‘Dancing with the Stars’ Takes on a Whole New Twist
Two galaxies perform an intricate dance in this new Hubble Space Telescope image. The
galaxies, containing a vast number of stars, swing past each other in a graceful
performance choreographed by gravity.
The pair, known collectively as Arp 87, is one
of hundreds of interacting and merging galaxies
known in our nearby universe. Arp 87 was originally
cataloged by astronomer Halton Arp in the mid-1960s.
Arp's Atlas of Peculiar Galaxies is a compilation
of astronomical photographs using the Palomar 200-inch
Hale and the 48-inch Samuel Oschin telescopes.
The resolution in the Hubble image shows exquisite
detail and fine structure that was not observable
when Arp 87 was first cataloged in the 1960's.
The two main players comprising Arp 87 are NGC 3808 on the right (the larger of the
two galaxies) and its companion NGC 3808A on the left. NGC 3808 is a nearly face-on
spiral galaxy with a bright ring of star formation and several prominent dust arms. Stars,
gas, and dust flow from NGC 3808, forming an enveloping arm around its companion.
NGC 3808A is a spiral galaxy seen edge-on and is surrounded by a rotating ring that
contains stars and interstellar gas clouds. The ring is situated perpendicular to the plane
of the host galaxy disk and is called a "polar ring."
As seen in other mergers similar to Arp 87, the corkscrew shape of the tidal material or
bridge of shared matter between the two galaxies suggests that some stars and gas drawn
from the larger galaxy have been caught in the gravitational pull of the smaller one.
The shapes of both galaxies have been distorted by their gravitational interaction with
Interacting galaxies often exhibit high rates of star formation. Many lines of evidence -
colors of their starlight, intensity of emission lines from interstellar gas, far-infrared
output from heated interstellar dust - support this fact. Some merging galaxies have the
highest levels of star formation we can find anywhere in the nearby universe.
A major aspect of this excess star formation could be properly revealed only when
Hubble turned its imaging capabilities toward colliding galaxies. Among the
observatory's first discoveries was that galaxies with very active star formation contain
large numbers of super star clusters - clusters more compact and richer in young stars
than astronomers were accustomed to seeing in our galactic neighborhood.
Arp 87 is in the constellation Leo, the Lion, approximately 300 million light-years away
from Earth. These observations were taken in February 2007 with the Wide Field
Planetary Camera 2. Light from isolated blue, green, red, and infrared ranges was
composited together to form this color image.
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)