Hundreds of star-forming nebulae discovered
Posted: 26 May 2010
A new census of star-forming activity in the Milky Way has revealed over 400 new nebulae, with the potential for many thousands more lurking in the shadows of our Galaxy. The findings were revealed today at the 216th American Astronomical Society meeting in Miami, Florida.An artist’s impression showing a) the fields of view that made up the H II region surveys, and b) how supernovae in the most abundant regions of star-formation are blowing gas up into the halo. Image: Bill Saxton/NRAO/AUI/NSF.
Using data acquired by the Very Large Array of radio telescopes in New Mexico, USA, and from the infrared Spitzer Space Telescope as part of GLIMPSE (Galactic Legacy Infrared mid-Plane Survey Extraordinaire, (find out more at: http://www.astro.wisc.edu/sirtf/), a team led by Dr Thomas Bania of Boston University made a list of objects that were potentially star-forming regions of ionised hydrogen (H II) gas. To confirm that they were so, Bania’s team followed up with the Green Bank radio telescope in West Virginia and searched for the distinctive radio frequencies coming from a process known as recombination, where free electrons recombine with hydrogen nuclei (which are just protons). This would prove that the gas in the objects is the ionised variety typical of star-forming nebulae. Bania’s team found that 95 percent of the candidate objects – 448 in total – turned out to be areas of starbirth, concentrated either in the spiral arms or at the tips of the bar that runs through the centre of the Galaxy. Twenty-five nebulae were also found further from the galactic centre than the Sun, whereas previously only two had been known to exist at such a distance.
“This new census will allow us to study the structure of the Galaxy and how it has evolved chemically from all those materials cooked up in the interiors of stars,” adds Bania. The question is, how do these materials get mixed up into the interstellar medium across the Galaxy? Analysis of H II regions at the tip of the galactic bar may hold the answers, says Felix Lockman of the National Radio Astronomy Observatory (NRAO), who was also presenting his team’s results, based on the PhD thesis of his colleague Alyson Ford of the University of Michigan, today in Miami.
“Something I’ve been interested in for a long time is the thickness of the ‘atmosphere’ of the Milky Way,” says Dr Felix Lockman of the National Radio Astronomy Observatory, referring to the diffuse interstellar medium that extends through the galactic disc and out into the Galaxy’s halo. In previous years he had discovered that interstellar hydrogen extending up into the halo was not a diffuse cloud, but was broken up into discrete and very dense blobs containing hundreds of solar masses. How did they get up there? Why don’t they dissipate?
A new Galactic All-Sky survey with the Parkes radio telescope in Australia found that many of these blobs, which measure in at around 600 solar masses each and span about 200 light years across, are above one of the tips of the galactic bar, whereas there are far fewer over the gaps between the spiral arms.
“They are sitting on top of enormous regions of star formation,” says Lockman. “Somehow the clouds in the halo reflect the underlying star-forming activity below.”
He postulates that ‘superbubbles’ blown into the interstellar gas by explosive supernovae in the star-forming regions sweep up the gas and transport it above the plane of the disc, like a supersonic lava lamp. Gradually these clouds, which contain all the debris and heavy elements (metals) spewed out by the supernovae, fall back onto the Milky Way’s disc. “We think this is how the metals get mixed into the disc,” says Lockman.
Past results had indicated that the rate of star formation in the Milky Way meant that on average between one and ten solar masses of gas is turned into new stars every year. Do all these newly discovered H II regions cause this figure to increase? Based on the density of the detected nebulae, the answer is not significantly. However, there is an important footnote to this. The surveys looked across only two small areas of the Galaxy, and extrapolating from their findings Bania estimates that there are between 2,000 and 25,000 H II nebulae in the Galaxy potentially detectable by the Green Bank telescope were they to expand their current survey. However, says Bania, this could just be the tip of the iceberg, and deeper surveys could turn up many more.
“Based on the observations it looks like there is a significant missing population of star-forming regions,” says Dr Loren Anderson of the Astrophysical Laboratory of Marseille, France, who also worked on the project. “We think there are three times as many than what we’ve found, but we can’t see them with the Green Bank telescope.” In any case, it would appear that the Milky Way Galaxy doesn’t have any plans to stop churning out new stars just yet.
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