Scientists at Case Western Reserve University have discovered a possible dwarf protogalaxy, and another dwarf galaxy, in the constellation Ursa Major.

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, also known the Pinwheel Galaxy or NGC 5457, is the dominant member in a group of 15 galaxies in Ursa Major.

“Most galaxies reside in such small-group environments, which means the factors shaping M101 are likely the same shaping most galaxies throughout the Universe,” the astronomers said.

“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 Prof Chris Mihos of Case Western Reserve, who led two studies published in the Astrophysical Journal – paper1 (arXiv.org version) & paper 2 (arXiv.org version).

“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, named G1425+5235 and G1355+5439, were identified as new dwarf galaxies in the group independent from M101 itself.

A follow-up analysis showed a faint patch of starlight associated with G1425+5235, confirming its status as a true dwarf galaxy with a population of both gas and stars.

But the same analysis found no stars inside G1355+5439.

“The object could be a proto dwarf-galaxy,” Prof 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 team 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 shapes and colors of the plumes suggest that they formed when a small galaxy passed by M101, and its gravity tugged stars and gas out from the bigger galaxy.

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Bibliographic information: J. Christopher Mihos et al. 2013. The Extended Optical Disk of M101. ApJ 762, 82; doi: 10.1088/0004-637X/762/2/82

J. Christopher Mihos et al. 2012. The HI Environment of the M101 Group. ApJ 761, 186; doi: 10.1088/0004-637X/761/2/186