The Large-scale Ionization Cones in the Galaxy

There is compelling evidence for a highly energetic Seyfert explosion (10(56-57) erg) that occurred in the Galactic center 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 C IV/C II, Si IV/Si II) observed by the Hubble Space Telescope reveal that some Magellanic Stream clouds toward 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 degrees from the south Galactic pole with an opening angle of roughly 60 degrees. For the Magellanic Stream at such large Galactic distances (D greater than or similar to 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated H alpha emission and H ionization fraction (x(e) greater than or similar to 0.5), enhanced C IV/C II and Si IV/Si II ratios, and high C IV and Si IV 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 Magellanic Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud-halo interaction) is too weak to explain the Magellanic Stream's ionization. Instead, the nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f(E) approximate to 0.1-1). Our time-dependent Seyfert flare models adequately explain the observations and indicate that 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 (approximate to 2-8 Myr).