The Universe is a violent place - as astronomers use increasingly sensitive
means and methods to study the diverse processes out there, they become aware of
the extraordinary forces acting in the space that surrounds us.
With larger telescopes and ever-more sophisticated instruments, new
information is gained about remote celestial objects and their behaviour. Among
the most intriguing ones are the radio galaxies which emit prodiguous
amounts of energy, in the form of fast-moving particles and intense
electromagnetic radiation.
One of these is known as 3C 445; it is located near the celestial
equator within the zodiacal constellation Aquarius (The Waterman), at a distance
of about 1 billion light-years. It most probably harbours a black hole at its
centre, more massive than the one at the centre of our own galaxy, the Milky Way
(ESO PR
19/02). This galaxy was first observed from Cambridge (United Kingdom)
in the 1950's and was listed as radio source no. 445 in the Third Cambridge
Catalogue (1959), hence the name.
Later observations revealed a strong outflow from this galaxy's active
centre, visible on radio maps as two opposite plasma jets with strong
synchrotron radiation ([2])
originating from rapidly moving electrons in the associated magnetic field
(image "a" in PR Photo 26/02).
Now, a trio of European astronomers [1]
have used two advanced instruments, ISAAC and FORS1 on the 8.2-m VLT ANTU
telescope at the ESO Paranal
Observatory (Chile) to obtain near-infrared images of these jets (images
"b" and "c" in PR Photo 26/02).
As can be clearly seen on the radio picture of 3C 445 obtained with
the NRAO Very Large Array
(VLA) radio facility ("a"), the plasma jets of fast particles emanating
from the galaxy ram into the surrounding intergalactic medium (mostly primordial
hydrogen), thereby producing two "shocks", both at a distance of
approximately 1.5 million light-years from the central galaxy and with
particularly strong synchrotron emission. With a total length of more than 3
million light-years, or no less than one-and-a-half times the distance from the
Milky Way to the Andromeda galaxy, this structure is indeed gigantic.
The region where the jets collide with the intergalactic medium are known as
"hot spots". Superposing the intensity contours of the radio emission
from the southern "hot spot" on a near-infrared J-band (wavelength 1.25 µm) VLT
ISAAC image ("b") shows three distinct emitting areas; they are even better
visible on the I-band (0.9 µm) FORS1 image ("c"). This emission is obviously
associated with the shock front visible on the radio image.
This is one of the first times it has been possible to obtain an
optical/near-IR image of synchrotron emission from such an intergalactic
shock and, thanks to the sensitivity and image sharpness of the VLT, the
most detailed view of its kind so far.
The central area (with the strongest emission) is where the plasma jet from
the galaxy centre hits the intergalactic medium. The light from the two other
"knots", some 10 - 15,000 light-years away from the central "hot spot", is also
interpreted as synchrotron emission. However, in view of the large distance, the
astronomers are convinced that it must be caused by electrons accelerated in
secondary processes at those sites.
The new images thus confirm that electrons are being continuously accelerated
in these "knots" - hence called "cosmic accelerators" - far from the galaxy and
the main jets, and in nearly empty space. The exact physical circumstances of
this effect are not well known and will be the subject of further
investigations.
The present VLT-images of the "hot spots" near 3C 445 may not have the same
public appeal as some of those beautiful
images that have been produced by the same instruments during the past
years. But they are not less valuable - their unusual importance is of a
different kind, as they now herald the advent of fundamentally new insights into
the mysteries of this class of remote and active cosmic objects.
Notes
[1]: The new results are described in a research paper,
"Particle Accelerators in the Hot Spots of Radio Galaxy 3C 445, Imaged with the
VLT" by M. Almudena Prieto (ESO, Garching, Germany), Gianfranco
Brunetti (Istituto de Radioastronomia del CNR, Bologna, Italy) and
Karl-Heinz Mack (Istituto de Radioastronomia del CNR, Bologna, Italy;
ASTRON/NFRA, Dwingeloo, The Netherlands; Radioastronomisches Institut der
Universität Bonn, Germany), that recently appeared in the research journal
Science (Vol. 298, pp. 193-195).
[2]: When electrons - which are electrically charged - move
through a magnetic field, they spiral along the lines of force. Electrons of
high energy spiral very rapidly, at speeds near the speed of light. Under such
conditions, the electrons emit highly polarized electromagnetic radiation. The
intensity of this radiation is related to the strength of the magnetic field and
the number and energy distribution of the electrons caught in this field. Many
cosmic radio sources have been found to emit synchrotron radiation - one of the
best examples is the famous Crab Nebula, depicted in ESO PR
Photo 40f/99.
