Jordi Miralda-Escudé

Com Funcionen Realment els Quàsars?

Els quàsars són els objectes més lluminosos que es coneixen a l'univers, i els trobem en el centre de les galàxies. Alguns quàsars brillen tant com fins a 100 o 1000 vegades la lluminositat de la nostra galàxia, la Via Làctia, i aquesta enorme quantitat de llum prové d'una regió tan petita que sembla puntual, just al centre de la galàxia hoste, inclús amb els telescopis més potents de que disposem. Alguns quàsars emeten un poderós raig de partícules, que es pot observar sovint per la seva emissió en ones ràdio o raigs X.

Un exemple de com es veu un quàsar distant quan el mirem amb el Telescopi Espacial Hubble i l'Observatori Chandra de raigs X el trobem en aquestes imatges de PKS 1127-145 , un d'aquests quàsars que és tan brillant que no podem ni tan sols veure la galàxia al voltant perquè la pròpia llum del quàsar enlluerna excessivament. Alguns quàsars no són tan lluminosos (reben altres noms estrafolaris tals com ``galàxies Seyfert''), i aleshores podem veure la galàxia amb un punt brillant en el centre, tal com en el cas de NGC 7742 , una galàxia espiral el disc de la qual veiem gairebé exactament de cara.

The first quasars were discovered in 1963, and shortly after their discovery astrophysicists theorized that they were perhaps being caused by very massive black holes. Black hole's are predicted by Einstein's theory of General Relativity, which is the theory that describes gravity as arising from the geometry and curvature of space-time. A black hole is a mass that has been compressed into such a very small region of space, and its gravity has become so strong that not even light can escape from it. When any object falls toward a black hole, it accelerates to near the speed of light before it finally disappears into it. A black hole is the perfect garbage disposal machine: if you throw any object into a black hole, you will never see it or hear anything about it again.

Even though we call black holes ``black'' because no light can escape from inside their event horizon, when a lot of matter is falling into a black hole it will accelerate near the speed of light as it comes close to the horizon. If several streams of matter are in different orbits around the black hole after having fallen to it from different directions, they will collide with each other at speeds close to the speed of light. An enormous amount of energy is released in such a collision, which can then be emitted in the form of light. A black hole can convert as much as 10 or 20% of the rest-mass energy of the matter that it captures into light energy, and become extremely luminous. This is a much better efficiency of energy production than the efficiency of nuclear fusion reactions that power stars like the Sun, which is less than 1%. So, that is the reason that astrophysicists thought of black holes as soon as they discovered quasars: with much less mass than a whole galaxy, it produces much more light than a whole galaxy.

A good website to learn the basics on Active Galaxies and Quasars .

The standard idea about quasars has been that a central black hole is surrounded by a gaseous accretion disk, and it continuously captures matter from this accretion disk to produce its luminosity. One problem with this idea has remained unsolved: how does the matter get to the accretion disk, and how did the black hole grow to contain such a large mass by simply accreting from this disk? To understand this problem, it is useful to think of some scales. A typical galaxy that is 100000 light years across might contain a black hole that has captured as much mass as one thousandth of all the mass of the stars in the galaxy. The galaxy might contain this much mass in stars within a region of 100 light-years from the center. The black hole itself (its event horizon where infalling matter accelerates to the speed of light) is only about 10 light-minutes across, and when the quasar is shining the accretion disk that is emitting the light is only about one light-day across. So, how did so much mass manage to get funneled into the center, from a radius of many light-years to a radius of one light-day?

One might think that the answer to this could be very simple: matter just falls to the black hole, so it gets more concentrated around it! But actually, when matter falls from a large distance, it always has some angular momentum and it moves in orbit around the center, instead of falling in directly. That is why matter in galaxies often forms disks. When the gas settles in a disk and cools by emitting radiation, it generally tends to form clumps, and these clumps keep fragmenting and collapsing until they form stars. But the gas in a disk never seems to flow in toward the center from a large distance in any galaxies. The problem is that astrophysicists have not found any good reason why this should be any different near the center of a galaxy: so we would expect that any matter in a disk that was several light-years from the center would simply have formed stars and stayed forever in orbit at a large radius, but instead it seems to have flowed in to accrete to the black hole. How did this happen?

New quasar model

The new idea that I am working on is that actually, when the matter that was eventually going to collapse into the black hole reached the central region of the galaxy that was several light-years across, it was a big gas cloud which did indeed turn into stars during a big burst of star formation, instead of directly forming a gas disk and remaining gaseous. Then, these stars managed to have their orbits become very highly eccentric, and when they passed near the center they went through a dense, small gas disk around the growing black hole. Every time the star goes through the disk, the shock slows down the star a little bit until its orbit gradually shrinks and becomes embedded inside the disk. In this way, many stars end up inside the disk and they are destroyed due to collisions with each other, with the end result being that their matter is dissolved through the disk. So, this is the way that matter is brought very close to the center, into the small accretion disk around the black hole: first the matter turns to stars, then the stars are captured by the disk, and while the quasar is active the rate of stellar captures results in an input of mass into the disk that balances the mass that accretes into the black hole from the disk. In this model, the disk is small enough that we can understand that its matter flows in toward the black hole, owing to viscosity processes; at larger radius, much of the mass is brought in by the stars being captured by the disk.

In a paper that I have published with Juna Kollmeier, a graduate student at Ohio State University, we proposed this model and we showed how it may explain another unsolved problem: the fact that the masses of black holes correlate very well with the velocity dispersion of the bulge or elliptical galaxy around them. This correlation was discovered around the year 2000, and you can find a good description of it in this astro-ph paper . In our model, there is a certain number of stars that must be present around the black hole in order that the rate at which they are captured is sufficient to supply mass to the accretion disk, at the rate that is necessary to make the quasar shine. If we want to have these number of stars, their total mass implies that they must move with a certain velocity dispersion. And we found that the correlation predicted by our model for the black hole mass and galaxy velocity dispersion matches reasonably well the observed relation.

At the present time I am working at understanding better the way in which the stars can be captured by the accretion disk, and the way that the accretion disk can remain in a steady-state configuration as it gains matter from the stars and loses it into the black hole. The stars first need to have their orbits modified to become highly eccentric. This can be achieved by the process of orbital relaxation. The exact way in which stars are brought to eccentric orbits depends on the relaxation process, which could be due to massive clusters or gas clouds passing through the central region of the galaxy and perturbing the orbits of the stars in that region. The gravity of the stellar system and of the gaseous accretion disk itself also introduces small perturbations to the gravitational field of the black hole that can change the way in which orbits can be modified to become highly eccentric. This process is essential to understand how stars are captured by the disk and whether the whole model can work.

A problem that this model is facing, which does not at this point have a clear solution, is related to the angular momentum of the stars that are captured by the disk. The stars that are captured by the accretion disk may tend to have a net specific angular momentum that is opposite to that of the disk. The reason is that the stars that move along their orbit in the opposite sense of rotation than the disk can be more easily captured, because the relative velocity of the collision with the gas disk will be faster (and so the stars can slow down more rapidly). If this is indeed true, then the stars would remove the angular momentum of the disk as they are captured, and the matter in the disk would have to rapidly accrete into the black hole. A steady-state solution where the disk is maintained as it captures more stars and accretes matter to the black hole could not exist. I am investigating this problem to see if it is actually true that captured stars have an average specific angular momentum opposite to the disk. The problem is actually highly complex because the stellar system around the accretion disk would normally have a small degree of rotation, which could constantly give an average angular momentum to the stars on nearly radial orbits that are undergoing the capture process. in order to rotate around the black hole.

Publication related to this project

  1. J. Miralda-Escudé and Juna Kollmeier 2005, ``Star Captures by Accretion Disks: A Possible Explanation of the M-sigma Relation'', Ap. J., 619, 30.
    You can also find the astro-ph preprint online.

    Jordi Miralda Escudé ( miralda@ieec.uab.es)

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