Frederic Bournaud, Stephanie Juneau, Emeric Le Floc'h, James Mullaney, Emanuele Daddi, Avishai Dekel, Pierre-Alain Duc, David Elbaz, Fadia Salmi, Mark Dickinson
We provide evidence for a correlation between the presence of giant clumps
and the occurrence of active galactic nuclei (AGN) in disk galaxies. Giant
clumps of 10^8-9 Msun are expected to arise from violent gravitational
instability in galaxies fed by rapid gas accretion, and it has been proposed
that this instability could feed supermassive black holes (BH). We use the
Mass-Excitation and Blue AGN diagrams to compare a sample of 14 clumpy,
unstable disks and a sample of 13 smoother, stable disks. The majority of
clumpy disks in our sample have a high probability (>50%) of containing AGN.
Their [Oiii]5007 emission line is strongly excited, inconsistent with
low-metallicity star formation, instead indicating Seyfert 2-type activity.
Stable disks rarely have such properties. Stacking ultra sensitive Chandra
observations (4Ms) reveals an X-ray excess in clumpy galaxies, which cannot be
solely due to star formation and confirms the presence of AGN much more
frequently than in stable disks. These AGN are probably substantially absorbed
but not necessarily Compton thick. The clumpy galaxies in our
intermediate-redshift sample have global properties typical of gas-rich disk
galaxies rather than mergers, which suggests that our findings also apply to
the physically-similar and numerous unstable disks at z~2. In this context, the
instability-driven BH feeding, with low average BH accretion rates (10^-3 to
10^-2 Msun/yr) and AGN luminosities (intrinsic Lx~10^42 erg.s^-1 with possible
bursts), could build up a large fraction of today's BH masses over a couple of
Gyr. Hence, violent instabilities at high redshift (giant clumps) are a much
more efficient driver of BH growth than the weak instabilities in nearby
spirals (bars). The observed evolution of violent instabilities with redshift
could explain the simultaneous downsizing of star formation and black hole
growth.
View original:
http://arxiv.org/abs/1111.0987
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