Unraveling the Mystery of Quasars: Are They Really Dying Out?

[Ramael]

Hi,

I was reading an article on physorg, as I often do, when I stumbled on something that both confused and perplexed me. The article, "Black holes' true power revealed by 'Russian doll' galaxy," referred to quasars and how they are "almost extinct today."

I'm well aware of hawking radiation, and how it slowly contributes to a loss of mass in black holes, but still this shouldn't cause the death of a black hole at this point in the universes life span yet, should it?

I noticed a similar reference in an article outside of physorg.com, and can't wrap my head around how quasars could possibly be going extinct. Am I missing something? Do even black wholes have a lifespan comparable to that of stars, or do strange phenomenon cause changes to a quasar at some point along its life?

I look forward to hearing some responses. :D

 

[Vagn]

A Quasar is when the central supermassive black hole of a galaxy is absorbing matter from its accretion disk, the matter heats up well into the millions of kelvin and emits high energy photons i.e. gamma rays and x-rays. Eventually all the matter in the accretion disk will be absorbed by the black hole, so that no more matter will fall into the black hole, which is when a quasar 'dies'. Of course eventually the supermassive black hole will evaporate by the process of Hawking radiation, but that is predicted to be about 10^100 years away yet.

 

[bombadil]

When it says Quasars are extinct today, it is merely referencing the observational fact that the number of quasars seems to peak at redshifts around $z\sim 1$ or 2 and at lower redshifts the number of quasars is much lower. Remember that objects with high redshifts are far away and objects with low redshifts are close by. Couple that with the fact that the light of far away objects takes a long time to reach us, and you have the following picture: the number of quasars was much higher a long time ago in the Universe's history (i.e. $z\sim 2$) than it is today (i.e. $z \sim 0$).

Why are the number of quasars decreasing as the Universe ages? Quasars are powered by interstellar gas falling on the supermassive black holes that reside at the center of galaxies. In the early universe, it's conceivable that the amount of available gas to fall on the central black hole was higher (perhaps because such gas is distributed differently in the present day universe since galaxies are older). It's also possible that to get significant amounts of gas to fall to the center a of a galaxy a galaxy-galaxy merger must occur. If this is the case, then more quasars are expected early in the universe because galaxies were closer together and thus had more collisions.