Physics & Astronomy Colloquium – Spring 2019 – Keel

Speaker: William C. Keel (University of Alabama)

Title: Tracing the History of Black-Hole Growth with Photoionization Echos

Abstract: Multiple energetic phenomena are associated with active galactic nuclei (AGN),
tracing the accretion of matter into supermassive black holes in galactic centers.
Discoveries by Galaxy Zoo participants led to identification of AGN whose host galaxies
are surrounded by ionized clouds as much as 30,000-150,000 light-years away. In many of
these, the nucleus falls far short of producing enough radiation to ionize these
clouds, suggesting that the AGN has faded by 5-100 times over the
light-travel timescale 30,000-200,000 years (sometimes with e-folding timescales of only
centuries). This behavior is preferentially seen in interacting or merging systems, and
may be associated with dramatic changes in the accretion structure around the central
black hole, or close binary black holes. Frequent evidence of circumnuclear outflow, and
sometimes triggered star formation, indicate that we may be dealing with switching the
mode of accretion energy rather than solely the accretion rate. This phenomenon may be
common in AGN, with implications for their demographics as well as accretion history.
Ongoing projects are extending this sample to wider areas of sky, fainter
and possibly older clouds, very nearby galaxies, and systems where the clouds
are superimposed on the host galaxy itself. The overall aim is to trace the
accretion histories of these black holes over 100,000-year spans and seek connections
between this behavior and feedback processes or behavior of the accretion disks.

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