The role of bars in AGN fueling in disk galaxies over the last seven billion years

Mauricio Cisternas, Kartik Sheth, Mara Salvato, Johan H. Knapen, Francesca Civano, Paola Santini

Published 2014-09-09Version 1

We present empirical constraints on the influence of stellar bars on the fueling of active galactic nuclei (AGN) out to z=0.84 using a sample of X-ray-selected AGN hosted in luminous face-on disk galaxies from the Chandra COSMOS survey. Using high-resolution Hubble Space Telescope imaging to identify bars, we find that the fraction of barred active galaxies displays a similar behavior as that of inactive spirals, declining with redshift from 71% at z~0.3, to 35% at z~0.8. With active galaxies being typically massive, we compare them against a mass-matched sample of inactive spirals and find that the AGN bar fraction is higher, with this enhancement being marginal at z>0.4, but becoming more pronounced at low redshift. The presence of a bar has no influence on the AGN strength, with barred and unbarred active galaxies showing equivalent X-ray luminosity distributions, though barred galaxies on average seem to show higher levels of central star formation. From our results, we conclude that the role of bars is restricted to providing the suitable conditions for BH fueling to occur, i.e., bring a fresh supply of gas to the central 100 pc. Beyond this point, the efficiency of the fueling process is independent of the large scale structure of a galaxy. At the high-redshift end, roughly 60% of active disk galaxies are unbarred. We speculate this to be related with the dynamical state of disks at higher redshifts which inhibits bar formation: being more gas-rich and turbulent than local spirals, these conditions could lead to instabilities and inflows, potentially providing the fuel supply for these AGN.