A spectacular head-on collision between two galaxies fueled the unusual
triangular-shaped star-birthing frenzy, as captured in a new image from
NASA's Hubble Space Telescope.
The interacting galaxy duo is collectively called Arp 143. The pair contains
the glittery, distorted, star-forming spiral galaxy NGC 2445 at right, along
with its less flashy companion, NGC 2444 at left.
Astronomers suggest that the galaxies passed through each other, igniting
the uniquely shaped star-formation firestorm in NGC 2445, where thousands of
stars are bursting to life on the right-hand side of the image. This galaxy
is awash in starbirth because it is rich in gas, the fuel that makes stars.
However, it hasn't yet escaped the gravitational clutches of its partner NGC
2444, shown on the left side of the image. The pair is waging a cosmic
tug-of-war, which NGC 2444 appears to be winning. The galaxy has pulled gas
from NGC 2445, forming the oddball triangle of newly minted stars.
"Simulations show that head-on collisions between two galaxies is one way of
making rings of new stars," said astronomer Julianne Dalcanton of the
Flatiron Institute's Center for Computational Astrophysics in New York and
the University of Washington in Seattle. "Therefore, rings of star formation
are not uncommon. However, what's weird about this system is that it's a
triangle of star formation. Part of the reason for that shape is that these
galaxies are still so close to each other and NGC 2444 is still holding on
to the other galaxy gravitationally. NGC 2444 may also have an invisible hot
halo of gas that could help to pull NGC 2445's gas away from its nucleus. So
they're not completely free of each other yet, and their unusual interaction
is distorting the ring into this triangle."
NGC 2444 is also responsible for yanking taffy-like strands of gas from its
partner, stoking the streamers of young, blue stars that appear to form a
bridge between the two galaxies.
These streamers are among the first in what appears to be a wave of star
formation that started on NGC 2445's outskirts and continued inward.
Researchers estimate the streamer stars were born between about 50 million
and 100 million years ago. But these infant stars are being left behind as
NGC 2445 continues to pull slowly away from NGC 2444.
Stars no older than 1 million to 2 million years are forming closer to the
center of NGC 2445. Hubble's keen sharpness reveals some individual stars.
They are the brightest and most massive in the galaxy. Most of the brilliant
blue clumps are groupings of stars. The pink blobs are giant, young, star
clusters still enshrouded in dust and gas.
Although most of the action is happening in NGC 2445, it doesn't mean the
other half of the interacting pair has escaped unscathed. The gravitational
tussle has stretched NGC 2444 into an odd shape. The galaxy contains old
stars and no new starbirth because it lost its gas long ago, well before
this galactic encounter.
"This is a nearby example of the kinds of interactions that happened long
ago. It's a fantastic sandbox to understand star formation and interacting
galaxies," said Elena Sabbi of the Space Telescope Science Institute in
Baltimore, Maryland.
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Space & Astrophysics