THE FERMI BUBBLES: SUPERSONIC ACTIVE GALACTIC NUCLEUS JETS WITH ANISOTROPIC COSMIC-RAY DIFFUSION

“The mass, momentum, total energy, and induction equations are solved by the Piecewise-Parabolic Method (PPM; Colella & Woodward (1984)) in the USM solver (Lee & Deane 2009; Lee 2012).”

“A.4 Nonlinear Shock Tube Test for a Composite of Thermal Gas and Cosmic Rays The Sod shock tube problem (Sod 1978) is a standard test for the accuracy of computational fluid codes, in particular the Riemann solver.”

“In order to incorporate cosmic rays in the simulations, we implemented a new CR module in the USM solver in FLASH4.”

“…extragalactic radio sources (e.g., Scheuer 1974; Blandford & Rees 1974) and CRfilled bubbles (e.g., Laing et al. 2006) observed in massive galaxies and galaxy clusters (McNamara & Nulsen 2007), though their gamma-ray emission cannot be easily detected due to limited sensitivity and resolution.”

“Assuming isotropic diffusion, typical values of the diffusion coefficient in the Galaxy is observationally constrained to be κiso ∼ (3 − 5) × 1028 cm2 s−1, which may depend on the magnetic field structure and CR energy (Strong et al. 2007).”

“For the simulated X-ray map, the X-ray emissivity in an energy range 1.4 − 1.6 keV is calculated using the MEKAL model (Mewe et al. 1985; Kaastra & Mewe 1993; Liedahl et al. 1995) implemented in the utility XSPEC (Arnaud 1996), assuming solar metallicity.”

“Furthermore, AGN jets are known to be responsible for radio synchrotron emission from extended extragalactic radio sources (e.g., Scheuer 1974; Blandford & Rees 1974) and CRfilled bubbles (e.g., Laing et al. 2006) observed in massive galaxies and galaxy clusters (McNamara & Nulsen 2007), though…”

“100 GeV, the IC cooling time of ∼ 100 GeV electrons poses a stringent upper limit to the age of the bubbles to be at most a few million years old (Su et al. 2010).”

“The energy input from these jets may have altered the properties of the multiphase gas in the Galactic bulge and halo, suppressed the accretion process, and regulated the co-evolution of the SMBH and the stellar bulge in the past.”

“However, the general success in reproducing the observed X-ray features provides a strong support for the hypothesis that both the Fermi bubbles and the ROSAT X-ray features originate from the same episode of jet activity from the central SMBH.”

“Furthermore, AGN jets are known to be responsible for radio synchrotron emission from extended extragalactic radio sources (e.g., Scheuer 1974; Blandford & Rees 1974) and CRfilled bubbles (e.g., Laing et al. 2006) observed in massive galaxies and galaxy clusters (McNamara & Nulsen 2007), though…”

“In many situations where f is nearly isotropic in momentum space, f can be solved as a function of only r, |p|, and t (Skilling 1975; Miniati et al. 2001).”

“…pair of gamma-ray jets recently discovered by Su & Finkbeiner 2012), the detection of the Fermi bubbles, together with indications of past activity (Koyama et al. 1996; Revnivtsev et al. 2004), suggest that the central SMBH may have regularly undergone cycles of jet activity, or even was at a much…”

“Jets from SMBH are generally thought to carry magnetic field, as it plays an important role in jet collimation and acceleration (see Ferrari (1998) and references therein), and as suggested by observations of lobes of radio galaxies (Owen et al. 2000; Kronberg et al. 2001; Croston et al. 2005).”

“Cosmic rays may stream through thermal gas in regions where the magnetic field is locally aligned, with streaming speed limited by the Alfvén speed (Skilling 1971; Kulsrud 2005).”

“Moreover, such strong wind would have dominated over the gravitational restoring force (McCarthy et al. 2008) and stripped a significant amount of Milky Way gas away.”

“In this paper we adopt a more simplistic approach, which is to treat cosmic rays as a second fluid and solve directly for the evolution of CR pressure pcr as a function of r and t (Drury & Voelk 1981; Jones & Kang 1990; Ryu et al. 2003; Mathews & Brighenti 2008; Rasera & Chandran 2008).”

“The population of CR electrons is invoked to explain the hard-spectrum microwave synchrotron radiation spatially-correlated with the Fermi bubbles observed by the Wilkinson Microwave Anisotropy Probe (WMAP), also known as the ‘WMAP haze’ (Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012).”

“Jets from SMBH are generally thought to carry magnetic field, as it plays an important role in jet collimation and acceleration (see Ferrari (1998) and references therein), and as suggested by observations of lobes of radio galaxies (Owen et al. 2000; Kronberg et al. 2001; Croston et al. 2005).”

“One may ask whether this is linked to the enhanced 511 keV radiation observed in the Galactic bulge (see a review by Prantzos et al. 2011).”

“One of the most important findings from the first two years of Fermi Gamma-ray Space Telescope observations are two large bubbles extending to ∼ 50◦ above and below the Galactic center (GC), with a width of ∼ 40◦ in longitude (Su et al. 2010; Dobler et al. 2010).”

“Jets from SMBH are generally thought to carry magnetic field, as it plays an important role in jet collimation and acceleration (see Ferrari (1998) and references therein), and as suggested by observations of lobes of radio galaxies (Owen et al. 2000; Kronberg et al. 2001; Croston et al. 2005).”

“21 Similarly, for the case of a hybrid fluid consisting of thermal gas and cosmic rays, an analytical solution of the shock tube problem has been derived by Pfrommer et al. (2006).”

“…from the decay of neutral pions produced during inelastic collisions between cosmic ray (CR) protons and thermal nuclei (the ‘hadronic’ model; Crocker & Aharonian 2011), and/or from inverse Compton (IC) scattering of photons in the interstellar radiation field (ISRF) and the cosmic…”

“In many situations where f is nearly isotropic in momentum space, f can be solved as a function of only r, |p|, and t (Skilling 1975; Miniati et al. 2001).”

“AGN are also considered to be a viable agent for distributing the fields into the intergalactic medium (IGM) and intracluster medium (ICM) by relativistic jets (e.g., Koide et al. 1999; Contopoulos et al. 2009; Sutter et al. 2012).”

“The population of CR electrons is invoked to explain the hard-spectrum microwave synchrotron radiation spatially-correlated with the Fermi bubbles observed by the Wilkinson Microwave Anisotropy Probe (WMAP), also known as the ‘WMAP haze’ (Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012).”

“…not necessarily aligned with the rotational axis of the galactic disks, and such misalignments are observed frequently in other galaxies (e.g., Middelberg et al. 2004; Giroletti et al. 2005; Gallimore et al. 2006; Kharb et al. 2010), and also recently found in the Milky Way (Su & Finkbeiner 2012).”

“The total pressure is ptot = (γ−1)eth+(γcr−1)ecr+B2/8π, where eth is the internal energy density of the gas, γ = 5/3 is the adiabatic index for ideal gas, and γcr = 4/3 is the effective adiabatic index of cosmic rays in the relativistic regime (Jubelgas et al. 2008).”

“…= 1 κ‖,i,j + 1 κ‖,i+1,j , 2 κ⊥ = 1 κ⊥,i,j + 1 κ⊥,i+1,j , (A9) and the transverse flux is sloped-limited in order to avoid negative CR energy densities when there is a large gradient (Sharma & Hammett 2007), ∂ecr ∂y = L { L [ ∂ecr ∂y ∣ ∣ ∣ ∣ i,j−1/2 , ∂ecr ∂y ∣ ∣ ∣ ∣ i,j+1/2 ] , L [ ∂ecr ∂y ∣…”

“Studies of ram pressure stripping of dwarf spheroidal galaxies orbiting the Milky Way give constraints to the value of ne,floor to be around 2.4 × 10−5 − 2.5 × 10−4 cm−3 (Blitz & Robishaw 2000; Grcevich & Putman 2009).”

“…recently discovered by Su & Finkbeiner 2012), the detection of the Fermi bubbles, together with indications of past activity (Koyama et al. 1996; Revnivtsev et al. 2004), suggest that the central SMBH may have regularly undergone cycles of jet activity, or even was at a much more active state…”

“Jets from SMBH are generally thought to carry magnetic field, as it plays an important role in jet collimation and acceleration (see Ferrari (1998) and references therein), and as suggested by observations of lobes of radio galaxies (Owen et al. 2000; Kronberg et al. 2001; Croston et al. 2005).”

“…not necessarily aligned with the rotational axis of the galactic disks, and such misalignments are observed frequently in other galaxies (e.g., Middelberg et al. 2004; Giroletti et al. 2005; Gallimore et al. 2006; Kharb et al. 2010), and also recently found in the Milky Way (Su & Finkbeiner 2012).”

“The fB is essentially a free parameter, and therefore in our simulations it is set such that the order of magnitude of the resulting magnetic field in the end of the simulations is consistent with observed values of & 50 µG at the GC (Crocker et al. 2010).”

“…of hot gas with temperature T ∼ 106 K extending out to the virial radius of the Milky Way halo with a scale height of a few tens of kpc (Blitz & Robishaw 2000; McCammon et al. 2002; Rasmussen et al. 2003; Fang et al. 2006; Bregman & Lloyd-Davies 2007; Grcevich & Putman 2009; Miller & Bregman 2012).”

“Because of the magnetic draping effect (Lyutikov 2006), the field lines are stretched and compressed in the draping layer, causing field alignment with the bubble surface as well as amplification of field strength.”

“…the main simulation code and is generated by 3D inverse Fourier transform (FFT) of magnetic field that, in k-space, has an amplitude given by (Ruszkowski et al. 2007) Bk ∝ k−11/6 exp(−(k/k1)4) exp(−k2/k), (12) where k = (k2x+k 2 y+k 2 z) 1/2, and k1 and k2 are the cutoff wavenumbers for…”

“Bending of radio jets in some cases can be explained by external ram pressure as galaxies move through an intracluster gas (e.g., Begelman et al. 1979; Balsara & Norman 1992).”

“The amount of cosmic rays that could be accelerated inside the shocks could be roughly estimated based on shock properties in our simulations and an assumed model for diffusive shock acceleration (Enßlin et al. 2007).”

“The anisotropic CR diffusion terms in the total energy and CR energy equations are updated explicitly using fluxes computed from the conservative ‘centered asymmetric differencing’ scheme (Parrish & Stone 2005; Sharma & Hammett 2007).”

“…to be due to some combination of fieldline wandering due to a random field component and magnetic field structure on scales less than the CR gyroradius, which allows the cosmic rays to migrate from one fieldline to another (Duffy et al. 1995; Jokipii 1999; Enßlin & Vogt 2003; Hauff et al. 2010).”

“Assumptions and Numerical Techniques We perform 3D MHD simulations of CR injections from the GC using the adaptive-mesh-refinement (AMR) code FLASH4 (Fryxell et al. 2000; Dubey et al. 2008).”

“…slower and wider jets are required to fulfill the constraints from the observed line ratios (§ 2.3), implying that the relativistic jets from the AGN may have slowed down a little by interacting with the interstellar medium during propagation from pc to kpc scales (e.g., Middelberg et al. 2004).”

“These values are unreasonably large for estimated properties of the Local Group (e.g., McConnachie et al. 2007; Cox & Loeb 2008).”

“…of hot gas with temperature T ∼ 106 K extending out to the virial radius of the Milky Way halo with a scale height of a few tens of kpc (Blitz & Robishaw 2000; McCammon et al. 2002; Rasmussen et al. 2003; Fang et al. 2006; Bregman & Lloyd-Davies 2007; Grcevich & Putman 2009; Miller & Bregman 2012).”

“This model is based on the magnetic ‘tower’ model of Li et al. (2006), in which the magnetic field is composed of a poloidal flux that presumably comes from the dynamo process in black hole accretion disks, as well as a toroidal component that may be generated by shearing of the poloidal flux lines…”

“…the detection of the Fermi bubbles, together with indications of past activity (Koyama et al. 1996; Revnivtsev et al. 2004), suggest that the central SMBH may have regularly undergone cycles of jet activity, or even was at a much more active state for the past 1-10 Myr than now (Totani 2006).”

“It is known that the Milky Way moves in the intragroup medium (IGM).”

“However, we note that there are still large uncertainties in the current constraints on the observed profiles of the hot gaseous halo (Fang et al. 2006; Miller & Bregman 2012).”

“This misalignment could either arise from an intrinsic effect, such as jet precessions (e.g., Falceta-Gonçalves et al. 2010), or could be due to jet interactions with the surrounding medium as the jet travels from pc to kpc scales (e.g., Fanaroff-Riley type I radio sources).”

“In this paper we adopt a more simplistic approach, which is to treat cosmic rays as a second fluid and solve directly for the evolution of CR pressure pcr as a function of r and t (Drury & Voelk 1981; Jones & Kang 1990; Ryu et al. 2003; Mathews & Brighenti 2008; Rasera & Chandran 2008).”

“These values are unreasonably large for estimated properties of the Local Group (e.g., McConnachie et al. 2007; Cox & Loeb 2008).”

“AGN are also considered to be a viable agent for distributing the fields into the intergalactic medium (IGM) and intracluster medium (ICM) by relativistic jets (e.g., Koide et al. 1999; Contopoulos et al. 2009; Sutter et al. 2012).”

“…not necessarily aligned with the rotational axis of the galactic disks, and such misalignments are observed frequently in other galaxies (e.g., Middelberg et al. 2004; Giroletti et al. 2005; Gallimore et al. 2006; Kharb et al. 2010), and also recently found in the Milky Way (Su & Finkbeiner 2012).”

“The hot gas distribution in the inner galaxy is also well fit by a galactic wind (Everett et al. 2008, 2010) emanating from a ring with inner galactocentric radius about 3 kpc.”

“…for radio synchrotron emission from extended extragalactic radio sources (e.g., Scheuer 1974; Blandford & Rees 1974) and CRfilled bubbles (e.g., Laing et al. 2006) observed in massive galaxies and galaxy clusters (McNamara & Nulsen 2007), though their gamma-ray emission cannot be easily…”

“…to be due to some combination of fieldline wandering due to a random field component and magnetic field structure on scales less than the CR gyroradius, which allows the cosmic rays to migrate from one fieldline to another (Duffy et al. 1995; Jokipii 1999; Enßlin & Vogt 2003; Hauff et al. 2010).”

“…the main simulation code and is generated by 3D inverse Fourier transform (FFT) of magnetic field that, in k-space, has an amplitude given by (Ruszkowski et al. 2007) Bk ∝ k−11/6 exp(−(k/k1)4) exp(−k2/k), (12) where k = (k2x+k 2 y+k 2 z) 1/2, and k1 and k2 are the cutoff wavenumbers for…”

“The population of CR electrons is invoked to explain the hard-spectrum microwave synchrotron radiation spatially-correlated with the Fermi bubbles observed by the Wilkinson Microwave Anisotropy Probe (WMAP), also known as the ‘WMAP haze’ (Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012).”

“Bending of radio jets in some cases can be explained by external ram pressure as galaxies move through an intracluster gas (e.g., Begelman et al. 1979; Balsara & Norman 1992).”

“In this paper we adopt a more simplistic approach, which is to treat cosmic rays as a second fluid and solve directly for the evolution of CR pressure pcr as a function of r and t (Drury & Voelk 1981; Jones & Kang 1990; Ryu et al. 2003; Mathews & Brighenti 2008; Rasera & Chandran 2008).”

“This estimate is likely only a lower limit, because the condition for the onset of RT instability becomes more stringent for inflating bubbles (Pizzolato & Soker 2006).”

“In this paper we adopt a more simplistic approach, which is to treat cosmic rays as a second fluid and solve directly for the evolution of CR pressure pcr as a function of r and t (Drury & Voelk 1981; Jones & Kang 1990; Ryu et al. 2003; Mathews & Brighenti 2008; Rasera & Chandran 2008).”

“Assuming the Fermi bubbles are tilted by α degrees from the rotational axis of the Galaxy, the required ram pressure from the wind (Burns et al. 1979) is Pram,wind = Pram,jet tanα, where Pram,jet = ρjv 2 jet is the ram pressure of the jets.”

“Here we present results of a 2D test of a sound wave propagating in a composite of thermal gas and cosmic rays (Rasera & Chandran 2008).”

“Since in a tangled field the effective diffusion coefficient (κiso) can be suppressed compared to that along the field lines (κ‖), likely by a factor of ∼ 3 (Tao 1995), we choose to vary the parallel diffusion coefficient in the range of (0.4− 1.2)× 1029 cm2 s−1.”

“At t = tFermi, the magnetic field within the shells is amplified to & 10 µG, comparable to the equipartition field strength inferred by radio polarization mea- 9 10 surements (Jones et al. 2012).”

“The latter is associated with the turbulent interstellar medium, which has a typical coherence length of ∼ 5 − 50 pc (see a recent review by Noutsos (2012)).”

“For both simulations, the initial tangled field has an average strength of ∼ 1 µG, which corresponds to plasma β = ptot/(B 2/8π) ∼ 1 in the ambient galactic halo (note that pcr = 0 for the ambient gas), in agreement with observations (e.g., Haverkorn & Heesen 2012).”

“Accordingly, the sound speed of the combined fluid of gas and cosmic rays, which is used when solving the Riemann problem in the PPM method as well as when computing the hydrodynamic timestep, is replaced by the effective sound speed (Miniati 2007), c′s = √ γpth + γcrpcr ρ .”

“AGN are also considered to be a viable agent for distributing the fields into the intergalactic medium (IGM) and intracluster medium (ICM) by relativistic jets (e.g., Koide et al. 1999; Contopoulos et al. 2009; Sutter et al. 2012).”

“In simulations following magnetic fields the diffusion is treated anisotropically with one or two orders of magnitude lower diffusion coefficients along the magnetic field lines (Hanasz et al. 2013, Pakmor et al. 2016, Yang et al. 2012).”

“The formation of the bubbles can be modeled by emission of a jet from the black hole (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012), a spherical outflow from the black hole (Zubovas et al. 2011), a wind from supernova explosions (Crocker & Aharonian 2011), or a sequence of shocks from…”

“Yang et al. (2012) are able to produce the observed O viii/O vii ratio and find that the shocked region of the bubbles is ∼108 K while the interior is 107–108 K.”

“We solve the magnetohydrodynamic (MHD) equations using the HLL3R method (Waagan 2009; Bouchut et al. 2010; Waagan et al. 2011), extended to a separate monoenergetic CR fluid similar to Yang et al. (2012).”

“In the extrinsic turbulence picture, i.e., f ≤ 1, the heating rate is −fuA · ∇pc Cosmic ray advection and coupling to the thermal gas is included in the simulations using the modules described in our earlier work (Yang et al. 2012, 2013).”

“Theoretically, models that incorporate self-regulated energy injections from the central supermassive black holes (SMBHs) successfully prevent run-away cooling (Sijacki et al. 2007; Cattaneo & Teyssier 2007; Dubois et al. 2010; Yang et al. 2012b).”

“…type of candidate positron source: It has been suggested that the Fermi bubbles are manifestations of recent jet activity of Sgr A* (Su & Finkbeiner 2012; Yang et al. 2012), and thus may be reminiscent of ordinary radio lobes in external galaxies (see A84, page 10 of 16 e.g. Mingo et al. 2012).”

“2 (Fryxell et al. 2000; Dubey et al. 2008) as extended to include cosmic rays (Yang et al. 2012; Yang et al. 2013; Yang & Ruszkowski 2017; Ruszkowski, Yang & Zweibel 2017) to solve the above equations.”

“Details of the CR physics module can be found in Yang et al. (2012) and Ruszkowski et al. (2017), where we discuss simulations the Fermi bubbles and CR-driven galactic winds, respectively.”

“…of a fairly recent (within a∼Myr timescale) explosive outburst (Su et al. 2010; Mertsch & Sarkar 2011; Zubovas et al. 2011; Guo & Mathews 2012; Yang et al. 2012; Zubovas & Nayakshin 2012; Fujita et al. 2013; Barkov & Bosch-Ramon 2014; Mou et al. 2014; Wardle & Yusef-Zadeh 2014) from the…”

“It is worth remarking that the expansion velocity that we derived conflicts with the order-of-magnitude higher values advocated in the literature (Guo & Mathews 2012; Guo et al. 2012; Zubovas & Nayakshin 2012; Yang et al. 2012; Lacki 2013).”

“This wind age is consistent with the timescale of energy injection that created the FBs, whether via AGN jets (Guo & Mathews 2012; Yang et al. 2012), feedback from nuclear star formation (Crocker & Aharonian 2011; Carretti et al. 2013; Lacki 2014), accretion flows onto SgrA∗ (Cheng et al. 2011; Mou…”

“The method of initializing a tangled field is illustrated in Yang et al. (2012a) and Yang et al. (2013), to which we refer the readers for details.”

“(72) includes CR diffusion and streaming. The diffusion term is usually written as r (k recr), where k is the diffusion coefficient. For isotropic CR diffusion, the diffusion term can be rewritten as [418] r(krecr) = r(kkbb ecr)+r[k?(I bb)ecr]. (76) Galaxies 2018, 6, 1 26 of 47 where k is the diffusion tensor, the kkan…”

“Following equations (18) and (19) from Yang et al. (2012), we can estimate the timescales for the growth of both instabilities.”

“…with AGN jets have characteristic total energy injection rates and ages of & 1044 erg s−1 and ≈1–3 Myr (e.g., Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012, 2013), whereas simulations producing the bubbles from weaker AGN winds suggest values of 1041–1042 erg s−1 and 5–10 Myr (e.g.,…”

“…from some episode of energy injection at the GC, possibly associated with nuclear star formation processes (Crocker & Aharonian 2011; Carretti et al. 2013), or with past activity of the central AGN (Cheng et al. 2011; Zubovas & Nayakshin 2012; Guo & Mathews 2012; Yang et al. 2012, hereafter Y12).”

“This is different from current simulations of Sgr A⋆ outbursts, which assume that the gas in these regions was initially halo gas (e.g., Guo & Mathews 2012; Yang et al. 2012).”

“Outflows triggered by star formation in the Galactic centre (GC) region (Crocker 2012; Lacki 2014) and by the black hole at the GC (Guo & Mathews 2012; Yang et al. 2012; Mou et al. 2014) have been proposed for the dynamical origin of the FBs.”

“, [21-24]) and others have a starburst outflow (e.”

“…energetic magnetized plasma (see the discussion in Elmouttie et al. 1998; Mingo et al. 2012), while in the case of our Galaxy, it is supported by general energetic arguments and numerical simulations reproducing well the observed properties of bubbles (e.g., Guo & Mathews 2012; Yang et al. 2012).”

“CR advection and coupling to the gas are included using the same methods as in Ruszkowski et al. (2017) (see Yang et al. 2012, 2013; Sharma et al. 2009) with the CR streaming flux denoted by Fcr and an associated CR heating of the gas denoted by Hcr.”

“Yang et al. (2012) studied the reduction of the diffusion coefficient for Fermi bubbles by the latter.”

“…MI transport (e.g., Crocker & Aharonian 2011; Mou et al. 2014; Crocker et al. 2015), leptonic transport (e.g., Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012, 2013, the latter two are abbreviated as Y12 and Y13, respectively), and in-situ acceleration models (e.g., Mertsch & Sarkar 2011;…”

“Numerous works followed that similarly investigated injecting a combination of thermal energy and momentum [61,111] as well as works that considered purely kinetic jets [62,68,70,71,110] that were able to inhibit cooling and match a range of observed cluster properties.”

“…were created by some large episode of energy injection in the GC, such as an active galactic nucleus (AGN)like outburst (e.g., Guo et al. 2012; Yang et al. 2012) or from nuclear starburst activity (e.g., Lacki 2014) in the past with an energy release of 1055–56 erg over 10Myr ago (Su et al.…”

“Yang et al. (2012) simulate the onset of a CR-driven outflow comparable to the Milky Way Fermi bubbles using anisotropic CR diffusion.”

“Thirdly, the velocity of the jet required in the Yang et al. (2012, 2013) is very low, only ∼ 0.03c.”

“Several models have been proposed to explain both the morphology and spectral properties of the FBs (Cheng et al. 2011; Mertsch & Sarkar 2011; Crocker & Aharonian 2011; Crocker 2012, 2013; Zubovas et al. 2011; Zubovas & Nayakshin 2012; Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012).”

“We also use an additional cosmic ray module that evolves cosmic rays as a second, relativistic fluid in addition to the usual thermal gas (Yang et al. 2012; Ruszkowski et al. 2017).”

“…et al. 1991; Everett et al. 2008; Girichidis et al. 2016; Ruszkowski et al. 2017; Wiener et al. 2017), ionize the interstellar medium (Grenier et al. 2015), and contribute to the formation of the Fermi Bubbles (Guo et al. 2012; Yang et al. 2012) and the galactic dynamo (Hanasz et al. 2009).”

“In other models, CRs are carried by jets or outflows generated by the super-massive black hole at the GC (Zubovas et al. 2011; Guo & Mathews 2012; Yang et al. 2012) or they are accelerated in the bubbles (Mertsch & Sarkar 2011; Cheng et al. 2011).”

“Simulations of the Fermi bubbles as overpressured structures driven by bursts of AGN activity into the ambient interstellar medium were developed by Guo & Mathews (2012), Guo et al. (2012), Yang et al. (2012), and Yang et al. (2013).”

“First, AGN driven models in which the bubbles originate from a past accretion activity of the MW central black hole over a time scale of 3–12 Myr, with a luminosity of ∼ 2 × 1041–1043 erg s−1 , either via an accretion wind (AGNW) (Zubovas, King & Nayakshin 2011; Zubovas & Nayakshin 2012; Mou et al. 2014, 2015) or via…”

“To explain, then, the large extent and energy content of the Bubbles some models (e.g., Zubovas & Nayakshin 2012; Guo & Mathews 2012; Yang et al. 2012, 2013; Barkov and Bosch-Ramon 2013) hypothesise recent activity of the super-massive black hole at the Galactic Center (GC) with very fast transport…”

“However, while structures such as the Fermi bubbles may be indicators of past activity (Guo & Mathews 2012; Yang et al. 2012), the majority of the emission seen from the inner ∼30 pc is due to the dense complex of star-formation.”

“Their origin and nature is an active area of observational and theoretical research (see Dobler et al. 2010; Su et al. 2010; Crocker & Aharonian 2011; Yang et al. 2012; Ackermann et al. 2014; Yang et al. 2014; Crocker et al. 2015, and references therein).”

“Other active galactic nucleus (AGN) related explanations are given by Guo & Matthews (2012), Yang et al. (2012), and Zubovas et al. (2011).”

“They may drive galactic outflows, help form large-scale structures like the Fermi Bubbles (Guo et al. 2012; Yang et al. 2012), and play a role in galactic dynamos (Hanasz et al. 2009, and references therein).”

“…- (i) high luminosity (∼ 1042−44 erg s−1) wind driven by the central black hole (Zubovas et al. 2011; Guo et al. 2012; Zubovas & Nayakshin 2012; Yang et al. 2012; Yang & Ruszkowski 2017) requiring the age of FBs to be tage ∼ few Myr, (ii) low luminosity (∼ 2 × 1041 erg s−1) wind driven by…”

“We notethat even more energy can be released in the GC, ∼W 1056 erg, if a giant molecular cloud is captured by the black hole (see Zubovas & Nayakshin 2012; Yang et al. 2012).”

“…structures may suggest past large-scaled activities in the Galactic Center, such as active galactic nucleus jet activity (e.g., Guo & Mathews 2012; Yang et al. 2012), non-relativistic outflow from the nucleus (e.g., Zubovas et al. 2011; Zubovas & Nayakshin 2012; Mou et al. 2014), or wind driven…”

“This problem can be overcome using models based on the jet activity of the central active galactic nucleus where jet speeds of over 104 km s−1 are readily achievable (Guo & Mathews 2012; Yang et al. 2012).”

“…can be provided either by a shock or shocks which are generated by tidal processes nearby the central black hole or by an MHD-turbulence which is excited behind a shock (for some aspects of the leptonic model see e.g. Cheng et al. 2011, 2014; Mertsch & Sarkar 2011; Yang et al. 2012, and others).”

“The magnetohydrodynamic equations are extended by a mono-energetic, relativistic cosmic-ray fluid (Yang et al. 2012) in the advection- 6 SImulating the Life-Cycle of molecular Clouds, http://hera.ph1.uni-koeln.de/∼silcc/ 2 diffusion approximation as described in Girichidis et al. (2014).”

“In this regard, numerical hydrodynamical simulations of the Fermi bubbles were performed in 2D by Guo & Mathews (2012) and Guo et al. (2012) and 3D by Yang et al. (2012).”

“Bubbles arising from jets have been simulated by several authors (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012); more recently, detailed simulations of the leptons in Active Galactic Nuclei (AGN) jets and the resulting inverse Compton and synchrotron signals have been carried out (Yang,…”

“Yang et al. (2012) successfully reproduced the morphology of the Fermibubbles and concluded that the suppression of magnetohydrodynamic (MHD) instabilities can be explained by a draped magnetic field layer.”

“…shape of the bubbles as observed in gamma-rays (Su et al. 2010; Ackermann et al. 2014) and used in the (M)HD simulations (Guo & Mathews 2012; Yang et al. 2012), leads us to employ toroidal coordinates (u, v, φ) which map to cartesian coordinates (x, y, z) through x = a sinh v cos φ cosh v −…”

“Competing theories for their origin involve either jets (or collimated outflows) from an active galactic nucleus (AGN) (75; 76; 77; 78), or the outflow from either a nuclear starburst or a prolonged high rate of star formation in the Galactic center (79; 80; 81; 82).”

“In one picture, the Fermi γ-ray bubbles are a relic of past AGN-like activity stimulated by accretion of stars or gaseous material onto the central supermassive black hole, Sgr A* (Zubovas, King and Nayakshin 2011; Cheng et al. 2011; Guo et al. 2012; Yang et al. 2012, 2013).”

“…cosmic rays (CRs) interacting with hot gas in the halo (Crocker & Aharonian 2011), to star bursts (Biermann et al. 2010), to AGN activity in our Galaxy (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012; Cheng et al. 2011), to originating from dark matter annihilation (Dobler et al. 2011).”

“The boundary of the Fermi bubbles is generally determined by the CD, because the magnetic field in the post-shock ISM is parallel to the CD, which can bound the CRs inside the CD (Yang et al. 2012), or outside but close to the CD (Mou et al. 2015).”