May 26, 2016

WEST LAFAYETTE, Ind. — A team of astronomers at Purdue University and the National Optical Astronomy Observatory have confirmed a rare discovery of the early formation of one of the most massive structures in the present-day universe, a galaxy cluster.

The structure, named PC 217.96+32.3A, is located 12 billion light years away in the constellation of Boötes. A paper detailing the findings of the structure was published in the Astrophysical Journal and is available online.

"Galaxy clusters are the largest and most massive structures in the universe comprising up to thousands of galaxies," said Kyoung-Soo Lee, a Purdue professor of physics and astronomy who first spotted the potential protocluster in 2014 and is part of the team that recently confirmed it. "What we have found is an actively forming cluster, or a 'protocluster' when the universe was only around 1.7 billion years old. We believe that this protocluster will easily evolve into a galaxy cluster with total mass more than a quadrillion times that of our sun by the present time. It is monstrous in size and a very rare find."

Gravity draws matter in the universe together. Some of this matter combines to form stars, and a collection of stars forms galaxies. Galaxies, in turn, can be drawn together into galaxy clusters.

 "Galaxy clusters observed in the present day universe have the oldest and most massive galaxies, but their formation process is not well known," Lee said. "Protoclusters provide useful cosmic laboratories where we can directly witness and study the formation process."

Lee and Arjun Dey, of the National Optical Astronomy Observatory, led the team that confirmed the protocluster using spectroscopic measurements from the Mayall telescope on Kitt Peak and the Keck II Telescope on Mauna Kea.

Matter in galaxies, such as atomic hydrogen and oxygen, emit light with a known wavelength. The effects of distance and movement on that wavelength are known, which allows for measurements that can reveal the distance a galaxy is from Earth and its relative motion. Using spectroscopy to capture and measure specific wavelengths of light, the team determined the distribution and dynamics of the galaxies within the protocluster.

 "Many of the faint galaxies in this patch lie at the same distance," Dey said in a statement. "They are clumped together due to gravity and the evidence suggests that the cluster is in the process of forming."

The team found that the majority of the member galaxies are concentrated in the protocluster's inner core, which has 14 times more galaxies than expected, Lee said.

The core is surrounded by a lower but still high-density region populated by galaxies and spanning roughly 10 million light years across. In addition, multiple smaller streams of galaxies are found near the cluster, possibly in the process of falling toward the core, and large voids surround the structure where no galaxy appears to reside.

The observed size and morphologies of the structure match the theoretical expectation for that of a protocluster, in which large structures coexist with spider web-like filaments and vast cosmic voids, Lee said.

The measurements also showed that the main body of the protocluster consists of two distinct groups moving at relative line-of-sight speed of 500 kilometers per second. This suggests that they are in the process of merging, which is consistent with the theoretical expectation that the largest galaxy clusters do not coalesce into their final stable form until much later, she said.

Roughly 30 million light years away from protocluster PC 217.96+32.3A, the team found another protocluster, thought to be 2-3 times lower in mass, named 217.96+32.3B.

The team theorizes that the two protoclusters will independently evolve into distinct clusters.

"The likelihood of finding two large structures of this magnitude within a small cosmic volume is extremely low, less than 2 percent," Lee said. "The two clusters may eventually evolve into a supercluster, which is a loosely bound group of galaxy clusters. Our own Milky Way is a part of such a structure known as the Virgo supercluster."

In addition to Lee and Dey, team members include Naveen Reddy of the University of California, Riverside; Hanae Inami of Observatoire de Lyon; Sungryong Hong of the University of Texas, Austin; Michael Cooper of the University of California, Irvine; Anthony H. Gonzalez, of the University of Florida, Gainesville; and Buell T. Jannuzi of the Steward Observatory at the University of Arizona, Tucson.

The National Optical Astronomy Observatory and NASA funded the research. Kitt Peak National Observatory and the National Optical Astronomy Observatory are operated by the Association of Universities of Research in Astronomy under a cooperative agreement with the National Science Foundation. The W.M. Keck Observatory is a scientific partnership between the NASA, the California Institute of Technology and the University of California, and is supported by the W.M. Keck Foundation. 

ABSTRACT 

Spectroscopic Confirmation of a Protocluster at z ≈ 3.786

Arjun Dey, Kyoung-Soo Lee, Naveen Reddy, Michael Cooper, Hanae Inami,

Sungryong Hong, Anthony H. Gonzalez, Buell T. Jannuzi 

We present new imaging and spectroscopic observations of the field in Boötes containing the z = 3.786 protocluster, PC 217.96+32.3, discovered by Lee et al (2014) and confirm that it is one of the most overdense and largest scale high-redshift structures known. Such overdense and physically large structures are rare even in the largest cosmological simulations. We used the MOSAIC1.1 imaging camera at the Mayall 4m telescope to image a 1.2◦ × 0.6◦ area (≈ 150 × 75 comoving Mpc) surrounding the core of the cluster and have uncovered 165 candidate Lyα emitting galaxies (LAEs) and 788 candidate Lyman Break galaxies (LBGs). There are at least 2 overdense regions traced by the LAEs, the largest of which shows an areal overdensity in its core (i.e., within a radius of 2.5 comoving Mpc) of 14±7 relative to the average spatial density of LAEs in the imaged field. Further, the average density of LAEs in the imaged field is twice that derived by other field LAE surveys. Spectroscopy at the W. M. Keck Observatory yielded redshifts for 164 galaxies (79 LAEs and 85 LBGs), 65 of which lie at the redshift of the protocluster structure at 3.785 ± 0.010. The velocity dispersion of galaxies near the core is found to be σ ≈ 350±40 kms−1, a value that is robust to selection effects.