Watching a cannibal galaxy dine
distance of about 11 million light-years. One of the most studied objects in
the southern sky, by 1847 the unique appearance of this galaxy had already
caught the attention of the famous British astronomer John Herschel, who
catalogued the southern skies and made a comprehensive list of nebulae.
Herschel could not know, however, that this beautiful and spectacular
appearance is due to an opaque dust lane that covers the central part of the
galaxy. This dust is thought to be the remains of a cosmic merger between a
giant elliptical galaxy and a smaller spiral galaxy full of dust.
Between 200 and 700 million years ago, this galaxy is indeed believed to
have consumed a smaller spiral, gas-rich galaxy — the contents of which
appear to be churning inside Centaurus A's core, likely triggering new
generations of stars.
First glimpses of the "leftovers" of this meal were obtained thanks to
observations with the ESA Infrared Space Observatory , which revealed a 16
500 light-year-wide structure, very similar to that of a small barred
galaxy. More recently, NASA's Spitzer Space Telescope resolved this
structure into a parallelogram, which can be explained as the remnant of a
gas-rich spiral galaxy falling into an elliptical galaxy and becoming
twisted and warped in the process. Galaxy merging is the most common
mechanism to explain the formation of such giant elliptical galaxies.
The new SOFI images, obtained with the 3.58-metre New Technology Telescope
at ESO's La Silla Observatory, allow astronomers to get an even sharper view
of the structure of this galaxy, completely free of obscuring dust. The
original images, obtained by observing in the near-infrared through three
different filters (J, H, K) were combined using a new technique that removes
the dark, screening effect of the dust, providing a clear view of the centre
of this galaxy.
What the astronomers found was surprising: "There is a clear ring of stars
and clusters hidden behind the dust lanes, and our images provide an
unprecedentedly detailed view toward it," says Jouni Kainulainen, lead
author of the paper reporting these results. "Further analysis of this
structure will provide important clues on how the merging process occurred
and what has been the role of star formation during it."
The research team is excited about the possibilities this new technique
opens: "These are the first steps in the development of a new technique that
has the potential to trace giant clouds of gas in other galaxies at high
resolution and in a cost-effective way," explains co-author João Alves.
"Knowing how these giant clouds form and evolve is to understand how stars
form in galaxies."
Looking forward to the new, planned telescopes, both on the ground and in
space, "this technique is very complementary to the radio data ALMA will
collect on nearby galaxies, and at the same time it poses interesting
avenues of research for extragalactic stellar populations with the future
European Extremely Large Telescope and the James Webb Space Telescope, as
dust is omnipresent in galaxies," says co-author Yuri Beletsky.
Previous observations done with ISAAC on the VLT (ESO 04/01) have revealed
that a supermassive black hole lurks inside Centaurus A. Its mass is about
200 million times the mass of our Sun, or 50 times more massive than the one
that lies at the centre of our Milky Way. In contrast to our own galaxy, the
supermassive black hole in Centaurus A is continuously fed by material
falling onto into it, making the giant galaxy a very active one. Centaurus A
is in fact one of the brightest radio sources in the sky (hence the "A" in
its name). Jets of high energy particles from the centre are also observed
in radio and X-ray images.
The new image of Centaurus A is a wonderful example of how frontier science
can be combined with aesthetic aspects. Fine images of Centaurus A have been
obtained in the past with ESO's Very Large Telescope (ESO PR Photo 05b/00)
and with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La
Silla.