The Role of Gravitational Instabilities in the Feeding of Supermassive Black Holes

Abstract

I review the recent progresses that have been obtained, especially through the use of high-resolution numerical simulations, on the dynamics of self-gravitating accretion discs. A coherent picture is emerging, where the disc dynamics is controlled by a small number of parameters that determine whether the disc is stable or unstable, whether the instability saturates in a self-regulated state or runs away into fragmentation, and whether the dynamics is local or global. I then apply these concepts to the case of AGN discs, discussing the implications of such evolution on the feeding of supermassive black holes. Nonfragmenting, self-gravitating discs appear to play a fundamental role in the process of formation of massive black hole seeds at high redshift ( 10–15) through direct gas collapse. On the other hand, the different cooling properties of the interstellar gas at low redshifts determine a radically different behaviour for the outskirts of the accretion discs feeding typical AGNs. Here the situation is much less clear from a theoretical point of view, and while several observational clues point to the important role of massive discs at a distance of roughly a parsec from their central black hole, their dynamics is still under debate.

Suppression of the accretion rate in thin discs around binary black holes

“…Our choice of αSS is equal to Farris et al (2014) and is consistent with the equivalent value extrapolated from MHD simulations by Shi et al (2012); Shi & Krolik (2015), although this result needs to be validated for thinner discs. Note that the main angular momentum transport process at ≈ 0.1 pc separation in an AGN disc might be associated with gravitational instabilities (Goodman 2003;Lodato 2012). For the thin discs in AGN, self-gravitating angular momentum transport is local and is expected to provide equivalent α ≈ 0.1 − 0.3, consistent with our choice (Cuadra et al 2009).…”

“…The black line in Figure 4 shows an interpolating function with a linear behaviour for H/R < 0.1 and constant ξ = 1 for H/R > 0.1. The typical values of H/R in AGN discs are believed to span H/R ∼ 10 −2 − 10 −3 (Shakura & Sunyaev 1973;Collin-Souffrin & Dumont 1990;Natarajan & Pringle 1998;Goodman 2003;Lodato 2012;Gerosa et al 2015) depending on the region of the disc examined and on the degree of self-gravity in the disc (Haiman et al 2009). If the law in equation ( 17) keeps holding also for these physical values of H/R, we expect to have ξ ∼ 0.1 − 0.01, implying that the accretion rate is suppressed up to a factor 10 2 with respect to the normal AGN activity.…”

How black holes stop their host galaxy from growing without AGN feedback

“…Star formation can only take place where the gas is unstable to gravitational collapse (Wang & Silk 1994). In this context the BH may also play an important role in the stability of the disc (Lodato 2012). The gas properties will change throughout the galaxy with some regions being more unstable than others.…”