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Australian dish charts where stars are born

The Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the nearest sizeable galaxy to our Milky Way, and is therefore a popular target for astronomers studying the evolution of stars.

ASTRONOMERS HAVE MAPPED in detail the star-forming regions of the nearest star-forming galaxy to our own, a step toward understanding the conditions surrounding star creation.

The researchers, led by University of Illinois astronomy professor Tony Wong—and including Associate Professor Sarah Maddison and PhD student Annie Hughes, both of the Swinburne University of Technology in Melbourne, Australia—have published their findings in the December issue of the Astrophysical Journal Supplement Series.

The Large Magellanic Cloud (LMC) is a popular galaxy among astronomers both for its nearness to our Milky Way and for the spectacular view it provides, a big-picture vista impossible to capture of our own galaxy.

“If you imagine a galaxy being a disc, the LMC is tilted almost face-on so we can look down on it, which gives us a very clear view of what’s going on inside,” Wong said.

Mopra dish

CSIRO's 22-metre-diameter Mopra radio telescope, located near Coonabarabran in NSW.

As the LMC is in the far southern sky, it is an ideal target for Australian telescopes. And indeed, the team used the CSIRO’s 22-metre-diameter radio telescope at Mopra, near Coonabarabran in north-central New South Wales.

Where are stars born?

Although astronomers have a working theory of how individual stars form, they know very little about what triggers the process or the conditions in space that are optimal for star birth.

Wong’s team focused on areas called molecular clouds, which are dense patches of gas—primarily molecular hydrogen—where stars are born. By studying these clouds and their relationship to new stars in the galaxy, the team hoped to learn more about how gas clouds turn into stars.

Using the Mopra dish, the astronomers mapped more than 100 molecular clouds in the LMC and estimated their sizes and masses, identifying regions with ample material for making stars. This seemingly simple task engendered a surprising find.

Conventional wisdom states that most of the molecular gas in a galaxy is apportioned to a few large clouds. However, Wong’s team found many more low-mass clouds than they expected—so many, in fact, that a majority of the dense gas may be sprinkled across the galaxy in these small molecular clouds, rather than clumped together in a few large blobs.

MAGMA image of the LMC

False-colour image of the Large Magellanic Cloud galaxy combining maps of neutral atomic hydrogen gas (red), hydrogen energised by nearby young stars (blue), and new data from Wong’s team which roughly show the locations of dense clouds of molecular hydrogen (green). It's thought that stars form within molecular hydrogen clouds.

Star formation widespread in the LMC galaxy

The large numbers of these relatively low-mass clouds means that star-forming conditions in the LMC may be relatively widespread and easy to achieve.

To better understand the connection between molecular clouds and star formation, the team compared their molecular cloud maps to maps of infrared radiation, which reveal where young stars are heating cosmic dust.

“It turns out that there’s actually very nice correspondence between these young massive stars and molecular clouds,” Wong said.

“We can say with great confidence that these clouds are where the stars form, but we are still trying to figure out why they have the properties they do,” he added.

Adapted from information issued by University of Illinois at Urbana-Champaign. Mopra photo courtesy CSIRO. MAGMA image of LMC courtesy Tony Wong, University of Illinois.

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Galaxies are running out of gas

A star-forming region

Compared to earlier cosmic epochs, galaxies these days are running of out of the gas raw material with which to make new stars. (Hubble Space Telescope image.)

THE UNIVERSE FORMS FEWER STARS than it used to, and a CSIRO study has now shown why—compared to the past, galaxies today have less gas from which to make stars.

Dr Robert Braun (CSIRO Astronomy and Space Science) and his colleagues used CSIRO’s Mopra radio telescope near Coonabarabran, NSW, to study far-off galaxies and compare them with nearby ones.

Light (and radio waves) from the distant galaxies takes time to travel to us, so we see the galaxies as they were between three and five billion years ago.

Galaxies at that stage of the Universe’s life appear to contain considerably more molecular hydrogen gas than comparable galaxies in today’s Universe, the research team found.

Stars form from clouds of molecular hydrogen. The less molecular hydrogen there is, the fewer stars will form.

The research team’s paper is in press in Monthly Notices of the Royal Astronomical Society.

Raw material for stars

Astronomers have known for at least 15 years that the rate of star formation peaked when the Universe was only a few billion years old and has declined steeply ever since.

“Our result helps us understand why the lights are going out,” Dr Braun said. “Star formation has used up most of the available molecular hydrogen gas.”

Mopra radio telescope

CSIRO's Mopra radio telescope near Coonabarabran in New South Wales.

After stars form, they shed gas during various stages of their lives, or in dramatic events such as explosions (supernovae). This returns some gas to space to contribute to further star formation.

“But most of the original gas—about 70%—remains locked up, having been turned into things such as white dwarfs, neutron stars and planets,” Dr Braun said.

“So the molecular gas is used up over time. We find that the decline in the molecular gas is similar to the pattern of decline in star formation, although during the time interval that we have studied, it is declining even more rapidly.”

Dark energy the demon

Ultimately, the real problem is the rate at which galaxies are “refuelled” from outside.

Gas falls into galaxies from the space between galaxies, the intergalactic medium. Two-thirds of the gas in the universe is still found in the intergalactic medium—the space between the galaxies—and only one third has already been consumed by previous star formation in galaxies, astronomers think.

“The drop-off in both gas availability and star formation seems to have started around the time that Dark Energy took control of the Universe,” Dr Braun said.

Up until that time, gravity dominated the Universe, so the gas was naturally pulled in to galaxies, but then the effect of Dark Energy took over and the Universe started expanding faster and faster.

This accelerating expansion has probably made it increasingly difficult for galaxies to capture the additional gas they need to fuel future generations of star formation, Dr Braun speculates.

Adapted from information issued by CSIRO; NASA, ESA, STScI/AURA.

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