Astronomers Find Wall of Galaxies Traversing the Hubble Deep Field

Astronomers have announced the discovery of a wall of distant galaxies crossing the Hubble Deep Field at a distance of some 4 billion light years from Earth. The report is being presented by Drs. Daniel Vanden Berk and Chris Stoughton of the Department of Energy’s Fermi National Accelerator Laboratory, Dr. Arlin Crotts of Columbia University, and Dr. David Tytler and David Kirkman of the University of California San Diego to the American Astronomical Society meeting in Atlanta, GA. The discovery supports earlier speculation that galaxies are distributed in wall or sheet-like structures throughout the universe.

The structure was identified near a region of the sky which surrounds and contains the Hubble Deep Field (HDF) — a small patch of sky which has been the focus of intense research since 1995 when the Hubble Space Telescope first took very deep images of it. Several teams of astronomers have made follow-up observations of galaxies in the HDF to determine their distances. A key finding from these studies is that more than half of the galaxies are found in a small number of groups, rather than being smoothly distributed in space. While the clustering of galaxies in the nearby universe has been known for decades, the HDF studies showed that this pattern appears to persist to much greater distances away fromus. However, since the HDF is a very small patch of the sky — less than 1% the area of the full moon — it was unclear just how big the groups of galaxies are and what they look like. Based on analogy to large galaxy structures seen in the local universe, a team led by Dr. Judith Cohen of the California Institute of Technology speculated that the galaxy groups are merely parts of much larger structures which traverse the HDF, similar the the so-called “Great Wall” of galaxies.

To test this prediction, Vanden Berk and colleagues used light from distant quasars to illuminate a wider area of sky surrounding the HDF. The conventional approach is to measure the distances to hundreds of faint galaxies to see where they lie with respect to the HDF galaxy groups. This takes a lot of time even with the world’s largest telescopes. Instead, the team looked for the shadows of galaxies against brighter more distant objects called quasars. Quasars are among the brightest objects in the universe, typically giving out as much light as hundreds or thousands of galaxies. When a foreground galaxy lies in front of a quasar, gas in the galaxy absorbs some of the quasar light at precise colors which depend on the redshift, and hence distance, to the absorbing galaxy. Each lineofsight to a quasar yields a catalog of absorbing galaxy redshifts, but the observations require less telescope time than measuring the galaxy light directly.

Quasars were found in a field centered on the HDF, but 400 times its area, using a combination of imaging and spectroscopy with the 0.76-meter (30-inch) Telescope and 2.7-meter (107-inch) Harlan J. Smith Telescope at the University of Texas McDonald Observatory, near Fort Davis, Texas. The 19 quasars identified to date provide a grid of background spotlights against which light-absorbing foreground galaxies can be detected. A preliminary set of 3 quasars were observed using spectrographs on the 10-meter (400-inch) Keck ITelescope on Mauna Kea, Hawaii, the 3.5-meter (140-inch) ARC Telescope at Apache Point Observa- tory, NewMexico, and the 2.4-meter (95-inch) MDM Telescope at Kitt Peak, Arizona. The spectrographs spread out the quasar light into many component colors. Small color ranges deficient in quasar light indicate the presence of an intervening galaxy which has blocked out some of the quasar light.

The study revealed a number of intervening galaxies, but more importantly, several of these galaxies were found to be at precisely the same distance as one of the largest groups in the HDF surveys, at a redshift of 0.559, or a distance of roughly 4 billion light years away from earth. The probability that these distance matches occurred by chance is very small, indicating that the galaxy groups and absorbing galaxies are parts of the same structure. The absorbing galaxies and HDF galaxy group are some 25 million light years apart. The simplest explanation is that the wall-like structure containing galaxies which traverses the Hubble Deep Field is at least 25 million light years across.

Tracing the matter in the universe through the absorption of light is “an efficient way to study not only the distribution of distant galaxies, but also the gas that will someday form galaxies, and may lie in giant wall-like structures that we see populated by galaxies today,” said Vanden Berk, who is a research associate at Fermilab. Theoretical models often predict the existence of wall or sheet-like structures of gas and galaxies, but their sizes and shapes, as well as when they are formed are largely unknown. “It is very exciting to see that the groups of galaxies we found directly have counterparts seen in absorption by gas and at large distances. The Universe appears to contain almost unimaginably huge networks of galaxies, and the material — gas — for making new ones. Galaxy formation is a messy, complicated process that is not at all well understood. Tracing out these structures in the Universe is essential to constructing an empirical picture of how the Universe evolved to its present state,” says Dr. David Hogg of the Institute for Advanced Study in Princeton, NJ, who is part of the team lead by Dr. Judith Cohen of Caltech. Further observations of both quasars and galaxies are needed to confirm the results and to see if galaxy walls are a common feature of the universe.

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