  We have analyzed the redshift-dependent fraction of galactic bars over
0.2<z<0.84 in 2,157 luminous face-on spiral galaxies from the COSMOS 2-square
degree field. Our sample is an order of magnitude larger than that used in any
previous investigation, and is based on substantially deeper imaging data than
that available from earlier wide-area studies of high-redshift galaxy
morphology. We find that the fraction of barred spirals declines rapidly with
redshift. Whereas in the local Universe about 65% of luminous spiral galaxies
contain bars (SB+SAB), at z ~0.84 this fraction drops to about 20%. Over this
redshift range the fraction of strong (SB) bars drops from about 30% to under
10%. It is clear that when the Universe was half its present age, the census of
galaxies on the Hubble sequence was fundamentally different from that of the
present day. A major clue to understanding this phenomenon has also emerged
from our analysis, which shows that the bar fraction in spiral galaxies is a
strong function of stellar mass, integrated color and bulge prominence. The bar
fraction in very massive, luminous spirals is about constant out to z ~ 0.84
whereas for the low mass, blue spirals it declines significantly with redshift
beyond z=0.3. There is also a slight preference for bars in bulge dominated
systems at high redshifts which may be an important clue towards the
co-evolution of bars, bulges and black holes. Our results thus have important
ramifications for the processes responsible for galactic downsizing, suggesting
that massive galaxies matured early in a dynamical sense, and not just as a
result of the regulation of their star formation rate.
