FOSSIL IMPRINT OF A POWERFUL FLARE AT THE GALACTIC CENTER ALONG THE MAGELLANIC STREAM
The Fermi satellite discovery of the gamma-ray emitting bubbles extending 50° (10 kpc) from the Galactic center has revitalized earlier claims that our Galaxy has undergone an explosive episode in the recent past. We now explore a new constraint on such activity. The Magellanic Stream is a clumpy gaseous structure free of stars trailing behind the Magellanic Clouds, passing over the south Galactic pole (SGP) at a distance of at least 50–100 kpc from the Galactic center. Several groups have detected faint Hα emission along the Magellanic Stream (1.1 ± 0.3 × 10−18 erg cm−2 s−1 arcsec−2) which is a factor of five too bright to have been produced by the Galactic stellar population. The brightest emission is confined to a cone with half angle θ1/2 ≈ 25° roughly centered on the SGP. Time-dependent models of Stream clouds exposed to a flare in ionizing photon flux show that the ionized gas must recombine and cool for a time interval To = 0.6 − 2.9 Myr for the emitted Hα surface brightness to drop to the observed level. A nuclear starburst is ruled out by the low star formation rates across the inner Galaxy, and the non-existence of starburst ionization cones in external galaxies extending more than a few kiloparsecs. Sgr A⋆ is a more likely candidate because it is two orders of magnitude more efficient at converting gas to UV radiation. The central black hole (M• ≈ 4 × 106M☉) can supply the required ionizing luminosity with a fraction of the Eddington accretion rate (fE ∼ 0.03–0.3, depending on uncertain factors, e.g., Stream distance) typical of Seyfert galaxies. In support of nuclear activity, the Hα emission along the Stream has a polar angle dependence peaking close to the SGP. Moreover, it is now generally accepted that the Stream over the SGP must be farther than the Magellanic Clouds. At the lower halo gas densities, shocks become too ineffective and are unlikely to give rise to a polar angle dependence in the Hα emission. Thus it is plausible that the Stream Hα emission arose from a “Seyfert flare” that was active 1–3 Myr ago, consistent with the cosmic ray lifetime in the Fermi bubbles. Sgr A⋆ activity today is greatly suppressed (70–80 dB) relative to the Seyfert outburst. The rapid change over a huge dynamic range in ionizing luminosity argues for a compact UV source with an extremely efficient (presumably magneto-hydrodynamic) “drip line” onto the accretion disk.