Bar Evolution over the Last 8 Billion Years: A Constant Fraction of Strong Bars in the GEMS Survey

One-third of present-day spirals host optically visible strong bars that drive their dynamical evolution. However, the fundamental question of how bars evolve over cosmological times has yet to be resolved, and even the frequency of bars at intermediate redshifts remains controversial. We investigate the frequency of bars out to z ~ 1 drawing on a sample of 1590 galaxies from the Galaxy Evolution from Morphologies and SEDs survey, which provides morphologies from Hubble Space Telescope Advanced Camera for Surveys (ACS) two-band images and accurate redshifts from the COMBO-17 survey. We identify spiral galaxies using three independent techniques based on the Sersic index, concentration parameter, and rest-frame color. We characterize bar and disk features by fitting ellipses to F606W and F850LP images, using the two bands to minimize shifts in the rest-frame bandpass. We exclude highly inclined (i>60^deg) galaxies to ensure reliable morphological classifications and apply different completeness cuts of M_V<=-19.3 and -20.6. More than 40% of the bars that we detect have semimajor axes a<0.5" and would be easily missed in earlier surveys without the small point-spread function of ACS. The bars that we can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in the range ~1.2-13 kpc. We find that the optical fraction of such strong bars remains at ~30%+/-6% from the present day out to look-back times of 2-6 Gyr (z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0) it certainly shows no sign of a drastic decline at z>0.7. Our findings of a large and similar bar fraction at these three epochs favor scenarios in which cold gravitationally unstable disks are already in place by z~1 and where on average bars have a long lifetime (well in excess of 2 Gyr). The distributions of structural bar properties in the two slices are, however, not statistically identical and therefore allow for the possibility that the bar strengths and sizes may evolve over time.