Whats The Importance Of Black Hole Repositioning? (Cosmology)

Based on a suite of smoothed particle hydrodynamics simulations with the swift code and a Bondi-Hoyle-Lyttleton subgrid gas accretion model, Yannick Bahe and colleagues investigated the impact of repositioning on SMBH growth and on other baryonic components through AGN feedback. They found that repositioning has direct physical consequences such as it promotes SMBH mergers and thus accelerates their initial growth. In addition, it raises the peak density of the ambient gas and reduces the SMBH velocity relative to it, giving a combined boost to the accretion rate that can reach many orders of magnitude. Their study recently appeared in Arxiv.

Energy feedback from active galactic nuclei (AGN) that are powered by supermassive black holes (SMBHs) at the centres of massive galaxies is an essential ingredient of galaxy formation simulations. The orbital evolution of SMBHs is affected by dynamical friction that cannot be predicted self-consistently by contemporary simulations of galaxy formation in representative volumes. Instead, such simulations typically use a simple “repositioning” of SMBHs, but the effects of this approach on SMBH and galaxy properties have not yet been investigated systematically.

Now, Yannick Bahe and colleagues investigated the impact of repositioning on SMBH growth by using hydrodynamical simulations and a Bondi-Hoyle-Lyttleton subgrid gas accretion model.

They showed that, across at least a factor ∼1000 in mass resolution, SMBH repositioning (or an equivalent approach) is a necessary prerequisite for AGN feedback; without it, black hole growth is negligible. They also showed that, limiting the effective repositioning speed to ≲ 10 km s¯1 delays the onset of AGN feedback and severely limits its impact on stellar mass growth in the centre of massive galaxies.

Projected SMBH positions (circles, colour-coded by 𝑚BH) are plotted over the corresponding gas density-temperature map (brightness representing surface density and hue temperature, increasing from pink to yellow) of the most massive 𝑧 = 0 halo (𝑀200c = 1.7 × 10¹³ M) in the 25 Mpc eagle-resolution simulations with the Default (left) and NoRepositioning (centre) models, respectively. © Yannick Bahe et al.

Finally, they shed light on three mechanisms through which repositioning affects SMBH growth, and hence AGN feedback. Firstly, it enables SMBH mergers – which lead to higher SMBH masses and hence increase the Bondi-Hoyle-Lyttleton accretion rate. Secondly, it moves SMBHs to regions of higher gas density, by up to several orders of magnitude. Thirdly, it (indirectly) slows SMBHs down by an order of magnitude with respect to their ambient gas.

“Our results suggest that a more sophisticated and/or better calibrated treatment of SMBH repositioning is a critical step towards more predictive galaxy formation simulations.”, they conclude.


Reference: Yannick M. Bahé, Joop Schaye, Matthieu Schaller, Richard G. Bower, Josh Borrow, Evgenii Chaikin, Folkert Nobels, Sylvia Ploeckinger, “The importance of black hole repositioning for galaxy formation simulations”, Arxiv, 2021. https://arxiv.org/abs/2109.01489


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