Images online at http://www.eso.org/outreach/press-rel/pr-2002/phot-26-02.html

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 micron) VLT ISAAC image (“b”) shows three distinct
emitting areas; they are even better visible on the I-band (0.9
micron) 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 Universitaet 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.

ESO PR Photo 26/02 may be reproduced, if credit is given to the
European Southern Observatory (ESO) and the US National Radio
Astronomy Observatory (NRAO).