GIANT GAMMA-RAY BUBBLES FROMFERMI-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND?

Abstract

Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50 degrees above and below the Galactic center, with a width of about 40 degrees in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN/dE ∼ E −2 ) than the IC emission from electrons in the Galactic disk, or the gamma-rays produced by decay of pions from proton-ISM collisions. There is no significant spatial variation in the spectrum or gamma-ray intensity within the bubbles, or between the north and south bubbles. The bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; the edges of the bubbles also line up with features in the ROSAT X-ray maps at 1.5 − 2 keV.We argue that these Galactic gamma-ray bubbles were most likely created by some large episode of energy injection in the Galactic center, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last ∼ 10 Myr. Dark matter annihilation/decay seems unlikely to generate all the features of the bubbles and the associated signals in WMAP and ROSAT; the bubbles must be understood in order to use measurements of the diffuse gamma-ray emission in the inner Galaxy as a probe of dark matter physics. Study of the origin and evolution of the bubbles also has the potential to improve our understanding of recent energetic events in the inner Galaxy and the high-latitude cosmic ray population.

The jet feedback mechanism (JFM) in stars, galaxies and clusters

“…Such outflows can efficiently interact and influence the ambient gas (section 4.3), in particular via the inflation of hot low density bubbles. The discovery of bipolar lobes even in spiral galaxies (e.g., Querejeta et al 2016), including in the Milky Way (Su et al 2010) that are likely to have been inflated by jets Mou et al 2015), and possibly in Andromeda (Pshirkov et al 2016), suggests that the feedback process in galaxies, even on scales of the bulge of spiral galaxies, might take place through the inflation of bubbles.…”

Fermi bubbles as a source of cosmic rays above 1015 eV

“…Thickness of the bubble walls ∆x necessary to produce observed gamma-ray flux (thick line) depending on the spectral index of relativistic electrons inside the bubble δ. Vertical dashed line marks the expected spectrum of electrons based on radio data [2]. Horizontal dashed line marks the expected thickness based on the spatial structure [30].…”

“…Despite their nature is still enigmatic the location of these objects indicates their connection with past or present activity in the center of our Galaxy. Different models relate the bubbles to starburst activity [15], single [20,21,30,32] or multiple [13] energy release events on a central black hole.…”

A possible origin of gamma rays from the Fermi Bubbles

“…The Fermi space telescope has recently made an exciting discovery of two large gamma-ray emission regions, the so-called Fermi Bubbles, above and below the Galactic center [1]. The bubbles, whose origin still remains mysterious, extend up to ∼ 50 • in Galactic latitude and are ∼ 40 • wide in Galactic longitude.…”

“…The bubbles are most likely created by large energy injection from Galactic center in the past [1], and therefore, their study provide valuable informations on the past activities of the Galaxy, particularly about the Email address: s.thoudam@astro.ru.nl (Satyendra Thoudam) Galactic center region. Their proximity also give us a unique opportunity to understand similar extended lobes present in other galaxies.…”