Damien Broderick, <firstname.lastname@example.org>, writes:
> I heard on the radio yesterday that a recent study has observed a
> particular X-ray spectrum profile associated with the cores of certain
> (young, very distant?) galaxies that's consistent with humungous black
> holes but not anything else. Sorry, no url available.
The May issue of Scientific American has an article on this subject. The difficulty is distinguishing black holes from compact objects like neutron stars. Suppose physics is wrong and you can have ultra-massive neutron stars; how would the emission spectrum differ from black holes? In most cases they are about the same. But the difference is that in some cases, with black holes, mass can fall below the event horizon before it has fully radiated away its heat. With neutron stars, it always radiates the heat, either before or after it hits the surface. Black holes end up with a different spectrum than with neutron stars in these situations, and this has apparently now been observed.