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The Question

(Submitted March 01, 1998)

How long is a Quasar's life and what happens when it dies?

The Answer

The question you asked about quasar lifetimes is an excellent one, but we only know this on the basis of theoretical arguments that are consistent with observational data; all quasars ever discovered (the first discovery was about 35 years ago) are still "there," so this is the only truly undisputed observational measurement of their lifetime. Let me describe what we know about quasars, and from this, I will give you some arguments for their lifetimes.

In general, quasars are relatively bright point sources; we believe that they are centers, or "nuclei" of galaxies. They show large redshifts, meaning that they are moving away from us at large velocities. In the currently accepted scenario that the Universe is expanding such that the velocity of expansion is roughly proportional to the distance from us to an object, this means that quasars are very distant objects, located often 1/2 way to the edge of the visible Universe. Two points are important here:

First, since quasars are relatively bright, yet very distant, so intrinsically, they must be extremely luminous - perhaps a thousand times more luminous than all stars in a galaxy put together. Yet this tremendous power has to arise in a region that is comparable in size to the Solar system, and we know this from the fact that their brightness varies on a relatively short time scales. Since no object can be larger than the distance light can travel over a time during which the object changes its brightness by, say, a factor of two - this implies that their tremendous light output arises in a relatively small volume. We believe that the best scenario is that quasars are powered by an infall of matter onto a very massive black hole, having a mass as large as a million to 100 million Suns.

However, there is only so much matter per unit time that a black hole can "swallow" - this is typically, for a 1 - 10 million solar mass black hole, about one solar mass per year. So, to the first order, lifetime of a quasar has to be at least one to ten million years.

Second point has to do with the number density of quasars as a function of their distance. At large distances - say, half-way to the edge of the Universe - there are many more quasars than we see at our local neighborhood. However, since light travels with finite speed, we observe these objects when there were about 1/2 as old as we are now. What happened to those quasars? Let's for the moment take the age of the Universe (meaning the time elapsed from the Big Bang) to be 12 billion years. At the distance corresponding to the time of roughly 9 billion years ago, we already see many galaxies and quasars, and we infer that the masses of their black holes are about 1-10 million solar masses. What happened to them? To account for those quasars of the past, we should have a lot of rather anonymous local galaxies harboring very massive but inactive black holes in their center.

It turns out that recent theoretical work implies that for a given number or grams of matter falling onto a black hole per second, the efficiency of conversion of the gravitational energy to radiation (light) drops as the mass of a black hole increases, and eventually, the quasar gets fainter and fainter even with the same rate of mass accretion. So, this can account for local quasars being dim. In fact, very recent work on the basis of the Hubble Space Telescope clearly indicates that there are many local galaxies (some researchers think as many as 50% of all galaxies!) have "quiet" black holes in their centers. This implies that the quasar phase is probably shorter than a few billion years.

This gives you a general idea: we don't know, but today's best estimates are that the quasar - meaning a luminous phase of an accreting supermassive black hole - probably lasts for a time span of 10 million to a few billion years.

Greg Madejski for Ask an Astrophysicist

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