Our Universe is large and we think of it as being mostly empty. But in
a previous Image of the week we presented Galaxies that were interacting with each
other.
Surprisingly for many of us, collisions between companion galaxies are
not unusual. Indeed, the average distance between two galaxies in the
intergalactic medium is such that mutual encounters are probable,
especially in groups and clusters. Collisions were even more frequent
in the past when the Universe was younger and smaller.
If, for long, galaxies had been considered as immutable
"Island-Universes", it has recently been found that they are in fact
subject to a strong evolution due to their environment, and collisions
play a major role in this process. Galaxy interactions cause major
morphological transformations, trigger violent starbursts and in
general contribute to strengthen the exchanges between the
interstellar and intergalactic medium.
This week's image, obtained with the CFHT, presents an example of a
spiral galaxy being torn apart by another elliptical galaxy. Both
galaxies are located close to the center of a cluster of galaxies,
Abell 1185. Number 105 in the Atlas of Peculiar Galaxies compiled by
H. Arp, this system has been nicknamed "The
Guitar", because of its suggestive morphology. The spectacular
jet-like tails that emanate from the galaxies are intriguing but
rather common in these interacting systems. They mainly consist of
stars that have been pulled out from the disks (more or less the
galaxies' mid-planes) of the parent galaxy through a purely
gravitational, tidal, effect. Before their final merging, colliding
galaxies may hence loose part of their stellar and gaseous
content. This material might afterwards recondense again in the
intergalactic medium and collapse to form second-generation
galaxies. Two such newly born baby galaxies can be seen in Arp 105 at
the extremity of each tidal tail, more than one hundred thousand
parsecs (325000 light years) from their parents for one of them. They
can be seen at the top and bottom of this week's image. Their
properties are very similar to classical dwarf galaxies. The northern
object (at the top of the image) is actually as luminous as the
Magellanic Clouds.
A complementary study has shown that both "tidal dwarf galaxies"
contain massive clouds of atomic hydrogen which has also been expelled
from the disk of their parents. This gas reservoir is recycled in the
tidal dwarf galaxies to form new generation of stars. The compact
knots in the tails of Arp 105 are such star-forming regions.
Therefore, even at galactic scales is recycling useful!
This week's image was obtained a few years ago, in January 1992, at
the Canada-France-Hawaii Telescope with MARLIN, a focal reducer
installed at Cassegrain focus. MARLIN offered optical imaging and
spectroscopic capabilities, with the help of focal plane masks, much
like MOS is doing today. In this wide V-band (550nm) direct image, the
field of view is 5.6' x 5.6' (190 kpc x 190 kpc). Spectra were also
obtained for the Guitar, they are presented in the paper cited above.
You can learn more about interacting galaxies by clicking here.
Technical description: