New Details of Milky Way Galaxy's Structure Revealed
Newly-discovered star-forming regions are concentrated at the end of the Milky Way's central bar and in the spiral arms.
Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

MIAMI ? A large number of star-forming areas in our Milky Way galaxy, previously unknown to astronomers, have been discovered.

These newfound regions are providing astronomers with important information about the galaxy's structure and are yielding new clues about the process of galaxy evolution.

The new star-forming regions were found to be concentrated at the end of the galaxy's central bar and in its spiral arms. A separate study also located an abundance of enormous hydrogen gas clouds in portions of the galaxy above the central bar's junction with a spiral arm ? a finding that was unexpected, said Felix J. Lockman of the National Radio Astronomy Observatory (NRAO), a member of the team that made the discovery.

The Milky Way's central bar structure cuts across the heart of the galaxy, where a supermassive black hole resides. It is a region packed with more stars than in the galaxy's outskirts.

The findings were presented here today at the 216th meeting of the American Astronomical Society.

Massive, young stars

The newfound star-forming regions, called H II regions, are areas in which hydrogen atoms are ionized, or stripped of their electrons, by the intense radiation of massive, young stars.

These so-called "OB" stars are much more massive than the sun, and are very luminous and bright, said Thomas Bania of Boston University, a member of the team that found the H II regions. The radiation from these hot, bright stars is sufficient to ionize the surrounding gas and produce an H II region that acts as a signpost for star formation.

"We can clearly relate the locations of these star-forming sites to the overall structure of the galaxy," Bania said. "Further studies will allow us to better understand the process of star formation and to compare the chemical composition of such sites at widely different distances from the galaxy's center."

The researchers used infrared and radio telescopes to find these regions, which are hidden from detection in the visible light range by the Milky Way's gas and dust. The astronomers were able to see all the way across the galactic disk, allowing them to detect a number of new sites.

"We found our targets by using the results of infrared surveys done with NASA's Spitzer Space Telescope and of surveys done with the National Science Foundation's (NSF) Very Large Array radio telescope," said Loren Anderson of the Astrophysical Laboratory of Marseille in France. "Objects that appear bright in both the Spitzer and VLA images we studied are good candidates for H II regions."

The astronomers then used the NSF's Robert C. Byrd Green Bank Telescope in West Virginia, an extremely sensitive radio telescope, to detect characteristic radio frequencies emitted by electrons as they recombined with protons to form hydrogen molecules.

Evidence for the process of recombination confirmed that the regions studied contained ionized hydrogen and were indeed H II regions.

"We have doubled the census of known star-forming regions," Bania said. "Combined with the previously-known sample of H II regions, we can see locations of star formations throughout the Milky Way galaxy."

Beyond the sun

Further analysis allowed the astronomers to determine that H II regions were concentrated at the end of the galaxy's central bar and in its spiral arms. Their analysis also showed that 25 of the regions are farther from the galaxy's center than our solar system, which sits about 26,000 light-years from the galactic center. The entire galaxy is about 100,000 light-years across. (A light-year is the distance that light can travel in one year ? about 6 trillion miles (10 trillion kilometers).)

These 25 extremely distant star-forming regions were detected in an area where only two were previously known. These regions that are beyond the solar circle could be an important key to unlocking mysteries of the Milky Way's chemical makeup, said Bania.

"Finding the ones beyond the solar orbit is important, because studying them will provide important information about the chemical evolution of the galaxy," he explained. "There is evidence that the abundance of heavy elements changes with increasing distance from the galactic center. We now have many more objects to study and improve our understanding of this effect."

Huge clouds

In a separate study, the discovery of enormous hydrogen clouds in portions of the Milky Way will also help astronomers better understand the process of galactic evolution.

"The properties of these clouds show clearly that they originated as part of the Milky Way's disk, and are a major component of our galaxy," Lockman said. "Understanding these clouds is important in understanding how material moves between the galaxy's disk and its halo, a critical process in the evolution of galaxies."

The detection of these clouds came as a surprise to the researchers, because of their immense size, mass and density.

The massive clouds consist of neutral hydrogen gas, with an average mass equal to that of approximately 700 suns. Their sizes vary greatly, but most are about 200 light-years across.

"They're also quite far above the Milky Way's disk," Lockman said. "And they're dense. These things should be falling like rocks. They're not like clouds on Earth. These things have densities hundreds or thousands of times more than their surroundings. How do they fit into the Milky Way's evolution?"

The astronomers studied 650 of these hydrogen clouds in two distinct and widely-separated regions of the galaxy. The clouds are between 400 and 15,000 light-years above or below the disk-like plane of the galaxy.

The Milky Way's disk contains most of the galaxy's stars and gas, and is surrounded by a "halo" of gas more distant than the clouds that the astronomers studied.

"These clouds were first detected with the National Science Foundation's Robert C. Byrd Green Bank Telescope, and are quite puzzling," Lockman said. "They are in a transitional area between the disk and the halo, and their origin has been uncertain."

Compare and contrast

The research team also used data from the Galactic All-Sky Survey, made with the Parkes radio telescope located in Australia.

By comparing the observations of the two regions, the astronomers saw that one region contained three times as many hydrogen clouds as the other. Furthermore, that region's clouds are, on average, twice as far above the galaxy's plane.

This dramatic difference, they believe, is because the region with more clouds lies near the tip of the galaxy's central bar, where the bar merges with a major spiral arm. This is an area of intense star formation and contains many young stars whose strong winds can propel gas away from the region.

The most massive stars will also explode as supernovas, blasting material outward. In the other region of clouds studied, star formation activity was found to be much more sparse.

"Somehow the clouds in the halo reflect the underlying star-forming activity below," Lockman said.

The researchers believe that the clouds are generated as a byproduct of the enormous amount of energy emitted into the galaxy's atmosphere ? called the interstellar medium ? from the explosions of supernovas.

The thick bubbles of energy created by the explosions then break up into the clouds that were observed by the researchers.

The lifetimes of these clouds and how they are held together remain unknown, said Lockman, but understanding their origin could provide clues about the Milky Way's atmosphere as a whole. ?

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