Fermi bubbles: the collimated outburst needed to explain forward-shock edges

Santanu Mondal, Uri Keshet, Kartick C. Sarkar, Ilya Gurwich

Research output: Contribution to journalArticlepeer-review

6 Scopus citations
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Abstract

The bipolar, non-thermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic Centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modelling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at z ∼ 100 pc heights in our model) with a half-opening angle θ 4°, a normalized velocity β-2 v/(0.01c) 0.4, and an energy E≳ 2β -22× 1055 erg, launched T≃ 3.3β-2-1 Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller E/(β-2θ)2, and a comparable T, and follow a ballistic stage that must reach a height zs 5 kpc.

Original languageEnglish
Pages (from-to)2581-2598
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Volume514
Issue number2
DOIs
StatePublished - 22 Apr 2022

Keywords

  • Galaxy: centre
  • Galaxy: halo
  • ISM: jets and outflows

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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