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Astronomers spy on galaxies in the raw

A CSIRO RADIO TELESCOPE has detected the raw material for making the first stars in galaxies that formed when the Universe was just three billion years old – less than a quarter of its current age. This opens the way to studying how these early galaxies make their first stars.

The telescope is CSIRO’s Australia Telescope Compact Array telescope near Narrabri, NSW. “It one of very few telescopes in the world that can do such difficult work, because it is both extremely sensitive and can receive radio waves of the right wavelengths,” says CSIRO astronomer Professor Ron Ekers.

The raw material for making stars is cold molecular hydrogen gas, called H2. It can’t be detected directly but its presence is revealed by a ‘tracer’ gas, carbon monoxide (CO), which emits radio waves.

The Spiderweb

In one project, astronomer Dr Bjorn Emonts (CSIRO Astronomy and Space Science) and his colleagues used the Compact Array to study a massive, distant conglomerate of star-forming ‘clumps’ or ‘proto-galaxies’ that are in the process of coming together as a single massive galaxy. This structure, called the Spiderweb, lies more than ten thousand million light-years away (at a redshift of 2.16).

The Spiderweb, imaged by the Hubble Space Telescope

MAIN IMAGE: The Spiderweb, imaged by the Hubble Space Telescope – a central galaxy (MRC 1138-262) surrounded by hundreds of other star-forming ‘clumps’. (Credit: NASA, ESA, George Miley and Roderik Overzier, Leiden Observatory.) INSET: In blue, the carbon monoxide gas detected in and around the Spiderweb. (Credit: B. Emonts et al, CSIRO/ATCA)

Dr Emonts’ team found that the Spiderweb contains at least sixty thousand million  times the mass of the Sun in molecular hydrogen gas, spread over a distance of almost a quarter of a million light-years. This must be the fuel for the star-formation that has been seen across the Spiderweb. “Indeed, it is enough to keep stars forming for at least another 40 million years,” says Dr Emonts.

Magnifying lens

In a second set of studies, Dr Manuel Aravena (European Southern Observatory) and colleagues measured CO, and therefore H2, in two very distant galaxies (at a redshift of 2.7).

The faint radio waves from these galaxies were amplified by the gravitational fields of other galaxies – ones that lie between us and the distant galaxies. This process, called gravitational lensing, “acts like a magnifying lens and allows us to see even more distant objects than the Spiderweb,” says Dr Aravena.

Dr Aravena’s team was able to measure the amount of H2 in both galaxies they studied. For one of the galaxies (called SPT-S 053816-5030.8), they could also use the radio emission to make an estimate of how rapidly the galaxy is forming stars – an estimate independent of the other ways astronomers measure this rate.

Antennae of CSIRO's Compact Array telescope

Dishes of the CSIRO’s Australia Telescope Compact Array near Narrabri in New South Wales. Photo: David Smyth

Upgraded telescope

The Compact Array’s ability to detect CO is due to an upgrade that has boosted its bandwidth – the amount of radio spectrum it can see at any one time – sixteen-fold (from 256 MHz to 4 GHz), and made it far more sensitive.

“The Compact Array complements the new ALMA telescope in Chile, which looks for the higher-frequency transitions of CO,” says Ron Ekers.

Adapted from information issued by CSIRO.

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Milky Way galaxy is a ‘snake pit’

CSIRO's Australia Telescope Compact Array

CSIRO's Australia Telescope Compact Array was used to make a map of galactic gas polarisation.

A PIT OF WRITHING SNAKES. That’s what the first picture of turbulent gas inside our Milky Way galaxy looks like.

Professor Bryan Gaensler of the University of Sydney, Australia, and his team used a CSIRO radio telescope in eastern Australia to make the ground-breaking image, published in the journal Nature today.

The space between the stars in our Galaxy is not empty, but is filled with thin gas that continually swirls and churns.

“This is the first time anyone has been able to make a picture of this interstellar turbulence,” said Professor Gaensler. “People have been trying to do this for 30 years.”

Turbulence makes the Universe magnetic, helps stars form, and spreads the heat from supernova explosions through the Galaxy

“We now plan to study turbulence throughout the Milky Way. Ultimately this will help us understand why some parts of the Galaxy are hotter than others, and why stars form at particular times in particular places,” Professor Gaensler said.

Spectacular image

Gaensler and his team studied a region of our Galaxy about 10,000 light-years away in the constellation Norma.

They used CSIRO’s Australia Telescope Compact Array near Narrabri, NSW, because “it is one of the world’s best telescopes for this kind of work,” as Dr Robert Braun, Chief Scientist at CSIRO Astronomy and Space Science explained.

The radio telescope was tuned to receive radio waves that come from the Milky Way. As these waves travel through the swirling interstellar gas, one of their properties—polarisation—is very slightly altered, and the radio telescope can detect this.

(Polarisationis the direction the waves “vibrate”. Light can be polarised—for instance, some sunglasses filter out light polarised in one direction while letting through other light.)

Gas turbulence map of part of the Milky Way

A map has been made of the gas in our Milky Way galaxy. The 'snakes' are regions of gas where the density and magnetic field are changing rapidly as a result of turbulence.

The researchers measured the polarisation changes over an area of sky and used them to make a spectacular image of overlapping entangled tendrils, resembling writhing snakes.

The “snakes” are regions of gas where the density and magnetic field are changing rapidly as a result of turbulence.

Best match

The “snakes” also show how fast the gas is churning — an important number for describing the turbulence.

Team member Blakesley Burkhart, a PhD student from the University of Wisconsin, made several computer simulations of turbulent gas moving at different speeds.

These simulations resembled the “snakes” picture, with some matching the real picture better than others.

By picking the best match, the team concluded that the speed of the swirling in the turbulent interstellar gas is around 70,000 kph—relatively slow by cosmic standards.

Adapted from information issued by CSIRO. Images courtesy B. Gaensler et al. (data: CSIRO/ATCA) and David Smyth, CSIRO.

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