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Australian Human Resources Institute Diversity Awards 2013 Finalist

Joss Bland-Hawthorn (SIfA, University of Sydney)

"Fossil imprint of a powerful flare at the Galactic Centre along the Magellanic Stream"

Abstract. The Fermi satellite discovery of the gamma-ray emitting bubbles extending 50 degrees (10 kpc) from the Galactic Centre 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 canonical distance of 55 kpc from the Galactic Centre. Several groups have detected faint H_alpha emission along the Magellanic Stream that is a factor of 5 too bright to have been produced by the Galactic stellar population. The brightest emission is confined to a cone with half angle ~25 degrees. The central black hole (M_BH ~ 4 x 10^6 M$_sun) can supply the required ionising luminosity with a fraction (f_E ~ 0.03-0.1) of the Eddington accretion rate, typical of Seyfert galaxies. After considering the time-dependent ionisation of the gas, the ionized Stream must recombine for a time interval T_o ~ 0.8-2.9 Myr to provide the H_alpha surface brightness. Thus it is conceivable the faint H_alpha emission along the Magellanic Stream arose from a Seyfert flash (i.e. flare) that was active up to 3 Myr ago. If the Stream over the SGP is as far out as 100 kpc, a higher rate of accretion (or beaming) by the Seyfert nucleus may be required to explain the observations. We discuss planned future tests of the Seyfert flare model.

Held in the AAO Meeting Room (Room 7, 1st Floor, Building 2) at 11:00 AM on Thursday, 13 June 2013