Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays

Abstract

Recently evidence has emerged for enormous features in the γ-ray sky observed by the Fermi-LAT instrument: bilateral 'bubbles' of emission centered on the core of the Galaxy and extending to around ±10 kpc above and below the Galactic plane. These structures are coincident with a non-thermal microwave 'haze' found in WMAP data and an extended region of X-ray emission detected by ROSAT. The bubbles' γ-ray emission is characterised by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity ∼ 4 × 10 37 erg/s, around one order of magnitude larger than their microwave luminosity while more than order of magnitude less than their X-ray luminosity. Here we show that the bubbles are naturally explained as due to a population of relic cosmic ray protons and heavier ions injected by processes associated with extremely long timescale ( > ∼ 8 Gyr) and high areal density star-formation in the Galactic center.PACS numbers: 98.35. Nq,98.62.Nx,98.70.Rz,98.35.Jk A recent analysis [1,2] of Fermi-LAT [3] γ-ray data has revealed two, enormous bubble-like emission features centered on the core of the Galaxy and extending to around ±10 kpc above and below the Galactic plane. At lower Galactic latitudes these structures are coincident with a non-thermal microwave 'haze' found in WMAP 20-60 GHz data [4,5] and an extended region of diffuse X-ray emission detected by ROSAT [6].A natural explanation of these structures [2] would be that they are due to the same population of highlyrelativistic ( > ∼ 50 GeV) cosmic ray (CR) electrons which synchrotron radiate at multi-GHz frequencies and simultaneously produce > ∼ 1 GeV γ-rays through the inverse Compton (IC) process. However, given the severe radiative energy losses experienced by electrons, the hard spectrum, uniform intensity, vast extension, and energetics of the bubbles render the origin of this particle population extremely mysterious (see fig. 1) [1,2,4,5,7]. In particular, transport of ≥TeV, IC-radiating electrons to the requisite distances from the plane would require velocities of > 0.03 c, too fast for a Galactic wind (this is a conservative lower limit as only electron energy losses on the 2.7 K CMB are accounted for). If diffusive, a diffusion coefficient of ∼ 10 31 cm 2 /s would be required for E e = 1 TeV, 1-2 orders of magnitude larger than the Galactic plane value. One might postulate an in-situ electron acceleration/injection process to surmount these difficulties [1] but here spectral considerations present a severe test. The ∼ E −2 γ γ-ray spectrum might be due to IC emission from a cooled ∼ E −3 e electron population but there is a robustly-detected [1] hardening in the γ-ray spectrum below ∼ 1 GeV. In order to produce such a break either a unique (over the age of the Galaxy) injection event of * Roland.Crocker@mpi-hd.mpg.de † Felix.Aharonian@dias.ir age ∼ 10 6 years or a sharp, ∼1 TeV low-energy hardening or cut-off in the injection spectrum of electrons is required. The former seems unlikely as there are no indications of su...

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…As far as γ-ray emission mechanism is concerned, there remains an uncertainty whether the inverse Compton scattering of cosmic microwave background photons by relativistic electrons is the source (Su et al 2010) or the interactions of high energy protons with protons in the medium (Crocker & Aharonian 2011). The high energy electrons or protons can either be accelerated in situ by internal shocks and turbulence (Mertsch & Sarkar 2011), or advected from the disc.…”

A possible origin of gamma rays from the Fermi Bubbles

“…Some models suggest that the cosmic rays might originate from activities at the Galactic center and transported into the bubbles by high-speed winds or jets [6,7,8], while others suggest they might be accelerated by shock waves or plasma turbulence present inside the bubbles [9,10,11].…”

The AGN Jet Model of the Fermi Bubbles

“…The leptonic scenario assumes that the gamma ray emission comes from inverse Compton scattering (ICS) of the ambient interstellar radiation field (ISRF) by the same population of hard electrons (Dobler et al 2010, Su et al 2010. Alternatively, in the hadronic scenario, the gamma ray emission results from CR protons, which collide inelastically with the ambient gas and produce neutral pions, which decay into gamma rays (Crocker & Aharonian 2011, Zubovas et al 2011, Mou et al 2015. With properly chosen CR spectrum, both the leptonic and hadronic scenarios fit gamma ray data quite well, while the latter seems to require an extra population of primary electrons to explain the observed microwave emission from the Fermi bubbles (Ackermann et al 2014).…”

“…Crocker & Aharonian (2011) suggested that the bubbles are powered by CR protons continuously injected by supernova explosions in the GC over the last few Gyrs. Crocker et al (2014) and Crocker et al (2015) further argue that the bubbles are inflated by a nuclear outflow driven by GC star formation over the last few 100 million years.…”