Literatures for AGN feedback w/ Hot accretion subgrid model


Sub-grid Model

  • Smith, Sijacki, Shen+17
    • This ideally needed dynamic range is beyond the reach of current state-of-the-art simulations, requiring the representation of the effects of unresolved processes by adopting so-called ‘sub-grid’ schemes. For large scale cosmolog- ical simulations, where the interstellar medium (ISM) is poorly resolved, these schemes must rely on dealing with stellar feedback at a high level of abstraction.
    • if the target of a simulation is a single galaxy, either in an idealised, isolated setup or in a cosmological ‘zoom-in’, then the higher resolution available enables the adoption of more explicit models of feedback, allowing investigations of how feedback arises on comparatively smaller scales to be carried out.


  • Wurster & Thacker 13
    • highest star formation rates (SFRs) and the activity from active galactic nuclei (AGNs) to occur in these most massive galaxies. However, observational evidence contradicts this, showing that in massive galaxies, the peak SFRs and peak AGN activity occurred at redshifts 1–2 (e.g. Madau et al. 1996; Shaver et al. 1996), and not today. This reduction in activity from z ∼ 2 to today was termed ‘downsizing’ by Cowie et al. (1996).One favoured explanation of downsizing is that during mergers, gas from the merger fuels both star formation and AGN activity (e.g. Sanders et al. 1988; Scannapieco, Silk & Bouwens 2005); the feedback from the increased AGN activity then blows away all the gas, leading to a red and dead galaxy (e.g. Springel, Di Matteo & Hernquist 2005)

Central SMBH & Mass Accretion

  • Almeida & Ricci 17
    • Over the past decades several pieces of observational evidence have shown that supermassive black holes (SMBHs; $M_{BH} \sim 10^{6−9.5}\,M_{\odot}$) are found at the center of almost all massive galaxies, and that those SMBHs play an important role in the evolution of their host galaxies1 during a phase in which they are accreting material and are observed as active galactic nuclei (AGN).
  • Barai, Gallerani, Pallottini+17
    • Active galactic nuclei (AGN) are believed to host supermassive black holes (SMBHs) at their centers (e.g., Rees 1984; Kormendy & Richstone 1995; Ferrarese & Ford 2005).
  • Cruz-Osorio, Sanchez-Salcedo, Lora-Clavijo 17
    • The Event Horizon Telescope (EHT)1 will be able to resolve the BH structures on event horizon scales, which may be crucial not only to test General Relativity (Broderick et al. 2014) but also to study accretion processes very close to the BH event horizon (Ricarte & Dexter 2015)
    • There are many ingredients that can change the mass accretion rate on to the BH with respect to the canonical value given in Equation (1) (Bondi Solution): the physics of stellar feedback, AGN feedback, the presence of small-scale structure, turbulence and magnetic fields in the multiphase interstellar medium, accretion-disc winds, the accretion of stars, and so on (e.g., Hopkins et al. 2016)

Galaxy Model

  • Gan+14: the total density distributions of ellipticals can usually be well described by an $r^{−2}$ profile over a large radial range (Rusin & Kochanek 2005; Czoske et al. 2008; Dye et al. 2008).
  • Talk by Dandan Xu: radial mass density profile almost isothermal: $\rho \sim r^{−2}$ (Koopmans+06; Auger+10; Sonnenfeld+13)

Secular Evolution

  • Suh+17:
    • the growth of the majority of star-forming galaxies have been regulated more by internal secular processes rather than by merger process (e.g. Elbaz et al. 2011; Rodighiero et al. 2011; Wuyts et al. 2011).
    • mergers do not dominate the triggering of AGN activity, at least for moderate-luminosity AGNs. Allevato et al. (2011, 2012, 2016) further point out that moderate-luminosity AGNs inhabit group-sized halos ($10^{13−13.5} M_{\odot}$), almost independent of redshift up to z ~ 5. This also implies that major mergers cannot be the main driver of the evolution of AGNs.
  • Du+17:
    • BH growth is unlikely to have been driven by significant mergers, at least since redshift z ∼ 2 (Gabor et al. 2009; Georgakakis et al. 2009; Cisternas et al. 2011; Schawinski et al. 2011; Kocevski et al. 2012; Fan et al. 2014)
  • Kocevski, Faber, Mozena+12
    • Galaxy mergers have long been espoused as a possible fueling mechanism given their effectiveness in dissipating angular momentum and funneling gas to the center of galaxies (Barnes & Hernquist 1991; Mihos & Hernquist 1996). This can drive both accretion onto the SMBH and growth of the stellar bulge, which would help explain the tight correlations observed between the two (e.g., Gebhardt et al. 2000; Ferrarese & Merritt 2000; Marconi & Hunt 2003; Haring & Rix 2004). … major galaxy mergers provide an attractive mechanism to both trigger AGN activity and help explain the coevolution observed between SMBHs and their hosts (Hopkins et al. 2008).
    • Dunlop et al. (2003) find that QSOs at z ∼ 0.2 are no more likely to exhibit structural disturbances when compared to a control sample of similar non-active galaxies
    • Grogin et al. (2005) and Pierce et al. (2007), using data from the Great Observatories Origins Deep Survey (GOODS;Giavalisco et al. 2004) and the All-wavelength Extended Groth strip International Survey (AEGIS; Davis et al. 2007), respectively, find that host galaxies at z ∼ 1 do not show disturbances or interaction signatures more often than their quiescent counterparts (see also Sanchez et al. 2004)
    • Gabor et al. (2009) and Cisternas et al. (2011) examined the host morphologies of AGNs selected in the Cosmic Evolution Survey (COSMOS; Scoville et al. 2007) and report that the disturbed fraction among active and quiescent galaxies at z ∼ 1 is not significantly different. Instead they find that a majority of AGNs at this redshift are hosted by disk galaxies that do not show strong distortions.
    • Schawinski et al. (2011) recently extended this work to z ∼ 2 by examining the light profiles of a relatively small number of AGNs in a portion of the GOODS-S field. They report that a majority of host galaxies at this redshift have morphologies best fit by low Sersic indices indicative of disk-dominated galaxies and suggest that the bulk of SMBH growth since z ∼ 2 must be driven by secular processes and not major mergers.
    • the hosts of moderate-luminosity AGNs are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z ∼ 2.
  • Di Matteo, Springel, Hernquist 05, Nature
    • we report simulations that simultaneously follow star formation and the growth of black holes during galaxy–galaxy collisions. We find that, in addition to generating a burst of star formation, a merger leads to strong inflows that feed gas to the supermassive black hole and thereby power the quasar. The energy released by the quasar expels enough gas to quench both star formation and further black hole growth.
  • Sanchez+17
    • in general, the simplistic picture that all AGN hosts present evidence of recent interactions is known not to be true for most Seyfert galaxies (e.g. Hunt & Malkan 1999), nor even for the stronger type-I QSOs (e.g. Sanchez et al. 2004; Bohm et al. 2013).


  • Hartwig, Voloteri, Dashyan 17:
    • There are several observations that indicate a correlation between the mass of the central SMBH and large scale properties of the host galaxy, such as the stellar veloc- ity dispersion σ, luminosity, or the bulge mass (Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000; Tremaine et al. 2002; Marconi & Hunt 2003; Haring & Rix 2004; Gultekin et al. 2009). These relations imply a possible co-evolution of the black hole (BH) and its host galaxy, and AGN feedback has been suggested to be responsible for regulating accretion on the BH and star formation in the galaxy, possibly guiding this correlation (see Heckman & Kauffmann 2011, for a review).
  • Mehdipour, Kaastra, Kriss+17
    • The observed associations between SMBHs and their host galaxies, such as the M-σ relation (Ferrarese & Merritt 2000), point to their co-evolution through a feedback mechanism.
  • Seifina, Titarchuk, Virgill 17
    • Various estimates of a BH mass, based on the rapid variability timescale, all give that 107 < MBH < 109 M⊙. This indirect constraints provide only an upper limit to the BH mass (see R88), for example, a minimum variability time of ∼ 100 s that sets an upper limit to the light-crossing time (Bloom et al. 2011; Burrows et al. 2011; Liang & Liu 2003) and well-known empirical relations between MBH and the host galaxy environment (Silk & Rees 1998; Jahnke & Maccio 2011). Based on optical luminosity of the host galaxy, Levan et al. (2011) evaluated a spheroidal mass, $M_{\star} \sim 10^{9-10} \, M_{\odot}$. Using the log-linear spheroidal mass and the BH mass relation of Bennert et al. (2011), Levan et al. found that likely a BH mass within $2\times10^{6} < M_{\rm BH} < 10^{7} M_{\odot}$ taking into account the relation by Graham (2012) instead of the relation,$M_{\star}−M_{\rm BH}$. The latter relation also provides a lower range of BH masses, $10^{5}\, M_{\odot} < M_{\rm BH} ≤ 10^{7} \, M_{\odot}$ which is more consistent with the BH mass, $\log{(M_{\rm BH}/M_{\odot})} = 5.5 \pm 1.1$ estimated by Miller & Gultekin (2011), based on empirical, so called fundamental plane relations between radio and X-ray luminosities of accreting BHs.
  • Wurster & Thacker 13
    • The two strongest correlations are the relationship between the black hole mass and the stellar velocity dispersion ($M_{BH}−\sigma$; e.g. Silk & Rees 1998; Ferrarese &Merritt 2000; Gebhardt et al. 2000; Tremaine et al. 2002; King 2003;Gultekin et al. 2009), and the black hole mass and the bulge mass ($M_{BH}−M_{b}$; Magorrian et al. 1998; McLure & Dunlop 2002; Marconi & Hunt 2003).
  • Kim & Fabbiano 13
    • The energy feedback may scale with the halo mass, if the DM halo determines the super-massive black-hole mass, as suggested by, e.g.,Booth & Schaye (2010) as a possible variation of the popular relation between the black-hole and bulge masses (however, see also Kormendy & Bender 2011, who pointed out that $M_{\rm BH}$ is not correlated directly with the DM halo particularly for bulgeless galaxies).

Accretion disc:

  • Kasliwal, Vogeley, Richards 16:
    • Low accretion rate coupled with low surface density results in the formation of advection- dominated accretion flows (Narayan & Yi 1994; Chen et al. 1995) where the cooling time of the accretion flow is much greater than the in-fall time. Such disks are optically thin and may exist in low-luminosity AGN. Some quasars, a few Seyfert 1s and the intermediate states of black-hole X-ray bi- naries (BHXRB) are thought to possess radiation pressure dominated ‘slim’ disks—H/r < 1 where H is the local disk height at radius r—with medium surface mass densities and very high accretion rates (Abramowicz et al. 1988). Such disks also have shorter inflow time as compared to the cooling time-scale, making the flow advective. The third stable solution occurs at high surface mass density but low accretion rate and results in the formation of classic ‘thin’ accretion disks that are optically thick and geometrically thin (H/r « 1) (Shakura & Sunyaev 1973; Frank, King & Raine 2002). Most AGN and BHXRBs in the high and soft states are thought to possess such disks (Blaes 2014).
  • Ryan, Ressler, Dolence+17:
    • Ohsuga et al. (2009) first demonstrated that radiation leads to thick/thin disk transitions in numerical models. Fragile & Meier (2009) found a cooling state inconsistent with either a pure RIAF or a thin disk, and compared it to a magnetically-dominated accretion flow in the inner disk.
    • Sadowski et al. (2017) addressed cooling in RIAFs with self-consistent electron heating and a gray M1 radiation closure, while Sadowski & Gaspari (2017) use a similar model except with constant proton-to-electron tempera- ture ratios to study the transition to radiatively efficient flows
    • we introduce a scheme that couples a global, albeit axisymmetric, model with elec- tron heating (Ressler et al. 2015) for the flow to a Monte Carlo radiation MHD scheme (Ryan et al. 2015), yield- ing a frequency-dependent, full transport solution to the equations of two-temperature relativistic radiation MHD.

AGN phase time scale

  • Zubovas 17:
    • AGN luminosity should decrease over a finite amount of time as well, perhaps of order $10^{4} − 10^{5}\,\rm yr$ (Schawinski et al. 2010; Ichikawa et al. 2016; Keel et al. 2017).
    • A typical AGN phase probably lasts only $t_q \sim 5 \times 10^{4} − 2 \times 10^{5} \rm yr$ (Schawinski et al. 2015; King & Nixon 2015), followed by a fading phase of duration $t_{d} \sim$ a few times $10^{4} \rm yr$ (Keel et al. 2017).

Star formation

  • Semenov+18
    • star formation occurs only in star-forming gas, defined using some condi- tions, e.g., that gas density (temperature) is larger (smaller) than some threshold, that gas within some region is gravita- tionally bound, that gas is in molecular phase, etc. (see, e.g., Hopkins et al. 2013a). Star-forming gas is then converted into stellar particles using a stochastic Poisson process with the rate

AGN activity - Star formation

  • Hopkins & Quataert 10
    • There is also compelling evidence that quasar activity is preceded and/or accompanied by a period of intense star formation in galactic nuclei (Sanders et al. 1988a,b; Kauffmann et al. 2003; Dasyra et al. 2007).

AGN Wind (i.e. mechanical feedback)

  • Pellegrini, Ciotti, Ostriker 12
    • aperture solid angle of the conical nuclear wind increases with increasing $l \equiv L_{\rm BH} / L_{\rm edd}$.
    • while radiative effects mainly work on the kiloparsec scale, mechanical feedback from the AGN winds is more concentrated and affects the ISM on the ∼100 pc scale.
  • Ghayuri 16:
    • broad-line region (BLR) cloud in three classes: (A) non disc (B) disc-wind (C) pure disc structure.We propose that clumpy structures in the brightest quasars belong to class A, fainter quasars and brighter Seyferts belong to class B, and dimmer Seyfert galaxies and all low-luminosity AGNs (LLAGNs) belong to class C
  • Hartwig, Voloteri, Dashyan 17:
    • Our current understanding of AGN-driven outflows is based on observations at various wavelengths. The dynamics of the ISM, which is a first indicator of AGN feedback, can be traced by dust, CO, C+, or Cii emission with radio telescopes, such as ALMA. The observations of gas kinematics of high-z galaxies is one of ALMA’s main science drivers and it can reach sub-kpc resolution at z = 6 (Wang et al. 2013; Trakhtenbrot et al. 2017). Radio observations reveal large molecular outflows with velocities of 100 km/s up to a few times 1000 km/s (Aalto et al. 2012; Cicone et al. 2014). Recently, Maiolino et al. (2017) report star formation inside a galactic outflow, another strong indication for the presence of an outflow of cold gas. In Mrk231, the closest and best studied quasar, we observe velocities of ∼ 1000 km/s and mass outflow rates of the order ∼ 1000 $M_{\odot}\,{\rm yr^{−1}}$ (Feruglio et al. 2010; Rupke & Veilleux 2011; Sturm et al. 2011). These outflows have to be powered by the AGN, because SN-driven outflows can only account for outflow velocities up to ∼ 600 km/s (Martin 2005; Sharma & Nath 2013). Besides these molecular outflows, the Fe K lines in the X-ray reveal highly ionised AGN outflows with mildly relativistic velocities of ∼ 0.1c (Chartas et al. 2002; Pounds et al. 2003; Reeves et al. 2003; Cappi 2006; Gofford et al. 2013). Some ultrafast outflows even have velocities up to ∼ 0.3c, but the majority has velocities around ∼ 0.1c (King & Pounds 2015).
    • Numerical 3D simulations of AGN-driven outflows demonstrate that the outflow takes the path of least resistance and propagates preferentially along the poles of the host galaxy (Gabor & Bournaud 2013, 2014; Costa et al. 2014, 2015; Roos et al. 2015; Bieri et al. 2017; Barai et al. 2017). This results in highly anisotropic outflows as found by Costa et al. (2014) and observationally confirmed by Cicone et al. (2015). However, 3D simulations remain expensive, have explored limited parameter space, and have not yet focused on low-mass or high redshift galaxies.
    • The transition from momentum- to energy-driven occurs at $\sim 2\times10^{−3}$ pc and the momentum-driven solution is less efficient in driving an outflow. (i.e. energy-driven when $t_{\rm flow} < t_{\rm Compton}$)
    • The assumption of $v_\rm{in}$ = 0.1c is supported by observations of the Doppler-shifted Fe K band absorption lines, which are observed in many local AGNs (Pounds et al. 2003; Tombesi et al. 2010; Tombesi 2016). Ostriker et al. (2010) provide a more detailed and complete discussion of the momentum driving and mechanical luminosity.
    • current observations of AGN do not show evidence of Compton cooling from a one tem- perature medium and weak constraints on a possible cooling from a two-temperature medium. This observation supports the theory of energy-driven winds on galactic scales that do not radiate away their thermal internal energy. This suggests that most AGN-driven outflows are energy-driven.
  • Barai, Gallerani, Pallottini+17
    • Bieri et al. (2016) executed radiation-hydrodynamical simulations of idealised gas-rich galaxy disks, where photons from a quasar interacts with the multiphase interstellar medium (ISM), and are found to generate powerful infrared-radiatively-driven outflows
  • Mehdipour, Kaastra, Kriss+17
    • The AGN outflowsmay play an important role in this feedback as they can impact star formation, chemical enrichment of the intergalactic medium, and cooling flows in galaxy clusters (e.g. review by Fabian 2012).
    • Winds of photoionised gas (warm absorbers -WA) are commonly observed in bright AGN through high-resolutionUV and X-ray spectroscopy (e.g. Crenshaw et al. 1999; Blustin et al. 2005). They often consist of multiple ionisation components, outflowing with velocities of typically few hundred km s−1. From an observational point of view, other kinds of winds with different properties from WAs have been found in the X-ray band: high-ionisation ultra-fast outflows (e.g. PDS 456, Reeves et al. 2009) and obscuring outflows (e.g. NGC 5548, Kaastra et al. 2014). Compared to the common WAs at pc-scale distances fromthe black hole (e.g. Kaastra et al. 2012), the obscuring outflow found in NGC 5548 is a faster and more massive wind closer to the accretion disk. It produces strong absorption of the X-ray continuum, in addition to appearance of blue-shifted and broad UV absorption lines. X-ray obscuration with associated UV line absorption has been seen also in Mrk 335 (Longinotti et al. 2013) and NGC 985 (Ebrero et al. 2016). Variable X-ray absorption is commonly found in type-I AGN; e.g. NGC 1365 (Rivers et al. 2015); PDS 456 (Matzeu et al. 2016); NGC 4151 (Beuchert et al. 2017); IRAS 13224-3809 (Parker et al. 2017).
  • Fiore, Feruglio, Shankar+17
    • The molecular gas depletion timescale and the molecular gas fraction of galaxies hosting powerful AGN driven winds are 3–10 times shorter and smaller than those of main sequence galaxies with similar star formation rate (SFR), stellar mass, and redshift. These findings suggest that, at high AGN bolometric luminosity, the reduced molecular gas fraction may be due to the destruction of molecules by the wind, leading to a larger fraction of gas in the atomic ionised phase.
    • AGN winds are, on average, powerful enough to clean galaxies from their molecular gas only in massive systems at z < 2, i.e. a strong form of co-evolution between SMBHs and galaxies appears to break down for the least massive galaxies.
    • several direct observation of ISM modifications by AGN winds have been collected so far. Cano-Diaz et al. (2012), Cresci et al. (2015), and Carniani et al. (2016) have found that AGNwinds and actively star-forming regions are spa- tially anti-correlated. Similarly, Davies et al. (2007) and Lipari et al. (2009) found little evidence for young (Myr) stellar populations in the < 1 kpc region of Markarian 231 where a powerful molecular outflow is observed (Feruglio et al. 2010, 2015).

Radiative Feedback

  • Pellegrini, Ciotti, Ostriker 12
    • sudden increase of the gas emission during outbursts is due to the increase in temperature and density in the central galactic regions (≃102−103 pc) caused by radiative gas heating (Compton and photoionization) and by compression due to direct and reflected shock waves produced by mechanical and radiative feedback which are associated with the AGN and the starburst.

Negative Feedback

  • Baron+17
    • Common sources of negative feedback are supernova- (SN) and AGN-driven winds, which can heat the gas via shocks, and photoionize it (Heckman, Armus & Miley 1990; Di Matteo, Springel & Hernquist 2005; Springel et al. 2005; Ciotti, Ostriker & Proga 2010; Feruglio et al. 2010; Cicone et al. 2014)
    • Kaviraj et al. (2007) … For galaxies less massive than $M = 10^{10}M_{\odot}$, SN-driven feedback is the main quenching mechanism, while galaxies above this mass are mainly quenched by an AGN.
  • Davies, Groves, Kewley+17
    • suppression and triggering of star formation by AGN feedback (e.g. Croft et al. 2006; Elbaz et al. 2009; Cano-Dıaz et al. 2012; Rauch et al. 2013; Cresci et al. 2015;Salome, Salome & Combes 2015)
  • Barai, Gallerani, Pallottini+17
    • AGN feedback should operate mostly in the negative form quenching star formation, as suggested by observations (e.g., Schawinski et al. 2006; Wang et al. 2007; Lanz et al. 2016), and simulations (e.g., Scannapieco, Silk & Bouwens 2005; van de Voort et al. 2011; Dubois et al. 2013; Tremmel et al. 2016). This feedback limits the formation of massive stellar systems, enabling simulations to reproduce the observed exponential cut-off at the bright- end of galaxy luminosity function (e.g., Croton et al. 2006; Silk & Mamon 2012), as opposed to the dark-matter halo mass function. Populations of old, passive (the red and dead) massive elliptical galaxies are observed at z ∼ 2 (e.g., Cimatti et al. 2004; Saracco et al. 2005; Whitaker et al. 2013). This suggests that quasar feedback was already suppressing star formation at high-z
  • Wylezalek, Zakamska 16:
    • suppression of star formation in regions of high velocity winds (Cano-D´ıaz et al. 2012; Brusa et al. 2015b; Cresci et al. 2015b; Carniani et al. 2016).
  • Pellegrini, Ciotti, Ostriker 12
    • AGN activity also terminates star formation because, at the end of each major accretion episode, feedback is strong enough to empty the galaxy of gas on the kiloparsec scale
  • Sanchez+17
    • quenching happens from inside-out involving both a decrease of the efficiency of the star formation and a deficit of molecular gas

SF Quenching

  • Nogueira-Cavalcante+17
    • Many astrophysical processes are well recognized to be agents that can either trigger star-formation (e.g., mergers, disk instabilities) − thus leading to an accelerated exhaustion of the gas content in a galaxy − or directly quench the star-forming activity in a galaxy by expelling or heating the necessary fuel for continued star formation (e.g., AGN feed-back).
    • In the case of massive galaxies, Nandra et al. (2007) studied a sample of X-rays sources and found that the majority of host galaxies are red, suggesting a scenario where the AGNs either cause or maintain the star formation quenching. Interestingly, Schawinski et al. (2009) found evidences of recent destruction of the molecular gas in a sample of faint active galactic nuclei, suggesting that even low-luminosity AGN episodes are able to quench star formation in galaxies. In the case of low-mass galaxies, supernovae winds have been shown to be capable of quenching star formation, expelling the gas from the interstellar medium (Lagos et al. 2013; Menci et al. 2005).
    • At early times (z∼1−2) Peng et al. (2010) suggest that fast processes (e.g., major mergers) play a very important role in the evolution of galaxies. However, Mendez et al. (2011), using quantitative morphological parameters, found that the green valley at 0.4<z<1.2 shows lower merger fractions than those measured for blue galaxies, suggesting that mergers are not important for quenching star formation in green valley galaxies at these redshifts. At later times (z ∼ 0), contrastingly, mergers become less common (Conselice et al. 2003), and slower processes that generally involve interactions between stars, gas clouds and the dark matter halo (e.g., disk in- stabilities, bars) may become more important (Spitzer & Schwarzschild 1951, 1953). However, Martin et al. (2007) found that ∼50% of the green valley galaxies at z∼0.1 show signs of AGN activity, although the AGN luminosity was not found to be correlated with the timescale of star formation quenching. Considering that AGN activity is usually associated with fast star formation quenching, as demonstrated by hydrodynamical simulations (Dubois et al. 2013; Sijacki & Springel 2006), faster processes may also play an important role in quenching star formation at lower redshifts.

Positive Feedback

  • Wylezalek, Zakamska 16:
    • other objects show enhancement of star formation through AGN feedback which can be caused by compression of gas clouds by AGN outflows leading to that gravitational collapse (Cresci et al. 2015b,a).
  • Pellegrini, Ciotti, Ostriker 12
    • the final mass of newly formed stars in the kiloparsec-scale cold shells originated by the AGN feedback during the outbursts is significantly larger than in pure cooling flow models. From this point of view, the AGN activity induces star formation.
  • Barai, Gallerani, Pallottini+17
    • AGN feedback can occasionally be positive, by inducing star-formation, and this aspect also plays an important role. AGN outflows can overpressure and compress clumpy gas clouds, triggering starbursts, as have been shown in theoretical and numerical studies (e.g., De Young 1989; Silk 2005; Zubovas et al. 2013), including cold molecular clumps condensing out in quasar outflows (Ferrara & Scannapieco 2016). Positive feedback has been observed in jet-induced star formation and radio-optical alignment (e.g., Chambers, Miley & van Breugel 1987; Zinn et al. 2013), as well as in Seyfert-like radio-quiet AGN (Cresci et al. 2015)

No relation between AGN - SF

  • Biernacki & Teyssier 17:
    • in very high resolution simulations of isolated galactic discs, Gabor&Bournaud (2014) and Roos et al. (2015) have demonstrated that AGN feedback has very little effect on the SF within the disk. The SMBH could in principle release as much as $10^{59}$ erg of energy ($E_{\rm SMBH} = 0.1\,M_{\rm SMBH} c^{2}$), largely exceeding binding energy of the galaxy ($E_{\rm gal} \approx M_{\rm gas} \sigma^{2}$, where $\sigma$ is the velocity dispersion).

Stellar Feedback

  • Angles-Alcazar, Faucher-Giguere, Quataert+17
    • bursty stellar feedback has strong implications for BH and AGN demographics, especially in the early Universe and for low-mass galaxies
    • stellar feedback regulates the gas reservoir in galactic nuclei, which can severely limit early BH growth
    • BH growth is more efficient at later times, when the nuclear stellar potential retains a significant gas reservoir, star formation becomes less bursty, and galaxies settle into a more ordered state, with BHs rapidly converging onto the scaling relation when the host reaches $M_{\rm bulge} \sim 10^{10} \, M_{\odot}$
    • Dwarf galaxies experience bursty star formation down to z = 0 (e.g. Anglés-Alcázar et al. 2016), suggesting that BH growth will also be inefficient in low-redshift dwarfs. In such galaxies, inefficient BH growth may correlate with the formation of dark matter cores and stellar size fluctuations driven by stellar feedback (e.g. Chan et al. 2015; El-Badry et al. 2016)
  • Rosas-Guevara+15
    • AGN feedback is assumed to be ineffective in low-mass haloes, where the gas cooling time is short compared to the sound-crossing time (White & Frenk 1991), and only to couple effectively in quasi-hydrostatic haloes ($M\gtrsim 10^{12} \,M_{\odot}$).
    • if the primary driver of the star formation rate in galaxies is the balance between outflows and inflows (i.e. the star formation rate of galaxies adjusts itself so that the inflow and outflow are in equilibrium). In the case of low-mass haloes, the AGN feedback loop is (assumed to be) ineffective and the balance between gas supply and outflow is set by the supernova (SN)-driven outflow rate. In higher mass haloes, the AGNs regulate the galaxy growth either by offsetting the cooling rate (Bower et al. 2006), or by puffing up the hot gas halo (Bower et al. 2008; McCarthy et al. 2011; Bower, Benson & Crain 2012) so that the cooling rate is reduced. In either case, the result is to suppress the mass of the cold gas and reduce the star formation rate in massive haloes, creating a break in the stellar mass function (SMF).

Overcooling Problem

  • where gas clouds cool too quickly and star formation happens too early (Silk et al. 2013).
  • Pontzen+16
    • The low level of black hole accretion can be sustained even when there is insufficient dense cold gas for star formation. Conversely, supernova feedback is too distributed to generate outflows in high-mass systems, and cannot maintain quenching over periods longer than the halo gas cooling time.

Temperature gradient

  • Pellegrini, Ciotti, Ostriker 12
    • Negative: Negative radial gradients, as shown by the model temperature during quiescence, are common among ellipticals, as revealed most recently by Chandra observations
    • Positive (hot gas-rich environment or jet):
      • positive outer gradient is shown by galaxies in high-density environments, suggesting the influence of circumgalactic hot gas.
      • weak radio AGNs distribute their heat locally and host negative inner temperature gradients, whereas more luminous radio AGNs heat the gas more globally through a jet or rising bubbles and produce a flat profile or a positive gradient

Global Outflow & CGM

  • Schneider, Robertson 16:
    • cool gas observed in outflows at large distances from the galaxy (> 1kpc) likely does not originate through the entrainment of cold material near the central star burst.
      • These early studies could reasonably ignore radiative cooling effects by limiting their studies to small clouds. In larger scale problems where the cooling time-scale is smaller than the dynamical time-scale, thermal energy losses must be included.
      • radiative cooling inhibits destruction of the dense material and extends the lifetime of the cloud relative to the adiabatic case
  • Maiolino+17, Nature
    • Although there exists observational evidence for star formation triggered by outflows or jets into their host galaxy, as a consequence of gas compression, evidence for star formation occurring within galactic outflows is still missing.
  • Bieri+16:
    • a quasar with a luminosity of $10^{46}\,{\rm erg \, s^{−1}}$ can drive large-scale winds with velocities of $10^{2}−10^{3}\,{\rm km \,s^{−1}}$ and mass outflow rates around $10^{3}\,M_{\odot} \,{\rm yr^{−1}}$ for times of order a few million years.
    • Infrared radiation is necessary to efficiently transfer momentum to the gas via multi-scattering on dust in dense clouds. —> wind launching
  • Barai, Gallerani, Pallottini+17
    • A strong manifestation of AGN feedback are AGN outflows observed in a wide variety of forms (reviews by Crenshaw, Kraemer & George 2003; Everett 2007). Some examples are: blue-shifted broad absorption lines in the ultraviolet and optical (Reichard et al. 2003; Rupke & Veilleux 2011), warm absorbers (Chartas, Brandt & Gallagher 2003; Krongold et al. 2007) and ultra-fast outflows in X-rays (Tombesi et al. 2013, 2015), molecular outflows and atomic outflows detected in the IR, sub-millimetre and millimetre wavelengths (Sturm et al. 2011; Cicone et al. 2014; Dasyra et al. 2015; Feruglio et al. 2015; Morganti et al. 2016), ionized gas in rest-frame optical (Kakkad et al. 2016)
  • Naab, Ostriker 17, ARA&A
    • how galactic outflows actually transport metal-enriched material into the circumgalactic medium (see, e.g., Oppenheimer & Dav´e 2006, Schaye et al. 2015). This is also a numerically challenging question, as the spatial resolution in the halos of galaxies is typically much lower than in the dense regions, and mixing processes are highly complex (e.g., Scannapieco &Br ¨uggen 2015).
  • Heckman, Borthakur, Wild+17
    • One of the major ways in which feedback occurs is via the outflows of gas driven from strongly star-forming galaxies by the energy and/or momentum injected by massive stars (see Heckman & Thompson 2017 for a recent review). These galactic winds are ubiquitous in star-forming galaxies at intermediate and high redshift (e.g. Weiner et al. 2009; Steidel et al. 2010; Erb et al. 2012; Kornei et al. 2012; Martin et al. 2012; Bor- doloi et al. 2014b; Rubin et al. 2014)
    • galactic winds can have dramatic effects. They may account for the large relative mass of metals in the IGM, for the evolving mass-metallicity relation for galaxies, for the expulsion of baryons from low-mass dark matter halos, and for the transport of low-angular momentum material from forming galaxies (Somerville & Dave 2015 and references therein).

Missing Baryon Problem

  • Farber, Ruszkowski, Yang+17
    • most galaxies are missing a large fraction of baryons compared to the cosmological average (Bell et al. 2003). Models matching observed luminosity functions to simulated halo mass functions find that 20% of the baryons are accounted for in $L_{\star}$ galaxies, and that this fraction decreases rapidly for both more and less massive galaxies (Guo et al. 2010). This suggests that the efficiency of converting baryons into stars is a strong function of halo mass. The discrepancies between halo and stellar properties, “the missing baryons problem,” constitutes an outstanding challenge in galaxy formation. Galactic winds can possibly solve the missing baryons problem by ejecting baryons out of galaxies. For galaxies more massive than $L_{\star}$, active galactic nuclei likely dominate the energetics of the outflows (e.g., Croton et al. 2006), while in less massive galaxies galactic winds are likely driven by stellar feedback (Larson 1974, Chevalier & Clegg 1985, Dekel & Silk 1986).

X-ray & X-ray Cavity

  • Pellegrini, Ciotti, Ostriker 12
    • X-ray surface brightness becomes flatter in shape with increasing time, since the emission level decreases mostly in the central galactic regions (within $R_{e}$). This important effect is produced by the nuclear outbursts which remove gas from the center.
    • bright ellipticals imaged with Chandra (e.g., Loewenstein et al. 2001) show a brightness profile that is quite flat within the central ~1 kpc, a feature impossible to reproduce with pure inflow models, while it resembles the profile of the “pre-burst” phase
    • These feedback events must leave signatures on the X-ray properties of the galaxies; indeed, the observed temperature and brightness profiles often cannot be fit with smooth profiles, such as those predicted for galactic winds or cooling flows (e.g., Sarazin 2012; Statler 2012)
  • Lobban, Porquet, Reeves+17
    • Active galactic nuclei (AGN) are routinely observed to emit strongly across the entire electromagnetic spectrum with the broad-band spectral energy distribution comprising both thermal and non-thermal emission components (Shang et al. 2011). Typically peaking at ultraviolet (UV) wavelengths, the dominant energy output of Seyfert galaxies is generally considered to arise from thermal emission from material in the inner parts of a geometrically-thin, optically-thick accretion disc surrounding the supermassive black hole (SMBH; Shakura & Sunyaev 1973). The location of the UV-emitting material is typically 10–1000 $r_{g}$ from the central black hole, depending on the properties of the accretion flow. This thermal UV emission is then thought to be responsible for producing the X-ray continuum, which is typically power-law in shape, via inverse-Compton scattering of soft thermal photons via an optically-thin ‘corona’ of hot ($T \sim 10^{9}$ K) electrons, typically within a few tens of $r_{g}$ from the black hole (Haardt &Maraschi 1993).
  • Wurster & Thacker 13
    • Large X-ray cavities in the hot gas halo around AGN(e.g. Boehringer et al. 1993; McNamara et al. 2000) suggest that some outflowevents can be quite powerful; these cavities are likely formed by jets from the AGN (e.g. McNamara & Nulsen 2007).


  • Fox, Davé 17, Book ''Gas Accretion onto Galaxies''
    • Gas around galaxies has been detected in observations in absorption in quasar sightlines and in emission in hydrogen Lyman-$\alpha$˛,OVI, and soft X-rays (e.g., Péroux et al. 2005; Turner et al. 2016; Steidel et al. 2011; Hayes et al. 2016; Anderson et al. 2015). However, it is difficult to determine whether this gas is accreting, outflowing, or static.
    • there are observations that show enhanced MgII absorption along the major and minor axis of galaxies, potentially showing evidence for biconical outflows along the minor axis and recycled wind accretion along the major axis (Kacprzak et al. 2012).
  • Heckman, Borthakur, Wild+17
    • The evolution of galaxies is largely driven by how and when they accrete gas and by how the feedback from newly formed stars and black holes regulates this accretion (see Somerville & Dave 2015 and references therein). In turn, the evolution of the inter-galactic medium will be affected by these same feedback processes which can photo-ionize, shock-heat, and chemically-enrich it (e.g. Menard et al. 2010). These flows into and out of galaxies will occur within the circum-galactic medium (CGM), a region extending out to roughly the galaxy virial radius.
    • galactic winds can have dramatic effects. They may account for the large relative mass of metals in the IGM, for the evolving mass-metallicity relation for galaxies, for the expulsion of baryons from low-mass dark matter halos, and for the transport of low-angular momentum material from forming galaxies (Somerville & Dave 2015 and references therein).


  • Sanchez+17
    • The strong magnetic fields due to the accelerated charged parti- cles moving fast in the accreation disk generate collimated radio jets and extended radio emission.
  • Koziel-Weirzbowska+17
    • At least 10% of active galactic nuclei (AGNs) is associated with radio sources powered by jets (Kellermann et al. 2016, and references therein)
  • Su, Liu & Zhang 17
    • There is a much higher detection rate (>50%) of radio loud nuclei in the LLAGNs than that (∼ 15%) in the luminous AGNs (Ho & Ulvestad 2001; Nagar et al. 2002).