University of Toronto
Title: Not quite a black hole: from quadratic gravity to gravitational wave echoes
Astrophysical black hole candidates might be horizonless ultracompact objects. Of particular interest is the plausible fundamental connection with quantum gravity. The puzzle is then why we shall expect Planck scale corrections around the horizon of a macroscopic black hole. Taking asymptotically free quadratic gravity as a possible candidate of UV completion of general relativity, I will show how the would-be horizon can be naturally replaced by a tiny interior with high curvature. The new horizonless 2-2-hole may then be the nearly black endpoint of gravitational collapse. In the era of gravitational wave astronomy, echoes in the post-merger phase may provide a great opportunity to probe horizonless ultracompact objects. Given the uncertainties associated with the echoes waveform, I will discuss using simple windowing methods to extract a signal from noise, where the time delay between echoes is the primary observable. Applying these methods to the LIGO data, tentative signals for echoes are found for multiple LIGO black hole merger events.
Smithsonian Astrophysical Observatory
Title: A Galaxy-Scale Fountain of Cold Molecular Gas
Pumped by a Black Hole
A new ALMA observation of the cool core brightest cluster galaxy in Abell 2597 reveals that a supermassive black holecan act much like a mechanical pump in a water fountain, driving a convective flow of molecular gas that drains into the blackhole accretion reservoir, only to be pushed outward again in a jet-driven outflow that then rains back toward the galaxy center from which it came. The ALMA data reveal "shadows" cast by giant molecular clouds falling on ballistic trajectories towards the black hole in the innermost hundred parsecs of the galaxy, manifesting as deep redshifted continuum absorption features. The black hole accretion reservoir, fueled by these infalling cold clouds, powers an AGN that drives a jet-driven molecular outflow in the form of a 10 kpc-scale, billion solar mass expanding molecular bubble. HST reveals that the molecular shell is permeated with young stars, perhaps triggered in situ by the jet. Buoyant X-ray cavities excavated by the propagating radio source may further uplift the molecular filaments, which are observed to fall inward toward the center of the galaxy from which they came, presumably keeping the fountain long-lived. I will discuss this specific result in the larger context of galaxies as a whole, as the results show that cold molecular gas can couple to black hole growth via both feedback and feeding, in alignment with "cold chaotic accretion" models for the regulation of star formation in galaxies.