The Large-scale Ionization Cones in the Galaxy

Abstract

Abstract There is compelling evidence for a highly energetic Seyfert explosion (1056–57 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 Hα 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 / , 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 Hα emission and H ionization fraction (x e ≳ 0.5), enhanced / and Si iv/Si ii ratios, and high 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 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/γ-ray bubbles in leptonic jet/wind models (≈2–8 Myr).