Journal Article

Non-thermal insights on mass and energy flows through the Galactic Centre and into the <i>Fermi</i> bubbles

R. M. Crocker

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 423, issue 4, pages 3512-3539
Published in print July 2012 | ISSN: 0035-8711
Published online July 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21149.x
Non-thermal insights on mass and energy flows through the Galactic Centre and into the Fermi bubbles

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We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ∼200 pc (in diameter) of the Galaxy. Our modelling is constrained, in particular, by the non-thermal radio continuum and γ-ray signals detected from the region. The modelling points to a current star formation rate of 0.04–0.12 M yr−1 at 2σ confidence within the region with best-fitting value in the range 0.08–0.12 M yr−1 which – if sustained over 10 Gyr – would fill out the ∼109 M stellar population of the nuclear bulge. Mass is being accreted on to the Galactic Centre (GC) region at a rate yr−1. The region’s star formation activity drives an outflow of plasma, cosmic rays and entrained, cooler gas. Neither the plasma nor the entrained gas reaches the gravitational escape speed, however, and all this material fountains back on to the inner Galaxy. The system we model can naturally account for the recently observed ≳106 M‘halo’ of molecular gas surrounding the Central Molecular Zone out to 100–200 pc heights. The injection of cooler, high-metallicity material into the Galactic halo above the GC may catalyze the subsequent cooling and condensation of hot plasma out of this region and explain the presence of relatively pristine, nuclear-unprocessed gas in the GC. This process may also be an important ingredient in understanding the long-term stability of the GC star formation rate. The plasma outflow from the GC reaches a height of a few kpc and is compellingly related to the recently discovered Fermi bubbles by a number of pieces of evidence. These include that the outflow advects precisely (i) the power in cosmic rays required to sustain the bubbles’γ-ray luminosity in saturation; (ii) the hot gas required to compensate for gas cooling and drop-out from the bubbles and (iii) the magnetic field required to stabilize the walls of these structures. Our modelling demonstrates that ∼109 M of hot gas is processed through the GC over 10 Gyr. We speculate that the continual star formation in the GC over the age of the Milky Way has kept the supermassive black hole in a quiescent state thus preventing it from significantly heating the coronal gas, allowing for the continual accretion of gas on to the disc and the sustenance of star formation on much wider scales in the Galaxy. In general, our investigations explicitly reveal the GC’s important role in the Milky Way’s wider stellar ecology.

Keywords: radiation mechanisms: non-thermal; cosmic rays; ISM: jets and outflows; ISM: supernova remnants; Galaxy: centre; galaxies: star formation

Journal Article.  25060 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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