July 26, 2017
Credit: HUBBLE LEGACY ARCHIVE, ESA, NASA AND BILL SNYDER.
Mari-Ela Chock, Communications Officer
W. M. Keck Observatory
Maunakea, Hawaii– The international Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) collaboration based at the University of California, Riverside has combined observations from several of the world’s most powerful telescopes, including W. M. Keck Observatory on Maunakea, Hawaii, to carry out one of the largest studies yet of molecular gas – the raw material which fuels star formation throughout the universe – in three of the most distant clusters of galaxies ever found, detected as they appeared when the universe was only four billion years old. Allison Noble, a postdoctoral researcher at the Massachusetts Institute of Technology, led this newest research from the SpARCS collaboration.
Results were recently published in The Astrophysical Journal Letters.
Clusters are rare regions of the universe consisting of tight groups of hundreds of galaxies containing trillions of stars, as well as hot gas and mysterious dark matter.
First, the research team used spectroscopic observations from the Very Large Telescope in Chile and Keck Observatory’s powerful Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) to confirm nearly a dozen galaxies were star-forming members of the three massive clusters.
"Keck Observatory’s MOSFIRE data were essential to proving conclusively that the 11 galaxies analyzed (two pairs) were indeed members of the three clusters and not foreground galaxies,” said Gillian Wilson, a professor of physics and astronomy at UC Riverside and the leader of the SpARCS collaboration.
Next, the researchers took images through multiple filters from NASA’s Hubble Space Telescope, which revealed a surprising diversity in the galaxies’ appearance, with some galaxies having already formed large disks with spiral arms.
One of the telescopes the SpARCS scientists used is the extremely sensitive Atacama Large Millimeter Array (ALMA) telescope capable of directly detecting radio waves emitted from the molecular gas found in galaxies in the early universe. ALMA observations allowed the scientists to determine the amount of molecular gas in each galaxy, and provided the best measurement yet of how much fuel was available to form stars.
The researchers compared the properties of galaxies in these clusters with the properties of “field galaxies” (galaxies found in more typical environments with fewer close neighbors). To their surprise, they discovered that cluster galaxies had higher amounts of molecular gas relative to the amount of stars in the galaxy compared to field galaxies. The finding puzzled the team because it has long been known that when a galaxy falls into a cluster, interactions with other cluster galaxies and hot gas accelerate the shut off of its star formation relative to that of a similar field galaxy (the process is known as environmental quenching).
“This is definitely an intriguing result,” said Wilson. “If cluster galaxies have more fuel available to them, you might expect them to be forming more stars than field galaxies, and yet they are not.”
Allison Noble, a SpARCS collaborator and this study’s leader, suggests several possible explanations: It is possible that something about being in the hot, harsh cluster environment surrounded by many neighboring galaxies perturbs the molecular gas in cluster galaxies such that a smaller fraction of that gas actively forms stars. Alternatively, it is possible that an environmental process, such as increased merging activity in cluster galaxies, results in the observed differences between the cluster and field galaxy populations.
“While the current study does not answer the question of which physical process is primarily responsible for causing the higher amounts of molecular gas, it provides the most accurate measurement yet of how much molecular gas exists in galaxies in clusters in the early universe,” Wilson said.
The SpARCS team has developed new techniques using infrared observations from NASA’s Spitzer Space Telescope to identify hundreds of previously undiscovered clusters of galaxies in the early universe.
In the future, they plan to study a larger sample of clusters. The team has recently been awarded additional time on ALMA, Keck Observatory, and the Hubble Space Telescope to continue investigating how the neighborhood in which a galaxy lives determines for how long it can form stars.
The Keck Observatory data were obtained as the result of a collaboration amongst Wilson and fellow UC faculty members Michael Cooper (UC Irvine) and Saul Perlmutter (UC Berkeley).
The Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), gathers thousands of spectra from objects spanning a variety of distances, environments and physical conditions. What makes this large, vacuum-cryogenic instrument unique is its ability to select up to 46 individual objects in the field of view and then record the infrared spectrum of all 46 objects simultaneously. When a new field is selected, a robotic mechanism inside the vacuum chamber reconfigures the distribution of tiny slits in the focal plane in under six minutes. Eight years in the making with First Light in 2012, MOSFIRE's early performance results range from the discovery of ultra-cool, nearby substellar mass objects, to the detection of oxygen in young galaxies only two billion years after the Big Bang. MOSFIRE was made possible by funding generously provided by the National Science Foundation and astronomy benefactors Gordon and Betty Moore. It is currently the most in-demand instrument at Keck Observatory.
About W. M. Keck Observatory
The W. M. Keck Observatory operates the most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California, and NASA.
The international SpARCS collaboration based at UC Riverside has made the best measurement yet of the amount of fuel available to form stars in clusters of galaxies located in the early universe.