Roland M. Crocker

Giant magnetized outflows from the centre of the Milky Way

The nucleus of the Milky Way is known to harbour regions of intense star formation activity as well as a supermassive black hole. Recent observations have revealed regions of γ-ray emission reaching far above and below the Galactic Centre (relative to the Galactic plane), the so-called ‘Fermi bubbles’. It is uncertain whether these were generated by nuclear star formation or by quasar-like outbursts of the central black hole and no information on the structures’ magnetic field has been reported. Here we report observations of two giant, linearly polarized radio lobes, containing three ridge-like substructures, emanating from the Galactic Centre. The lobes each extend about 60 degrees in the Galactic bulge, closely corresponding to the Fermi bubbles, and are permeated by strong magnetic fields of up to 15 microgauss. We conclude that the radio lobes originate in a biconical, star-formation-driven (rather than black-hole-driven) outflow from the Galaxy’s central 200 parsecs that transports a huge amount of magnetic energy, about 1055 ergs, into the Galactic halo. The ridges wind around this outflow and, we suggest, constitute a ‘phonographic’ record of nuclear star formation activity over at least ten million years.

A lower limit of 50 microgauss for the magnetic field near the Galactic Centre

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of ∼100 parsecs (pc), fields of ∼1,000 microgauss (μG; refs 1–3) have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas, the contested ‘very hot’ plasma, and the magnetic field) suggests fields of ∼100 μG over ∼400 pc size scales. Finally, assuming equipartition, fields of only ∼6 μG have been inferred from radio observations for 400 pc scales. Here we report a compilation of previous data that reveals a downward break in the regions non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least ∼50 μG on 400 pc scales, lest the synchrotron-emitting electrons produce too much γ-ray emission, given other existing constraints. Other considerations support a field of 100 μG, implying that over 10% of the Galaxys magnetic energy is contained in only ≲0.05% of its volume.

THE HIGH-ENERGY, ARCMINUTE-SCALE GALACTIC CENTER GAMMA-RAY SOURCE

Employing data collected during the first 25 months’ observa tions by the Fermi -LAT, we describe and subsequently seek to model the very high energy ( > 300 MeV) emission from the central few parsecs of our Galaxy. We analyse, in particular, the morphological, spec tral and temporal characteristics of the central source, 1FGL J1745.6-2900. Remarkably, the data show a clear, stati stic lly significant signal at energies above 10 GeV, where the Fermi -LAT has an excellent angular resolutio n comparable to the angular resolution of HESS at TeV energies. This not only reduces dramatically the cont amination both from the diffuse background and the nearby gamma-ray sources, but also makes meaningful the joint analysis of the Fermi and HESS data. Our analysis does not show statistically significant variab ility of 1FGL J1745.6-2900. Using the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the coinci de t, TeV source HESS J1745-290, we show that the spectrum of the central γ-ray source is inflected with a relatively steep spectral reg ion matching between the flatter spectrum found at both low and high energi es. We seek to model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in particular, co smic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies (given a reasonable energy-dependence of the assumed diffu sion coefficient) can well explain the spectral phenomenology. In general, however, we find considerable de g neracy between different parameter choices which will only be broken with the addition of morphological information thatγ-ray telescopes cannot deliver given current angular resolution limits. We argue that a fut ure analysis done in combination with higherresolution radio continuum data holds out the promise of bre aking this degeneracy. Subject headings: Galaxy: center — synchrotron radiation: cosmic rays — molec ular clouds: general

Wild at Heart: The particle astrophysics of the Galactic Centre

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 regions 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 H 2 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/μG < 400 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the regions 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.

Radio and gamma-ray constraints on dark matter annihilation in the Galactic center

We determine upper limits on the dark matter (DM) self-annihilation cross section for scenarios in which annihilation leads to the production of electron---positron pairs. In the Galactic center, relativistic electrons and positrons produce a radio flux via synchroton emission, and a gamma-ray flux via bremsstrahlung and inverse Compton scattering. On the basis of archival, interferometric and single-dish radio data, we have determined the radio spectrum of an elliptical region around the Galactic center of extent 3\ifmmode^\circ\else\textdegree\fi{} semimajor axis (along the Galactic plane) and 1\ifmmode^\circ\else\textdegree\fi{} semiminor axis and a second, rectangular region, also centered on the Galactic center, of extent

The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016

1.6\ifmmode^\circ\else\textdegree\fi{}\ifmmode\times\else\texttimes\fi{}0.6\ifmmode^\circ\else\textdegree\fi{}

γ-rays and the far-infrared–radio continuum correlation reveal a powerful Galactic Centre wind

. The radio spectra of both regions are nonthermal over the range of frequencies for which we have data: 74 MHz--10 GHz. We also consider gamma-ray data covering the same region from the EGRET instrument (about GeV) and from HESS (around TeV). We show how the combination of these data can be used to place robust constraints on DM annihilation scenarios, in a way which is relatively insensitive to assumptions about the magnetic field amplitude in this region. Our results are approximately an order of magnitude more constraining than existing Galactic center radio and gamma-ray limits. For a DM mass of

Interpretation of radio continuum and molecular line observations of Sgr B2: free-free and synchrotron emission, and implications for cosmic rays

{m}_{\ensuremath{\chi}}=10\text{ }\text{ }\mathrm{GeV}

A Dynamical Model for Gas Flows, Star Formation, and Nuclear Winds in Galactic Centres

, and an Navarro-Frank-White profile, we find

The Fermi Bubbles Revisited

⟨{\ensuremath{\sigma}}_{A}v⟩\ensuremath{\le}\mathrm{\text{few}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}