Dark Matter in 3-D

Light travels at a finite speed, so looking out into the distance is equivalent to looking back through time. Combining a set of slices at fixed distances (top) gives a 3-D map (bottom) that is like a geological core sample of the universe. Evolving over time from right to left, the distribution of dark matter becomes increasingly clumpy.

An international team led by Caltech scientists has made a three-dimensional map of dark matter that offers a first look at its distribution. Dark matter, which makes up most of the universe’s mass but neither emits nor reflects light, has so far eluded direct detection or even a definitive explanation for its makeup. But a cosmic quirk called “gravitational lensing,” first predicted by Einstein, allows the invisible stuff to be traced out.

The light rays from distant galaxies are deflected where space is curved by the gravitational influence of dark matter, making the shapes of background galaxies appear distorted. So postdoc Richard Massey and JPL scientist Jason Rhodes carefully measured those shapes to infer the distribution of foreground structures. Since gravitational lensing is sensitive to all mass, it reveals the location of otherwise invisible features—
including one concentration of dark matter a trillion times more massive than the sun, around a previously unknown cluster of galaxies.

The 3-D map, which was unveiled at the January meeting of the American Astronomical Society and also appeared in the January 18 issue of Nature, reveals a gelatinous network of cosmological filaments that grew over time, intersecting to form massive structures containing clusters of galaxies. According to lead author Massey and coauthor Richard Ellis, the Steele Family Professor of Astronomy, this provides the best evidence yet that normal matter coalesces to form galaxies only inside the preexisting scaffolding of dark matter.

The map was derived from the Hubble Space Telescope’s widest survey of the universe, led by Nick Scoville, Caltech’s Moseley Professor of Astronomy. The Cosmic Evolution Survey (COSMOS) consists of 575 slightly overlapping views of the universe requiring nearly 1,000 hours of observations—the largest project ever undertaken with the Hubble.

Scattered through the COSMOS images are some half-million distorted galaxies whose distances were measured to high accuracy—using color data from the Subaru telescope in Hawaii—as part of COSMOS’s research on large-scale structures.

Reprinted by permission from MacMillan Publishers Ltd: Massey, et al., Nature, vol. 445, pp. 286–290, January 18, 2007.

In this rendering of the entire COSMOS field, the contours show the total mass of both visible and dark matter. Ordinary matter grows inside a dark matter scaffolding: the galaxy mass distribution is shown in blue and number density in yellow; the two combine to become green. Red shows X-ray emission from hot, dense gas in the centers of dense clusters of galaxies. This view covers nearly two square degrees of sky, or roughly nine times the area of the full moon.

The resulting map stretches halfway back to the beginning of the universe, showing how dark matter started out smooth and grew increasingly clumpy as it continued to collapse. These observations will guide theorists grappling with how large cosmic structures evolved under the relentless pull of gravity, and may illuminate the role of “dark energy”—a sort of negative gravitational force that is believed to influence how dark matter clumps.

According to Scoville, stars in the galaxies in the densest cosmic structures of the early universe are generally found to be older than those in galaxies in more rarified environments, indicating that the galaxies in the denser regions formed first and that the mass accumulated in a bottom-up fashion. By contrast, those galaxies with ongoing star formation today dwell in less populated cosmic filaments and voids.

“Both the maturity of the stellar populations and the ‘downsizing’ of star formation in galaxies vary strongly with the epoch when the galaxies were born, as well as their dark-matter environment,” says Scoville. His team’s findings will appear in a future issue of The Astrophysical Journal. Other Caltech participants in this COSMOS research on large-scale structures include postdocs Peter Capak and Mara Salvato, Member of the Professional Staff Patrick Shopbell, and Kartik Sheth of the Infrared Processing and Analysis Center. —RT