The quasars are among the most mysterious and fascinating celestial
objects known. They are very far galaxies, the farthest we know of, that
emit an enormous amount of energy from the nuclear region. Their discovery,
only a few decades ago, was accidental, as it often happens in astronomy.
They appear at the telescope like bright spots, with a star-like appearance.
Their spectrum however is different from that of the stars: the spectral
lines are remarkably shifted towards the red. This is due to the fact
that they are extra galactic objects, billions of light years far, and
therefore subject to a very high redshift.
Due to their distance, the optical luminosity of quasars is very low:
in fact, it decreases proportionally to the square distance of the source.
Most of the radiation emitted by quasars is part of the radio and infrared
bands, as opposed to the normal galaxies, that emit prevalently in the
optical band. These objects were first noticed thanks to their radio emission.
In the fifties, the first radio astronomers had classified many radio sources,
the nature of which was unknown at the time; nevertheless, the resolving
power of the radio telescopes at the time was very low, and did not
allow to determine exactly the position of the radio sources, and to establish
the nature of the source. At the beginning of the sixties, the new radio
telescopes allowed to obtain the exact position of some of these sources;
once the position was known, the optical counterpart of these sources could
be studied. Some were identified as nebulae,
the remains of a supernova or radio
galaxies. In 1960 it was discovered that 3C 48, an object of the radio
sources catalogue, corresponded to a blue object, very weak, apparently
a star. Even its spectrum was unusual, with many emission lines. Later,
other radio sources of this kind were discovered, but it was thought that
they were peculiar stars with radio emission. Their spectral
lines did not resemble any known line. Subsequently, other compact
radio sources were identified as objects with a stellar appearance, and
this category was called "quasar" (abbreviation of "quasi-stellar radio
sources").
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Image of a quasar at a distance of approximately 9 billion light years (at the centre). The object on its right is a star near to us. The bottom-left object is an elliptic galaxy, that appears to be near the quasar, but actually is 2 billion light years far. The quasar, despite its huge distance, appears very bright and has a stellar aspect, because it emits an enormous amount of energy in a rather small region of its nucleus. (HST) |
The extra galactic nature of quasars was discovered only in 1963 by
the astronomer Maarten Schmidt. He realized that the spectral lines emitted
by the 3C 273 radio source had wavelengths spaced just like the lines of
Hydrogen. However, they were shifted towards the red, instead of being
in the optical band. Supposing that the redshift was due to the Doppler
effect, this meant that 3C 273 was moving away from us at 48,000 Km/s,
too high a speed for a star: it must have been a source external to our
galaxy, and a very distant one, too.
After this discovery, other radio sources and quasars were examined,
and the verdict was the same for all: they were extra galactic objects
billions of light years far! Calculating, on the distance and apparent
luminosity, their intrinsic luminosity, it was discovered that these objects
irradiate with an enormous power, hundreds of times more than the brighter
galaxies.
Studying the luminosity of quasars, it was discovered that it increases
and decreases, periodically at times, with time scales of days or
weeks. These variations indicate that the source of such huge luminosity
must be very small: a few light-days. This happens because the speed of
light is the maximum speed the information can reach; if the dimensions
of the source were greater than the distance travelled by light in a given
time interval, its parts could not "exchange information", in other words
interact, in that temporal scale, and would not be correlated. So, there
would not be any luminosity variations, which are due to reciprocal variations
of the various parts of the source.
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Quasars are present in various types of galaxies, from the normal to the highly disturbed ones, as you can see in these images of Hubble Space Telescope. They appear like stars because their luminosity is able to obscure that of the galaxy that contains them. Left, quasars contained in normal galaxies. Centre, in interacting galaxies. Right, in peculiar galaxies. (HST) |
Studying the quasars with more powerful instruments, a very weak emission surrounding the compact sources was discovered. It is the optical counterpart of quasars, that were identified as galaxies. The nature of quasars was therefore partly unveiled: very far galaxies with very small and powerful active nuclei. They are hardly visible because of the enormous distance.
But what is the central engine of quasars? Not the nuclear reactions that take place inside the stars: even summing up the contributions of all the stars in a galaxy, you would not obtain the power of a quasar, and, overall, it could not be concentrated in such a small region. It is believed that this kind of objects is supplied by a huge black hole in the nucleus of the galaxy, like the Seyfert galaxies of the radio galaxies. Around it, an accretion disc of gas and stars in rapid rotation around its axis; from the disc matter would continuously fall on the black hole, producing a very powerful radiation. If the black hole had a mass increase slightly bigger than that of the Sun every year, the observed luminosity could be explained.
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The relation between the galactic interactions and the presence of quasars has long been suggested. The images of Hubble Space Telescope confirmed this relation. Some quasars, like those you can see here, are part of galaxies colliding with other nearby galaxies, or even merging with them. Left, the tidal structure of a galaxy associated with a bright quasar at a distance of a billion and a half light years from the Earth. It was distorted by the gravitational interaction with a nearby galaxy. Right, the same image with a different contrast level. The quasar and the galaxy are separated by 11 thousand light-years only, approximately the same distance between the Sun and the centre of the Milky Way. (HST) |
One of the most important reasons for the astronomers to study quasars lies in their enormous distance: the farthest quasars are 13 billion light-years far and even more! Since the speed of light is finite, this means that even the quasar radiation we observe today was emitted billions of years ago: these objects appear today as they were in the first billions of years of the life of the Universe... Some times we say that quasars can represent the "childhood" of galaxies. However, it is not clear whether all galaxies went through a phase of this kind, in the course of their life, or if they are peculiar objects.
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Right, the structure of the barred spiral galaxy that contains the 1229+204 quasar. The collision with a dwarf galaxy rich in gas has probably concentrated large amounts of gas in its nucleus, and this gas is probably the fuel for the engine of the quasar, maybe a huge central black hole. Moreover, the collision induces the intense formation of new stars in the galaxy, as a consequence of the acceleration and the compression of the gas, and of the subsequent gravitational collapse. The regions of recent stellar formation are those in blue. (HST) |
