BY DR EMILY BALDWIN
Posted: 08 April, 2009
A team of international astronomers has completed the most detailed survey of galaxies in the nearby Universe, which not only maps their location, but details their direction and speed of motion, too.
By monitoring the way in which galaxies pull on each other’s gravity, astronomers can map the gravitational forces at work in the local Universe. This helps to shed light on the way that matter - both visible and invisible - is distributed throughout the Universe.
“Light can be obscured, but you can’t hide gravity,” says Heath Jones, lead scientist of the Six-Degree Field Galaxy Survey (6dFGS). The team is made up of researchers from Australia, the UK and the USA and the project was carried out with the UK’s 1.2 metre Schmidt Telescope located in eastern Australia and operated by the Anglo-Australian Observatory.
The clustering pattern of about 100,000 nearby galaxies, revealed by the 6dF Galaxy Survey. Each galaxy is shown as a dot. The Milky Way Galaxy is at the centre of the pattern. Image: Dr Chris Fluke, Centre for Astrophysics and Supercomputing, Swinburne University of Technology.
“The whole point of this survey is one: measuring redshifts to measure the geography of the local Universe in the Southern Hemisphere, and two: to measure the distance to 10,000 galaxies when previously in the Southern Hemisphere there has been data for about 1,000 galaxies,” explains John Lucey of the University of Durham.
The survey has scanned some eighty percent of the southern skies, revealing the locations of clusters and strings of galaxies, as well as over 500 voids. “We knew about a lot of the major structures before, but how you get the filaments linking them together, we didn’t know that at all well.” This irregular layout of matter will help astronomers to measure the “peculiar” velocities of the galaxies.
“When you want to measure the peculiar velocity you have to measure the distances to the galaxies,” Lucey tells Astronomy Now. “The Universe is expanding uniformly but if you have a big lump of matter in the Universe it slows the expansion around the dense regions. Similarly if you have a void, things are moving out of the void. So if you measure those peculiar velocities you can measure where the matter is in the Universe on a very large scale.”
The UK Schmidt Telescope. Image: Shaun Amy.
One particularly interesting point about the Southern Hemisphere, says Lucey, is the phenomenon known as the Great Attractor, a region of space towards which galaxies appear to be accelerating. “One of the key goals of this survey is to measure many more peculiar velocities over the southern hemisphere so we can really definitively say what the Great Attractor is, because there is a lot of debate as to whether it is a local phenomenon that is not that far away, or is due to a more distant object.”
Some people have suggested that the large supercluster Shapley, is the culprit. “By measuring peculiar velocities you can map those sorts of things,” says Lucey, and this is something that will be keeping the team busy for the months and years ahead.
Measuring the peculiar velocities relies on measuring the width of spectral lines in a galaxy. This requires the high-resolution spectrograph purpose-built for the 6dFGS survey, which allows 150 spectra to be taken simultaneously, and therefore covers the majority of the Southern Hemisphere in a reasonable amount of time. The survey has taken almost a decade since its conception to the delivery of the first results.
Click here to find out more about the 6dF Galaxy Survey.