Two-phase model for black hole feeding and feedback

MNRAS 437, 2404–2411 (2014) doi:10.1093/mnras/stt2059 Advance Access publication 2013 November 26 Two-phase model for black hole feeding and feedback Sergei Nayakshin‹ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK Accepted 2013 October 22. Received 2013 September 24; in original form 2013 June 7 ABSTRACT Key words: accretion, accretion discs – black hole physics – stars: formation – galaxies: evolution – quasars: general. 1 I N T RO D U C T I O N We first review the current state of the analytical active galactic nucleus (AGN) feedback models in Section 1.1, and their relation to the observations. We then discuss two important challenges to these models that arose recently from microphysics of shocks, observations and numerical simulations in Sections 1.2 and 1.3. The scope of this paper and a brief description of the solution to these challenges are discussed in Section 1.5. 1.1 Spherically symmetric models of AGN feedback 1.1.1 Observations and energy-driven feedback The mass Mbh of supermassive black holes (SMBH) residing in the centres of many galaxies is observed to correlate strongly with properties of the host. For example, Mbh  1.5 × 4 M (Ferrarese & Merritt 2000; Gebhardt et al. 2000), where 108 σ200 σ 200 = σ /200 km s−1 and σ is the one-dimensional velocity dispersion of the stars in the host, σ = (GMbulge /2Rb )1/2 , where Mbulge and Rb are the bulge mass and the effective radius, respectively. Furthermore, Mbh  1.6 × 10−3 Mbulge (Häring & Rix 2004), although a more recent census of classical bulge systems show a higher Mbh /Mbulge ratio by a factor of a few (Kormendy & Ho  E-mail: 2013). More recent observations show correlations of Mbh with other properties of the host (Graham 2004; Ferrarese et al. 2006; Cattaneo et al. 2009). Barred galaxies show underweight black holes (Graham 2008; Hu 2008; Kormendy, Bender & Cornell 2011), possibly indicating that SMBH growth is fuelled not by planar inflows but rather by a ‘direct’ deposition of cold clouds from the bulge (Nayakshin, Power & King 2012). Pre-dating these observations, Silk & Rees (1998) envisioned that SMBH may influence their host galaxies strongly despite being a tiny fraction of the total mass. They showed that energy-conserving outflows from growing SMBH could expel all the gas in the host galaxy, terminating SMBH and galaxy growth. This model assumes that primary outflow from the SMBH does not cool when shocked in the interaction with the ambient gas. Quantitatively, however, the theory predicts Mbh ∝ σ 5 and requires a surprisingly inefficient coupling between the power output of SMBH and the host. Let us write the energy passed from the outflow to the gas in the host as e Mbh c2 . Requiring e Mbh c2 ∼ fg Mbulge σ 2 , and fg ∼ 0.1 is the fractional mass of the gas in the bulge, we find that e ∼ 5 × 10−5 to yield Mbh ∼ 10−3 Mbulge at σ = 200 km s−1 . Such inefficiency is puzzling. For comparison, the radiative power output of SMBH gives efficiency of the order of  r ∼ 0.1 (Shakura & Sunyaev 1973). In fact, the recent study of Kormendy & Ho (2013) excluded pseudobulges and systems currently undergoing mergers from the sample, focusing only on the classical bulges, and obtained the SMBH to bulge mass ratio of ∼0.005 rather than ∼0.0015 favoured by earlier studies. This further lowers the estimate of the energy coupling  C 2013 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society We study effects of active galactic nucleus (AGN) feedback outflows on multiphase inter stellar medium (ISM) of the host galaxy. We argue that supermassive black hole (SMBH) growth is dominated by accretion of dense cold clumps and filaments. AGN feedback outflows overtake the cold medium, compress it, and trigger a powerful starburst – a positive AGN feedback. This predicts a statistical correlation between AGN luminosity and star formation rate at high luminosities. Most of the outflow’s kinetic energy escapes from the bulge via low-density voids. The cold phase is pushed outward only by the ram pressure (momentum) of the outflow. The combination of the negative and positive forms of AGN feedback leads to an M−σ relation similar to the result of King. Due to porosity of cold ISM in the bulge, SMBH influence on the low density medium of the host galaxy is significant even for SMBH well below the M−σ mass. The role of SMBH feedback in our model evolves in space and time with the ISM structure. In the early gas rich phase, SMBH accelerates star formation in the bulge. During later gas poor (red-and-dead) phases, SMBH feedback is mostly negative everywhere due to scarcity of the cold ISM. Two-phase model for feedback between the UFO and the bulge to e ∼ 2 × 10−5 . In contrast, numerical simulations reproducing the observed correlations require efficiencies of the order of e ∼ 5 × 10−3 (Di Matteo, Springel & Hernquist 2005). Concluding, it appears that energy-driven feedback models simply produce too much energy in the outflow; these models must invoke, somewhat arbitrarily, a tiny energy coupling factor to the bulge, e ∼ (2−5) × 10−5 . It is not clear why this factor would be constant from one galaxy to another, and therefore why a tight correlation between Mbh and Mbulge (Kormendy & Ho 2013) would exist at all in this framework. King (2003) proposed a more detailed AGN feedback model which is able to account naturally for most of the relevant observations to date. In this model, SMBH outflows start from the innermost region of the accretion discs, and escape the region at velocity comparable to the local escape velocity, e.g. v out ∼ 0.1c. Such outflows were actually observed in quasar PG 1211+143 (King & Pounds 2003; Pounds et al. 2003). The outflows carry a momentum flux Ṁvout ∼ LEdd /c, comparable to the escaping radiation momentum flux when the SMBH luminosity is at the Eddington limit, LEdd . Pleasingly, the kinetic energy carried by the ultrafast outflow (UFO) is (v out /2c)LEdd , which is equivalent to  ∼ 5 × 10−3 , naturally accounting for the empirical results of Di Matteo et al. (2005). Furthermore, observational support for widespread existence of and a significant power carried by the UFOs has since become available (e.g. Tombesi et al. 2010a,b; Pounds & Vaughan 2011). The key characteristic of this model is that it operates in both momentum-conserving and energy-conserving regimes at different times. King (2003) has shown that close to the SMBH, shocked UFO wind suffers significant Inverse Compton (IC) losses on the AGN radiation field. Equalling the IC cooling time to the flow time, 1/2 one finds the IC cooling radius, Ric ∼ 0.5 kpc M8 σ200 , where 8 M8 = Mbh /10 M (King, Zubovas & Power 2011). The outflow 2 , within R  Ric , is losing most of its kinetic energy, (1/2)Ṁvout and is in the momentum-conserving regime. The ambient gas in this regime is affected mainly by the physical push from the UFO. C (...truncated)