Galactic milestone to measure expansion

of the Universe

BY KEITH COOPER

ASTRONOMY NOW

Posted: 8 June, 2009

A new milestone in the effort to measure accurate distances to galaxies will be of huge assistance in the battle to understand the nature of dark energy, according to new results presented at a meeting of the American Astronomical Society in California on Monday.

Using a variety of radio telescopes including the Very Large Baseline Array and the Green Bank Telescope in the USA, and the Effelsberg Radio Telescope in Germany, a team of astronomers were able to make direct measurements of a disc of material circling the black hole in the centre of the galaxy UGC 3789. Water masers – molecules of water that amplify radio waves, similar in concept to a laser – helped strengthen the signal, allowing the scientists to measure the angular diameter of the disc. Then, through simple geometry, they were able to deduce that UGC 3789 is 160 million light years from Earth. This method of distance measurement had previously been used in 1999 on the galaxy NGC 4258, which is much closer at 23 million light years.

Water masers in a disc around the central black hole of NGC 4258 allowed astronomers to accurately measure its distance in 1999. Now the same method has been used for an even more distant galaxy, UGC 3789. Image: NRAO/AUI/Gerald Cecil and Holland Ford.

When Edwin Hubble attempted the first galaxy distance survey between 1925 and 1936, he used what we call the distance ladder method. Nearby galaxies such as the Andromeda Galaxy had their distances measured via the precise period–luminosity relationship of Cepheid variable stars. Hubble estimated the distances to more distant galaxies by studying their supergiant stars, which he found to all have the same maximum luminosity. For galaxies so far away that he couldn’t resolve individual stars, Hubble estimated their distance based on their overall luminosity, comparing that with closer galaxies for which he already had a distance estimate.

This new result simply continues the work that Hubble began. Now that we know the exact distance to UGC 3789, we can use this galaxy to calibrate distances to even more faraway galaxies. The next rung on the ladder is anticipated soon; during the same presentation, efforts to measure a similar disc in the even more distant galaxy NGC 6323 were announced.

“The very high sensitivity of the telescopes allows making such images of galaxies even beyond 300 million light years,” says Cheng-Yu Kuo, a member of the science team from the University of Virginia.

Knowing the exact distances to galaxies is vital if we want to better understand how the Universe is expanding. Again, we hark back to the revolutionary work done by Edwin Hubble during the first half of the twentieth century. The Hubble Constant is a measure the rate at which the Universe is expanding. The most recent, accurate measurement of the Hubble Constant is 72 ± 8 kilometres per second per megaparsec. One megaparsec is 3.26 million light years, and what this value of the Hubble Constant is telling us is that if we could divide the entire Universe up into ‘little’ boxes, each a megaparsec in diameter, each box would be expanding by 72 kilometres every second.

Astronomers are constantly striving to refine the Hubble Constant even further, because a more accurate Hubble Constant will help in efforts to understand the nature of dark energy, which is causing the expansion of the Universe to accelerate. We can gain a better value for Hubble’s Constant if we know the precise positions of galaxies as they are dragged along with the expansion.

“The new measurement is important because it demonstrates a one-step, geometric technique for measuring distances to galaxies far enough to infer the expansion rate of the Universe,” says James Braatz of the National Radio Astronomy Observatory, who presented the results in California at the 214th American Astronomical Society Meeting. NGC 4258 was too close to measure the expansion; UGC 3789 is much better placed, and the more distant we can accurately measure galaxies, the better, for the expansion is more pronounced on larger scales.