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Gravitational lensing weighs 70 galaxies and infers

dark matter
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 23, 2008

In the largest ever single collection of gravitational lens galaxies, an international team of astronomers have helped settle a long-standing debate over the relationship between mass and luminosity in galaxies, and used the results to infer the presence of dark matter.

The results are the culmination of the two and a half year Sloan Lens ACS (SLACS) Survey, part of the Sloan Digital Sky Survey (SDSS), which uses a 2.5 metre telescope to measure the precise distances to millions of galaxies and stars. The SDSS allowed astronomers to identify an impressive collection of 70 galaxies through the challenging gravitational lensing technique, which requires the precise alignment of two galaxies – perhaps separated by several thousand light years – along the same line of sight.

The large panel shows the Hubble Space Telescope image of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue. The smaller right-hand panels show the components of a model of this image: top, a model for how the more distant background galaxy would appear in the absence of the lensing effect; centre, a smooth model for the brightness of the more nearby massive galaxy; bottom, the appearance of the background galaxy when its image is distorted by the gravity of the nearer galaxy. Image: A. Bolton (UH/IfA) for SLACS and NASA/ESA.

“Since strong gravitational lensing is a relatively rare phenomenon, one needs a very large sample of galaxies to begin with,” says Dr Adam Bolton of the University of Hawaii, lead author of two papers which will appear in the Astrophysical Journal in August and September.  “Then, one needs some sort of effective way of sorting through them to find the one in 500 or so that actually might be a lens. Finally, one needs a way to confirm the candidates securely.”

The team took advantage of the SDSS for the first and second steps of this strategy by searching the spectra of a few hundred thousand galaxies, looking for the signature of second galaxies at greater distances along the same lines of sight. For the final confirmation, they used the Hubble Space Telescope’s Advanced Camera for Surveys (ACS) to observe and record the details of the gravitationally lensed galaxies.

“If the galaxies are precisely aligned, the appearance is that of a perfect ring or a symmetric cross,” Bolton tells Astronomy Now. A complete ring is known as an Einstein ring, after Albert Einstein who was one of the first scientists to predict the existence of gravitational lenses back in the 1930s. Einstein ring images can be up to 30 times brighter than the image of the distant galaxy would be in the absence of the lensing effect. “As the alignment becomes less precise,” continues Bolton, “the appearance will be an asymmetric ring, or a bright arc plus fainter counter-arcs, and at even greater misalignment, we see a double-image system, with a brighter image farther from the centre and a fainter counter-image closer to the centre. Finally, at even greater misalignment, the strong lensing effect disappears, and we only see one image of the background galaxy, though that image may still be mildly distorted.”

For each lens, the astronomers measured the apparent sizes of the Einstein rings on the sky using the Hubble images, and measured the distances to the two galaxies of the aligned pair using Sloan data. The combination of these measurements allowed them to deduce the mass of the nearer galaxy, a feat that has not been achieved in this detail in any other lens surveys of this scale.

“Within the central regions that we measure with strong gravitational lensing, these galaxies have masses ranging from a few tens of billions of times the mass of the Sun, up to a trillion times the mass of the Sun,” says Bolton. “Including the regions further out, their masses are at least three times greater than that, and probably even larger still. Specifying the total mass of a galaxy is actually a bit of a fuzzy problem, since we can never really locate its outer edge precisely.”

A colour-enhanced mosaic of Hubble Space Telescope images of a selection of gravitational-lens galaxies discovered by the SLACS survey. In each case, the massive foreground galaxy is seen in yellow to red, and the distorted features of the more distant background galaxy are seen in blue. Image: A. Bolton (UH/IfA) for SLACS and NASA/ESA.

Although astronomers have long recognised a relationship between the brightnesses, sizes, and characteristic internal stellar velocities of elliptical galaxies, this relationship can be explained in a number of ways, either through a difference in the ‘mass-to-light ratio’ between galaxies of different masses, or through fundamental differences in the arrangement of mass within the galaxies, because the degree of concentration of mass towards the centre of a galaxy could vary systematically between galaxies of different masses.

“Since it is in general difficult to observe the actual galaxy masses directly, this ambiguity has persisted,” Bolton tells Astronomy Now. “With our study, we measured masses directly via strong lensing, and we have been able to resolve the puzzle in favour of the former explanation, at least for galaxies of medium to large mass.”

But the achievement doesn’t end there. The masses of the galaxies, combined with the measurements of size, brightness and velocity, can be used to infer the presence of dark matter, the mysterious, invisible material that makes up most of the matter in the Universe.

“First, if we consider a circle of increasing radius centred on the lensing galaxy, we see several different indications that the mass enclosed within that circle increases more rapidly than the starlight enclosed within it,” explains Bolton. “Thus some form of dark matter must make an increasing relative contribution at larger distances from the galaxy's centre. Second, in going from galaxies of intermediate mass to galaxies of high mass, we see a clear increase in the ‘mass-to-light’ ratio that is hard to explain without requiring some form of dark matter that contributes the extra mass in the more massive galaxies.”

In the last 30 years, since the first gravitational lens example was discovered, many more lenses have been identified, but their scientific potential has always been limited by the disparate assortment of known examples. The new SLACS Survey results has significantly changed this situation by identifying a single large and uniformly selected sample of strong lens galaxies, that promise to form the basis of many further studies.

An animation of the gravitational lensing effect can be found at: http://www.ifa.hawaii.edu/info/press-releases/Bolton7-08/lensing_animation.html

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