I was going to comment on Ton 6-1-8, but the Phoenix A you mentioned, holy 100 billion Solar Masses Batman!, I was not aware of it's discovery, thank you so much for dropping that info, I must go quench my knowledge of Phoenix A
I'm not very good at this - can somebody please help? I'm trying to calculate the Schwarzschild radius of a black hole of 30 billion solar masses. The result I get is on the order of 10 light years! That seems high. Does anyone have a simple formula?
So 30 solar masses has an event horizon radius of 90 Km, and a 30 billion solar mass black hole would have an event horizon radius of 90 billion kilometers. A light year is 9,460,730,777,119.56 km (see https://www.metric-conversions.org/length/lightyears-to-kilometers.htm ), so that event horizon becomes 90,000,000,000 / 9,460,000,000,000 = 9 / 946 = 0.0095 light year = ~ 1/100 light-year.