RSSArchive for June, 2013

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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Close encounter could reveal planets

NASA’s Hubble Space Telescope will have two opportunities in the next few years to hunt for Earth-sized planets around the red dwarf star Proxima Centauri. The opportunities will occur in October 2014 and February 2016 when Proxima Centauri, the star nearest to our Solar System, passes in front of two other stars. Astronomers plotted Proxima Centauri’s precise path and predicted the two close encounters using data from Hubble.

Red dwarfs are the most common class of stars in our Milky Way galaxy; there are about 10 for every star like our Sun. Red dwarfs are less massive than other stars, and because lower-mass stars tend to have smaller planets, they are ideal places to go hunting for Earth-sized planets.

Previous attempts to detect planets circling Proxima Centauri have not been successful. But astronomers believe they may be able to detect smaller Earth-sized planets, if they exist, by looking for ‘microlensing’ effects during the two rare stellar alignments.

The projected motion of the red dwarf star Proxima Centauri

The projected motion of the red dwarf star Proxima Centauri (green line) over the next decade, as plotted from Hubble Space Telescope observations (the path appears looped due to Earth’s motion around the Sun. In 2014 and 2016 Proxima Centauri will pass almost in front of two background stars, affording astronomers a rare opportunity to study the warping of space by Proxima’s gravity. The amount of warping will be used to calculate a precise mass for Proxima Centauri and look for the gravitational footprint and any planets orbiting the star. Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews), and Digitized Sky Survey (STScI/AURA/UKSTU/AAO)

Microlensing occurs when a foreground star (the ‘lens’) passes close to our line of sight to a more distant background star (the ‘source’). The appearance of the background star may be distorted, brightened and multiplied depending on the alignment between the foreground lens and the background source.

These microlensing events, which range in duration from a few hours to a few days, will enable astronomers to precisely measure the mass of Proxima Centauri. Getting a precise determination of mass is critical to understanding a star’s temperature, diameter, intrinsic brightness and longevity.

Astronomers will measure the mass by examining images of each of the background stars to see how far the stars appear to be shifted from their real positions in the sky. The shifts will be the result of Proxima Centauri’s gravitational field warping space. The degree of shift can be used to measure Proxima Centauri’s mass; the greater the shift, the greater the mass. If the red dwarf has any planets, their gravitational fields will produce a second small position shift.

Diagram explaining microlensing as Proxima Centauri appears to pass close to a background star

The upcoming conjunction between the nearest star to our Sun, Proxima Centauri, and a distant background star. Proxima’s gravitational field distorts space like a funhouse mirror and bends the path of light from the background star. The result is that the apparent position of the star will shift slightly during Proxima Centauri’s passage, as seen in the upper right diagram. If an unseen planet is orbiting Proxima Centauri, the star’s apparent position will be further offset, as seen at lower right. Credit: A. Feild (STScI)

At a distance of 4.2 light-years from Earth, Proxima Centauri is just 0.2 light-year from the more distant binary star Alpha and Beta Centauri. These three stars are considered part of the triple-star system, though Proxima Centauri evolved in isolation from the two Sun-like companion stars.

Because Proxima Centauri is so close to Earth, the area of sky warped by its gravitation field is larger than for more distant stars. This makes it easier to look for shifts in apparent stellar position caused by this effect. However, the position shifts will be too small to be perceived by any but the most sensitive telescopes in space and on the ground. The European Space Agency’s Gaia space telescope (due for launch later this year) and the European Southern Observatory’s Very Large Telescope in Chile might be able to make measurements comparable to Hubble’s.

Adapted from information issued by NASA and the Space Telescope Science Institute.

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