Our Galaxy is expected to contain about 100 million black holes, the vast majority of which should be isolated, with no companion star. Yet, until recently, not a single isolated black hole had been unambiguously detected. After a brief review of the subject, I will describe astrometric microlensing, a powerful technique for detecting isolated stellar-mass black holes. Using this technique, we made the first unambiguous detection of an isolated stellar-mass black hole. The superb astrometric capability of the Hubble Space Telescope allowed us to measure the tiny deflection of background starlight caused by the black hole and, from this deflection, determine its mass. The measured mass of about 7 solar masses, together with the absence of detectable light from the lens, confirms its black-hole nature. I will discuss the implications of this discovery, our ongoing efforts to identify additional isolated black holes, and the prospects for detecting a large population of these elusive objects with the Nancy Grace Roman Space Telescope.
Speaker: Kailash Sahu, Space Telescope Science Institute
Host: Lena Murchikova
CIERA Astrophysics
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