With more and more detections of radio sources in the sky, the search of their optical counterparts, in addition to the discovery of radio galaxies, also yielded another class of objects -- the quasars.
The radio source 3C 48 was first found to have a 16th-magnitude star-like
optical counterpart with a spectrum
showing broad emission lines which could not be identified at that
time.
Later another radio source 3C 273 was found to have also such a weird
optical counterpart.
They are called quasi-stellar radio sources -- quasars.
In 1963, a Dutch astronomer Maarted Schmidt recognized that those lines were
Balmer lines of hydrogen so strongly
redshifted that they were difficult to identify.
[The spectrum of 3C 273]
3C 273 has a redshift of 0.158 (440h-1 Mpc)
3C 48 has a redshift of 0.367 (900h-1 Mpc)
Quasars are distant and powerful !!!
Quasars are generally blue; excess in UV.
Further surveys found many 'quasars', but radio-quiet.
So they are called quasi-stellar objects -- QSOs.
QSOs are sometimes refered to
the whole class; less than 10% of QSOs are quasars. (Occasionally, the name 'quasar' is
used to refer the whole class.)
Quasars with small redshifts are often in galaxy clusters, in which galaxies are faint and with a similar redshift.
[Host galaxies of quasars]
Quasars have jets and radio lobes.
[X-ray image of 3C 273]
[4C 32.69]
[The 3C 273 superluminal motion; blobs moving away at about 1 mas per year]
[The superluminal motion]
Quasars have also absorption lines. Often observed is Lyman series.
[The spectrum of quasar PKS 2000-030, z = 3.773]
What causes the Lyman α forest?
| Object | Luminosity (ergs/sec) |
| Sun | 4 x 1033 |
| Milky Way galaxy | 1044 |
| Seyfert galaxies and radio galaxies | 1043 - 1045 |
| Quasars and QSOs | 1045 - 1049 |