Wild at Heart: the particle astrophysics of the Galactic Centre

Abstract

We consider the high‐energy astrophysics of the inner ∼200 pc 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 ∼150 pc 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 ∼10 kpc), 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 511 keV 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.

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

“…The magnetic field may be exceedingly important near the Galactic center, where the field seems to be particularly strong (Tsuboi et al 1986;Sofue et al 1987;Yusef-Zadeh & Morris 1987;Chuss et al 2003;Crocker et al 2010Crocker et al , 2011Ferrière 2010;Bally et al 2014). Recent measurements of the magnetic field in G0.253+0.016 find a strong ordered magnetic field component with several mG, roughly following the large-scale bending of the cloud (Pillai et al 2015).…”

Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 III. Helical magnetic fields in the nuclear outflow

“…The supernova rate in NGC 253 lies between 0.03 and 0.3 yr −1 (Ulvestad & Antonucci 1997;Lenc & Tingay 2006) and is thus much larger than for the Galactic centre with (0.02−0.08) × 10 −2 yr −1 (Crocker et al 2011a). Assuming that the outflow is driven by the starburst, the energy input in NGC 253 is thus a factor of 100 higher than in the Milky Way.…”

Recent star formation in the inner Galactic bulge seen by ISOGAL

“…Assuming that all objects in our CMZ sample have formed over an average time span of 1 Myr, we can convert the total mass of YOs (∼77 000 M ) to an average star formation rate of 0.08 M yr −1 . Previous studies (Crocker et al 2011;Yusef-Zadeh et al 2009) published a range of values for the star formation rate in the CMZ from 0.08-0.15 M yr −1 . An et al (2011) make a rough estimate of the star formation rate based on the sources they have in common with the Yusef-Zadeh et al source sample, deriving a value of 0.07 M yr −1 .…”