Quasar observations and distance estimates

Quasars were first discovered as bright sources of radio emission. When astronomers identified their optical counterparts—the objects seen at the same positions in visible light—their spectra showed unusually large redshifts.

Redshift and distance

A spectrum contains lines at wavelengths that can be compared with the wavelengths at which those lines were emitted. If the observed lines are shifted towards longer wavelengths, the spectrum is redshifted. The redshift $$z$$ is calculated from the wavelength of a spectral line:

Here, $$\lambda_{\mathrm{observed}}$$ is the wavelength measured at Earth and $$\lambda_{\mathrm{emitted}}$$ is the wavelength at emission. A larger shift relative to the emitted wavelength gives a larger redshift.

The large cosmological redshifts of quasars indicate that they are extremely distant. They are among the most distant objects whose distances can be measured. At large redshifts, converting redshift into a distance estimate requires a cosmological model. The simple Hubble-law approximation is appropriate only at sufficiently small redshifts.

Distance and quasar luminosity

The energy flux $$F$$ is the energy received per unit area per unit time. Once a quasar’s distance has been estimated, its observed flux can be used to estimate its luminosity $$L$$: its total power output.

If the quasar emits equally in all directions, its luminosity is related to the observed flux by:

The quantity $$d_L$$ is the luminosity distance, the distance used in this flux–luminosity relationship. For a given observed flux, a greater luminosity distance implies a greater luminosity: the energy reaching Earth represents a smaller share of the total emitted power.

Quasars are so distant that their measured fluxes imply exceptionally high luminosities, even though the brightness measured at Earth is much less than their total power output.

Quasar central engine

A quasar is an exceptionally luminous centre of a galaxy, powered by material falling towards a supermassive black hole. The black hole itself does not emit the light that makes the quasar visible: the radiation comes from matter around it.

Gas falling towards the black hole forms a rotating accretion disc. As material moves inward through the disc, gravitational energy is converted into thermal energy. The hot material emits radiation, producing the quasar’s immense luminosity.

A supermassive black hole powers a quasar when it is supplied with enough infalling material to make the galaxy’s centre exceptionally luminous. This luminous centre is an active galactic nucleus; the quasar phase depends on the supply of material to the black hole.