gas replenished from i ntergalactic space. Irregular galaxies evolve moderately fast for z < 1. The predictions of the model are compared with the observation of g alaxy number counts and redshift distributions for the B, I, and K col or bands. We show that K-band observations are largely controlled by s pheroids, which make them particularly suitable to study cosmology. We argue that Omega = 1 models are disfavored, unless the basic assumpti ons of the present model are abandoned. The K-band observations reach quite high redshift: for instance, observations at K = 23 mag may expl ore 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 irre gular galaxies, spheroids making a very small contribution. It is show n that young irregular galaxies cause the steep slope of the counts. T he fraction of irregular galaxies increases with decreasing brightness : at B = 24 mag, they contribute as much as spiral galaxies. Thus, ''t he faint blue galaxy problem'' is solved by invoking young galaxies. T his interpretation is corroborated by a comparison of our prediction w ith 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 gaseo us at higher redshift, and hence star formation is much more active th an today. Consistency is also shown with the constraint on the luminos ity evolution from a Mg II quasar absorption-line-selected sample. We estimate that two-thirds of the baryons in stars are stored in spheroi ds and one-third in disks, only less than 10% being in irregular galax ies. The amount of baryons in disk stars is increasing, since they for m to Omega(b) similar to 0.001, which just offsets the decrease of neu tral gas toward the present epoch, as inferred from quasar absorption- line surveys.