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Cited by 120Astrophysics and Cosmic PhenomenaAstrophysical Phenomena and ObservationsGamma-ray bursts and supernovae

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

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Crocker, Roland M., Bicknell, Geoffrey V., Taylor, Andrew M., Carretti, Ettore·2015·The Astrophysical Journal
DOI 10.1088/0004-637x/808/2/107
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Abstract

The Galactic center's giant outflows are manifest in three different, nonthermal phenomena: (1) the hard-spectrum, γ -ray "Fermi bubbles" emanating from the nucleus and extending to ; (2) the hard-spectrum, total-intensity microwave (∼20–40 GHz) "haze" extending to in the lower reaches of the Fermi bubbles; and (3) the steep-spectrum, polarized, "S-PASS" radio (∼2–20 GHz) lobes that envelop the bubbles and extend to . We find that the nuclear outflows inflate a genuine bubble in each Galactic hemisphere that has the classical structure, working outward, of reverse shock, contact discontinuity (CD), and forward shock. Expanding into the finite pressure of the halo and given appreciable cooling and gravitational losses, the CD of each bubble is now expanding only very slowly. We find observational signatures in both hemispheres of giant, reverse shocks at heights of ∼1 kpc above the nucleus; their presence ultimately explains all three of the nonthermal phenomena mentioned above. Synchrotron emission from shock-reaccelerated cosmic-ray electrons explains the spectrum, morphology, and vertical extent of the microwave haze and the polarized radio lobes. Collisions between shock-reaccelerated hadrons and denser gas in cooling condensations that form inside the CD account for most of the bubbles' γ -ray emissivity. Inverse Compton emission from primary electrons contributes at the 10%–30% level. Our model suggests that the bubbles are signatures of a comparatively weak but sustained nuclear outflow driven by Galactic center star formation over ≳few × 10 8 yr.

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Crossref · 10.1088/0004-637x/808/2/107

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  • Either the diffusion has a steep dependence on the electron energy or the diffusion does not play a role, in which case the 2.3 GHz emitting electrons are simply advected out from the Galaxy over a time-scale of t_sync,cool ∼ 25 Myr.

    “Therefore, either the diffusion has steep dependence on the electron energy (perhaps due to how it 'leaks' from the FBs; Crocker et al. 2015) or the diffusion does not play a role at all, in which case, the 2 . 3 GHz emitting electrons are simply advected out from the Galaxy over a time-scale of t sync , cool ∼ 25 Myr.”

    From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Crocker et al. 2015

  • The age of the bubbles can be much lower, ≲300 Myr, if the p-p interaction occurs in denser clouds of about 0.1 cm^-3 inside the bubbles or at the contact discontinuity.

    “The age of the bubbles can be much lower ( ≲ 300 Myr) if the p -p interaction occurs in denser clouds ( ∼ 0 . 1 cm -3 ), probably condensed out of the low-density medium, inside the bubbles or at the contact discontinuity (de Boer & Weber 2014; Crocker et al. 2015; Mou et al. 2015).”

    From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Crocker et al. 2015

  • Although the p-p channel can successfully explain the γ-ray spectrum from the Fermi bubbles, the synchrotron emission from secondary leptonic particles fails to explain the spectrum or intensity of the microwave haze emission.

    “Although the p -p channel can successfully explain the γ -ray spectrum from the FBs, the synchrotron emission from the secondary leptonic particles fails to explain the spectrum or the intensity of the microwave haze emission (Ackermann et al. 2014; Crocker et al. 2015; Cheng et al. 2015a).”

    From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Crocker et al. 2015

  • The dynamical time in such a case is t_dyn ∼110 Myr.

    “The dynamical time in such a case is t dyn ∼ 110 Myr (Crocker et al. 2015).”

    From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Crocker et al. 2015

  • A similar bending of the polarized radio ridges near the waist of the FBs also indicates a slow rise of these ridges over a time scale of ≳10 Myr.

    “A similar bending of the polarized radio ridges near the waist of the FBs also indicates a slow rise of these ridges over a time scale of ≳ 10 Myr (Crocker et al. 2015).”

    From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Crocker et al. 2015

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Authors
Crocker, R. M., Bicknell, G. V., Taylor, A. M., & Carretti, E.
Affiliations
  • Australian National University (AU)
  • Dublin Institute For Advanced Studies (IE)
  • Commonwealth Scientific and Industrial Research Organisation (AU)
ORCID
Venue
The Astrophysical Journal
Publisher
American Astronomical Society
Published
2015-07-24
Volume
808
Issue
2
Pages
107
ISSN
  • 0004-637X (ISSN-L)
  • 1538-4357
DOI
10.1088/0004-637x/808/2/107
OpenAlex
W2119218368
Cited by
120
References
98
Citations by year
2026: 5 · 2025: 7 · 2024: 11 · 2023: 11 · 2022: 13 · 2021: 12 · 2020: 7 · 2019: 10 · 2018: 15 · 2017: 13 · 2016: 13 · 2015: 3
Type
other
Language
en
Open access
Licence
Locations
  • The Astrophysical Journal (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • ANU Open Research (Australian National University) (submittedVersion)
  • arXiv (Cornell University)
Funders
Australian Research Council
Topics
  • Physical Sciences › Physics and Astronomy › Nuclear and High Energy Physics › Astrophysics and Cosmic Phenomena
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysical Phenomena and Observations
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Gamma-ray bursts and supernovae
Keywords
Fermi bubbles, reverse shock, synchrotron radiation, inverse Compton emission, contact discontinuity, cosmic ray acceleration, hadron collisions, Galactic halo, stellar feedback, gamma-ray emission

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