Chuck Bednar for redOrbit.com – Your Universe Online
Astronomers from Case Western Reserve University have discovered new features in the first known spiral galaxy, a system that has been studied for more than 150 years, according to new research published in the latest edition of the Astrophysical Journal Letters.
The researchers reported finding faint plumes extending from the northeast and south of M51a, which is a nearby spiral galaxy also known as the “Whirlpool Galaxy.” Analysis of a new image made using a 20-hour exposure revealed new details of the nearly 120,000 light-years long linear northwest plume, as well the absence of stars one part of the southeast tail, they explained.
“These features can be used in future modeling to understand the history of M51, when it and its companion galaxy first started to interact,” said Aaron Watkins, a PhD student in the university’s department of astronomy and lead author of the study. Working along with Watkins on the study were Case Western astronomy professor Chris Mihos and observatory manager Paul Harding.
M51a was the first spiral galaxy ever located, having been originally identified and sketched by William Parsons, the Earl of Rosse, in 1845. It and M51b, its smaller companion galaxy, are both located approximately 31 million light years away in the constellation Canes Venatici.
“No professional astronomer we know of has ever taken such a deep image of this galaxy,” said Watkins. The images used in the new study were captured using Case Western’s Burrell Schmidt telescope at the Kitt Peak National Observatory near Tucson in 2010 and 2012, the authors said.
Watkins and his colleagues aimed their telescope at M51 on moonless evenings in February, March, and April of those years, exposing its digital camera to the galaxy’s light at 20-minute intervals. They recalibrated the equipment in between exposures. For a total of 10 hours, the light was filtered to reveal younger stars, and for another 10 hours, it was filtered to reveal older stars.
Those images were then combined to create the final picture.
The northwestern plume of M51a was first observed in the 1970s, but technological limits at the time limited the amount of detail that could be detected. Astronomers found that it was made up primarily of older, redder stars and had only small patches of gas. Based on the age of the stars and the length of the plume, it was likely formed as the result of an interaction between an outer disk of M51 with another galaxy 200 million years ago or more.
On the other hand, the southern plume has no morphological similarities with the surrounding parts of M51, and is completely devoid of gas. The plume has comparatively few stars, and as a result, little mass and total light. The researchers suggest that the plume may be the remains of a third satellite or body in the M51 system.
Watkins said that the northeast plume has about the same total light as the southern one, and while it may be an extension of the north side of the galaxy, that cannot be confirmed at this time. Previous research into the southeastern gas tail believed it was stretched out during an interaction with another galaxy, but this new analysis found no stars.
While that is an unusual trait for a tail such as this, the study authors said that it can serve as a test for future interaction models. They are now investigating different ways to study the galaxy, with the goal of gathering more detail from the faint plumes. Watkins believes that the northwest plume is bright enough to be studied further using the Hubble space telescope.
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