This galaxy looks a lot like our own Milky Way galaxy. But while our galaxy is actively forming lots of new stars, this one is birthing stars at only half the rate of the Milky Way. It’s been mostly quiet for billions of years, feeding lightly on the thin gas in intergalactic space.
This is the spiral galaxy UGC 2885. It’s about 2.5 times wider than our galaxy and is about 230 light years away in the constellation Perseus. The galaxy’s nickname is “Rubin’s Galaxy” after American astronomer Vera Rubin. (This is another posthumous honor for Rubin: the LSST (Large Synoptic Survey Telescope) was just renamed in her honor. It’s now the Vera C. Rubin Observatory.)
It’s not just the star-formation process that’s slow in this galaxy. UGC 2885 isn’t feeding on any smaller neighboring galaxies, so its central black hole is starved of in-falling gas. It’s something like a stellar retirement community.
But the most puzzling thing about this massive galaxy is how it got so large.
Benne Holwerda of the University of Louisville, Kentucky, observed this galaxy with NASA’s Hubble Space Telescope. Holwerda nicknamed the galaxy Rubin’s Galaxy. His work follows and is inspired by Rubin’s work, which centered on measuring the rotation of galaxies and determining their masses. That in turn provided evidence for the existence of dark matter.
“My research was in a large part inspired by Vera Rubin’s work in 1980 on the size of this galaxy,” said Holwerda. “We consider this a commemorative image. This goal to cite Dr. Rubin in our observation was very much part of our original Hubble proposal.”
This new Hubble image of Rubin’s Galaxy is part of results that Holwerda presented at the American Astronomical Society’s Annual Meeting. Holwerda’s research is focused on the enormous size of the galaxy.
“How it got so big is something we don’t quite know yet,” said Holwerda in a press release. “It’s as big as you can make a disk galaxy without hitting anything else in space.”
UGC 2885 is fairly isolated in space, and that isolation might provide a clue to its unblemished shape. Galaxies are often misshapen due to interactions with other galaxies. But UGC 2885 is alone in its neighborhood, and retains its pristine spiral shape. The structure of the arms and the disk is almost perfect, and it has none of the tidal tails that disrupted galaxies exhibit.
The galaxy is also a barred spiral, even though astronomers couldn’t see the bar at first. This is unusual, because astronomers think the bar in barred spirals is caused by gravitational interactions with neighboring galaxies and satellite galaxies, none of which exist in proximity to UGC 2885.
To grow this large, it either had to consume smaller, surrounding galaxies, or it had to slowly accrete intergalactic gas over a long period of time. Whatever the case, it took its time growing this large. “It seems like it’s been puttering along, slowly growing,” Holwerda said.
Holwerda and the other astronomers on the team are examining the galaxy’s halo, and counting the globular star clusters in it. If they find an excessive number of globular clusters in the halo, it’s evidence that over billions of years smaller galaxies fell into Rubin’s Galaxy, leaving these clusters in the halo.
Future telescopes will be able to see more clearly into the galaxy and find clues to its growth. The James Webb will launch soon (!!!) and can see into the infrared with extreme sensitivity. It’ll also be able to examine the globular cluster population. NASA’s WFIRST (Wide Field Infrared Survey Telescope) will do an even better job of peering into the halo and studying the globular clusters.
“The infrared capability of both space telescopes would give us a more unimpeded view of the underlying stellar populations,” said Holwerda.
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