All lights brought out of the dark; Dark matter as the original root of the universe; Cataract in a gravitational lens may be a tiny galaxy; via humanrescueintermedia

Cataract in a gravitational lens may be a tiny galaxy
By Matthew Francis | Published January 19, 2012 6:25 AM
http://arstechnica.com/science/news/2012/01/cataract-in-a-gravitational-lens-may-be-a-tiny-galaxy.ars

Below: Hubble Space Telescope infrared image of Einstein ring B1938+666; right: Einstein ring in radio light


The Universe seems to be built from the bottom up: small structures combine to make larger and larger objects. In the best models cosmologists have developed, dark matter is the architect, providing the gravitational foundations on which the gas and dust that form stars can collect. Large-scale dark matter simulations of the Universe produce results that match the observed populations of large galaxies such as Milky Way. But they also predict far too many low-mass galaxies compared to what we’ve seen in population surveys.

One possible resolution to the problem is that at least some low-mass galaxies may be lacking much ordinary matter. If a galaxy has few or no stars and little gas, it won’t produce much light, making it difficult to observe. Nevertheless, all mass has a gravitational effect that can reveal itself under certain circumstances. A recent analysis by S. Vegetti, D. J. Lagatutta, J. P. McKean, M. W. Auger, C. D. Fassnacht, and L. V. E. Koopmans found a small anomaly in a gravitational lens that may be a small satellite galaxy that’s too faint to be seen via direct observation.

The Milky Way, M31 (Andromeda Galaxy), and many other observed galaxies have satellite galaxies surrounding them. Some of these are relatively large and bright (like the Large and Small Magellanic Clouds), but many are very faint and relatively low mass, such as the Sagittarius dwarf elliptical galaxy. Standard dark matter models predict large numbers of these dwarf satellites, far more than we’ve actually observed.

Mass affects the path of light, as a consequence of the general theory of relativity. For a sufficiently large mass, the light’s shift may be sufficiently large that we can measure it, and it can produce lensed images of the original light source. In gravitational lensing, the lens is a galaxy or galaxy cluster lying between Earth and a distant source, itself typically a galaxy. If the lens is directly in the line of sight, the image of the source galaxy can be distorted into an Einstein ring, a circular image of the source. By studying the shape and other characteristics of the image, observers can reconstruct details about both the lens and the source galaxies.

A particular lens system, JVAS B1938+666, is a distant elliptical galaxy that produced a bright Einstein ring image of an even more remote source galaxy. Independent observations by the W. M. Keck Telescope’s Near Infrared Camera 2 and the Near Infrared Camera aboard the Hubble Space Telescope provided the basis to reconstruct the mass distribution in the lens galaxy. The analysis by Vegetti and colleagues found an anomaly in the results; explaining it requires either an extra bit of mass in the lens or some intervening dust to block the light from parts of the image. However, dust absorbs light in a particular way, changing the spectrum of the image, and the team failed to find the appropriate signature.

That leaves the most likely culprit being another clump of mass that isn’t part of the main lens galaxy. Subtracting a reasonable model for the elliptical galaxy leaves the image pattern that would be produced by this hypothetical clump of mass. If you assume that this lensing isn’t from an unconnected galaxy with a smaller effect, then the clump of mass can be characterized fairly easily apart from the larger elliptical galaxy. Starting with assumptions taken from standard dark matter simulations, the researchers attempted to fit the location and mass of the clump, and found something consistent with an object in the same mass category as the Sagittarius dwarf galaxy.

Although the research letter published today in Nature calls this dwarf galaxy candidate “dark” both in the title and description (implying no emitted light), the authors do point out that they can only put a ceiling on the luminosity on the object. That maximum limit is still brighter than several of the Milky Way’s dwarf satellite galaxies. In other words, it’s premature to declare that a dark galaxy has been found, even if the mass estimate holds up under further investigation.

Nevertheless, the presence of this candidate has other important implications for dark matter models. The lens galaxy (and hence the perturbing lump of matter) is about 3 billion parsecs away, meaning the light we observe from it was emitted when the Universe was roughly half its current age. (Recall that the Universe is expanding, so the galaxy is carried farther from us by the expansion even as the light travels between it and Earth.) Detecting any small galaxy at that distance is not only an impressive feat, but also gives us some hints about the history of the formation of galaxies farther back in time. Since the candidate dwarf galaxy was found using parameters from dark matter simulations, the authors argue that the simulations may not be as far off as they have seemed.

Nature, 2012. DOI: 10.1038/nature10669 (About DOIs).

Photograph by Hubble Space Telescope

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