6.3 Where are they now?

At the beginning of this course we asked whether active galaxies really are in a class of their own or whether most galaxies go through an active stage at some point in their lives. We can shed some light on this by looking for evidence that active galaxies evolve.

The first question is where AGNs came from. No-one knows how supermassive black holes formed and the question is intimately tied up with the origins of galaxies which is itself a vigorously debated topic. But it is likely that close interactions and collisions between galaxies were much more common than they are now, and such disturbances played an important part in providing material to feed a growing black hole and to stimulate AGN activity. Even today, active galaxies are more likely than normal galaxies to be within the gravitational influence of a companion galaxy - about 15% of Seyferts have companions compared with 3% of normal galaxies - and you have seen examples such as Centaurus A (see Figure 23) which seem to be the result of a recent merger.

Next we can ask how long AGNs live. As indicated earlier, we observe distant objects not as they are today, but as they were at the time their light was emitted. As electromagnetic radiation takes 3.2 million years to travel one megaparsec, even the relatively nearby quasar, 3C 273, is seen as it was some 2.5 billion years ago, and those with the highest observed redshifts are seen perhaps only a billion years after the beginning of the Universe. So by studying the most remote quasars and comparing them with closer ones, it should be possible to see if they have changed over the lifetime of the Universe.

Astronomers have worked out the numbers of quasars in a given volume of space for different redshifts. When the expansion of the Universe is taken into account, the number density of quasars seems to have reached a maximum around a redshift of 2-3 about 10 billion years ago and has been declining sharply ever since. Indeed, quasars were something like 10³ times more common then than they are now. This suggests that the quasar phenomenon is short-lived, by cosmic standards. Where have they all gone?

• Bearing in mind what you already know about quasars, what would you expect a 'dead' quasar to look like?

• As a quasar is believed to be an AGN within an otherwise normal galaxy, a dead quasar would look like a normal galaxy without an AGN.

• How could you tell whether a normal galaxy once had a quasar inside it?

• Look in the nucleus! If the black hole model is correct, dead quasars will leave a supermassive black hole behind them.

So if quasars are indeed powered by supermassive black holes, it should be possible to find the 'relic' black holes in our local region of space, even where there are no obvious AGNs. If a galaxy was once a quasar the black hole will still be there; it is, after all, rather difficult to dispose of an object of 10⁸M_⊙.

In the last section you learned about the rotation studies used to measure the masses of black holes in AGNs - M87 holds the record at about 3 billion M_⊙. The same methods have been used to examine the centres of normal galaxies and one result has been a dark object with a mass of about 2 × 10⁶M_⊙ residing at the centre of the Milky Way.

There is even more compelling evidence that M31 (the Andromeda Galaxy), which is the nearest big spiral to the Milky Way, contains an object of 3 × 10⁷M_⊙. Even its small elliptical companion, M32, hides an object of 2 × 10⁶M_⊙. Several more otherwise normal galaxies, most of them not far from the Milky Way, appear to possess supermassive objects, and the closer the observations get to the centre, the more confident astronomers are that these concentrations of mass are indeed black holes.

The modern view is that many, perhaps most, galaxies contain supermassive black holes, though we know that some do not (another nearby spiral, M33, has been shown to have no supermassive black hole, or at least nothing more massive than 3000M_⊙). The ubiquity of supermassive black holes means that it is possible that many of the galaxies that we observe as 'normal' at the present time might have gone through an active stage in the past. It should be stressed however that there is no definite proof that this scenario is correct.

The idea that extinct (or perhaps, dormant) quasars might be lurking quite close to us is intriguing and also, perhaps, alarming. One important question is why the quasars died. It cannot simply be because of a lack of fuel. As you saw earlier, less than one solar mass a year is needed to fuel a typical AGN. This is a relatively small amount and could easily be provided by the host galaxy. However, in order to fall into the central black hole, any surrounding gas clouds must also lose angular momentum. You saw earlier that very close to the black hole, material can only spiral inwards because of the viscosity of the gas in the accretion disc. The mechanism by which more distant orbiting clouds may spiral in towards the centre of an active galaxy is still something of a mystery. However, it seems likely that whatever process operates to cause material to spiral inwards, it will be the clouds that are closest to the AGN that will be most strongly affected. Thus it has been suggested that as time passes the AGN may 'sweep clean' the gas from its immediate environment. If, as is expected, this gas is not replenished from clouds that are on orbits further away from the AGN then the mass accretion rate will drop, and the active galaxy will fade over time.

However this is not the end of the story, since if the central regions of the galaxy are disturbed - perhaps by a galactic collision or merger - then it is possible that the gas supply to the black hole could be temporarily restored and the AGN could then spring back into life. This may be what is currently happening in the case of the Centaurus A (Figure 23), which we have seen is a galaxy that appears to have undergone a recent merger. This scenario seems plausible, but is extraordinarily difficult to test in detail. However if this view of how AGN are fuelled is correct, then it is possible, although perhaps not very likely, that one day the black hole at the centre of the Milky Way could begin to accrete matter and start shining like a quasar.