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Crocker R.M.,Max Planck Institute For Kernphsik | Jones D.I.,Max Planck Institute For Kernphsik | Aharonian F.,Max Planck Institute For Kernphsik | Aharonian F.,Dublin Institute for Advanced Studies | And 3 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost starburst-like region is ultimately driven dominantly by ongoing massive star formation. We show that this region's radio continuum (RC) emission is in relative deficit with respect to the expectation afforded by the far-infrared-radio continuum correlation (FRC). Likewise we show that the region's γ -ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km s-1) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed ~kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic field falls in the range ~100-300 μG and that - in the time they remain in the region - GC cosmic rays do not penetrate into the region's densest molecular material. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

Crocker R.M.,Max Planck Institute For Kernphsik | Aharonian F.,Max Planck Institute For Kernphsik | Aharonian F.,Dublin Institute for Advanced Studies
Physical Review Letters | Year: 2011

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 from the Galactic plane. These structures are coincident with a nonthermal microwave "haze" and an extended region of x-ray emission. The bubbles' γ-ray emission is characterized by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity 4×1037erg/s, around 1 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 time scale (≥ 8 Gyr) and high areal density star formation in the Galactic center. © 2011 American Physical Society.

Crocker R.M.,Max Planck Institute For Kernphsik
Monthly Notices of the Royal Astronomical Society | Year: 2012

We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ~200pc (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 0.04-0.12M ⊙yr -1 at 2σ confidence within the region with best-fitting value in the range 0.08-0.12M ⊙yr -1 which - if sustained over 10Gyr - would fill out the ~10 9M ⊙ 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 ≳10 6M ⊙'halo' of molecular gas surrounding the Central Molecular Zone out to 100-200pc 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 ~10 9M ⊙ of hot gas is processed through the GC over 10Gyr. 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. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Crocker R.M.,Max Planck Institute For Kernphsik
Proceedings of the International Astronomical Union | Year: 2011

The Galactic centre-as the closest galactic nucleus-holds both intrinsic interest and possibly represents a useful analogue to starburst nuclei which we can observe with orders of magnitude finer detail than these external systems. The environmental conditions in the GC-here taken to mean the inner 200 pc in diameter of the Milky Way-are extreme with respect to those typically encountered in the Galactic disk. The energy densities of the various GC ISM components are typically ∼two orders of magnitude larger than those found locally and the star-formation rate density ∼three orders of magnitude larger. Unusually within the Galaxy, the Galactic centre exhibits hard-spectrum, diffuse TeV (=10 12 eV) gamma-ray emission spatially coincident with the region's molecular gas. Recently the nuclei of local starburst galaxies NGC 253 and M82 have also been detected in gamma-rays of such energies. We have embarked on an extended campaign of modelling the broadband (radio continuum to TeV gamma-ray), non-thermal signals received from the inner 200 pc of the Galaxy. On the basis of this modelling we find that star-formation and associated supernova activity is the ultimate driver of the region's non-thermal activity. This activity drives a large-scale wind of hot plasma and cosmic rays out of the GC. The wind advects the locally-accelerated cosmic rays quickly, before they can lose much energy in situ or penetrate into the densest molecular gas cores where star-formation occurs. The cosmic rays can, however, heat/ionize the lower density/warm H 2 phase enveloping the cores. On very large scales (∼10 kpc) the non-thermal signature of the escaping GC cosmic rays has probably been detected recently as the spectacular "Fermi bubbles" and corresponding "YWMAP haze". © 2012 International Astronomical Union.

Crocker R.M.,Max Planck Institute For Kernphsik | Jones D.I.,Max Planck Institute For Kernphsik | Aharonian F.,Max Planck Institute For Kernphsik | Aharonian F.,Dublin Institute for Advanced Studies | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We consider the high-energy astrophysics of the inner ∼200pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to (i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the region's non-thermal radio and TeV γ-ray emission; (ii) drive a powerful wind that advects non-thermal particles out of the inner Galactic Centre; (iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm envelope detected throughout the Central Molecular Zone; (iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ∼150pc scales above and below the plane (the Galactic Centre lobe); and (v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼10kpc), generate the recently identified Wilkinson Microwave Anisotropy Probe (WMAP) haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/μ G < 40 0 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region's dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this starburst-like environment - act to substantially modify the conditions of star formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511keV line emission. We present extensive appendices reviewing the environmental conditions in the Galactic Centre, deriving the star formation and supernova rates there, and setting out the extensive prior evidence that exists, supporting the notion of a fast outflow from the region. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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