Wavelength and Redshift Dependence of Bulge/Total Light Ratios in   Galaxies
  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. Models are presented 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 that bulge-to-total ($=$ B/T) light ratios for local galaxies increase significantly from U through I-bands with the rate of increase slightly depending on the relative ages of the two components. Hence, simultaneous decomposition of galaxy profiles in several bands can give direct information about the relative ages of bulges and disks and help constrain galaxy formation scenarios. The redshift evolution of B/T-ratios in various bands U, B, V, I, H is calculated accounting both for cosmological and evolutionary corrections. 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 ${\rm z=0}$ to ${\rm z \gta 1}$ but still are shown to increasingly overestimate B/T ratios at higher redshifts. This is a robust result irrespective of the respective ages of the bulge and disk stellar components and implies that the scarcity of bulge-strong systems at ${\rm z \geq 0.8}$ reported for HDF and Hawaiian Deep Field galaxies is further enhanced. 
Jochen Schulz, Uta Fritze-v. Alvensleben, Klaus J. Fricke   (Universit\"atssternwarte, G\"ottingen, Germany)