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 \approx 0.5 - 2 from a simultaneous consideration of colours 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 pre-existing bulges mostly at z \approx 1 - 2 ; irregular galaxies as seen today formed by aggregation of clouds at z \approx 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 ( irregulars ) 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 colour bands .
We show that K band observations are largely controlled by spheroids , which make them particularly suitable to study cosmology .
We argue that \Omega = 1 models are disfavoured , 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 irregulars , 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 irregulars ) 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 2/3 of the baryons in stars are stored in spheroids and 1/3 in disks , only < 10 % being in irregular galaxies .
The amount of baryons in disk stars is increasing , as they form to \Omega _ { b } \sim 0.001 , which just offsets the decrease of neutral gas towards the present epoch , as inferred from quasar absorption line surveys .