HST has opened the possibility to decompose the surface brightness profiles of galaxies up to significant redshifts and look-back times into { r ^ { 1 / 4 } - } bulge and exponential disk components .
This should allow to study the redshift evolution of bulge and disk luminosity contributions and discriminate between the different formation scenarios for these galaxy components currently discussed , i.e .
decide if star formation in bulges and disks started at the same time or was delayed in either of the two components .
An indispensable prerequisite for the comparison of bulge-to-disk ratios of galaxies at different redshifts is to properly account for cosmological band shift and evolutionary effects . We present evolutionary synthesis models for both components and add their spectra in various proportions to obtain the full range of local galaxies ’ B-band bulge-to-total light ratios .
Bulge star formation is assumed to occur on a short timescale of 10 ^ { 9 } yr , disk star formation proceeds at a constant rate .
We study the evolution of the relative light contributions of both components backward in time and , for a given cosmological model , as a function of redshift .
This allows us to see how far back into the past the locally well-established correlation between galaxy morphologies and spectral properties can hold .
To cope with the present uncertainty about the formation epochs of bulge and disk components we present models for three scenarios : bulges and disks of equal age , old bulges and delayed disk star formation , and old disks with subsequent bulge star formation . We quantitatively show the wavelength dependence of bulge-to-total ( = B/T ) light ratios for local galaxies .
The different star formation timescales for bulge and disk components lead to B/T ratios that significantly increase from U through I-bands ( by factors 4 – 6 for weak bulge systems \sim Sc ) with the rate of increase slightly depending on the relative ages of the two components . The redshift evolution of B/T-ratios in various bands U , B , V , I , H is calculated accounting both for cosmological and evolutionary corrections assuming a standard cosmology ( { H _ { 0 } = 65 ,~ { } \Omega _ { 0 } = 0.1 ,~ { } \Lambda _ { 0 } = 0 } ) .
In particular , for the two scenarios with old bulges and old or younger disks , the redshift evolution of B/T-ratios is dramatic in every band and both for galaxies ending up at { z \sim 0 } with low and high B-band B/T light ratios .
Our results clearly show that it does not make any sense to compare B/T ratios measured in one and the same band for galaxies at different redshifts without fully accounting for evolutionary and cosmological effects .
These , unfortunately , significantly depend on the relative ages of the two components and , hence , on the galaxy formation scenario adopted .
We also show that simultaneous decomposition of galaxy profiles in several bands can give direct information about these relative ages and constrain formation scenarios for the different galaxy components . Of the wavelength bands we explore ( U , B , V , I , H ) , the I- and H-bands show the smoothest redshift evolution and , hence , are best suited for a first order comparison of galaxies over the redshift range from { z = 0 } to { z \stackrel { > } { \scriptstyle \sim } 1 } .
Our robust result that – irrespective of the respective ages of the bulge and disk stellar components – I-band B/T-ratios apparently increase with increasing redshift for all galaxy types with present B/T > 0.1 implies that the scarcity of bulge-strong systems at { z \geq 0.8 } reported by and for HDF and Hawaiian Deep Field galaxies is further enhanced .