Why are active galactic nuclei and host galaxies misaligned?

Philip F Hopkins,Lars Hernquist,Christopher C Hayward,Desika Narayanan

doi:10.1111/j.1365-2966.2012.21449.x

Abstract

It is well-established observationally that the characteristic angular momentum axis on small scales around AGN, traced by radio jets and the putative torus, is not well-correlated with the large-scale angular momentum axis of the host galaxy. In this paper, we show that such misalignments arise naturally in high-resolution simulations in which we follow angular momentum transport and inflows from galaxy to sub-pc scales near AGN, triggered either during galaxy mergers or by instabilities in isolated disks. Sudden misalignments can sometimes be caused by single massive clumps falling into the center slightly off-axis, but more generally, they arise even when the gas inflows are smooth and trace only global gravitational instabilities. When several nested, self-gravitating modes are present, the inner ones can precess and tumble in the potential of the outer modes. Resonant angular momentum exchange can flip or re-align the spin of an inner mode on a short timescale, even without the presence of massive clumps. We therefore do not expect that AGN and their host galaxies will be preferentially aligned, nor should the relative alignment be an indicator of the AGN fueling mechanism. We discuss implications of this conclusion for AGN feedback and BH spin evolution. The misalignments may mean that even BHs accreting from smooth large-scale disks will not be spun up to maximal rotation, and so have more modest radiative efficiencies and inefficient jet formation. Even more random orientations are possible if there is further, un-resolved clumpiness in the gas, and more ordered accretion may occur if the inflow is slower and not self-gravitating.

Highlights

Understanding accretion is critical for inferring the origin of the supermassive black hole (BH) population (Soltan 1982; Salucci et al 1999; Shankar et al 2004; Hopkins, Narayan & Hernquist 2006b)
One critical long-standing puzzle is the consistent observational finding that there is little or no correlation between the angular momentum axis of material accreting on to the BH and the axis of the host galaxy. This has been observed with a number of different tracers, e.g. radio jets or obscuring active galactic nuclei (AGN) ‘torii’ defining the plane along which material flows into the inner accretion disc
Feedback is assumed to generate a non-thermal sound speed that depends on the local star formation rate, and the gas density; the results shown span a wide range in this ‘effective sound speed’ without any strong dependence on the exact value

Summary

Introduction

Understanding accretion is critical for inferring the origin of the supermassive black hole (BH) population (Soltan 1982; Salucci et al 1999; Shankar et al 2004; Hopkins, Narayan & Hernquist 2006b). Models widely invoke some form of feedback from AGN to explain the origin of the BH–host relations, rapid quenching of star formation in bulges, the colour–magnitude relation and the cooling flow problem (e.g. Silk & Rees 1998; King 2003, 2005; Di Matteo, Springel & Hernquist 2005; Springel, Di Matteo & Hernquist 2005; Hopkins et al 2008; Hopkins & Elvis 2010; Croton et al 2006, and references therein). This has been observed with a number of different tracers, e.g. radio jets (expected to align with the axis of the BH spin or inner accretion disc, but see Natarajan & Pringle 1998) or obscuring AGN ‘torii’ defining the plane along which material flows into the inner accretion disc (see e.g. Keel 1980; Lawrence & Elvis 1982; Ulvestad & Wilson 1984; Schmitt et al 1997; Simcoe et al 1997; Kinney et al 2000; Gallimore et al 2006; Zhang et al 2009). The nuclear disc is misaligned with the larger scale disc/galaxy inflows; but the latter must be the origin of the former, so this is not trivially expected

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Conclusion