Wednesday, February 2, 2011

Stellar Objects Part 6: Quasars

Stellar Objects Part 6: Quasars

Quasars are the black hole giants of the universe lurking within active galaxies.


A quasar is a compact region in the center of an active galaxy surrounding a supermassive black hole. An active galaxy is a galaxy whose galactic center is a black hole that is accreting mass and spewing out radiation. A quasar's radius is thought to be between 10 and 10,000 times the radius of the black hole's event horizon and it is powered by the accretion disc that surrounds the black hole. It is the brightest and most powerful object in the universe. This is an artist's concept of what a growing quasar might look like.



Quasars tend to inhabit young galaxies (galaxies that still have lots of gas and dust to "feed" the black hole centered within) and can emit up to a thousand times the energy output of the entire Milky Way galaxy. About 200,000 quasars have been identified so far.

The first quasar was identified in 1963 by the astronomer Maarten Schmidt. He also determined that most quasars are extremely redshifted, placing them at distances of around 13 billion light years away. This means they are found only in the most remote, and therefore oldest, parts of the universe. The radiation emitted from quasars spans the full electromagnetic spectrum, from X-rays to the far infrared, peaking in the ultraviolet-optical range, making them bright to optical telescopes despite their incredible distance. This range of radiation indicates that the atoms within the quasar's accretion disc are not just extremely hot but highly irradiated as well; this distinguishes quasars from stars. The closest quasar is 3C 273, located in the constellation Virgo, a quasar visible with an amateur telescope. It has a mass of about 886 Suns and it sports a large visible jet (about 10% of quasars are endowed with one). It's relatively close, at 2.4 billion light years away. 







This is an X-ray image of how it looks through the Chandra X-ray observatory.






The radiation from a quasar is from outside the event horizon. It is energy created by the gravitational stress and friction of material accelerating toward the black hole. There are several nearby galaxies, ours included, that are centered on black holes but not quasars. Currently, most astrophysicists believe a black hole lies at the center of every large galaxy but only a small fraction of these black holes emit powerful radiation and are categorized as quasars as a result. A quasar may be ignited or re-ignited when a fresh source of matter falls toward a galactic black hole and creates a rotating accretion disk, as a result of the infalling material's angular momentum. A typical quasar consumes about 10 solar masses a year. The brightest known quasars consume 1000 solar masses a year. Some physicists predict a quasar will form when Andromeda galaxy eventually collides with our Milky Way, in about 4 billion years. When a quasar consumes all the nearby matter, its accretion disk dissolves and the active galaxy becomes an ordinary galaxy.

Microquasars are small cousins of quasars. Unlike galactic black holes, they are radio-emitting X-ray binary stars. They are named after quasars because they share some characteristics such as strong emission of radiation, often a pair of radio jets, and an accretion disk, although this disk can surround either a black hole or a neutron star, either of them being only a few solar masses. This compact mass object accretes mass from a rotating normal star. This is an artist's impression of the microquasar, SS 433.



Something changed in the universe over the last couple of billions of years so that quasars are generally no longer created. There were very few quasars during the first billion years of the universe but as many as thousands of times more quasars in the 2-billion year old universe compared to today, suggesting an epoch of quasar formation occurred just after the time the first stars (thought to be massive) formed. Some theorists suggest that the early universe contained a much higher percentage of matter in the form of neutral hydrogen and helium molecular gases than the universe does today. This matter rapidly fed primordial black holes created during one of the early universe's phase transitions, in particular the phase transition of light pseudo-Goldstone bosons. Clusters of accreting primordial black holes formed centers of dark matter condensation and as a result, supermassive black holes evolved, accompanied by early quasar activity.

Finally, we explore mysterious Worm Holes, next.

2 comments:

  1. I'm confused by the statement that quasar 3C 273 has a mass of 187 suns. Is that just the accretion disc?
    If, so, how was the number determined?

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    Replies
    1. How to measure a Quasar's mass:

      This is a great question! But first I need to apologize for an error I made in reporting this quasar's mass! Its correct mass is 886 ± 187 solar masses, not 187 solar masses.

      Quasars have only been known for about 40 years. The mass of 3C 273 (http://en.wikipedia.org/wiki/3C_273) was measured using a new technique called reverberation mapping (http://en.wikipedia.org/wiki/Reverberation_mapping), developed around 2004 (http://arxiv.org/abs/astro-ph/0407538) to measure black hole masses. The optical spectrum of light you get from a quasar (or black hole) contains both a continuous spectrum and an emission line spectrum. The continuum spectrum is produced at the center of the quasar where gas (in the accretion zone) is being heated up and pulled into the black hole. The emission lines come from orbiting gases further out that are excited by the continuum radiation. There are always changes going on in the continuum zone (close to the interior black hole) – outflows, jets, winds and changes in radiation pressure) and there is a lag between these changes in the continuum spectrum and resulting changes in the emission line spectrum from gas further out. You can measure this delay (http://astro.berkeley.edu/~louis/astro292_eliot/reverb.pdf) by measuring the width of something called the broad line region of the spectrum. It is created at a specific radius from the center of the black hole and it indicates Doppler motion. From this, you get a measure of the velocity of the gas at the broad line region. The velocity depends on the gravitational pull by the quasar or black hole. It includes all the mass that is being pulled in and has been pulled in but not the mass of the gas that is orbiting the quasar.

      It's not easy to do. You need a lot of sampling and you need to make assumptions about the shape of the radius mentioned above. There can also be contamination of the broad line region of the spectrum by other emission lines, etc., and gravitational lensing can distort the measurements.

      It is certainly much harder to estimate a quasar's mass than a typical star's mass, which can be directly measured if it is part of a binary system or estimated from its temperature and luminosity and plugging that information into a stellar interior model (because the dynamics of stars are fairly well understood).

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