1.4. Matter Content of the Milky Way
As we have been reminded at this meeting, one form of dark matter has been discovered, namely MACHOs . It is still unclear what fraction of the microlensing events is due to MACHOs in our galactic halo, and what fractions are due to objects in either the Magellanic Clouds or our own (extended) galactic disc. If all the observed microlensing events originate from our galactic halo, as much as 8 % to 40 % of it could be composed of MACHOs. However, it now seems clear that MACHOs cannot constitute the bulk of the halo of the Milky Way .
A topic discussed at length at this meeting has been the composition of the central object in our galaxy, Sagittarius A*, that weighs ~ 3 × 106 solar masses , as seen in Fig. 5. Although this is normally presumed to be a black hole, but this has not been established. We know from the observation of adjacent stellar orbits that the central mass must be concentrated within a small radius. Curvature has been observed in the orbits of some nearby stars, as seen in Fig. 6, and the rate of precession of these orbits promises to become a useful tool for measuring how much of the mass of Sagittarius A* is extended.
A point in favour of the black hole interpretation of Sagittarius A* is that it has been observed to flare in X-rays, exhibiting large luminosity variations over very short time scales . Matter falling into a black hole is expected to emit X-rays, and the rapid time variation indicates that the central engine of Sagittarius A* must be small. However, Sagittarius A* is not very bright, and it has been argued that this poses a so-called `blackness problem' which motivates considering other models. However, as was discussed here , the blackness of Sagittarius A* is not necessarily a problem, since there are advective flow models that appear well able to reproduce its observed brightness .
As alternatives to the black hole hypothesis, balls of condensed bosons or fermions [23, 24, 25, 26] have been proposed as alternative models for Sagittarius A*. The latter model postulates a neutral, weakly-interacting fermion weighing about 15 KeV, and we have seen here detailed simulations of the evolution with time of a ball made out of such fermions , as seen in Fig. 7. This could not be a conventional neutrino, because the oscillation experiments tell us that they are degenerate to within 10-2 eV2, and Tritium -decay experiments tell us that the e mass is less than about 2.5 eV. Moreover, astrophysical and cosmological data suggest a similar upper limit on all the neutrino species. We also heard how such a fermion might also constitute the halo of the Milky Way .
Figure 7. Illustration of the formation of a fermion ball , showing the initial infall and the subsequent bouncing of material, some of which escapes while most falls back.
A potential `smoking gun' for the black-hole interpretation of Sagittarius A* is a Fe emission line at about 8.6 KeV, that is expected to be smeared by the gravitational redshift near the horizon. An effect consistent with this expectation has been reported. On the other hand, radiative decay of a fermion weighing 15 to 17 KeV could also produce photons in the same energy range!
In my view, there is no reason to abandon the conservative black-hole paradigm for Sagittarius A*, and Occam's razor prompts me to favour it. On the other hand, such a paradigm must be challenged constantly, and it is good to have a rival model that we can use to benchmark the success of the black-hole paradigm.