Case Western Reserve University astronomer Chris Mihos leads a team of Ohio researchers recently awarded nearly 20 hours of observing time on the Hubble Space Telescope to study the outskirts of the nearby spiral galaxy M101.
Access to Hubble is extremely competitive, with only about one in five proposals being awarded observing time and research funding. Their observations, to be made during a 10-day window this fall or next—the only time Hubble will be in position to focus on all the features the team seeks—will help determine how galaxies form over time.
The study is motivated by recent observations of M101 by Mihos and his collaborators, including CWRU Observatory Manager Paul Harding, using CWRU's Burrell Schmidt telescope in Kitt Peak, Arizona. Those observations revealed very faint, diffuse blue light from young stars in the galaxy's extreme outer regions – signs that the galaxy is continuing to grow over time.
They also saw that the galaxy's outer regions were extremely distorted. The distortions, coupled with computer simulations by recently graduate astronomy major Sean Linden, raise the possibility that if M101 collided with one of its companion galaxies 200-300 million years ago, the collision could have tugged stars and gas out of M101, leading to the formation of new stars.
"But there's a problem," Mihos said. "From ground-based telescopes, what we see is light from millions of stars all blended together. Based on that, it's hard to say unequivocally what happened at what time. We need to see the individual stars to work out the details and really understand what's going on."
So Mihos, Harding, CWRU astronomy graduate student Aaron Watkins and Youngstown State University's Patrick Durrell and John Feldmeier, associate professors of physics and astronomy, applied for time on Hubble to peer closer. Being above the atmosphere and possessing more powerful optics, the multi-billion-dollar telescope will enable them to see nearly 10,000 individual faint stars.
The colors and brightness of these individual stars, from very old red stars to young and bright blue stars, indicate not only age, but also chemical makeup. These details will help the researchers determine whether new stars have always been forming in the galaxy's outskirts or whether a recent interaction with a companion galaxy triggered transient changes.
The team will also try to determine if M101, which is about 25 million light years away, has a halo of faint stars like that in the Milky Way. Astronomers believe that the halos of spiral galaxies are built through an ongoing process of accretion: As a spiral galaxy grows with time, smaller galaxies fall in and are torn apart by its gravity, leaving their stars orbiting in an extended halo.
But some scientists have suggested M101 has no halo -- which, if true, would throw a wrench in the current understanding of galaxy formation.
"If it really has no halo, then it's hard to see how it fits in our standard picture of galaxy formation," Mihos said. "We'd have to come up with scenarios for building a big spiral galaxy without the accretion that normally forms a halo. That could be hard to do."
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