Widespread galactic cannibalism in Stephan’s Quintet revealed by CFHT

Full field of view of the CFHT-MegaCam image in optical
wavelengths (strong color saturation and contrast reveal
the nature of the various components). Credit: CFHT,
Pierre-Alain Duc (Observatoire de Strasbourg) &
Jean-Charles Cuillandre (CEA Saclay/Obs. de Paris)

An extremely deep multi-band optical image from the
Canada-France-Hawaii Telescope (CFHT, Hawaii. USA) casts a
new light on the formation process of the famous group of 5
colliding galaxies. The image reveals structures undetected
thus far, in particular a very extended red halo composed of
old stars, and centered on an elliptical galaxy, NGC 7317,
which had been ignored in previous studies on the dynamics of
the global collision. These results are published in the
Monthly notices of the Royal Astronomical Society by a
team from the Observatoire Astronomique de Strasbourg
(France), CEA Saclay (France) and the Lund Observatory

The wide field image captured with the 380 megapixel camera
called MegaCam is focused on the nearby galaxy NGC 7331. The
image exhibits several galactic and extragalactic features,
some very extended and dim, including filaments of interstellar
dust in the foreground (galactic cirrus). The scientists’
attention was however captured by the condensation of galaxies
in the field, much further beyond NGC 7331: the famous
Stephan’s Quintet named after the French astronomer Édouard
Stephan who was the first to observe it in 1878.

Stephan’s Quintet is a compact group of 5 spiral and elliptical
galaxies (excluding a spiral in appearance related to the
system but actually present in the foreground, at the same
distance as NGC 7331). The Hubble Space Telescope immortalized
this region after observations of the group became one of the
telescope’s iconic images. Stephan’s Quintet is the poster
child for studies on the collective evolution of galaxies
subjected to a range of effects such as interactions and slow
collisions creating gravitational stellar streams, high speed
galactic collision, gas ramming, starbursts and creation of
intergalactic stellar systems.

Stephan’s Quintet in true colors as featured in the
CFHT/Coelum 2018 calendar. NGC 7317 is the lower right member
of the group. Credit: CFHT/Coelum, Jean-Charles Cuillandre
(CFHT/CEA Saclay/Obs. de Paris) & Giovanni Anselmi (Coelum)

Due to its unique features, Stephan’s Quintet has been widely
observed across the entire electromagnetic spectrum, and has
been the subject of many complex numerical simulations. The
team detected a red halo composed of old stars centered on a
galaxy, NGC 7317. NGC 7317 was thought to be in a stable state
or recently arrived near the group. The detection of red stars
implies the contrary, that this galaxy has been interacting for
a very long time with the other members of the group.
Interactions such as the one seen in these observations are
called galactic cannibalism. Galactic cannibalism occurs when
the gravitational forces from a larger galaxy or group of
galaxies slowly tear apart a smaller galaxy. Characteristic
features of galactic cannibalism are streams or halos of stars
orbiting the larger galaxy, like the halo of red stars seen
around NGC 7317. A first implication is that Stephan’s Quintet
is far older than currently admitted. The models of formation
and evolution of this emblematic system will have to be
revised. This global case of galactic cannibalism should
eventually lead to the formation of a giant .

This new result illustrates the current renewed interest in the
scientific field for deep imaging on nearby galaxies. Many
observing programs, including several developed at CFHT which
is particularly well suited for such studies, aim at decoding
the past history of through the detection in
their direct environment of faint extended features, a
technique known as galactic archeology.

Each year since 2000, CFHT produces in collaboration with the
Italian editor Coelum the Hawaiian Starlight calendar based on
beautiful images of the sky captured by MegaCam. These images
result from special observations obtained through CFHT
director’s discretionary time when atmospheric conditions, in
particular the stability of the atmosphere, are not suitable
for regular observations. Some of these images sometimes turn
out to be of great scientific interest: such is the case here
for Stephan’s Quintet.

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More information: Pierre-Alain Duc et al. Revisiting
Stephan’s Quintet with deep optical images, Monthly Notices
of the Royal Astronomical Society: Letters
DOI: 10.1093/mnrasl/sly004 , On
Arxiv: https://arxiv.org/abs/1712.07145