Bright pink nebulae almost completely encircle a spiral
galaxy in this NASA/ESA Space Telescope image of NGC 922. The ring structure
and the galaxy’s distorted spiral shape result from a smaller galaxy scoring a
cosmic bullseye, drumming the centre of NGC 922 some 330 million years ago.
NGC 922 clearly reveals itself not to be a normal spiralgalaxy. The spiral arms are disrupted, a stream of stars extends out towards
the top of the image, and a bright ring of nebulae encircles the core.
Observing with NASA’s Chandra X-ray Observatory reveals more chaos in the form
of ultraluminous X-ray sources dotted around the galaxy.
NGC 922’s current strange form is a result of a space
bullseye millions of years ago. A smaller galaxy, catalogued as 2MASXI
J0224301-244443, plunged right through the heart of NGC 922 and shot out the
other side. In wide-field views of the NGC 922, the small interloper can be
still be seen shooting away from the scene of the crash.
As the small galaxy passed through the middle of NGC 922, it
set up ripples that disrupted the clouds of gas, and triggered the formation of
new stars whose radiation then lit up the residual gas. The bright pink colour
of the resulting nebulae is a trait sign of this process, and it is caused by
excited hydrogen gas (the dominant element in interstellar gas clouds). This procedure
of excitation and release of light by gases is similar to that in neon signs.
In theory, if two galaxies are aligned just right, with the
small one passing through the centre of the larger one, the ring of nebulae
should form a perfect circle, but more often the two galaxies are slightly off
kilter, leading to a circle that, like this one, is noticeably brighter on one
side than the other.
These objects, called collisional ring galaxies, are
relatively rare in our cosmic neighbourhood. Although galaxy collisions and
mergers are commonplace, the precise alignment and ratio of sizes necessary to
form a ring like this is not, and the ring-like phenomenon is also thought to
be relatively short-lived.
The chances of seeing one of these galaxies nearby is
therefore quite low. Despite the immense number of galaxies in the Universe,
this is one of only a handful known in our cosmic neighbourhood (the Cartwheel
Galaxy, see potw1036a, being the most famous example). Observations of the more
distant Universe (where we see further into the past) show that these rings
were more common in the past, however.
Hubble’s image of NGC 922 consists of a series of exposures
taken in visible light with Hubble’s Wide Field Camera 3, and in visible and
near-infrared light with the Wide Field and Planetary Camera 2.
Notes
The Hubble Space Telescope is a project of international
cooperation between ESA and NASA.
A version of this image was entered into the Hubble’s Hidden
Treasures image processing competition by contestant Nick Rose.
Image credit: NASA, ESA
Acknowledgement: Nick Rose
0 comments:
Post a Comment
Note: only a member of this blog may post a comment.