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Prof. Sarah Gallagher

University of Western Ontario

Date of Live Presentation: Mon, 17-Jun-2024
Location: UNB Fredericton


The Biggest Blowhards: Windy Supermassive Black Holes


Supermassive black holes reside in the centres of every massive galaxy including our own Milky Way. In relatively brief spurts, black holes grow as luminous quasars through the infall of material through an accretion disk. Remarkably, the light from the accretion disk can outshine all of the stars in the host galaxy by a factor of a thousand, and this radiation can also drive energetic outflows. Mass ejection in the form of winds appears to be as fundamental to quasar activity as accretion, and can be directly observed in many objects with broadened and blue-shifted UV emission and absorption features. Applying unsupervised and hierarchical clustering algorithms on quasar spectra, we can match windy quasars with specific emission-line properties sensitive to the shape of the ionizing continuum. Beyond the dust sublimation radius, radiation pressure is still important, but high energy photons from the central engine can now push on dust grains. This physics underlies the dusty wind picture for the putative obscuring torus. I'll describe our model of the dusty wind and evaluate its successes and shortcomings in accounting for observed properties of quasars such their mid-infrared power and the fraction of hidden objects.

Short bio

Sarah Gallagher is currently an Associate Professor in the Department of Physics and Astronomy at the University of Western Ontario. Prior to that, she was an Assistant Research Astronomer at the University of California, Los Angeles. In 2006, she completed a Spitzer Postdoctoral Fellowship entitled Understanding Quasar Outflows: Evolution or Orientation? Her research focuses on investigating the nature of winds from luminous quasars (accreting supermassive black holes at the centers of distant galaxies) using observatories covering the infrared to the X-ray, including two of NASA's Great Observatories, Spitzer and Chandra. Gallagher received her Ph.D. in Astronomy and Astrophysics from Penn State where she was also a member of the Chandra ACIS Instrument team. Her thesis, entitled The View through the Wind: X-ray Observations of Broad Absorption Line Quasars, incorporated X-ray data from three observatories: ROSAT, ASCA, and Chandra. Gallagher spent a year at MIT working with the Chandra X-ray gratings group before going to UCLA in 2003. Before graduate school, she was an undergraduate at Yale University and a Physics teacher for two years at the Holderness School where she also coached soccer, ran a girls' dormitory, and led winter camping trips.

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