Quasars
W
e have seen that once we move beyond the Local
Group, all the galaxies are racing away from us at
ever-increasing speeds. There is a definite link between
distance and recessional velocity, so that once we know
the velocity – which is given by the red shift in the
spectral lines – we can work out the distance. The most
remote ‘normal’ galaxies so far found lie at least 10,000
million light-years from us, but we know of objects which
are even further away. These are the quasars.
The quasar story began in the early 1960s. By that
time there had been several catalogues of radio sources
in the sky, several of which had been carried out at
Cambridge – but in general the radio emitters did not
correspond with visible objects, and in those far-off days
radio telescopes were not capable of giving really accurate
positions. One source was known as 3C-273, or the 273rd
object in the third Cambridge catalogue of radio sources.
For once Nature came to the astronomers’ assistance.
3C-273 lies in a part of the sky where it can be hidden or
occulted by the Moon, and this happened on 5 August- At the Parkes radio astronomy observatory in New
South Wales, observers timed the exact moment when the
radio emissions were cut off. Since the position of the
Moon was known, the position of the radio source could
be found. It proved to be an ordinary bluish star.
The results were sent to the Palomar Observatory in
California, where Maarten Schmidt used the great Hale
reflector to take an optical spectrum of the source. The
result was startling. 3C-273 was not a star at all, but some-
thing much more dramatic. The red shift of the spectral
lines indicated a distance of 3,000 million light-years, and
it followed that the total luminosity was much greater than
that of an average galaxy, even though the appearance
was exactly like that of a star. Other similar discoveries
followed, and it became clear that we were dealing with
objects of entirely new type. At first they were called
QSOs (Quasi-Stellar Radio Sources), but it then emerged
that by no means all QSOs are strong radio emitters, and
today the objects are always referred to as quasars.
Because the quasars are so powerful, they can be seen
across distances even greater than for normal galaxies.
According to the best estimate, we can now reach out
to at least 13,000 million light-years, so that we are see-
ing these quasars as they used to be when the universe
was young. There are none anywhere near the Local
Group, and it may be that no quasars have been formed
since the comparatively early history of the universe as
we know it.
Quasars are now known to be the cores of very active
galaxies, and it seems virtually certain that they are pow-
ered by massive central black holes. In many cases it is
now possible to see the companion galaxies. Very recent
research seems to suggest that quasars are born in environ-
ments where two galaxies are violently interacting or even
colliding. It is possible that a quasar may remain active for
only a limited period on the timescale of the universe, and
many large galaxies may go through a ‘quasar stage’
which does not last for very long.
There are also the BL Lacertae objects, named after
the first-discovered member of the class (which was
originally taken for an ordinary variable star, and given
a variable-star designation). Probably a BL Lac is simply a
quasar which we see at a narrow angle, perhaps by looking
straight down one of the jets.
The remoteness of the quasars means that they can
be used to study interstellar and intergalactic material. A
quasar’s light will have to pass through this material
before reaching us, and the material will leave its imprint
on the quasar spectrum; we can tell which lines are
due to the quasar and which are not, because the non-
quasar lines will not share in the overall red shift. Also,
we are becoming increasingly aware of what is termed
the ‘gravitational lensing effect’. If the light from a
remote object passes near a massive object en route, the
light will be ‘bent’, and the result may be that several
images will be formed of the object in the background;
if the alignment is not perfect there will still be detectable
effects. A good example is G22370305. Here we have
a galaxy 400 million light-years away, behind which is a
quasar lying at 8000 million light-years. The light from
the quasar is split, producing four images surrounding the
image of the lensing galaxy. This is often termed
the Einstein Cross, because it was Albert Einstein, in
his theory of relativity, who first predicted that such
effects could occur.
And yet there is some reason for disquiet. Objects at
equal distances from us must have the same red shift –
assuming that the shifts themselves are pure Doppler
effects. Halton Arp, formerly at the Mount Wilson
Observatory and now working in Germany, has found
that there are pairs, and groups of objects (quasar/quasar,
quasar/galaxy, galaxy/galaxy) which are connected by
visible ‘bridges’ and must therefore be associated, but
which have completely different red shifts. If so, then
the red shifts are not pure Doppler effects; there is an
important non-velocity component as well, so that all our
measurements of distance beyond our immediate neigh-
bourhood are unreliable. Arp goes so far as to suggest that
quasars are minor features ejected from comparatively
nearby galaxies.
Arp is certainly not alone in his views; he is strongly
supported by Dr Geoffrey Burbridge, Sir Fred Hoyle and
others. While at present this is very much a minority view,
it has to be taken very seriously indeed. If it proves to
be correct, then many of our cherished ideas will have
to be abandoned. It would, indeed, result in a revolution in
thought more radical than any since the 1920s, when
Hubble and Humason first showed that the ‘spiral nebulae’
are galaxies in their own right. Time will tell.ATLAS OF THE UNIVERSE
Quasar 3C-273. This
was the first quasar to be
identified, and is also the
brightest; its magnitude
is 12.8. It lies in Virgo. No
other quasar is brighter
than magnitude 16. Quasar HE 1013-2136,
imaged with the Kueyen
mirror of the VLT. The quasar
is embedded in a complex
structure with two arc-like
and knotty tails extending in
different directions; these
tails seem to result from
interactions between the
quasar host galaxy and one
or more of the close
companion galaxies. The
longer, southern tail extends
for 150,000 light-years, 1^1 / 2
times the diameter of our
own Galaxy.F Atl of Univ Phil'03stp 3/4/03 5:43 pm Page 202