Galaxy Zoo: Observing Secular Evolution Through Bars
  In this paper, we use the Galaxy Zoo 2 dataset to study the behavior of bars
in disk galaxies as a function of specific star formation rate (SSFR), and
bulge prominence. Our sample consists of 13,295 disk galaxies, with an overall
(strong) bar fraction of $23.6\pm 0.4\%$, of which 1,154 barred galaxies also
have bar length measurements. These samples are the largest ever used to study
the role of bars in galaxy evolution. We find that the likelihood of a galaxy
hosting a bar is anti-correlated with SSFR, regardless of stellar mass or bulge
prominence. We find that the trends of bar likelihood and bar length with bulge
prominence are bimodal with SSFR. We interpret these observations using
state-of-the-art simulations of bar evolution which include live halos and the
effects of gas and star formation. We suggest our observed trends of bar
likelihood with SSFR are driven by the gas fraction of the disks; a factor
demonstrated to significantly retard both bar formation and evolution in
models. We interpret the bimodal relationship between bulge prominence and bar
properties as due to the complicated effects of classical bulges and central
mass concentrations on bar evolution, and also to the growth of disky
pseudobulges by bar evolution. These results represent empirical evidence for
secular evolution driven by bars in disk galaxies. This work suggests that bars
are not stagnant structures within disk galaxies, but are a critical
evolutionary driver of their host galaxies in the local universe ($z<1$).
