3.9.3.3 - Quasars

Discovery of quasars

Early radio surveys found bright sources whose optical counterparts looked point-like, resembling stars. They were called quasi-stellar radio sources, shortened to quasars. “Quasi-stellar” describes their appearance, not their physical identity.

The decisive evidence came from optical spectra. In 1963 the apparently unfamiliar emission lines of 3C 273 were recognised as ordinary hydrogen lines at substantially longer wavelengths. Matching a pattern of lines establishes a reliable redshift; one unexplained line is less convincing. A cosmological interpretation placed the object far beyond nearby stars.

The first identifications were radio-bright, but not all quasars are strong radio emitters. A small angular image also does not by itself prove a small physical size: a very distant extended object can look tiny. Rapid changes in luminosity give an additional clue to a compact emitting region, because light needs time to cross a large source. The combination of spectrum, brightness and variability is much stronger than appearance alone.

Redshift and cosmological distance

The wavelength ratio gives a redshift without first needing a velocity:

For example, a rest line at $486\,\mathrm{nm}$ seen at $729\,\mathrm{nm}$ gives $z=0.500$. The wavelength has increased by 50%, not by a factor of 0.5. Cosmological redshift records stretching during the light's journey through expanding space. In an adopted cosmological model a large redshift indicates great distance and a long lookback time.

AQA describes quasars as among the most distant measurable objects: their high luminosities make spectra measurable over enormous distances. This is not a claim that quasars always hold the record for the most distant object of any kind.

When a simple distance estimate is appropriate

For a sufficiently small redshift, use positive recession speed $v\approx zc$ and then $d\approx v/H_0$. At large quasar redshifts, these are not a general exact distance recipe. A question may explicitly ask for a rough estimate using them; state the approximation rather than treating a result such as $zc>c$ as a measured local speed.

For a low-redshift active nucleus with $z=0.020$ and $H_0=65\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, use $c=3.00\times10^5\,\mathrm{km\,s^{-1}}$ to obtain $v\approx6000\,\mathrm{km\,s^{-1}}$ and $d\approx92\,\mathrm{Mpc}$. The matching velocity units are essential.

Accretion onto a supermassive black hole

A supermassive black hole provides a deep gravitational potential well, but light does not emerge from inside its event horizon. The luminous material is outside it. Gas spirals inward through an accretion disc: interactions within the gas convert gravitational potential energy into internal energy and radiation.