From: Andy Soos, ENN
Published January 11, 2013 12:45 PM

Pinwheel Galxy Distortion

The Pinwheel Galaxy (also known as Messier 101 is a face-on spiral galaxy distanced 21 million light-years away in the constellation Ursa Major. Peering deep into the dim edges of a distorted pinwheel galaxy in the constellation Ursa Major, astronomers at Case Western Reserve University and their colleagues have discovered a faint dwarf galaxy and another possible young dwarf caught before it had a chance to form any stars. Within the faint trails of intergalactic traffic, the researchers also found more evidence pointing to two already known dwarf galaxies as probable forces that pulled the pinwheel-shaped disk galaxy known as M101 out of shape.


M101 is the dominant member in a group of 15 galaxies in Ursa Major. M101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light-years it is seventy percent larger than the Milky Way. It has a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses.

On photographs M101 can be seen to be asymmetrical on one side. It is thought that in the recent past (speaking in galactic terms) M101 underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplified the density waves in the spiral arms of M101. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.

"We created the deepest image ever taken of M101 and followed it up with the most sensitive survey of gas clouds surrounding the galaxy," said Chris Mihos, an astronomy professor at Case Western Reserve and lead author of both papers. "Compared to what is seen in the Hubble Space Telescope image, the galaxy’s disk is much larger and we can see very large, faint plumes of stars and streamers of gas in its outskirts."

Perhaps most surprisingly, the researchers discovered two new clouds of hydrogen gas in the M101 group, more distant and distinct from M101’s own supply of gas. The gas clouds, formally named G1425+5235 and G1355+5439, were nicknamed Skipper and Gilligan by the team, and identified as new dwarf galaxies in the group, independent from M101 itself. A follow-up analysis of images from the Sloan Digital Sky Survey showed a faint patch of starlight associated with Skipper, confirming its status as a true dwarf galaxy with a population of both gas and stars.

But the same analysis found no stars inside Gilligan. Gilligan could be a proto dwarf-galaxy, Mihos said, where the density of gas inside the cloud was too low for gravity to squeeze the gas down and form stars. "We’ll follow up. There’s a gas cloud but no stars yet. People have seen a few starless clouds before, but they’ve always associated with gas from a larger galaxy. This is different — it has nothing to link it to the other galaxies in the group. It may be one of the first true protogalaxies ever discovered."

As galaxies move within galaxy groups, they may sideswipe one another or even run into each other head-on. These intergalactic traffic jams leave behind telltale signatures in the galaxies’ stars and gas. In the faint light around M101, the researchers discovered such evidence of a sideswipe in the galaxy’s past: a distorted plume of starlight reaching far to the northeast of the galaxy, and a second plume extending to the east.

The northeast plume of the pinwheel is bluer than any other region of the galaxy, indicating it is made from younger stars. "We think it was born about 250 million years ago. It has the right colors," Mihos said. "If the material in the blue plume was pulled out of an interaction with another galaxy, the interaction was probably 250 million years ago."

For further information see M101.

Pinwheel image via Wikipedia.

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