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Weighing 70 Galaxies

SLACS Survey strong lens object SDSSJ1430

This Hubble Space Telescope image shows a lensed Einstein ring of the background galaxy enhanced in blue. The size of this image on the sky is about 300 times smaller than the full Moon. Right: top, model of the more distant background galaxy without the lensing effect; center, a smooth brightness model of the nearby massive galaxy; bottom, the appearance of the background galaxy when its image is distorted by the gravity of the nearer galaxy causing an Einstein ring. Image by Adam Bolton (UH IfA) for SLACS and NASA/ESA


Adam Bolton
Adam Bolton
Photo by M. L. Bolton.

An international team of astronomers, including IfA's Adam Bolton, has recently announced a finding that helps to settle a long-standing debate over the relationship between mass (the amount of matter) and luminosity (brightness) in galaxies.

The team achieved this result by compiling the largest-ever single collection of "gravitational lens" galaxies--70 in all. A gravitational lens is a phenomenon similar to a terrestrial mirage, but it occurs on a scale of many thousands of light-years. When two galaxies happen to be precisely aligned with one another in the sky, the gravitational field of the nearer galaxy distorts the image of the more distant galaxy into multiple arc-shaped images or even into a complete ring, known as an "Einstein ring." These 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.

The discovery represents the culmination of the Sloan Lens ACS (or SLACS) Survey. The gravitational lenses were originally identified using data from the Sloan Digital Sky Survey, a major project that has used a dedicated 2.5-meter telescope in New Mexico to measure precise distances to nearly one million distant galaxies and quasars throughout one quarter of the sky. To observe and measure the details of the Einstein ring images, the SLACS astronomers then took advantage of the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope, which delivers pictures of unparalleled sharpness.

"The SLACS collection of lenses is especially powerful for science," said Bolton, lead author of two papers describing these latest results, which were published in the Astrophysical Journal in August and September. "For each lens, we measured the apparent sizes of the Einstein rings on the sky using the Hubble images, and we measured the distances to the two galaxies of the aligned pair using Sloan data. By combining these measurements, we were able to deduce the mass of the nearer galaxy."

In other lens surveys of this scale, distances to the lens and background galaxies--and hence the lens galaxy masses--have not been measured precisely.

By considering these galaxy masses along with measurements of their sizes, brightnesses, and stellar velocities, the SLACS astronomers were able to infer the presence of dark matter in addition to the visible stars within the galaxies. Dark matter is the mysterious, unseeable material that is the majority of matter in the Universe. And with such a large number of lens galaxies across a range of masses, they found that the fraction of dark matter relative to stars increases systematically when going from galaxies of average mass to galaxies of high mass.

The existence of gravitational lenses was first predicted by Albert Einstein in the 1930s, but the first example was not discovered until the late 1970s. In the 30 years since then, many more lenses have been discovered, but their scientific potential has been limited by the disparate assortment of known examples. The SLACS Survey has significantly changed this situation by discovering a single large and uniformly selected sample of strong lens galaxies. The SLACS collection promises to form the basis of many further scientific studies.

Other members of the SLACS collaboration are L. V. E. Koopmans (Kapteyn Instituut, Groningen, The Netherlands), Tommaso Treu (University of California, Santa Barbara), Raphael Gavazzi (Institut d'astrophysique de Paris), Leonidas A. Moustakas (JPL/Caltech), and Scott Burles (MIT).

SLACS mosaic

A color-enhanced mosaic of Hubble Space Telescope images of 60 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. The images are arranged from upper left in order of increasing distance of the foreground galaxy from Earth. SLACS: The Sloan Lens ACS Survey. Credit: A. Bolton (UH/IfA) for SLACS and NASA/ESA. For an online animation of the Einstein ring effect, see