Sparkling Stephan’s Quintet
Copyright: ESA/XMM-Newton (X-rays); ESA/Herschel/PACS, SPIRE (infrared); SDSS (optical)
The Stephan’s Quintet of galaxies was discovered by astronomer
Édouard Stephan in 1877. At the time, however, he reported the discovery
of ‘new nebulae’, as the concept of other galaxies beyond our Milky Way
was only formalised in the 1920s.
This image combines
observations performed at three different wavelengths, with ESA’s
Herschel and XMM-Newton space observatories as well as with ground-based
telescopes, to reveal the different components of the five galaxies.
Stephan’s
Quintet is one of the most spectacular galactic groups known, but only
four galaxies from the originally discovered quintet are physically
linked – the other was later discovered to be much closer to us. NGC
7320, the galaxy in the lower part the image, lies about 40 million
light-years from us, rather than the 300 million light-years of the
others.
One of them is the bright source above NGC 7320 in this
view, two are the intertwined galaxies immediately to the right of image
centre, and the fourth is the round patch towards the lower-right
corner.
Later, it was discovered that an additional galaxy, hidden
beyond the left edge of this image, sits at a similar distance to these
four galaxies, reinstating the group as a quintet.
By observing
these galaxies in infrared light with Herschel – shown in red and yellow
– astronomers can trace the glow of cosmic dust. Dust is a minor but
crucial ingredient of the interstellar matter in galaxies, which
consists mainly of gas and provides the raw material for the birth of
new generations of stars.
One galaxy stands out in the infrared light: the nearby NGC 7320, a spiral galaxy busy building new stars.
Shown
in white, the optical light observed from ground-based telescopes
reveals the shapes of the four distant galaxies, which exhibit tails and
loops of stars and gas. These intricate features are an effect of their
mutual gravitational attraction.
The intense dynamical activity
of the distant group is also portrayed in the distribution of diffuse
hot gas, which shines brightly in X-rays and was detected by XMM-Newton.
Represented
in blue, the hot gas appears to sit mostly between the four colliding
galaxies. It is likely a mixture of primordial gas predating the
formation of the galaxies and intergalactic gas that has been stripped
off the galaxies or expelled during their interactions.
A hint of a
shockwave from the interaction of these four galaxies is visible as an
almost vertical blue structure on the right of the image centre. This
structure of hot gas also seems to trace a filament of infrared-bright
dust that might have been heated by the shock.
At the top end of the shock, the infrared view reveals stars forming both within and outside the galaxies.
A
faint tail of stars, gas and dust extends towards the left, leading to a
dwarf galaxy glowing in infrared – the red and yellow object at the tip
of the tail.
Further to the left, a dense concentration of hot
gas is also visible in blue at the end of the tail, although it is
unclear whether it belongs to the galactic group or is a foreground
source.
Source: ESA
