New Cosmic “Yardstick” Could Help Understand Dark Energy

Article written: 8 Jun , 2009
Updated: 24 Dec , 2015
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A new method for measuring large astronomical distances is providing researchers with a cosmic yardstick to determine precisely how far away distant galaxies are. This could also offer a way to help determine how fast the Universe is expanding, as well as the nature of the mysterious Dark Energy that pervades the Universe. “We measured a direct, geometric distance to the galaxy, independent of the complications and assumptions inherent in other techniques. The measurement highlights a valuable method that can be used to determine the local expansion rate of the Universe, which is essential in our quest to find the nature of Dark Energy,” said James Braatz, of the National Radio Astronomy Observatory (NRAO), who spoke today at the American Astronomical Society’s meeting in Pasadena, California.

Braatz and his colleagues used the National Science Foundation’s Very Long Baseline Array (VLBA) and Robert C. Byrd Green Bank Telescope (GBT), and the Effelsberg Radio Telescope of the Max Planck Institute for Radioastronomy (MPIfR) in Germany to determine that a galaxy dubbed UGC 3789 is 160 million light-years from Earth. To do this, they precisely measured both the linear and angular size of a disk of material orbiting the galaxy’s central black hole. Water molecules in the disk act as masers to amplify, or strengthen, radio waves the way lasers amplify light waves.

The observation is a key element of a major effort to measure the expansion rate of the Universe, known as the Hubble Constant, with greatly improved precision. That effort, cosmologists say, is the best way to narrow down possible explanations for the nature of Dark Energy. “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,” said Braatz.
Dark Energy was discovered in 1998 with the observation that the expansion of the Universe is accelerating. It constitutes 70 percent of the matter and energy in the Universe, but its nature remains unknown. Determining its nature is one of the most important problems in astrophysics.

“Measuring precise distances is one of the oldest problems in astronomy, and applying a relatively new radio-astronomy technique to this old problem is vital to solving one of the greatest challenges of 21st Century astrophysics,” said team member Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA).

The work on UGC 3789 follows a landmark measurement done with the VLBA in 1999, in which the distance to the galaxy NGC 4258 — 23 million light-years — was directly measured by observing water masers in a disk of material orbiting its central black hole. That measurement allowed refinement of other, indirect distance-measuring techniques using variable stars as “standard candles.”

The measurement to UGC 3789 adds a new milepost seven times more distant than NGC 4258, which itself is too close to measure the Hubble Constant directly. The speed at which NGC 4258 is receding from the Milky Way can be influenced by local effects. “UGC 3789 is far enough that the speed at which it is moving away from the Milky Way is more indicative of the expansion of the Universe,” said team member Elizabeth Humphreys of the CfA.

Following the achievement with NGC 4258, astronomers used the highly-sensitive GBT to search for other galaxies with similar water-molecule masers in disks orbiting their central black holes. Once candidates were found, astronomers then used the VLBA and the GBT together with the Effelsberg telescope to make images of the disks and measure their detailed rotational structure, needed for the distance measurements. This effort requires multi-year observations of each galaxy. UGC 3789 is the first galaxy in the program to yield such a precise distance.

Team member Cheng-Yu Kuo of the University of Virginia presented an image of the maser disk in NGC 6323, a galaxy even more distant than UGC 3789. This is a step toward using this galaxy to provide another valuable cosmic milepost. “The very high sensitivity of the telescopes allows making such images of galaxies even beyond 300 million light years,” said Kuo.

Source: AAS


5 Responses

  1. Manu says

    A very frustrating story…
    Just how did they do it??
    How do water masers help?
    Pliiiiz do tell us!
    It IS, after all, “the oldest – and biggest – problem in astronomy”!

  2. Torbjorn Larsson OM says

    Good question. I googled this on NGC 4258 from -99:

    The newest observations were focused on maser “spots” on the near edge of the disk, where orbital motion shifts their position in the sky, though by an extremely small amount. The VLBA, however, was able to detect this extremely small movement, called “proper motion” by astronomers. This motion was detected by observing the galaxy at 4- to 8-month intervals over more than three years.

    “By knowing the speed at which the gas is orbiting and then measuring its motion across the sky, we can use plain old trigonometry to calculate the distance,” Greenhill said. He added, however, that “you need a bit of luck to be able to do this. So far, we know of only 22 galaxies with water masers in their nuclear regions that also are relatively nearby. Then, the geometry of the disk, relative to Earth, has to be right to allow us to make such a measurement”

  3. Manu says

    Thanks a lot!
    This is incredibly fantastic: observing motion in another galaxy…

  4. Member
    IVAN3MAN says

    Err… minor nitpick, Nancy. There’s a typo in the last line of the first paragraph: it should be spoke, not “spokr”. ๐Ÿ™‚

  5. Jon Hanford says

    Great story, Nancy, on this potentially independent technique for determining the distance of certain galaxies. This could provide astronomers with a crucial independent determination of a galaxy’s distance, separate from its redshift. And certainly, UGC 3789 is far enough away from us to be considered in the ‘Hubble flow’. And just to be clear, the galaxy used to illustrate this story is indeed UGC 3789 from the Palomar Digital Sky Survey. Just great to see this maser-method technique used to independently confirm redshift distance measurements.

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