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Astronomers Discover New Standard Candle

A way to measure the distance of active galactic nuclei could change the way astronomers think about the Universe and how it is expanding.

One of the trickiest problems in astronomy is the measure of distance.

In theory, distance should be simple to work out. If you know the intrinsic brightness of an object, a simple measure of its apparent brightness will tell you how far away it is (since brightness falls as an inverse square of its distance).

So in astronomy, the problem of distance is intimately linked to the problem of knowing an object’s intrinsic brightness.

But that’s hard. There’s simply no way to tell the intrinsic brightness of most stars and galaxies and so no way to work out their distance.

Astronomers have found a couple of exceptions, however. One is the Cepheid variable, a star whose brightness is linked to the rate at which its luminosity pulses. So if you know the pulsation period, you can work out the intrinsic brightness.

Another is type 1a supernova, which all explode with about the same mass and so have the same intrinsic brightness.

These so-called standard candles are the rulers that astronomers use to measure out distance in the universe. As such, they are extremely valuable.

Today, Darach Watson at the Dark Cosmology Centre at the University of Copenhagen in Denmark and a few pals, say they’ve come up with an entirely new kind of standard candle that measures the distance to active galactic nuclei.

Active galactic nuclei are galaxies with a central supermassive black hole that emits intense radiation. When this radiation hits nearby gas clouds, it ionises them causing them to emit a characteristic light of their own.

In recent years, astronomers have found that they can see both the emissions from the supermassive black hole as well as the emissions from the gas clouds. These are obviously related but the time it takes for radiation to reach the cloud means that changes here lag those in the supermassive black hole.

This delay, which can be measured with a technique called reverberation mapping, is then clear measure of the radius of the cloud.

But since the flux of the radiation from the black hole drops as an inverse square law, the brightness of these clouds also depends on their radius.

So a good measure of their radius also gives an indication of their intrinsic brightness.

Now Watson and co have used this technique to measure the distance to 38 active galactic nuclei at distances of up to z=4. That’s significantly further than is possible with type 1a supernova, whose distance cannot be accurately measured beyond z=1.7.

To say that this is interesting is putting it mildly. When Cepheid variables were identified as standard candles in the early part of the 20th century , Edwin Hubble used them to show that the Universe was expanding.

When type 1a supernovas were identified as standard candles in the early 1990s, astronomers used them to discover that the expansion of the Universe is accelerating.

So what of the prospects for this new method? Active galactic nuclei are among the brightest objects in the universe. Astronomers can see them at distances of up to about z=7, which corresponds to just 750 million years after the Big Bang.

An accurate way to determine their distance is sure to have profound implications.

Ref: arxiv.org/abs/1109.4632: A New Cosmological Distance Measure Using AGN

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