New stars from old gas surprise astronomers

The unexpected discovery comes thanks to instruments aboard NASA's Galaxy Evolution Explorer (GALEX) spacecraft which are sensitive to the ultraviolet radiation emitted by newly formed stars.

The team, led by astronomer David Thilker of Johns Hopkins University, included Barry Madore and Mark Seibert of the Carnegie Observatories. "This demonstrates the tremendous power of observing the ultraviolet from space," said Seibert. "By discovering star formation in what is likely a new class of dwarf galaxy the Galaxy Evolution Explorer observatory is certainly living up to its name."

The Leo Ring, discovered in 1983 by radio astronomers, is a cloud of hydrogen and helium gas orbiting two galaxies in the constellation Leo. The cloud is nearly invisible in optical wavelengths, and since its discovery astronomers had searched for stars within it without success. The launch of the GALEX spacecraft in 2003 with ultra-sensitive UV detectors has made it possible to explore new wavelengths and detect ultraviolet emissions from the cloud's star-forming regions, which the astronomers interpret to be small galaxies known as dwarf galaxies.

Previous measurements of the masses and velocities of clumps of hydrogen within the Leo Ring suggest it lacks any significant dark matter component, an aspect that distinguishes these newly discovered sites of star formation from other known dwarf galaxies. Since in current cosmological models galaxies form in association with a massive "halo" of dark matter, this suggests that the new galaxies formed through a distinct and not yet understood process.

Credit: NASA/JPL-Caltech/DSS

The unique ultraviolet vision of NASA's Galaxy Evolution Explorer reveals, for the first time, dwarf galaxies forming out of nothing more than pristine gas likely leftover from the early universe. Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way.

The forming dwarf galaxies shine in the far ultraviolet spectrum, rendered as blue in the call-out on the right hand side of this image. Near ultraviolet light, also obtained by the Galaxy Evolution Explorer, is displayed in green, and visible light from the blue part of the spectrum here is represented by red. The clumps (in circles) are distinctively blue, indicating they are primarily detected in far ultraviolet light.

The faint blue overlay traces the outline of the Leo Ring, a huge cloud of hydrogen and helium that orbits around two massive galaxies in the constellation Leo (left panel). The cloud is thought likely to be a primordial object, an ancient remnant of material that has remained relatively unchanged since the very earliest days of the universe. Identified about 25 years ago by radio waves, the ring cannot be seen in visible light.

Only a portion of the Leo Ring has been imaged in the ultraviolet, but this section contains the telltale ultraviolet signature of recent massive star formation within this ring of pristine gas. Astronomers have previously only seen dwarf galaxies form out of gas that has already been cycled through a galaxy. It has been enriched with metals -- elements heavier than helium -- produced as stars evolve.

The visible data come from the Digitized Sky Survey of the Space Telescope Science Institute in Baltimore, Md. The Leo Ring visible image (left) represents the survey's blue, red, and infrared bands with the colors blue, green, and red. The overlay indicating the location of hydrogen gas in the Leo Ring is based on observations made at the Arecibo Observatory in Puerto Rico.

Given the immense size of the Leo Ring it is unlikely that the gas comprising it has been cycled through or extracted from the central galaxies. It is conceivable that this gas has been untouched since the beginning of the Universe. If so, the newly formed dwarf galaxies could be almost pure hydrogen and helium and lack heavier elements ("metals" in astronomical terminology). The Leo Ring and its newly discovered ultraviolet dwarf galaxies would then provide astronomers with a nearby opportunity to see how galaxy formation probably proceeded in the very early Universe.

The new type of dwarf galaxy may have been common in the early Universe, when clouds of pristine gas would have been more abundant. The discovery opens a window on the process of star formation in primordial gases not yet enriched with heavy elements.

"The next phase is to follow up these objects with deep imaging and spectroscopy from the ground," said Carnegie astronomer Barry Madore, who is a co-author on the paper and a co-investigator on the GALEX mission."An observing run on Carnegie's 6.5m Baade telescope is already scheduled for this spring."

Source: Carnegie Institution of Washington
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