Feedback from supermassive and intermediate-mass black holes at galaxy centers using cosmological hydrodynamical simulations

Abstract Accretion of matter onto central Black Holes (BHs) in galaxies liberates enormous amounts of feedback energy, which affects the environment from pc to Mpc scales. These BHs are usually Supermassive BHs (SMBHs: mass ⩾106 M⊙) existing at the centers of active galactic nuclei (AGN), which are widely observed through their multi-wavelength emission at all cosmic epochs. Relatively recently, Intermediate-Mass BHs (IMBHs: mass = 100−106 M⊙) have started to be observed hosted in Dwarf Galaxy (DG) centers. Some of the central IMBHs in DGs show signatures of activity in the form of low-luminosity AGN. We have performed Cosmological Hydrodynamical Simulations to probe SMBHs in high-z quasars (Barai et al. 2018), and IMBHs in DGs (Barai & de Gouveia Dal Pino 2019). Our simulations employ the 3D TreePM SPH code GADGET-3, and include metal cooling, star formation, chemical enrichment, stellar evolution, supernova feedback, AGN accretion and feedback. Analyzing the simulation output in post-processing, we investigate the growth of the first IMBHs and the first SMBHs, as well as their impact on star-formation.


Introduction
Active galactic nuclei (AGN) emit enormous amounts of energy powered by the accretion of gas onto their central supermassive black holes (SMBHs) (e.g., Rees 1984). Feedback from AGN are believed to strongly influence the formation and evolution of galaxies (e.g., Richstone et al. 1998). A strong manifestation of AGN feedback are AGN outflows observed in a wide variety of forms (e.g., Crenshaw, Kraemer & George 2003).
Quasars are very powerful AGN existing more commonly at high-z than in the local Universe (e.g., Fan 2006). In the host galaxy of the quasar SDSS J1148 + 5251 at z = 6.4, Maiolino et al. (2012) detected broad wings of the [CII] line tracing a massive outflow with velocities up to ±1300 km/s. The physical mechanisms by which quasar outflows affect their host galaxies remain as open questions. SMBHs of mass 10 9 M are observed to be in place in luminous quasars by z ∼ 6, when the Universe was less than 1 Gyr old (e.g., Wu et al. 2015). It is difficult to understand how these early SMBHs formed over such short time-scales, and there are open issues with various plausible scenarios (e.g., Matsumoto et al. 2015).
Black holes are usually observed to belong to two populations: stellar-mass (M BH 10 − 100M ) BHs, and supermassive (M BH 10 6 M ) BHs. By natural extension, there should be a population of Intermediate-Mass Black Holes (IMBHs: with mass between 100 − 10 6 M ) in the Universe. Analogous to SMBHs producing AGN feedback, the IMBHs should also have feedback. AGN feedback mechanism has recently started to be observed in low-mass galaxies (e.g., Marleau et al. 2017;Penny et al. 2017). The concordance ΛCDM cosmological scenario of galaxy formation presents multiple challenges in the dwarf galaxy mass range: e.g. core versus cusp density profile, number of DGs.
Recently Silk (2017) made an exciting claim that the presence of IMBHs at the centers of Dwarf Galaxies (DGs) can potentially solve the problems. Early feedback from these IMBHs output energy and affect the gas-rich DGs at z = 5 − 8, can quench star-formation and reduce the number of DGs.
In this work we present results of the growth and feedback of SMBHs in AGN and IMBHs in DGs. We focus on negative BH feedback effects where star-formation is quenched (e.g., Scannapieco, Silk & Bouwens 2005;Schawinski et al. 2006).

Numerical Method and Simulations
The initial conditions at z = 100 are generated using the MUSIC † software (Hahn & Abel 2011). We use a modified version of the TreePM (particle mesh) -SPH (smoothed particle hydrodynamics) code GADGET-3 (Springel 2005) to perform our cosmological hydrodynamical simulations.
BHs are collisionless sink particles (of mass M BH ) in our simulations. A BH (of initial mass M BHseed ) is seeded at the center of each galaxy more massive than a total mass M HaloMin , which does not contain a BH already. We test different values of minimum halo mass and seed BH mass in the range: M HaloMin = (10 6 − 10 7 )M , and M BHseed = (10 2 − 10 3 )M . The sub-resolution prescriptions for gas accretion onto BHs and kinetic feedback are adopted from (Barai et al. 2014(Barai et al. , 2016.
We execute 4 Zoomed-In cosmological hydrodynamical simulations, with characteristics listed in Table 1, all the runs incorporating metal cooling, chemical enrichment, SF and SN feedback. The first run has no AGN included, while the latter three explore different AGN feedback models. We perform cosmological hydrodynamical simulations of small-sized boxes with periodic boundary conditions, to probe dwarf galaxies at high redshifts. We execute 10 simulations, with characteristics listed in Table 2.

Black Hole Accretion and Growth
The redshift evolution of the most-massive SMBH mass in the three AGN runs of the zoomed-in cosmological simulations is plotted in Fig. 1 -left panel. Each SMBH starts as a seed of M BH = 10 5 M , at z ∼ 14 in the runs AGNcone and AGNsphere (z ∼ 10 in AGNoffset). The subsequent growth is due to merger with other SMBHs and gas accretion. The dominant mode of SMBH growth occurs over the redshifts z = 9 − 6 in runs AGNcone and AGNsphere, corresponding to Eddington-limited gas accretion where Eddington ratio = 1. TheṀ BH has a power-law increase, and the SMBH mass increases † MUSIC -Multi-scale Initial Conditions for Cosmological Simulations: https://bitbucket. org/ohahn/music 186 P. Barai Yes h −1 × 10 6 10 2 2000 BHs2h7e7 Yes 5h −1 × 10 7 10 2 2000 BHs3h1e7 Yes 1 × 10 7 10 3 2000 BHs3h2e7 Yes 2 × 10 7 10 3 2000 BHs3h3e7 Yes 3 × 10 7 10 3 2000 BHs3h4e7 Yes 4 × 10 7 10 3 2000 BHs3h4e7v5 Yes 4 × 10 7 10 3 5000 BHs3h5e7 Yes 5 × 10 7 10 3 2000 BHs4h4e7 Yes 4  by a factor ∼10 3 . The final properties reached at z = 6 depends on the simulation; e.g. M BH = 4 × 10 9 M andṀ BH = 100M /yr in run AGNcone (red curve). There is variability of theṀ BH , whereby it fluctuates by a factor of up to 100. The SMBH grows 10 times more massive at z = 6 in the AGNcone case than in the AGNsphere run. This is because more gas can inflow along the perpendicular direction to the bi-cone, and accrete onto the SMBH. We find that first IMBHs are seeded at different cosmic times depending on the value of minimum halo mass for BH seeding, M HaloMin . The seeding epoch varies between z ∼ 22 to z ∼ 16 in our periodic-box cosmological simulations, when the first halos reach M halo = h −1 × 10 6 M to M halo = 5 × 10 7 M . The redshift evolution of the most-massive IMBH mass in these periodic-box simulation runs is plotted in Fig. 1 -right panel. Each IMBH starts from an initial seed of M BH = 10 2 M in the runs named BHs2*, 10 3 M in the runs named BHs3*, and 10 4 M in the runs named BHs4*. The subsequent mass growth is due to merger with other IMBHs (revealed as vertical rises in M BH ), and gas accretion (visualized as the positive-sloped regions of the M BH versus z). The mostmassive BH, considering all the runs, has grown to M BH = 2 × 10 6 M at z = 5 in run BHs3h4e7 (brown curve in Fig. 1 -right panel).

Star Formation
Stars form in the simulation volume from cold dense gas. The star formation rate (total SFR in the simulation box) versus redshift of the four zoomed-in cosmological simulations is displayed in Fig. 2 -left panel. The SFR rises with time in all the runs initially, and continues to increase in the noAGN case without a SMBH. The SFR in run AGNoffset is almost similar to that in the run noAGN, because the SMBHs are too small there to generate enough feedback. A similar outcome happens in the runs AGNcone and AGNsphere at z 8, when the SMBHs are too small.
The models suppress SF substantially from z ∼ 8 onwards, when the SMBHs have grown massive and generate larger feedback energy. Thus, we find that SMBHs need to grow to M BH > 10 7 M , in order to suppress star-formation, even in massive galaxies (of M = 4 × 10 10 M , and specific-SFR = 5 × 10 −9 yr −1 ).
The Star Formation Rate Density (SFRD in units of M yr −1 Mpc −3 , counting stars forming in the whole simulation box) versus redshift of the periodic-box cosmological simulation runs is displayed in Fig. 2 -right panel. The SFRD rises with time in the SN run (blue curve) initially from z ∼ 15, reaches a peak at z ∼ 4 (the peak epoch of star-formation activity in the Universe), and decreases subsequently over z ∼ 4 − 2. The presence of a IMBH quenches star formation by accreting some gas in, ejecting some gas out of the halo as outflows, and/or heating the gas. The models suppress SF substantially from z ∼ 8 onwards, when the IMBHs have grown massive. We find that IMBHs need to grow to M BH > 10 5 M , in order to suppress star-formation.
The red curve (run BHs2h1e6) already quenches SF as early as z ∼ 8. This is because the IMBH has already grown to M BH ∼ 5 × 10 5 M at that epoch, more massive than all the other runs. As another example, the brown (run BHs3h4e7) and royal-blue (run BHs3h5e7) curves quench SF from z ∼ 4.5 to z ∼ 3.5. This is the epoch when the IMBH masses in these runs increase from M BH = 10 5 M to M BH = 10 6 M (as can be seen from Fig. 1 -right panel).

Conclusions
Gas accretion onto central supermassive black holes of active galaxies and resulting energy feedback, often manifested as AGN outflows, is an important component of galaxy evolution. We investigate outflows in quasar-host galaxies at z 6 by performing cosmological hydrodynamical simulations. We simulate the 2R 200 region around a 2 × 10 12 M halo at z = 6, inside a (500 Mpc) 3 comoving volume, using the zoomed-in technique.

P. Barai
We find that, starting from 10 5 M seeds SMBHs can grow to 10 9 M in cosmological environments. During their growth, SMBHs accrete gas at the Eddington accretion rate over z = 9 − 6, for 100s of Myr. At z = 6, our most-massive SMBH has grown to M BH = 4 × 10 9 M . Fast (v r > 1000 km/s), powerful (Ṁ out ∼ 2000M /yr) outflows of shock-heated gas form at z ∼ 7, and propagate up to hundreds kpc. Star-formation is quenched over z = 8 − 6. The outflow mass is increased (and the inflow is reduced) by ∼ 20%.
Intermediate-mass black holes (mass between 100 − 10 6 M ) have started to been observed at the centers of dwarf galaxies. We perform cosmological hydrodynamical simulations of (2h −1 Mpc) 3 comoving boxes with periodic boundary conditions, to probe dwarf galaxies and central IMBHs at high redshifts. We conclude that IMBHs at DG centers grow from 10 2 − 10 3 M to 10 5 − 10 6 M by z ∼ 4 in cosmological environments. Star formation is quenched when the IMBHs have grown to M BH > 10 5 M . Our conclusions, based on numerical simulation results, support the phenomenological ideas made by Silk (2017). IMBHs at the centers of dwarf galaxies can be a strong source of feedback to quench star-formation and generate outflows. At the same time, these IMBHs form the missing link between stellar-mass and SMBHs.