A UNIFIED MODEL OF THE FERMI BUBBLES, MICROWAVE HAZE, AND POLARIZED RADIO LOBES: REVERSE SHOCKS IN THE GALACTIC CENTER’S GIANT OUTFLOWS

“…SN 2.8 10 erg s SFR 0.08 yr , (2.3) GC SF GC 51 40 1 GC 1 ´ ´ = ´ æ è çççç ö ø ÷÷÷÷÷ - - where a single core-collapse SN requires the formation of ∼90Me in stars (for a Kroupa 2001 initial mass function if the least massive star to explode has zero-age main-sequence mass of 8Me), we assume…”

“Values Le/LCR 0.1 are disfavored on theoretical grounds (Bell 1978), on the basis of analysis of the Galaxy’s global cosmic-ray and nonthermal radiation budget (Strong et al. 2010), and from observations and analysis of nonthermal radiation associated with giant cluster shocks (Pinzke et al. 2013;…”

“Our model generalizes the usual treatment of a wind-driven bubble (e.g., Weaver et al. 1977), in that the bubble is not assumed to be strongly overpressured with respect to the ambient medium and is also subject to significant optically thin radiative cooling.”

“Currently, star formation activity in the Galactic center is concentrated east of l = 0; there is a preponderance of molecular gas on this side of the region, and Sagittarius B2, one of the Galaxy’s most active star-forming giant molecular clouds, resides there (Morris & Serabyn 1996).”

“For IC emission, γ-rays at Eγ = 1 GeV are upscattered by primary electrons of energy E 640e GeV(Eγ/1 GeV)1/2(Eγ,0/ 6.3 × 10−4 eV)−1/2 adopting an energy for the photon background characteristic of the CMB. Finally, proton primaries of hadronic γ-rays in the observed range of 1–100 GeV have energies covering the approx…”

“…grounds (Bell 1978), on the basis of analysis of the Galaxy’s global cosmic-ray and nonthermal radiation budget (Strong et al. 2010), and from observations and analysis of nonthermal radiation associated with giant cluster shocks (Pinzke et al. 2013; Brunetti & Jones 2014, and references therein).”

“Cosmic-ray transport downstream of the shock is a combination of the ∼100–150 km s−1 flow of the shocked, downstream plasma and cosmic-ray streaming along the field lines at the Alfvén velocity (Kulsrud & Pearce 1969; Skilling 1971) of ∼240 km s−1.”

“The total cooling luminosity, Lcool shell, of the compression shell, R3, should not exceed the upper limit on the bolometric luminosity of the X-ray bulge, which, conservatively, is 1040 erg s−1 (Snowden et al. 1997; Almy et al. 2000).”

“These values are typical for determinations of the cosmic-ray acceleration efficiency of the Galaxy (expressed as a fraction of the input mechanical power from SNe;Hillas 2005; Strong et al. 2010), in giant, star-formation-driven outflows from other systems (e.g., NGC 253: Zirakashvili & Völk 2006), and for giant shoc…”

“Cosmic-ray transport downstream of the shock is a combination of the ∼100–150 km s−1 flow of the shocked, downstream plasma and cosmic-ray streaming along the field lines at the Alfvén velocity (Kulsrud & Pearce 1969; Skilling 1971) of ∼240 km s−1.”

“This is close to the radius of the central molecular zone (CMZ) molecular torus, 80–100 pc (Molinari et al. 2011).”

“…results indicate that the mass efflux in the outflow is Ṁ ∼ 0.03–0.1Me yr−1, rather close to the current rate of nuclear star formation (Crocker 2012; Longmore et al. 2013, and references therein), and in turn close to the long-time-averaged nuclear SFR.15 Moreover, on the basis of the…”

“…have been discovered in total-intensity radio and microwave emission, polarized radio and microwave emission, and X-ray emission (Bland-Hawthorn & Cohen 2003; Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012a; Ade et al. 2013; Carretti et al. 2013; Kataoka et al. 2013; Tahara et al. 2015).”

“For IC emission, γ-rays at Eγ = 1 GeV are upscattered by primary electrons of energy E 640e GeV(Eγ/1 GeV)1/2(Eγ,0/ 6.3 × 10−4 eV)−1/2 adopting an energy for the photon background characteristic of the CMB. Finally, proton primaries of hadronic γ-rays in the observed range of 1–100 GeV have energies covering the approx…”

“…of the bubbles have been discovered in total-intensity radio and microwave emission, polarized radio and microwave emission, and X-ray emission (Bland-Hawthorn & Cohen 2003; Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012a; Ade et al. 2013; Carretti et al. 2013; Kataoka et al. 2013;…”

“Other inputs to our modeling are the parameterization of the Galaxy’s gravitational potential (Breitschwerdt et al. 1991) and the plasma cooling function (Dopita et al. 2013).”

“The Fermi bubbles are enigmatic, giant γ-ray structures emanating north and south from the nucleus of the Milky Way (Dobler et al. 2010; Su et al. 2010; Su & Finkbeiner 2012; Ackermann et al. 2014; Yang et al. 2014) and extending to heights above the Galactic plane approaching 8 kpc. Features…”

“An important ingredient of our scenario is that recent work indicates that the gas density and pressure in the halo, especially in the vicinity of the bubbles, are larger than has often been assumed in the past (e.g., Stocke et al. 2013; Miller & Bregman 2014).7 5.”

“A constant-volume emissivity model would produce a center-brightened morphology, and the original instantiation of the hadronic model (Crocker & Aharonian 2011) was criticized on this ground.”

“This finding complements the idea that (some part of) the NPS is a signature of Galactic nuclear activity (Sofue 1977, 2000; Bland-Hawthorn & Cohen 2003; Totani 2006) and perhaps even related directly to the Fermi bubbles (Guo & Mathews 2012; Kataoka et al. 2013; Mou et al. 2014).”

“Indeed, Weaver et al. (2000) and Strickland et al. (2002) demonstrate that fitting to X-ray plasma emission from starburst outflows often leads to unphysically low metallicity estimates.”

“…have been discovered in total-intensity radio and microwave emission, polarized radio and microwave emission, and X-ray emission (Bland-Hawthorn & Cohen 2003; Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012a; Ade et al. 2013; Carretti et al. 2013; Kataoka et al. 2013; Tahara et al. 2015).”

“…emerges as r V n n v t v n n 2 70 500 km s 0.004 cm 0.1 cm pc, (2.6) 0 FB tot FB tot GC wind cool wind 1 1 2 tot FB 3 1 2 tot GC 3 1 2 p æ è ççç ö ø ÷÷÷ ´ æ è çççç ö ø ÷÷÷÷ æ è çççç ö ø ÷÷÷÷ - - - - - where we normalize ntot GC to a value informed by GC X-ray observations (Muno et al. 2004).”

“Indeed, there is diffuse, polarized 23 GHz emission evident over most of the solid angle of the bubbles (see Figure 3 of Carretti et al. 2013).”

“…center wind, with a kinetic power ∼1040 ergs s−1, is relatively weak, compared to, say, the ∼1042 erg s−1 wind from the dwarf starburst galaxy M82 (Shopbell & Bland-Hawthorn 1998; Cooper et al. 2008), andin the case of the Galactic center, gravity reduces the velocity of the wind on scales ∼kpc.”

“…determinations of the cosmic-ray acceleration efficiency of the Galaxy (expressed as a fraction of the input mechanical power from SNe;Hillas 2005; Strong et al. 2010), in giant, star-formation-driven outflows from other systems (e.g., NGC 253: Zirakashvili & Völk 2006), and for giant shocks in…”

“…be rather directly and robustly determined from the fact that the total stellar mass of the nuclear bulge (the stellar population coincident with the central molecular zone) is ∼109 Me,which has been formed over a timescale 1010 yr (Serabyn & Morris 1996; Figer et al. 2004). outwardly moving CD.”

“…grounds (Bell 1978), on the basis of analysis of the Galaxy’s global cosmic-ray and nonthermal radiation budget (Strong et al. 2010), and from observations and analysis of nonthermal radiation associated with giant cluster shocks (Pinzke et al. 2013; Brunetti & Jones 2014, and references therein).”

“…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…”

“Broadly speaking, the Bubbles might be the signatures of a fairly recent (within∼Myr timescale) explosive outburst (Su et al. 2010; Zubovas et al. 2011; Mertsch & Sarkar 2011; Zubovas & Nayakshin 2012; Guo & Mathews 2012; Yang et al. 2012; Fujita et al. 2013; Wardle 1 Research School of Astronomy…”

“…of the Galaxy (expressed as a fraction of the input mechanical power from SNe;Hillas 2005; Strong et al. 2010), in giant, star-formation-driven outflows from other systems (e.g., NGC 253: Zirakashvili & Völk 2006), and for giant shocks in galaxy clusters (Brüggen et al. 2012; Pinzke et al. 2013).”

“Such a multitemperature distribution is inferred, e.g., from the coincident X-ray and Hα emission observed from filaments around the bright central galaxies in clusters (e.g., Fabian et al. 2011) and is common to cooling flows.”

“…processes associated with the inner Galaxy (Thoudam 2013) or Galactic nucleus, e.g., tidal disruption events that occur there every 104–105 yr (Cheng et al. 2011, 2015) or the concentrated and sustained star formation activity (e.g., Lacki 2014) that occurs throughout the nuclear region…”

“Broadly speaking, the bubbles might be the signatures 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.…”

“The ∼50 km s−1 relative speed (van der Marel et al. 2012) of this motion implies a rather gentle effect that requires long timescales/low bubble gas velocities to take effect, consistent with the ∼100–150 km s−1 flow velocities we determine below for gas in the bubbles (see Yang et al. 2012, where…”

“…have been discovered in total-intensity radio and microwave emission, polarized radio and microwave emission, and X-ray emission (Bland-Hawthorn & Cohen 2003; Finkbeiner 2004; Dobler & Finkbeiner 2008; Dobler 2012a; Ade et al. 2013; Carretti et al. 2013; Kataoka et al. 2013; Tahara et al. 2015).”

“…through the plane, and, moreover, interaction and mixing between bubble matter and the surrounding halo gas around the CD may catalyze the condensation of halo gas so that, overall, more gas fountains back to the plane than is lofted upward(Marinacci et al. 2010), some of it low metallicity.”

“…that approaches 1039 erg s−1, and that the particles are delivered into the halo but not too far above the disk, these shocks are interesting candidates (see Parizot 2014) for the main accelerators of Galactic cosmic rays in this energy range (see Cheng et al. 2012; Lacki 2014; Taylor et al. 2014).”

“The nucleus directly accelerates a population of hardspectrum primary electrons that is advected from the region before it can lose much energy radiatively in situ (Crocker et al. 2011a, 2011b; Crocker 2012; Carretti et al. 2013).”

“…in a flow driven into the halo by star formation occurring in the Milky Way’s molecular ring (at galactocentric radius of ∼4 kpc) and/or the bar (Breitschwerdt et al. 1991, 2002; Everett et al. 2008, 2010; Kretschmer et al. 2013; de Boer & Weber 2014); halo density models that assume…”

“It has long been argued on the basis of stellar polarization data that the NPS is a structure of the local ISM (within a few × 102 pc; Bingham 1967; Berkhuijsen et al. 1971).”

“…be rather directly and robustly determined from the fact that the total stellar mass of the nuclear bulge (the stellar population coincident with the central molecular zone) is ∼109 Me,which has been formed over a timescale 1010 yr (Serabyn & Morris 1996; Figer et al. 2004). outwardly moving CD.”

“…k m s k ´ ´ = g Here, in the first line, the first 3/2 prefactor (approximately) corrects for the presence of heavy ions among beam and target nuclei (Mori 1997), 1/3 comes from the relative multiplicity of π0 among all daughter pions, fbol is the fraction of the bolometric luminosity emitted…”

“This finding complements the idea that (some part of) the NPS is a signature of Galactic nuclear activity (Sofue 1977, 2000; Bland-Hawthorn & Cohen 2003; Totani 2006) and perhaps even related directly to the Fermi bubbles (Guo & Mathews 2012; Kataoka et al. 2013; Mou et al. 2014).”

“…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 central supermassive…”

“…in the Milky Way’s molecular ring (at galactocentric radius of ∼4 kpc) and/or the bar (Breitschwerdt et al. 1991, 2002; Everett et al. 2008, 2010; Kretschmer et al. 2013; de Boer & Weber 2014); halo density models that assume hydrostatic equilibrium may not convey the full story in the inner…”

“The nucleus directly accelerates a population of hardspectrum primary electrons that is advected from the region before it can lose much energy radiatively in situ (Crocker et al. 2011a, 2011b; Crocker 2012; Carretti et al. 2013).”

“…Fermi bubbles are enigmatic, giant γ-ray structures emanating north and south from the nucleus of the Milky Way (Dobler et al. 2010; Su et al. 2010; Su & Finkbeiner 2012; Ackermann et al. 2014; Yang et al. 2014) and extending to heights above the Galactic plane approaching 8 kpc. Features…”

“This finding complements the idea that (some part of) the NPS is a signature of Galactic nuclear activity (Sofue 1977, 2000; Bland-Hawthorn & Cohen 2003; Totani 2006) and perhaps even related directly to the Fermi bubbles (Guo & Mathews 2012; Kataoka et al. 2013; Mou et al. 2014).”

“However, we adopt Equation (2.3) as our fiducial value for the injected power (see Lacki 2014).”

“The population of H I clouds entrained into the nuclear outflow detected by McClure-Griffiths et al. (2013) and the high-speed, warm ionized material found by Fox et al. (2014) together constitute plausible evidence that exactly such distributed mass loading is taking place in the nuclear outflow,…”

“In Section 1 we described UV absorption evidence (Keeney et al. 2006; Zech et al. 2008) for higher-altitude warm ionized material in the Galactic bulge that is participating in a nuclear fountain flow.”

“Some of this material (Zech et al. 2008) appears significantly supersolar in metallicity.”

“E( GeV) yr1- , ignoring non-synchrotron losses (particularly IC) for illustrative purposes and assuming pitch-angle scattering on timescales short with respect to the loss times (e.g., Reynolds 2009).”

“The NPS exhibits total-intensity and polarized, nonthermal radio, and microwave emission (Sofue & Reich 1979; Sun et al. 2014; Vidal et al. 2014), thermal X-ray emission (Snowden et al. 1997; Willingale et al. 2003), and ∼GeV γ-ray emission (Casandjian et al. 2009; Ackermann et al. 2014).”

“In particular, the NPS(Bolton & Westfold 1950) emerges north from the Galactic plane at l ∼ 20°–30° and rises to heights above b ∼ 80°, merging with the Loop I radio structure (Large et al. 1962; Haslam et al. 1982).”

“…by star formation occurring in the Milky Way’s molecular ring (at galactocentric radius of ∼4 kpc) and/or the bar (Breitschwerdt et al. 1991, 2002; Everett et al. 2008, 2010; Kretschmer et al. 2013; de Boer & Weber 2014); halo density models that assume hydrostatic equilibrium may not convey…”

“…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 central supermassive black hole with their γ-ray…”

“Indeed, Weaver et al. (2000) and Strickland et al. (2002) demonstrate that fitting to X-ray plasma emission from starburst outflows often leads to unphysically low metallicity estimates.”

“…Dobler & Finkbeiner 2008; Dobler 2012a; Ade et al. 2013) in total-intensity data up to Galactic latitudes b 35 ∣ ∣ , above which it becomes severely attenuated, though there has been a claim (Dobler 2012b) that faint, haze-like emission is detected up to the latitudinal extremities of the bubbles.”

“It has long been argued on the basis of stellar polarization data that the NPS is a structure of the local ISM (within a few × 102 pc; Bingham 1967; Berkhuijsen et al. 1971).”

“The total cooling luminosity, Lcool shell, of the compression shell, R3, should not exceed the upper limit on the bolometric luminosity of the X-ray bulge, which, conservatively, is 1040 erg s−1 (Snowden et al. 1997; Almy et al. 2000).”

“Furthermore, the fact that the γ-ray spectrum of the bubbles does not steepen (Ackermann et al. 2014)—rather, it seems to harden (Selig et al. 2014; Yang et al. 2014)—with increasing 5 Future Fellow.”

“It has long been argued on the basis of stellar polarization data that the NPS is a structure of the local ISM (within a few × 102 pc; Bingham 1967; Berkhuijsen et al. 1971).”

“The nucleus directly accelerates a population of hardspectrum primary electrons that is advected from the region before it can lose much energy radiatively in situ (Crocker et al. 2011a, 2011b; Crocker 2012; Carretti et al. 2013).”

“Some of us have recently shown, however, that accumulation of hadronic cosmic rays into gas condensations formed by local thermal instability in the bubbles can significantly reduce the timescale associated with the hadronic scenario to ∼few × 108 yr (Crocker et al. 2014).”

“…basis of various independent analyses of X-ray, radio polarization, and other data, that it is actually a Galactic-scale feature (Kataoka et al. 2013; Fang & Jiang 2014; Sun et al. 2014; Sofue 2015; Tahara et al. 2015) or perhaps a confusion of two structures, one local and one at Galactic scales.”

“…processes associated with the inner Galaxy (Thoudam 2013) or Galactic nucleus, e.g., tidal disruption events that occur there every 104–105 yr (Cheng et al. 2011, 2015) or the concentrated and sustained star formation activity (e.g., Lacki 2014) that occurs throughout the nuclear region…”

“The Galactic Center Spur (GCS;Sofue et al. 1989; Carretti et al. 2013; see Figure 13) is a highly collimated total- and polarized-intensity radio continuum feature that extends in a northwest direction from just east of the Galactic center to Galactic latitudes of b ∼ 25°.”

“…there are linear γ-ray and polarized radio arcs that run concentrically between the NPS and the eastern edge of the northern bubble (Su et al. 2010; Jones et al. 2012); fainter radio and X-ray spurs have also been claimed as the northwestern, southeastern, and southwestern complements of the NPS…”

“This outflow, however, is not powerful enough—given the strong gravitational field, the thermal pressure of the overlying halo gas, and the occurrence of gas cooling—to break out of the Galaxy.”

“…Dobler & Finkbeiner 2008; Dobler 2012a; Ade et al. 2013) in total-intensity data up to Galactic latitudes b 35 ∣ ∣ , above which it becomes severely attenuated, though there has been a claim (Dobler 2012b) that faint, haze-like emission is detected up to the latitudinal extremities of the bubbles.”

“…processes associated with the inner Galaxy (Thoudam 2013) or Galactic nucleus, e.g., tidal disruption events that occur there every 104–105 yr (Cheng et al. 2011, 2015) or the concentrated and sustained star formation activity (e.g., Lacki 2014) that occurs throughout the nuclear region…”

“The Quintuplet, at ∼4Myr old, is likely to have already hosted a number of very massive core-collapse SNe, and there is tantalizing evidence that some such SNe have occurred (Sofue 2003).”

“…explain the spectrum of the microwave haze with such secondaries since IC and synchrotron losses would be expected, in a single-zone model, to steepen the steady-state secondary distribution such that it is inconsistent with the hard haze emission (e.g., Ackermann et al. 2014; Cheng et al. 2014b).”

“…(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 central supermassive black hole with their γ-ray emission…”

“There are counterparts to this feature evident in γrays (where the feature appears as an edge particularly evident in 10–30 GeV Fermi data; see de Boer & Weber 2014, Figure 3 (a); see also R. Yang 2015, private communication) and in Hα emission.13 At 2.3 GHz, this feature is likely the result of…”

“…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 central supermassive black hole with their γ-ray emission originating in inverse Compton (IC) collisions of a…”

“…public release, Alex Hill for providing information about Hα emission in the southern bubble as observed by the Wisconsin H-Alpha Mapper (WHAM) southern sky survey (Haffner et al. 2010), and Ruizhi Yang for providing processed, preliminary maps of the Fermi-detected emission in the southern bubble.”

“Second, observations of the so-called “Fermi Bubbles” (see, e.g., Crocker et al. 2015 and references therein) indicate that powerful winds from the Galactic Center region have evacuated large bubbles extending to z-distances of ∼8 kpc on both sides of the Galactic plane.”

“5 The scenario in which slow but sustained (& 108 − 109 yr) energy injection from protracted star formation activity in the GC is responsible for the Fermi Bubbles (e.g., Crocker and Aharonian 2011; Crocker et al. 2015) appears to be inconsistent with the energy injection rate inferred from the X-ray properties of the…”