  We studied the chronology of galactic bulge and disc formation by analysing
the relative contributions of these components to the B-band rest-frame
luminosity density at different epochs. We present the first estimate of the
evolution of the fraction of rest-frame B-band light in galactic bulges and
discs since redshift z~0.8. We performed a bulge-to-disc decomposition of
HST/ACS images of 3266 galaxies in the zCOSMOS-bright survey with spectroscopic
redshifts in the range 0.7 < z < 0.9. We find that the fraction of B-band light
in bulges and discs is $(26 \pm 4)%$ and $(74 \pm 4)%$, respectively. When
compared with rest-frame B-band measurements of galaxies in the local Universe
in the same mass range ($10^{9} M_{\odot}\lessapprox M \lessapprox 10^{11.5}
M_{\odot}$), we find that the B-band light in discs decreases by ~30% from
z~0.7-0.9 to z~0, while the light from the bulge increases by ~30% over the
same period of time. We interpret this evolution as the consequence of star
formation and mass assembly processes, as well as morphological transformation,
which gradually shift stars formed at half the age of the Universe from
star-forming late-type/irregular galaxies toearlier types and ultimately into
spheroids.
