Active and dormant black holes in galactic nuclei

BHs of all sizes are very common in the universe. There are numerous known stellar-sized BHs in our galactic neighborhood, and in several nearby galaxies, with masses in the range 3–30 M⊙. These are the remnants of core collapse in type-II supernovae (SNs) with very massive progenitors of at least 20 or perhaps even 30 M⊙. Such objects are found in binary systems that are also strong X-ray sources, probably the result of accretion onto the BH. A direct causal connection between the SN explosion and the remnant BH has been established in several cases. A well-known example is the strong X-ray source in M100 with a location that coincides with that of a 1979 SN explosion (SN 1979C). As of 2011, this is the youngest known BH.

Active supermassive BHs, in galactic centers, have been known since the early discovery of QSOs in the 1960s. However, the idea that most galaxies, especially those with dynamically relaxed bulges, contain dormant supermassive BHs in their centers took much longer to develop. Detailed studies of the stellar velocity field and gas motion in about 60 nearby galaxies suggest the existence of such objects. Moreover, there seems to be a strong correlation between the mass of the bulge, its luminosity, and the mass of the BH. Because the physical sizes of the two masses are very different, and the ratio of masses is very large (about 100—4000; see Chapter 8), it is difficult to find a mechanism that will link the two. Understanding these relationships has become an area of intensive research in astronomy and is discussed later in this book.