Galaxy Zoo: Evolution of the bar fraction over the last eight billion years from HST COSMOS

Thomas Melvin (ICG, Portsmouth)

We measure the redshift evolution of the bar fraction in a sample of 2,380 visually selected disk galaxies found in Cosmic Evolution Survey (COSMOS) Hubble Space Telescope (HST) images. The visual classifications used to identify both the disk sample, and the presence of stellar bars were provided by citizen scientists via the Galaxy Zoo: Hubble (GZH) project. We find that the overall bar fraction increases by a factor of 2, from 11±2% at z=1.0 to 22±5% at z=0.4. Splitting our volume limited sample of massive disk galaxies into three bins of stellar mass, we find that the increase in bar fraction is most prominent in the highest mass bin, while the lower mass discs in our sample show a more modest evolution. Our results are consistent with a picture in which massive disc galaxy evolution begins to be affected by slow (secular) internal process at z=1.

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