Quasi-Stellar Objects - ``QSOs'' or ``Quasars''

Radio astronomy got going during the 1940s and 50s, and although in its early days position accuracy was poor, by the early 1960s a number of radio sources had been identified with apparently stellar (point-like) sources. The assumption was that they were stars, and yet their optical spectra were unlike those of any star previously observed. They were called quasi-stellar objects, and are now generally called QSOs or quasars.

The breakthrough came in 1963 when Marten Schmidt realised that the spectra of QSOs were highly redshifted in that atomic lines of rest wavelength $\lambda_0$ were actually seen at wavelength

$$\lambda_\mathrm{obs} = \lambda_0 (1 + z)$$

where z had the same value for all lines from a given object and is known as its redshift. By analogy with the Doppler shift (section 3.1.2)

$$\lambda_\mathrm{obs} = \lambda_0 \left( 1 + \frac{V_R}{c}\right),$$

the redshift can be interpreted as a radial velocity V_R = cz. In fact this only applies at small z, as one should really use the relativistic Doppler effect; moreover the ``radial velocity'' is really just the stretching out of distances in the Universe. The current record is $z \approx 5$.

Now in the next lecture I will look more at the expansion of the Universe, but locally it can be described by a speed that increases with distance at a rate of about $65\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$. The first QSO to be measured, called 3C273, had z = 0.158, or $V = 47,000\,\mathrm{km}\,\mathrm{s}^{-1}$. This translates to a distance of $730\,\pc$. Thus Schmidt's realisation of the redshifted nature of quasars immediately put them far outside our Galaxy, and in fact far outside the local group of galaxies. At the same time it implied vast luminosities, with the brightest quasars having luminosities of up to $10^{14}\,\mathrm{L}_\odot$. Such a luminosity is 100 times larger than that of a large galaxy. Whatever they are, they are not stars!

More remarkably still, the brightness of quasars can change in a matter of weeks. This is extraordinary because we don't expect astrophysical objects to be able to change more quickly than the time taken for light to travel across them. Thus even if all the stars in M31 went out at once, we would actually see those closest to us go out first, and it would be tens of thousands of years before the most distant followed suit. The only way round this is for a special coordination of changed directed specifically at us, which seems unlikely. Therefore the brightness changes of quasars implies that their huge luminosity comes from a region of less than $0.01\,\pc$ in size! The combination of vast luminosity and small size allows them to outshine their host galaxies and appear stellar, although in recent years some host galaxies of quasars have been imaged, most notably with the Hubble Space Telescope because good resolution is essential.