Originally posted to sci.astro
Warning: long posting ahead. The interpretation of the centres of radio galaxies happens to be kind of a hot topic at the moment, so I will do what I can to explain what we (or at least I) think the `bright central object' in NGC 4261 is, but it will take a bit of background for the folks on s.p.r.
First off, let me repeat Pete Newman's point; we don't do `unambiguous proofs of existence' in science. The evidence for a black hole in the nuclei of NGC 4261 and other nearby active galaxies like M87 is that we see high-velocity orbits for line-emitting gas, implying a large, compact central mass. If GR is correct, then such a large compact mass is certain to take the form of a black hole; but it could be that for some reason GR is not correct in the very strong gravity regime, and what is actually there is something different from what we canonically call a `black hole'. Whether you choose to call this a black hole or not is really a matter of semantics, but we do not have incredibly detailed agreement with GR's predictions in any of these cases of supposed black holes, because of the lack of accuracy of our observations (a point I'll come back to in a moment). [There are some rather nice observations that appear to show general-relativistic effects in the nucleus of one active galaxy (see Tanaka et al 1995, Nature 375 659) but we obviously need more such observations, and will have to wait for the next generation of X-ray satellites to get them.] I'm afraid that NASA press releases claiming the `confirmation' of black holes from observations like the ones that started this thread are just plain wrong. They certainly do not constitute a test of GR, since it's the assumption that GR is right in this regime that allows us to call them black holes in the first place.
Now; on the origins of the `bright object' in NGC 4261. This object is a radio galaxy, 3C270, and a large amount of the total energy it puts out is via a pair of radio lobes powered by twin jets which extend back into the optical nucleus. (There used to be a nice radio-optical overlay on the STScI site, though I haven't been able to find it on a casual look.) When you look at such objects with very long baseline radio interferometry, which has a resolution of milliarcseconds and so probes scales of parsecs at the distances of galaxies like NGC 4261 [note that these scales are still much larger than the Schwarzchild radii of the black holes GR predicts at the centres, although they are considerably smaller than the scales probed by HST] you typically see a one-sided radio jet (one jet very much brighter than the other), though the jets are pretty much symmetrical on the largest (100-kpc) scales. For various reasons we believe that the one-sidedness of the jets are illusory, caused by special-relativistic Doppler effects as a consequence of relativistic bulk velocities in the jets. This means that when you look at radio galaxies in the radio on arcsecond scales, and see a bright central core, you are not seeing emission from the (putative) black hole at all but from material some way away from it; but, importantly for the rest of my argument, the brightness of that core is determined by relativistic beaming as well as by any sort of intrinsic jet power, and, in fact, relativistic beaming of the sort we expect can change the observed brightness of a radio core of a given luminosity by about three orders of magnitude depending on whether the jet is pointing towards us or is in the plane of the sky.
So, when we see bright objects in the cores of radio galaxies like NGC 4261, we need to ask whether they're related to the jet. The HST snapshot survey of 3CR radio galaxies has shown that almost all nearby radio galaxies have these optical nuclei. Recently Chiaberge, Capetti & Celotti (1999 A&A in press and astro-ph 9907064) and, independently, a colleague and I (Hardcastle & Worrall 1999 MNRAS submitted) have found that the optical fluxes of these optical cores are strongly correlated with the arcsecond-scale fluxes of the radio cores (and, in our paper, with the X-ray cores too; see also Hardcastle & Worrall 1999 MNRAS in press, astro-ph/9907034). Because of the relativistic beaming effects I described above, this correlation would not exist if the optical or X-ray cores were, say, something coming from the accretion disc; we would still expect some correlation in a model in which the accretion disc is feeding the black hole which is powering the jet, but the scatter introduced by relativistic beaming would essentially wash it out completely in small samples like ours. Therefore, the optical emission originates in the jet, and it seems very likely that it is optical synchrotron emission, i.e. just a higher-energy version of the same process that produces the radio jets.
So (in my view) the best current answer is: the central object is bright because it has jets, as has already been suggested. It is not the (presumed) black hole itself that's radiating in the optical, but the jets associated with it.
I would love to stay and try to answer the inevitable followup question, `why, if it's a black hole, is there relativistic _outflow_ nearby?' but I'm afraid I have to go home and pack for my eclipse trip to Hungary tomorrow. (I shall be away for a week, and so miss followups to this; if anyone says anything they particularly want me to see, please e-mail.)