4 Black holes at the centres of ordinary galaxies
4.1 The Milky Way
Figure 1 showed two spiral galaxies: NGC 5548, which has an active nucleus, and NGC 3277 which does not. If we accept that AGN are the result of accretion on to supermassive black holes at the centres of the galaxies which harbour them, it is natural to ask the question whether galaxies like NGC 3277, which do not have active nuclei, are without central supermassive black holes, or simply without significant amounts of accretion on to their central black holes. Recent observations have made it clear that in some cases at least, apparently ordinary non-AGN galaxies seem to harbour supermassive black holes at their centres.
Perhaps most dramatic is the evidence for a supermassive black hole at the centre of our own Milky Way Galaxy. It is impossible to study the centre of our Galaxy in optical light because there is lots of gas and dust in the plane of the Galaxy which obscures our view of the central regions. At other wavelengths, however, the optical depth is less, and it has long been known that the centre of our Galaxy harbours a compact radio source which is called Sgr A* (pronounced ‘sadj ay-star’), and is shown in Figure 7. Apart from Sgr A*, the radio emission from the centre of our Galaxy is diffuse and filamentary. The stars near the centre of the Galaxy are not visible because they are not strong radio sources. The infrared (IR) view shown in Figure 8 is very different. The IR image shown in Figure 8 is diffraction-limited, and gives a resolution of 0.15 arcsec. The blobs are individual stars within 0.02 pc of the Sgr A* radio source, whose position is marked with the small cross at the centre of Figure 8.
Figure 8 is one frame of the short animation you will see in Activity 3. The animation shows a series of high spatial resolution infrared images of the centre of our Galaxy, which were taken during the 1990s. The motions of individual stars are clearly apparent. By measuring these motions, the strength of the gravitational field experienced by the stars can be deduced. This is analogous to the determination of the Sun's gravitational field (and hence the Sun's mass) by studying the orbits of the planets in the Solar System. As you will see in the animation, however, the stars at the centre of the galaxy are not neatly aligned in a plane analogous to the ecliptic in the Solar System. Instead, the stars follow randomly oriented orbits, and the virial theorem rather than Kepler's third law is used to deduce the gravitational field. The motions you will see in Activity 3 require the presence of a dark body with mass 2.45 ± 0.4 × 106M at the centre of our Galaxy. This dark central body is almost certainly a black hole.