The discovery of dark objects with masses MBH 106 to 109.5 M in galactic nuclei is secure. But are they BHs? Proof requires measurement of relativistic velocities near the Schwarzschild radius, rs 2 MBH / (108 M) AU. Even for M 31, rs ~ 8 x 10-7 arcsec. HST spectroscopic resolution is only 0".d1. The conclusion that we are finding BHs is based on physical arguments that BH alternatives fail to explain the masses and high densities of galactic nuclei.
The most plausible BH alternatives are clusters of dark objects produced by ordinary stellar evolution. These come in two varieties, failed stars and dead stars. Failed stars have masses m* 0.08 M. They never get hot enough for the fusion reactions that power stars, i.e., the conversion of hydrogen to helium. They have a brief phase of modest brightness while they live off of gravitational potential energy, but after this, they could be used to make dark clusters. They are called brown dwarf stars, and they include planetary mass objects. Alternatively, a dark cluster could be made of stellar remnants - white dwarfs, which have typical masses of 0.6 M; neutron stars, which typically have masses of ~ 1.4 M, and black holes with masses of several M. Galactic bulges are believed to form in violent starbursts, so massive stars that turn quickly into dark remnants would be no surprise. It is not clear how one could make dark clusters with the required masses and sizes, especially not without polluting the remaining stars with more metals than we see. But in the absence of direct proof that the dark objects in galactic nuclei are BHs, it is important to examine alternatives.
However, dynamical measurements tell us more than the mass of a potential BH. They also constrain the maximum radius inside which the dark stuff must live. Its minimum density must therefore be high, and this rules out the above BH alternatives in our Galaxy and in NGC 4258. High-mass remnants such as white dwarfs, neutron stars, and stellar BHs would be relatively few in number. The dynamical evolution of star clusters is relatively well understood; in the above galaxies, a sparse cluster of stellar remnants would evaporate completely in 108 yr. Low-mass objects such as brown dwarfs would be so numerous that collision times would be short. Stars generally merge when they collide. A dark cluster of low-mass objects would become luminous because brown dwarfs would turn into stars.
More exotic BH alternatives are not ruled out by such arguments. For example, the dark matter that makes up galactic halos and that accounts for most of the mass of the Universe may in part be elementary particles that are cold enough to cluster easily. It is not out of the question that a cluster of these could explain the dark objects in galaxy centers without getting into trouble with any astrophysical constraints. So the BH case is not rigorously proved. What makes it compelling is the combination of dynamical evidence and the evidence from AGN observations. This is discussed in the previous article.
For many years, AGN observations were decoupled from the dynamical evidence for BHs. This is no longer the case. Dynamical BH detections are routine. The search itself is no longer the main preoccupation; we can concentrate on physical questions. New technical developments such as better X-ray satellites ensure that progress on BH astrophysics will continue to accelerate.