Quasars
A quasar (quasi-stellar object) is the most luminous class of active galactic nucleus, powered by accretion of gas onto a supermassive black hole of roughly 108–1010 solar masses, with bolometric luminosities of about 1038 to 1041 watts that can exceed the light of all the stars in the host galaxy. Quasars were most abundant about 10 billion years ago.
Curious
In the early 1960s, radio astronomers found star-like points of light that were strong radio sources. In 1963, Maarten Schmidt showed that one of them, 3C 273, has spectral lines shifted by 15.8% toward the red, which meant it lay billions of light-years away and had to be extraordinarily luminous. These objects were named quasi-stellar radio sources, or quasars.
We now understand them as a bright phase of active galactic nuclei. Gas spirals into the central supermassive black hole through an accretion disc, heating to extreme temperatures. Because their light has travelled so long, distant quasars show us the young universe, and they are used as lamps to probe the gas between galaxies.
Exploring
| Property | Typical value |
|---|---|
| Black hole mass | 108–1010 M☉ |
| Luminosity | 1038–1041 W |
| Peak abundance | z ≈ 2–3 (about 10 Gyr ago) |
| Redshift of 3C 273 | 0.158 |
Deep Dive
Power source. Accretion converts a fraction η of rest-mass energy into radiation, with η typically about 0.1 (compared with 0.007 for hydrogen fusion). The luminosity L = ηṁc² means a luminous quasar swallows the equivalent of a few solar masses per year. Matching L to the Eddington limit gives a growth (Salpeter) time of about 45 million years for η = 0.1, so black holes of 109 solar masses seen only 700 million years after the Big Bang (quasars at redshifts above 7 have been found) are hard to explain with standard growth from stellar-mass seeds; heavy seeds from direct collapse or super-Eddington accretion are proposed, and the question is unresolved.
Unified picture. In the unified model of AGN, observed differences between quasars, Seyfert galaxies and radio galaxies depend largely on viewing angle and on whether relativistic jets are present, with a dusty torus obscuring the nucleus from some directions. This is a successful but not complete account; accretion rate, black hole spin and environment also matter.
Cosmic role. Quasar activity peaked when the universe was 2–3 billion years old and has declined since. The close correlation between black hole mass and host-galaxy bulge properties suggests that feedback from quasars regulates galaxy growth (see Galaxy Formation); the strength of this feedback is still debated. Absorption lines in quasar spectra (the Lyman-alpha forest) trace intergalactic gas and the structure of the universe (see Large-Scale Structure).
Concept Relationship Map
Requires: active galactic nuclei, accretion discs, supermassive black holes
Enables: probes of the early universe, galaxy-feedback models
Commonly confused with: ordinary stars (they only look point-like)
Sources
- Schmidt, M. (1963). Nature, 197, 1040.
- Salpeter, E.E. (1964). “Accretion of Interstellar Matter by Massive Objects.” The Astrophysical Journal, 140, 796–800.
- Netzer, H. (2013). The Physics and Evolution of Active Galactic Nuclei. Cambridge University Press.