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.