Cosmic Ballet or Devil's Mask?: Very Large Telescope Witnesses CloseInteraction in Galaxy Family (Forwarded)

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO Website at URL:
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For immediate release: 28 April 2004

ESO Press Photo 12/04

Cosmic Ballet or Devil's Mask?

Very Large Telescope Witnesses Close Interaction in Galaxy Family

Stars like our Sun are members of galaxies, and most galaxies are themselves
members of clusters of galaxies. In these, they move around among each other in
a mostly slow and graceful ballet. But every now and then, two or more of the
members may get too close for comfort -- the movements become hectic, sometimes
indeed dramatic, as when galaxies end up colliding.

ESO PR Photo 12/04
Cosmic Ballet or Devil's Mask -- Galaxy Triplet NGC 6769-71 (VLT MELIPAL + VIMOS)

ESO PR Photo 12/04 shows an example of such a cosmic tango. This is the superb
triple system NGC 6769-71, located in the southern Pavo constellation (the
Peacock) at a distance of 190 million light-years.

This composite image was obtained on April 1, 2004, the day of the Fifth
Anniversary of ESO's Very Large Telescope (VLT). It was taken in the imaging
mode of the VIsible Multi-Object Spectrograph (VIMOS) on Melipal, one of the
four 8.2-m Unit Telescopes of the VLT at the Paranal Observatory (Chile). The
two upper galaxies, NGC 6769 (upper right) and NGC 6770 (upper left), are of
equal brightness and size, while NGC 6771 (below) is about half as bright and
slightly smaller. All three galaxies possess a central bulge of similar
brightness. They consist of elderly, reddish stars and that of NGC 6771 is
remarkable for its "boxy" shape, a rare occurrence among galaxies.

Gravitational interaction in a small galaxy group

NGC 6769 is a spiral galaxy with very tightly wound spiral arms, while NGC 6770
has two major spiral arms, one of which is rather straight and points towards
the outer disc of NGC 6769. NGC 6770 is also peculiar in that it presents two
comparatively straight dark lanes and a fainter arc that curves towards the
third galaxy, NGC 6771 (below). It is also obvious from this new VLT photo that
stars and gas have been stripped off NGC 6769 and NGC 6770, starting to form a
common envelope around them, in the shape of a Devil's Mask. There is also a
weak hint of a tenuous bridge between NGC 6769 and NGC 6771. All of these
features testify to strong gravitational interaction between the three galaxies.
The warped appearance of the dust lane in NGC 6771 might also be interpreted as
more evidence of interactions.

Moreover, NGC 6769 and NGC 6770 are receding from us at a similar velocity of
about 3800 km/s -- a redshift just over 0.01 -- while that of NGC 6771 is
slightly larger, 4200 km/s.

A stellar baby-boom

As dramatic and destructive as this may seem, such an event is also an
enrichment, a true baby-star boom. As the Phoenix reborn from its ashes, a
cosmic catastrophe like this one normally results in the formation of many new
stars. This is obvious from the blueish nature of the spiral arms in NGC 6769
and NGC 6770 and the presence of many sites of star forming regions.

Similarly, the spiral arms of the well-known Whirlpool galaxy (Messier 51) may
have been produced by a close encounter with a second galaxy that is now located
at the end of one of the spiral arms; the same may be true for the beautiful
southern galaxy NGC 1232 depicted in another VLT photo (PR Photo 37d/98).

Nearer to us, a stream of hydrogen gas, similar to the one seen in ESO PR Photo
12/04, connects our Galaxy with the LMC, a relict of dramatic events in the
history of our home Galaxy. And the stormy time is not yet over: now the
Andromeda Galaxy, another of the Milky Way neighbours in the Local Group of
Galaxies, is approaching us. Still at a distance of over 2 million light-years,
calculations predict that it will collide with our galaxy in about 6,000 million
years!

More stunning images obtained with the Very Large Telescope can be found on the
Top 20 page,
http://www.eso.org/outreach/gallery/vlt/images/Top20/

Technical information: PR Photo 12/04 is a photo-composite showing the triplet
of galaxies NGC 6769, 6770 and NGC 6771, as observed with the VIMOS instrument
on Melipal, one of the four 8.2-m Unit Telescopes of ESO's Very Large Telescope.
The images were obtained on the morning of April 1, 2004 during twilight in
three different wavebands, each of which is here being represented by a
different colour: U (around 380 nm; associated to the Blue channel) ; B ( 425 nm
; Green channel) and V (550 nm ; Red channel). The total exposure time in the U,
B and V bands is, respectively, 20, 6 and 12 minutes. The mean seeing was around
0.9 arcsec. The field shown measures 6.1 x 6.8 arcmin. North is up and East is left.

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