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/γ-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 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 s 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° from the south Galactic pole with an opening angle of roughly 60°. For the Magellanic Stream at such large Galactic distances (D ≥ 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated H? emission and H ionization fraction (x_e ≥ 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 ∼0.1-1). Our timedependent 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/?-ray bubbles in leptonic jet/wind models (∼2-8 Myr).