Wavelength and Redshift Dependence of Bulge/Total Light Ratios in Galaxies

HST has opened the possibility to decompose the surface brightness profiles of galaxies up to significant redshifts and look-back times into r− 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 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 ∼ 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 (H0 = 65, Ω0 = 0.1, Λ0 = 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 ∼ 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 Iand 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 ∼ 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 ≥ 0.8 reported by Marleau & Simard (1998) and Aguerri & Trujillo (2002) for HDF and Hawaiian Deep Field galaxies is further enhanced.