Black hole spins and gas dynamics

J1649+2635: 4th jet seen in a spiral galaxy. SDSS.

Yesterday I went to a talk by Massimo Dotti on linking the spin evolution of black holes to gas dynamics, and I thought it would be interesting to write about this.



How do BH spins evolve?
They evolve through

1.  BH mergers (new spin for the remnant black hole)
and
2.  gas accretion

Gas accretion may be a) coherent, when the angular momentum of the accreted gas is in the same plane as the spin of the black hole. It then fairly quickly results in a maximally spinning black hole. Or b) chaotic. Then the BH accretes small gas clouds of random orientation. This spins down the black hole. Or c) hybrid. The BH is most likely to suffer a combination of coherent and chaotic accretion. The spin is a vector that changes during the accretion process.

Why should we care about black hole spin?

1. Spin influences Black Hole (BH) growth - generally, high spin slows down accretion

2. Affects the occupation fraction of black holes in galaxies, e.g., a black hole may be kicked out of a galaxy - see Chandra observation of a black hole ejection.

3. encodes information about the fueling process of the black hole and the evolution of the host galaxy

4. May be linked to jet production. The jet could spin down the black hole. The process that produces the jet is not understood. It is believed that a fast spinning accretion disc produces a powerful magnetic field, which is in contact with the black hole. The spinning black hole drags the magnetic field, winding it in a cone. The jet extracts energy from the black hole spinning it down.

Matching observations of BH Spin (Sesana et al. 2014)

So far BH spin is measured in only spiral galaxies that are still accreeting today. The smaller spins have large errors. They are all measured via K-alpha line fitting. Overall, the picture is consistent with a hybrid gas dynamics as expected.

 What next?
 Observe more jets like J2345-0449 (Bagchi et al. 2014),  in spiral galaxies for fast spinning central black holes. Most jets are currently observed in elliptical galaxies. Measure the spin of more black holes. Launch LISA.

What about Pulsar Timing?
Pulsar timing arrays observe black holes that are, generally, too far from merger to constrain the spin of the final black hole. The spin of the individual black holes cannot be measured as far as I understand.

What about LIGO?

Stellar mass black holes also power relativistic jets, but the nature of the jets is likely different, where the jets are more likely to be powered by the disc and not by the spin of the black hole (e.g., Diaz Trigo et al. 2013). This remains to be tested as LIGO sees sources that can also be observed electromagnetically.