Discovery and observational properties

Carl Seyfert reported in 1943 that a small fraction of galaxies have a very bright nucleus, which is a source of broad emission lines produced by atoms in a wide range of ionisation stages. These nuclei appear unresolved, i.e. they are stellar in appearance.

After World War II, radio surveys discovered a very bright radio source, called Cygnus A. Given the accurate radio-position of the source, the optical counterpart was identified with a peculiar-looking cD-galaxy, whose centre is encircled by a ring of dust.

The spectral lines in the optical spectrum of Cygnus A are redshifted to $\Delta\lambda/\lambda_{\rm laboratory}=0.057$. In terms of a Doppler shift, this corresponds to a recession velocity of 16000km s$^{-1}$. From this, one can estimate the distance to Cygnus A^11.1, $d\sim 240{\hbox{\rm Mpc}}$, and hence compute the implied luminosity of the radio source, which turned out to be roughly ten times the total energy output (i.e. over all wavelengths) of the MW.

In 1960, Thomas Matthews and Alan Sandage were trying to find the optical counterpart of another bright radio-source, called 3C 48^11.2. They found a 16th magnitude star like object, with a very curious spectrum, displaying broad emission lines that they could not identify with any known element or molecule. Sandage put it very scientifically: `This thing is exceedingly weird'. In 1963, another such weird object, 3C 273 turned-up. Given they looked like stars, in that they were point like (unlike galaxies which are extended), they were called quasi-stellar objects, or QSOs.

In the same year, the Dutch astronomer Maarten Schmidt recognised that the pattern of lines in 3C 273 was similar to the Balmer series^11.3 of the Hydrogen atom - but only if they were redshifted to the (improbably large ?) value of $\Delta\lambda/\lambda=0.16$, implying a recession velocity of $0.16c$ and a distance of 630Mpc. The distance to 3C 48 was even greater: a recession velocity of $0.367c$ and a distance^11.4of 1800Mpc.

For a distance of $d=630{\hbox{\rm Mpc}}$ to 3C 273, the distance modulus $m-M=5\log(d/10)\approx 39$. The apparent magnitude of 3C 273 is $m=12.8$ (in the V-band), hence the absolute magnitude is $M=-26$. Since the Sun's absolute magnitude $M_\odot=+5.$-ish, it implies that 3C 273 is $10^{0.4\times 31}\sim 10^{12}$ times brighter than the Sun, or 100 times as bright as the entire MW. Give or take a factor of a few. What kind of object can be so small yet so tremendously powerful?

Since then, many more QSOs have been discovered, we know several hundred thousands of them by now. The current^11.5 record holder has $\Delta\lambda/\lambda=6.3$. In terms of recession velocity, this is 6 times the speed of light (!), showing that interpreting such a redshift in terms of Doppler shift is not a good idea^11.6. The luminosity of the brighter QSOs can range up to $10^5$ times the luminosity of the MW.

The shear luminosity of QSOs alone already suggests we are looking at something unusual. In addition, some of these QSOs have radio-lobes which are tens of times the size of a galaxy. What is the engine that powers them?