Wednesday, August 16, 2006

G.L. Granato on "Mutual Feedback between SF and nuclear activity"

More on feedback, but while the last talk was mainly about feedback from stars, there are also AGN, i.e. supermassive black holes (SMBH) in the centers of galaxies, that radiate very brightly, when mass falls into them.

There are beautiful simulations by Di Matteo and Springel from last year, where they compare the merging of two disk galaxies with and without feedback from the quasar and find it to differ significantly. Text, images and movies here.

The AGN feedback mechanism can at the same time explain the relation between the mass of the black hole and the bulge of the galaxy (because it's the feedback that shuts down both BH-growth and SF at the same time) and the quick reddening of galaxies because the decline in SF is so quick.

As in stellar feedback, it is not clear how exacly and how much of the engergy is deposited in the gas. Radiation pressure, heating and kinematical flows are the candidates.

Much SF promotes the formation of the SMBH by producing the seed black holes and also by pushing material into it. It takes around 500 Myr in Granatos models for the SMBH to form and feedback from it to kick in and have an effect on the surroundings, cleaning it from gas. This is what we see as quasars at high (but not highest) redshifts and this phase is rather short. What is left afterwards is a "red and dead" galaxy that does no longer form stars.

As main conclusion, Granato stresses the mutual link between SF and the AGN that has to be included into models. The comparisons of his models with observations were in impressive agreement, but I have no idea how many free parameters are fitted here. This was just answered in the question-session: it's around 6-7.

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