The erratic dynamical life of black hole seeds in high-redshift galaxies

Abstract

The dynamics of black hole seeds in high redshift galaxies is key to understand their ability to grow via accretion and to pair in close binaries during galactic mergers. To properly follow the dynamics of black holes we develop a physically motivated model to capture unresolved dynamical friction from stars, dark matter and gas. We first validate the model and then we use it to investigate the dynamics of seed black holes born at z ∼ 9 in dwarf proto-galaxies. We perform a suite of zoom cosmological simulations with spatial resolution as high as 10 pc and with a stellar and dark matter mass resolution of 2×10 3 M and 2×10 5 M respectively. We first explore the dynamics of a seed black hole in the galaxy where it is born and show that it is highly erratic if the seed mass is less than 10 5 M . The dynamics is dominated by the stellar component, whose distribution is irregular and patchy, thus inducing stochasticity in the orbits: the black hole may be anywhere in the proto-galaxy. When this dwarf merges into a larger galaxy, it is paramount to simulate the process with very high spatial and mass resolution in order to correctly account for the stripping of the stellar envelope of the satellite black hole. The outcome of the encounter could be either a tight binary or, at least temporary, a wandering black hole, leading to multiple black holes in a galaxy, each inherited from a different merger.

Unveiling early black hole growth with multifrequency gravitational wave observations

“…Callegari et al 2009;Fiacconi et al 2013;Capelo et al 2015;Tamburello et al 2017;Pfister et al 2017;Tamfal et al 2018, and references therein). In zoomed-in high redshift (z ∼ 9) simulations of dwarf protogalaxies, dynamical friction against stars is found to be the main process of BH orbital decay for ∼ 10 5 M ⊙ seeds, while erratic dy-namics is seen below this mass, implying either rapid decay or BH wandering/ejection and the presence of multiple BHs in a galaxy, each inherited from a different merger (Pfister et al 2019). Interestingly, at high redshifts (z > 6) and for BHs of ∼ 10 6 M ⊙ , global or bar-induced torques in some cases appear to be more efficient than dynamical friction in promoting BH binary formation on timescales comparable to the local Hubble time at those redshift (Bortolas et al 2020).…”

Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function

“…Tremmel et al 2015;Pfister et al 2017). For this, we follow the approach of Pfister et al (2019) to model the dynamical friction exerted both by the gas and by the collisionless particles (stars and DM), which we do not resolve directly in our simulation. For the dynamical friction exerted by the gas, the matter lagging behind the BH induces a drag force (Ostriker 1999), that we model following Dubois et al (2013).…”

“…In this work, we take ρ DF,th = 50 cm −3 . For the dynamical friction caused by the collisionless particles (stars and DM), we use the implementation of Pfister et al (2019): the (negative) acceleration of the gas is again caused by matter lagging behind the BH, and is a function of the BH mass, velocity, and of the detailed distribution of stars and DM within 4∆x of the BH. We note that the implementation for collisionless particles is similar to that of Tremmel et al (2015).…”

The lifetime of binary black holes in Sérsic galaxy models

“…The black hole dynamics is by far more complex to describe, as halos are non-relaxed systems and subjected to repeated, multiple interactions. Gas clumps can make the dynamics stochastic, and star formation and feedback can change the underlying background, broadening significantly the lifetime distribution for the binaries in the LISA-relevant mass range (Bellovary et al 2019;Pfister et al 2019). Delay times between halo-halo mergers and black hole mergers can be computed using semi-analytical models which include also the formation of triple systems as a vehicle for the formation and coalescence of these light black holes (Bonetti et al 2019).…”