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We're all used to hearing about the superb angular resolution of space telescopes and the largest ground-based instruments, but only rather infrequently are we reminded that there is an instrument capable of producing astronomical images hundreds of times sharper.
The instrument in question in the Very Long Baseline Array (VLBA), a network of 10 radio dish antennae 25 metres in diameter, in various locations between Hawaii and the Caribbean, that work together as though they were part of the surface of a single continent-sized telescope. Given the very long "base" (from which the name derives) the angular resolution is extremely high and can be exploited in various fields of astrophysics.
The marvelous potential of the VLBA is demonstrated nicely by two recent measurements. The first comes from a study of the galaxy NGC 6264, for which the distance has been determined directly, from geometrical measurements, to be 450 million light years, with an uncertainty of only 9%. This is the most distant object for which a direct distance has been measured (rather than inferred) and the accuracy is unprecedented for an object at such a distance. The previous record was a 2009 determination for a galaxy 160 million light years away.
The VLBA was used to image what are called 'masers', clouds of gas that orbit near the centre of some galaxies and emit very intense radio emission at a very precise frequency. By measuring the angular distance of these clouds from the galaxy centre, their velocity (via redshift of the spectral line) and how much they move over a period of a few months, their physical distance in light years from the galaxy centre can be determined.
Having this distance determined, both as an angular separation and in light years, allows the distance of the galaxy from us to be determined.
This kind of measurement, independent of the assumptions that plague other distance determinations, is of fundamental importance for explaining things like the nature of dark energy and its role in the expansion of the Universe.
Directly measuring the distances to ever more distant galaxies will reduce the errors caused by, for example, local motions of galaxies, in determining the expansion rate of the Universe.
The second series of measurements concerns our own galaxy, in which the VLBA has measured the distances of a number of star forming regions (indicated in the above image). These reveal that the Milky Way is rotating faster than previously thought. This also implies that its mass is greater, and in fact is similar to that of the Andromeda galaxy, long considered to be more massive than our own.
Further measurements of the Andromeda galaxy, already underway, will determine precisely the speed and direction in which this galaxy is moving, enabling predictions about its future encounter with our own system.
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