A West Virginia University physicist and his colleagues have discovered hundreds of previously-unknown sites of massive star formation in the Milky Way; including the most distant such objects yet found in our home Galaxy. Ongoing studies of these objects promise to give crucial clues about the structure and history of the Milky Way.
Loren Anderson, assistant professor of physics at West Virginia University, and his colleagues, Thomas Bania of Boston University and Dana Balser at the National Radio Astronomy Observatory, found regions where massive young stars or clusters of such stars are forming. These regions, which astronomers call HII (H-two) regions, serve as markers of the Galaxy’s structure, including its spiral arms and central bar.
HII regions are ionized zones around very massive stars. The stars powering HII regions are more than 20 times the mass of the Sun. Anderson has created a catalogue of all these regions that allows scientists to better characterize the statistical properties of HII regions, trace Galactic structure, determine differences in star formation properties in a variety of environments, compare our Galaxy with other galaxies in the Universe, and examine the impact of evolved HII regions in triggering the creation of second generation stars.
“The problem to this point had been that was very difficult to get a complete sample because we did not have a survey of the whole sky that could find all of the HII regions in the galaxy,” Anderson explained. “NASA launched the WISE satellite a few years ago and the data were released this past March. The WISE all-sky survey at infrared wavelengths, where these HII regions emit a lot of energy, allowed me to compile a complete sample of HII regions in the Galaxy for the first time.”
Anderson reports that data from WISE shows about 2,000 new HII-region candidates that the team is studying. The three men presented their work to the American Astronomical Society’s meeting in Long Beach, California.
“We’re vastly improving the census of our Galaxy, and that’s a key to understanding both its current nature and its past history, including the history of possible mergers with other galaxies,” Bania said.
The astronomers are using the National Science Foundation’s (NSF) Robert C. Byrd Green Bank Telescope (GBT) in Greenbank, West Virginia and Arecibo Telescope in Puerto Rico, and data from NASA’s Spitzer and WISE (Widefield Infrared Survey Explorer) satellites. They plan to expand the effort to include Australian radio telescopes.
The effort began with a survey of the Milky Way using the GBT. Anderson and his colleagues looked for HII regions by seeking faint emission of hydrogen atoms at radio wavelengths that are unobscured by the dust in the Galaxy’s disk. By detecting these emissions, dubbed radio recombination lines, or RRLs, the GBT survey more than doubled the number of known HII regions in the Milky Way. They continued that work using the Arecibo Telescope, finding additional objects, including the largest HII region yet known, nearly 300 light-years across.
Data from previous surveys with radio and infrared telescopes, including Spitzer and WISE, helped to guide the new search. Later work analyzed similar emissions of helium and carbon atoms.
“The great sensitivity of the GBT and the Arecibo telescope, along with advanced electronics, made our new surveys possible,” Balser said.
The work so far has helped refine astronomers’ understanding of the Galaxy’s structure. They found concentrations of star formation in poorly-understood distant spiral arms and at the end of the Galaxy’s central bar.
Another major focus of the surveys is to study chemical variations in different regions of the Galaxy. Variations in the abundance of elements heavier than hydrogen can trace the history of star formation, and also indicate regions possibly containing material incorporated into the Galaxy through mergers with other galaxies throughout its history.
“We’ve already been surprised to learn that the thin, tenuous gas between the stars is not as well-mixed as we thought,” Balser said. “Finding areas that are chemically different from their surroundings can point to where gas clouds or smaller galaxies may have fallen into the Milky Way,” he added.
“Just as geologists traverse the landscape, mapping different rock types to reconstruct the Earth’s history, we’re working to improve the map of our Galaxy to advance our understanding of its structure and its history,” Bania said.
This research was made possible by a grant from NASA to Loren Anderson worth $255,083.
For more information contact Loren Anderson at Loren.Anderson@mail.wvu.edu.