Synthetic bulge+disk model for a spiral galaxy of age 13.7 Gyr (a–f) and 4 Gyr (g–l) simulated out to z = 3. The input SEDs to the simulation take only stellar emission into account and assume zero intrinsic extinction and attenuation by the intergalactic medium. a: reduced surface brightness δμ′ (z) for a bulge of solar metallicity forming with an exponentially decreasing SFR for an e-folding timescale, τ, of 0.5 Gyr and 1 Gyr (solid and dashed curves, respectively). b: δμ′ (z) for a disk of Z_⊙/5 forming with a constant SFR. c: bulge-to-disk surface brightness contrast, δμ_BD (δμ′ (bulge)–δμ′ (disk)). d and e: ObsF color of the bulge and the disk. Values at z = 0 correspond to the true (rest-frame) color. f: bulge-to-disk color contrast, δC_BD (the color of the bulge minus the color of the disk). The layout of panels g–l is identical to that of panels a–f with the only difference being the addition of simulations for a 0.6 Gyr old stellar disk (dotted curves) in panels h and k.