The large-scale ionization cones in the Galaxy

Abstract

There is compelling evidence for a highly energetic Seyfert explosion (10^{56-57} erg) that occurred in the Galactic Centre a few million years ago. The clearest indications are the x-ray/gamma-ray "10 kpc bubbles" identified by the Rosat and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e. elevated H-alpha emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular CIV/CII, SiIV/SiII) observed by the Hubble Space Telescope reveal that some Stream clouds towards both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative "ionization cones" from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15 deg from the South Galactic Pole with an opening angle of roughly 60 deg. For the Stream at such large Galactic distances (D > 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated H-alpha emission and H ionization fraction (X_e > 0.5), enhanced CIV/CII and SiIV/SiII ratios, and high CIV and SiIV column densities. Wind-driven "shock cones" are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Stream. The nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f_E ~ 0.1-1). Our time-dependent Seyfert flare models adequately explain the observations and indicate the Seyfert flare event took place T_o = 3.5 +/- 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the x-ray/gamma-ray bubbles in leptonic jet/wind models (2-8 Myr).

21 pages; 15 figures; 2 movies (this version matches published version)

References in corpus (27)

• The mass distribution and gravitational potential of the Milky Way
• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation
• Are the Magellanic Clouds on their First Passage about the Milky Way?
• The Spectrum and Morphology of the Fermi Bubbles
• Fueling Low-Level AGN Activity Through the Stochastic Accretion of Cold Gas
• The Origin of the Magellanic Stream and Its Leading Arm
• Super-Critical Growth of Massive Black Holes from Stellar-Mass Seeds
• Giant Magnetized Outflows from the Centre of the Milky Way
• The COS/UVES Absorption Survey of the Magellanic Stream. III: Ionization, Total Mass, and Inflow Rate onto the Milky Way
• The Source of Ionization along the Magellanic Stream
• Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way's Biconical Nuclear Outflow
• Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble
• Numerical Study of Turbulent Mixing Layers with Non-Equilibrium Ionization Calculations
• Radio Variability in Seyfert Nuclei
• The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo
• Ultrafaint Dwarf Galaxies - the lowest mass relics from before reionization
• The many streams of the Magellanic Stream
• A Genetic Approach to the History of the Magellanic Clouds
• The Magellanic System: the puzzle of the leading gas stream
• MHD Turbulent Mixing Layers: Equilibrium Cooling Models
• Evaporation and condensation of spherical interstellar clouds. Self-consistent models with saturated heat conduction and cooling
• A New Computational Fluid Dynamics Code I: Fyris Alpha
• Revealing the Ionization Properties of the Magellanic Stream using Optical Emission
• Detection of non-thermal hard X-ray emission from the "Fermi bubble" in an external galaxy
• On Constraining the Growth History of Massive Black Holes via Their Distribution on the Spin-Mass Plane
• Geometrodynamical Distances to the Galaxy's Hydrogen Streams

Cited by in corpus (5)

• Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets
• Observation of Acceleration of HI Clouds Within the Fermi Bubbles
• Mapping Outflowing Gas in the Fermi Bubbles: a UV Absorption Survey of the Galactic Nuclear Wind
• Kinematics of the Magellanic Stream and Implications for its Ionization
• Discovery of High-Velocity H$\alpha$ Emission in the Direction of the Fermi Bubble