A new Hubble space telescope image of a "barred" spiral galaxy provides a crisp look at the reddish horizontal arm that bisects NGC 2903 -- a favorite spring observing target of astronomers.
With a magnitude brighter than 10 in the constellation Leo, NGC 2903 is fairly easy to find and identify in a small telescope. However, only large-aperture telescopes or long-exposure photographs can reveal its intricate spiral structure and the bar across it.
NGC 2903's swirling whirlpool of stars spans 80,000 light-years -- slightly less than our own Milky Way -- and is located at a distance of some 25 million light-years. NGC 2903 is one of the more conspicuous northern objects that Charles Messier missed when compiling his catalogue of nebulous objects, so leaving its discovery to William Herschel.
This colorful image, obtained by the Wide Field and Planetary Camera 2 (WFPC2) onboard Hubble, lays bare the fine detail in the central part of the galaxy's bar. The image is dominated by the bar running diagonally just above the center of the image. It is the structure with a slightly reddish glow lying within the bluish spiral arms. The reddish color arises from large amounts of dust in the bar.
Belly up to the bar
Bars in spiral galaxies seem to be ubiquitous in our local universe. Up to two-thirds of all spirals contain bars. Astronomers have long suspected that the bar systems that dominate the appearance of some spiral galaxies provide an efficient mechanism for fueling star births at their centers.
A team of astronomers from the United Kingdom, Australia and Spain studied this new picture to test out the theory of bars and star birth.
The core of NGC 2903 is known for its complex, speckled appearance, full of "hot spots," said Almudena Alonso-Herrero from the University of Hertfordshire in the U.K. As the telescope resolved the "hot spots" in the center into individual stars and star clusters for the first time, it became clear that most of the star-forming action does not actually take place in these hot spots.
"The most striking feature in the Hubble images is that star formation seems to occur in nearby large regions of ionized hydrogen instead," said Alonso-Herrero in a prepared statement. "These star-forming regions are distributed in a mighty 2,000 light-year wide ring around the center of the galaxy, in a circumnuclear ring" (seen as whitish glow around the center in the image).
Circumnuclear rings are also seen in other galaxies and are often interpreted as being due to interstellar gas falling in toward their centers.
"We believe that the ring of newly born stars around the core of NGC 2903 is created because the bar acts as a transport mechanism, tunneling gas inwards," said Almudena Alonso-Herrero.
"Bars seem to be extremely efficient in triggering the formation of stars and they act as funnels for the flow of material from the outer parts of galaxy disks towards their centers."
Hubble's close-up view also shows other interesting details in the galaxy's center: huge dust lanes and lots of young stars are gathered in hot blue clusters sprinkled all over the spiral arms. NGC 2903 bears a close resemblance to the Milky Way, which is also believed to be a barred spiral galaxy. Barred spirals are excellent laboratories to study the processes that trigger star formation, and bars may be responsible for providing the gaseous fuel being gobbled up by massive central black holes in so-called active galaxies.
A paper on the findings will be published in a forthcoming issue of Monthly Notices of the Royal Astronomical Society. Alonso-Herrero worked on the paper with Stuart D. Ryder of the Anglo-Australian Observatory, Australia, and Johan H. Knapen of the Isaac Newton Group of Telescopes, Spain and the University of Hertfordshire.
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