The ngEHT's Role in Measuring Supermassive Black Hole Spins

While supermassive black-hole masses have been cataloged across cosmic time, only a few dozen of them have robust spin measurements. By extending and improving the existing Event Horizon Telescope (EHT) array, the next-generation Event Horizon Telescope (ngEHT) will enable multifrequency, polarimetric movies on event-horizon scales, which will place new constraints on the space-time and accretion flow. By combining this information, it is anticipated that the ngEHT may be able to measure tens of supermassive black-hole masses and spins. In this white paper, we discuss existing spin measurements and many proposed techniques with which the ngEHT could potentially measure spins of target supermassive black holes. Spins measured by the ngEHT would represent a completely new sample of sources that, unlike pre-existing samples, would not be biased towards objects with high accretion rates. Such a sample would provide new insights into the accretion, feedback, and cosmic assembly of supermassive black holes.

Automated summary

While supermassive black-hole masses have been cataloged, only a few dozen of them have reliable spin measurements. The next-generation Event Horizon Telescope (ngEHT) will enable multifrequency, polarimetric movies which can provide new constraints on space-time and accretion flow and potentially measure the spins of tens of supermassive black holes. The spins measured by ngEHT would create a new sample that is unbiased towards high accretion rate objects and could yield new insights into the cosmic assembly and behavior of supermassive black holes.