Showing posts with label protocluster galaxies. Show all posts
Showing posts with label protocluster galaxies. Show all posts

Wednesday, May 22, 2019

Star Formation in Young Galaxies Not Affected by Environment

Figure 1: Photo of the proto-cluster field from about 11 billion years ago (redshift z = 2.5) taken with MOIRCS on the Subaru Telescope. The insets are high-resolution narrow-band images of individual star-forming galaxies taken with IRCS+AO188. (Credit: NAOJ)

A team of astronomers used the Subaru Telescope to observe a proto-cluster of galaxies in the early Universe and found that the galaxies in it are forming stars in the same manner as isolated galaxies in the same era. This suggests that the galactic environment does not have a large influence on star formation in young galaxies.

Galaxies grow by forming new stars. By looking at where new stars are forming in young galaxies in the early Universe, astronomers can model how they will evolve into modern galaxies. A team led by Tomoko Suzuki, a post-doctoral researcher at Tohoku University, used the Subaru Telescope to observe a proto-cluster of galaxies from 11 billion years ago in the constellation Serpens. Using an Adaptive Optics (AO) system to correct for the blurring effect of Earth's atmosphere they successfully mapped the galaxies with a resolution of 0.2 arcsec (corresponding to 20/0.7 vision). Regions where young stars are forming are a different color than normal stars, so by using special filters to separate the colors, the team was able to observe both the stellar structure and the star-forming regions.

The observations show that on average for the more massive star-forming galaxies in the proto-cluster, the star-forming regions are more extended than the existing stellar structure. This means that the galaxies are growing by adding stars to their peripheries, rather than to their cores. This same pattern of star formation has been observed in isolated galaxies in sparsely populated regions in the same era. This result suggests that star formation in the early Universe is largely independent of galactic environment.

"The distribution of the star-forming region within galaxies is key information to understand the physical processes occurring in galaxies. We need to investigate not only the averaged structures but also the structure of the star-forming region within individual galaxies for more detailed studies." says Suzuki. "The next generation instrument ULTIMATE-Subaru will allow us to trace the individual structural growth of a large number of young galaxies in various environments."

These results will be published in Publications of the Astronomical Society of Japan (T. L. Suzuki, Y. Minowa, Y. Koyama, T. Kodama, M. Hayashi, R. Shimakawa, I. Tanaka, K.-i. Tadaki, "Extended star-forming region within galaxies in a dense proto-cluster core at z=2.53", 2019, Publications of the Astronomical Society of Japan, XX, XX). A preprint is available here. This study is supported by JSPS KAKENHI Grant Number JP18H03717.




Friday, April 27, 2018

Ancient Galaxy Megamergers

Artist’s impression of ancient galaxy megamerger
 
Images of a galaxy protocluster from SPT, APEX and ALMA



Videos

ESOcast 157 Light: Ancient Galaxy Pileups (4K UHD)
ESOcast 157 Light: Ancient Galaxy Pileups (4K UHD)

Artist’s impression of ancient galaxy megamerger
Artist’s impression of ancient galaxy megamerger


ALMA and APEX discover massive conglomerations of forming galaxies in early Universe

The ALMA and APEX telescopes have peered deep into space — back to the time when the Universe was one tenth of its current age — and witnessed the beginnings of gargantuan cosmic pileups: the impending collisions of young, starburst galaxies. Astronomers thought that these events occurred around three billion years after the Big Bang, so they were surprised when the new observations revealed them happening when the Universe was only half that age! These ancient systems of galaxies are thought to be building the most massive structures in the known Universe: galaxy clusters.

Using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX), two international teams of scientists led by Tim Miller from Dalhousie University in Canada and Yale University in the US and Iván Oteo from the University of Edinburgh, United Kingdom, have uncovered startlingly dense concentrations of galaxies that are poised to merge, forming the cores of what will eventually become colossal galaxy clusters.

Peering 90% of the way across the observable Universe, the Miller team observed a galaxy protocluster named SPT2349-56. The light from this object began travelling to us when the Universe was about a tenth of its current age.

The individual galaxies in this dense cosmic pileup are starburst galaxies and the concentration of vigorous star formation in such a compact region makes this by far the most active region ever observed in the young Universe. Thousands of stars are born there every year, compared to just one in our own Milky Way.

The Oteo team discovered a similar megamerger formed by ten dusty star-forming galaxies, nicknamed a “dusty red core” because of its very red colour, by combining observations from ALMA and the APEX.

Iván Oteo explains why these objects are unexpected: “The lifetime of dusty starbursts is thought to be relatively short, because they consume their gas at an extraordinary rate. At any time, in any corner of the Universe, these galaxies are usually in the minority. So, finding numerous dusty starbursts shining at the same time like this is very puzzling, and something that we still need to understand.”

These forming galaxy clusters were first spotted as faint smudges of light, using the South Pole Telescope and the Herschel Space Observatory. Subsequent ALMA and APEX observations showed that they had unusual structure and confirmed that their light originated much earlier than expected — only 1.5 billion years after the Big Bang.

The new high-resolution ALMA observations finally revealed that the two faint glows are not single objects, but are actually composed of fourteen and ten individual massive galaxies respectively, each within a radius comparable to the distance between the Milky Way and the neighbouring Magellanic Clouds.

"These discoveries by ALMA are only the tip of the iceberg. Additional observations with the APEX telescope show that the real number of star-forming galaxies is likely even three times higher. Ongoing observations with the MUSE instrument on ESO’s VLT are also identifying additional galaxies,” comments Carlos De Breuck, ESO astronomer.

Current theoretical and computer models suggest that protoclusters as massive as these should have taken much longer to evolve. By using data from ALMA, with its superior resolution and sensitivity, as input to sophisticated computer simulations, the researchers are able to study cluster formation less than 1.5 billion years after the Big Bang.

"How this assembly of galaxies got so big so fast is a mystery. It wasn’t built up gradually over billions of years, as astronomers might expect. This discovery provides a great opportunity to study how massive galaxies came together to build enormous galaxy clusters," says Tim Miller, a PhD candidate at Yale University and lead author of one of the papers.




More Information

This research was presented in two papers, “The Formation of a Massive Galaxy Cluster Core at z = 4.3”, by T. Miller et al., to appear in the journal Nature, and “An Extreme Proto-cluster of Luminous Dusty Starbursts in the Early Universe”, by I. Oteo et al., which appeared in the Astrophysical Journal.


The Miller team is composed of: T. B. Miller (Dalhousie University, Halifax, Canada; Yale University, New Haven, Connecticut, USA), S. C. Chapman (Dalhousie University, Halifax, Canada; Institute of Astronomy, Cambridge, UK), M. Aravena (Universidad Diego Portales, Santiago, Chile), M. L. N. Ashby (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA), C. C. Hayward (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA; Center for Computational Astrophysics, Flatiron Institute, New York, New York, USA), J. D. Vieira (University of Illinois, Urbana, Illinois, USA), A. Weiß (Max-Planck-Institut für Radioastronomie, Bonn, Germany), A. Babul (University of Victoria, Victoria, Canada) , M. Béthermin (Aix-Marseille Université, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France), C. M. Bradford (California Institute of Technology, Pasadena, California, USA; Jet Propulsion Laboratory, Pasadena, California, USA), M. Brodwin (University of Missouri, Kansas City, Missouri, USA), J. E. Carlstrom (University of Chicago, Chicago, Illinois USA), Chian-Chou Chen (ESO, Garching, Germany), D. J. M. Cunningham (Dalhousie University, Halifax, Canada; Saint Mary’s University, Halifax, Nova Scotia, Canada), C. De Breuck (ESO, Garching, Germany), A. H. Gonzalez (University of Florida, Gainesville, Florida, USA), T. R. Greve (University College London, Gower Street, London, UK), Y. Hezaveh (Stanford University, Stanford, California, USA), K. Lacaille (Dalhousie University, Halifax, Canada; McMaster University, Hamilton, Canada), K. C. Litke (Steward Observatory, University of Arizona, Tucson, Arizona, USA), J. Ma (University of Florida, Gainesville, Florida, USA), M. Malkan (University of California, Los Angeles, California, USA) , D. P. Marrone (Steward Observatory, University of Arizona, Tucson, Arizona, USA), W. Morningstar (Stanford University, Stanford, California, USA), E. J. Murphy (National Radio Astronomy Observatory, Charlottesville, Virginia, USA), D. Narayanan (University of Florida, Gainesville, Florida, USA), E. Pass (Dalhousie University, Halifax, Canada), University of Waterloo, Waterloo, Canada), R. Perry (Dalhousie University, Halifax, Canada), K. A. Phadke (University of Illinois, Urbana, Illinois, USA), K. M. Rotermund (Dalhousie University, Halifax, Canada), J. Simpson (University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh; Durham University, Durham, UK), J. S. Spilker (Steward Observatory, University of Arizona, Tucson, Arizona, USA), J. Sreevani (University of Illinois, Urbana, Illinois, USA), A. A. Stark (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA), M. L. Strandet (Max-Planck-Institut für Radioastronomie, Bonn, Germany) and A. L. Strom (Observatories of The Carnegie Institution for Science, Pasadena, California, USA).


The Oteo team is composed of: I. Oteo (Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK; ESO, Garching, Germany), R. J. Ivison (ESO, Garching, Germany; Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK), L. Dunne (Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK; Cardiff University, Cardiff, UK), A. Manilla-Robles (ESO, Garching, Germany; University of Canterbury, Christchurch, New Zealand), S. Maddox (Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK; Cardiff University, Cardiff, UK), A. J. R. Lewis (Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK), G. de Zotti (INAF-Osservatorio Astronomico di Padova, Padova, Italy), M. Bremer (University of Bristol, Tyndall Avenue, Bristol, UK), D. L. Clements (Imperial College, London, UK), A. Cooray (University of California, Irvine, California, USA), H. Dannerbauer (Instituto de Astrofíısica de Canarias, La Laguna, Tenerife, Spain; Universidad de La Laguna, Dpto. Astrofísica, La Laguna, Tenerife, Spain), S. Eales (Cardiff University, Cardiff, UK), J. Greenslade (Imperial College, London, UK), A. Omont (CNRS, Institut d’Astrophysique de Paris, Paris, France; UPMC Univ. Paris 06, Paris, France), I. Perez–Fournón (University of California, Irvine, California, USA; Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain), D. Riechers (Cornell University, Space Sciences Building, Ithaca, New York, USA), D. Scott (University of British Columbia, Vancouver, Canada), P. van der Werf (Leiden Observatory, Leiden University, Leiden, The Netherlands), A. Weiß (Max-Planck-Institut für Radioastronomie, Bonn, Germany) and Z-Y. Zhang (Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK; ESO, Garching, Germany).


ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 15 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a strategic partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.




Links



Contact:

Axel Weiss
Max-Planck-Institut für Radioastronomie
Bonn, Germany
Tel: +49 228 525 273
Email: aweiss@mpifr-bonn.mpg.de

Carlos de Breuck
ESO
Garching, Germany
Tel: +49 89 3200 6613
Email: cdebreuc@eso.org

Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Source: ESO/News

Thursday, May 26, 2016

A Young Mammoth Cluster of Galaxies Sighted in the Early Universe

The newly discovered protocluster of galaxies located in the Bootes field of the NOAO Deep Wide-field Survey.. Green circles identify the confirmed cluster members. Density contours (white lines) emphasize the concentration of member galaxies toward the center of the image. The patch of sky shown is roughly 20 arcminutes x 17 arcminutes in size. The cluster galaxies are typically very faint, about 10 million times fainter than the faintest stars visible to the naked eye on a dark night. The inset images highlight two example members that glow in the Ly-alpha line of atomic hydrogen. The protocluster is massive, with its core weighing as much as a quadrillion suns. The protocluster is likely to evolve, over 12 billion years, into a system much like the nearby Coma cluster of galaxies, shown in the image below. Credit: Dr. Rui Xue, Purdue University. Hi-res image

Coma Cluster image from the Sloan Digital Sky Survey
Credit: Dustin Lang and SDSS Collaboration 


Astronomers have uncovered evidence for a vast collection of young galaxies 12 billion light years away. The newly discovered “proto-cluster” of galaxies, observed when the universe was only 1.7 billion years old (12% of its present age), is one of the most massive structures known at that distance. The discovery, made using telescopes at Kitt Peak National Observatory in Arizona and the W. M. Keck Observatory on Mauna Kea, has been reported in the Astrophysical Journal.

“The protocluster will very likely grow into a massive cluster of galaxies like the Coma cluster, which weighs more than a quadrillion suns,” said Purdue University astrophysicist Dr. Kyoung-Soo Lee, who initially spotted the protocluster and is one of the authors in this study. Clusters this massive are extremely rare: only a handful of candidates are known at such early times. The new system is the first to be confirmed using extensive spectroscopy to establish cluster membership.

The team, led by Dr. Lee (Purdue University) and Dr. Arjun Dey of the National Optical Astronomy Observatory, used the Mayall telescope on Kitt Peak to obtain very deep images of a small patch of sky, about the size of two full moons, in the constellation of Bootes. The team then used the Keck II Telescope on Mauna Kea to measure distances to faint galaxies in this patch, which revealed the large grouping. “Many of the faint galaxies in this patch lie at the same distance,” say Dr. Dey. “They are clumped together due to gravity and the evidence suggests that the cluster is in the process of forming.”

Matter in the universe organizes itself into large structures through the action of gravity. Most stars are in galaxies, which in turn collect in groups and clusters. Galaxy clusters are commonly observed in the present-day universe and contain some of the oldest and most massive galaxies known. The formation and early history of these clusters is not well understood. The discovery of young proto-clusters allows scientists to directly witness and study their formation. The prevalence of massive clusters in the young universe can help constrain the size and expansion history of the universe.

The team is now searching larger areas of sky to uncover more examples of such young and massive protoclusters. “The discovery and confirmation of one distant and very massive protocluster is very exciting,” said Dr. Naveen Reddy, an astrophysicist at the University of California at Riverside and a coauthor of the study, “but it is important to find a large sample of these so we can understand the possibly varied formation history of the population as a whole.”

The other members of the team are Dr. Michael Cooper (University of California, Irvine), Dr. Hanae Inami (Observatoire de Lyon), Dr. Sungryong Hong (University of Texas, Austin), Dr. Anthony Gonzalez (University of Florida), and Dr. Buell Jannuzi (University of Arizona).

Reference:Spectroscopic Confirmation of a Protocluster at z=3.786,” Arjun Dey, Kyoung-Soo Lee, Naveen Reddy et al., 2016 May 20, Astrophysical Journal

preprint: http://arxiv.org/abs/1604.08627

Kitt Peak National Observatory and the National Optical Astronomy Observatory are operated by the Association of Universities for Research in Astronomy under a Cooperative Agreement with the National Science Foundation. The W. M. Keck Observatory is a scientific partnership between the National Aeronautics and Space Administration, the California Institute of Technology and the University of California, and made possible by the generous financial support of the W. M. Keck Foundation. The research was funded by the National Aeronautics and Space Administration and by NOAO.


Media Contact:

Dr. Joan Najita
National Optical Astronomy Observatory
950 N Cherry Ave
Tucson AZ 85719 USA
+1 520-318-8416
E-mail: najita@noao.edu


Science Contacts

Dr. Kyoung-Soo Lee
Purdue University
Tel: 765-494-3047
email: soolee@purdue.edu

Dr. Arjun Dey
National Optical Astronomy Observatory
Tel: 520-318-8429
email: dey@noao.edu


Monday, April 27, 2015

Astronomers Find New Details about Star Formation in Ancient Galaxy Protoclusters

Figure 1: Pseudo-color composite image of PKS 1138-262 region, derived from Hubble Space Telescope's ACS/WFC data archive (F814W and F475W). This region is one of the target protoclusters observed by MOIRCS on Subaru Telescope. (Credit: NAOJ/HST)

Figure 2: Mass-growth history expected of massive cluster of galaxies that have about 10^15 solar masses present day. Red spots are from this study. Black and grayer are for other massive cluster of galaxies, studied by the author's team and other research teams, respectively. (Credit: NAOJ)

Figure 3: Plot of stellar mass of the galaxies versus metallicity of gas in them. Gray and pale blue curves are for the present-day (nearby) galaxies and field galaxies at 11 billion years ago, respectively. Red is the current study about the proto-cluster of galaxies. The galaxies in the proto-clusters clearly show higher metallicity compared with the ones in the general fields at about the same time of the history in the universe. (Credit: NAOJ)


Figure 4: Illustrations of the metal enrichment processes (chemical evolution) in the field galaxies and the proto-cluster galaxies. Left (figure 4a) is for the galaxies in the general fields while the middle and the right (figures 4b and 4c, respectively) show their model that explain the unique enrichment processes of the heavy elements in the galaxies in the proto-clusters. (Credit: NAOJ)


Ongoing studies of distant galaxy protoclusters using the Multi-Object Infrared Camera and Spectrograph (MOIRCS) instrument on the Subaru Telescope is giving astronomers a closer look at the characteristics of star-forming regions in galaxies in the early universe. A team of astronomers from the National Astronomical Observatory of Japan (NAOJ) and SOKENDAI (Graduate University of Advanced Studies, Japan) are tracking velocity structures and gaseous metallicities in galaxies in two protoclusters located in the direction of the constellation Serpens. These appear around the radio galaxies PKS 1138-262 (at a redshift of 2.2, Figure 1) and USS 1558-003 (at a redshift of 2.5). The clusters appear as they would have looked 11 billion years ago, and the team concluded that they are in the process of cluster formation that has led to present-day galaxy clusters.

The MOIRCS near-infrared spectrograph is very effective for studies focused on the distant, early universe because strong emission lines from star-forming galaxies are redshifted from the optical to the near-infrared regime. This gives astronomers unique insights into these activities. (Note 1)

Based on the MOIRCS data, the team estimated that both protoclusters have a weight of about 10^14 solar masses (Figure 2). These follow the typical mass growth history of the today's most massive clusters, such as the 'Coma Cluster.' That makes the two protoclusters ideal laboratories for exploring early phasesof galaxy formation in a unique clustered environment.

The metallicity of the gases in the protocluster galaxies was studied using multiple spectral lines emitted from them. The result shows their gaseous metallicities are chemically enriched compared with those of galaxies in the general fields (Figure 3). Metals (elements heavier than hydrogen and helium) are created in the interiors of stars as they evolve and then released into surrounding gas through supernova explosions or stellar winds (often referred to as chemical evolution; Figure 4a).

The difference in gaseous metallicity between protoclusters and general fields suggests that star-formation histories and/or gas inflow/outflow processes should be different in the protocluster regions. The result also suggests that galaxy formation has already been influenced by environmental conditions in the era that star-formation activities are the most active across the universe. This would be an early phase of strong environmental effects seen in the present galaxy clusters.

In order to explain the metallicity excess in the protoclusters, the team members focused attention on the environmental effects of inflow and outflow mechanism on the galaxy formation process. Recent works report that inflow and outflow activities were most significant eleven billion years ago (at redshift ~2), and were about a hundred times more active relative to those in the local universe.

Clusters of galaxies are large self-gravitating systems in which galaxies and ionized gas are bound by massive amounts of dark matter. In such unique, dense environments, galaxies move at a speed of about 1000 kilometers per second. Due to this high speed, the galaxies are exposed to high pressure from intercluster medium. As a result, the outer regions with relatively poor metallicity are stripped. It is like the strong air resistance of air a bicycle rider experiences. In this case, the gaseous metallicities become higher because the chemical enrichment process takes place mainly in metal-rich central regions (Figure 4b). Another possibility is that the surrounding high-pressure, inter-cluster medium prevents outflowing gas from escaping from the galaxies (Figure 4c). This also results in higher gaseous metallicities of the cluster galaxies.

The research team concludes that the metallicity excess in the protocluster regions results from unique phenomena occurring in the cluster environment. The PI of this research, Mr. Rhythm Shimakawa of NAOJ and SOKENDAI (Note 2), is determined to continue studying the detailed physical properties of individual forming galaxies in the protoclusters to find clear evidence that proves this hypothesis.

This article is based on results from two research papers published in the Monthly Notices of the Royal Astronomical Society:

Rhythm Shimakawa, Tadayuki Kodama, Ken-ichi Tadaki, Ichi Tanaka, Masao Hayashi and Yusei Koyama, "Identification of the progenitors of rich clusters and member galaxies in rapid formation at z > 2", Volume 441, Issue 1, p.L1-L5, published in June 11, 2014,
and
Rhythm Shimakawa, Tadayuki Kodama, Ken-ichi Tadaki, Masao Hayashi, Yusei Koyama, Ichi Tanaka "An early phase of environmental effects on galaxy properties unveiled by near-infrared spectroscopy of protocluster galaxies at z>2", Volume 448, Issue 1, p.666-680, published in March 21, 2015.

This research is supported in part by a Grant-in-Aid for the Scientific Research (Nos. 21340045 and 24244015) by the Japanese Ministry of Education, Culture, Sports, Science and Technology.

Authors:

  • Rhythm Shimakawa (Subaru Telescope, National Astronomical Observatory of Japan [NAOJ]/SOKENDAI(Graduate University for Advanced Studies))
  • Tadayuki Kodama (Optical and Infrared Astronomy Division, NAOJ/SOKENDAI)
  • Ichi Tanaka (Subaru Telescope, NAOJ)
  • Kenichi Tadaki (Max-Planck-Institute fur Extraterrestrische Physic, Germany)
  • Masao Hayashi (Optical and Infrared Astronomy Division, NAOJ)
  • Yusei Koyama (Institute of Space Astronomical Science, Japan Aerospace Exploration Agency)

Notes:

  1. See the Web release by M. Hayashi, "Discovery of an Ancient Celestial City Undergoing Rapid Growth: A Young Protocluster of Active Star-Forming Galaxies".
  2. Mr. Rhythm Shimakawa received the first "SOKENDAI Future Scientist Award" for his research "Environmental effects on galaxy formation: When and how did spiral and elliptical galaxies diverge?", which includes the two papers referred in this article.

Saturday, May 05, 2012

Subaru Telescope Discovers the Most Distant Protocluster of Galaxies

Using the Subaru Telescope, a team of astronomers led by Jun Toshikawa (The Graduate University for Advanced Studies, Japan), Dr. Nobunari Kashikawa (National Astronomical Observatory of Japan), and Dr. Kazuaki Ota (Kyoto University) has discovered the most distant protocluster of galaxies ever found--one that existed less than one billion years after the Big Bang. Since protoclusters are ancestors of today's massive clusters of galaxies, this discovery of a protocluster in the early Universe advances our understanding of how large-scale structures form and how galaxies evolve.

The nearby or "local" universe, an area that extends about 380 million light-years away from Earth, contains many galaxy clusters, i.e., gravitationally bound groups of about 100 to more than 1000 galaxies. These clusters are connected with each other and make up a huge network of galaxies called the "large-scale structure" of the Universe. Such configurations raise fundamental questions: When and how did these structures form in the history of the Universe?

Astronomers think that the Universe started out as an almost homogeneous mass that spread uniformly. Small fluctuations in the initial mass distribution increased by gravity over the 13.7 billion years of the Universe's age and produced the recent array of clusters. Because clusters contain a larger number of old and massive galaxies than those found in isolated galaxies, astronomers speculate that developing clusters may significantly affect the evolution of their member galaxies. Therefore, understanding the details of cluster formation (Note 1) is an essential step in addressing key issues of structure formation and galaxy evolution. A necessary part of this process is an investigation of all stages of cluster formation from beginning to end, which is why the current team gave particular emphasis to studying the birth of clusters.

The team focused on this phase of cluster formation by searching very distant galaxies that existed in the early Universe. Such observations present challenges for a couple of reasons. First, the light from more distant galaxies is faint and difficult to detect. Second, protoclusters in the early Universe are rare. The use of the Subaru Telescope allowed the team to overcome these difficulties. The telescope not only has an 8.2 m primary mirror with large light-gathering power but also offers the advantage of the Subaru Prime Focus Camera (Suprime-Cam) with a wide-field imaging capability. These features are particularly beneficial for discovering faint and rare objects in the distant Universe.

The team chose to observe the Subaru Deep Field, a 0.25 square-degree-wide field in the northern sky near the constellation Coma Barenices. The Subaru Deep Field is one of the most suitable regions for finding protoclusters in the early Universe; the area is not only deep and wide but has been intensively observed with the Subaru Telescope, which has detected very faint galaxies. When the team searched for distant galaxies in the Subaru Deep Field and investigated their distribution, they found a region with a surface number density five times greater than the average (Fig. 1).

Figure 1: Left panel: The distribution of galaxies 12.7 billion years ago. White circles represent the galaxies, and their size represents the luminosity of the galaxies. The color contour represents the density; the redder the color, the higher the density. The reddest region appears in the lower part of the figure. (Credit: NAOJ). Right panel: An enlargement from the map that shows the area around the cluster. (Credit: NAOJ)

The astronomers then used Subaru's Faint Object Camera and Spectrograph (FOCAS) to conduct a spectroscopic observation, which confirmed that most of the galaxies located in the highly dense region lay in a narrow area in the line-of-sight. This concentration of galaxies could not be explained by chance. On the basis of their observations with the Subaru Telescope, the team confirmed the existence of a protocluster 12.72 billion years ago (Fig. 2)--the most distant protocluster found with its distance established by spectroscopic observations (Note 2). The astronomers were able to directly observe this cluster of galaxies at an early stage in galaxy evolution, when structures were beginning to form in the early Universe. This discovery will be an important step on the way to understanding structure formation and galaxy evolution.

Figure 2: A close-up of the central region of the protocluster. Objects circled in red are galaxies 12.7 billion light-years away. (Credit: NAOJ) .

Although the team also investigated the properties of the galaxies in the protocluster (Note 3), they did not find a significant difference between the protocluster galaxies and other galaxies in the field. The astronomers speculate that the characteristic features of cluster galaxies in the nearby Universe occurred in later stages of cluster development, not during their birth (Note 4). Close examination of the internal structure of the protocluster showed that it could consist of subgroups of galaxies, merging together to form a more massive cluster (Note 5).

The team will continue their research with the Subaru Telescope's forthcoming Hyper-Suprime Camera (HSC), which has an imaging capability with a field of view seven times wider than Suprime-Cam. The astronomers expect to use HSC to reveal how many protoclusters existed in the early Universe and to provide a better picture of protoclusters in general. Toshikawa summarized the team's intent: "By continually working to find such distant protoclusters, we can understand cluster formation more clearly."


Reference:

These results were published in the May 1, 2012, edition of the Astrophysical Journal. This research was supported by The Japan Society for the Promotion of Science through Grant-in-Aid for Scientific Research 23340050. The authors of the paper are:
Jun Toshikawa, The Graduate University for Advanced Studies, Japan
Nobunari Kashikawa, National Astronomical Observatory of Japan
Kazuaki Ota, Kyoto University
Tomoki Morokuma, University of Tokyo
Takatoshi Shibuya, The Graduate University for Advanced Studies, Japan
Masao Hayashi, National Astronomical Observatory of Japan
Tohru Nagao, associate professor, Kyoto University
Linhua Jiang, University of Arizona
Matthew A. Malkan, University of California
Eiichi Egami, University of Arizona
Kazuhiro Shimasaku, University of Tokyo
Kentaro Motohara, University of Tokyo
Yoshifumi Ishizaki, The Graduate University for Advanced Studies, Japan


Notes:

1. Protoclusters more than 12.7 billion light-years away also closely relate to issues of cosmic reionization.

2. Prior to this research, Ouchi et al. used the Subaru Telescope and discovered the most distant protocluster ever found in 2005. In 2012 Trenti et al. found a protocluster candidate beyond 12.7 billion light-years ago, but spectroscopic observations have not confirmed their distances.

3. This research investigated luminosities and star formation rates.

4. Investigation of other properties such as mass, age and color is necessary to conclude whether the properties of protoclusters are different from those of field galaxies.

5. The interesting structure in Fig.1 is elongated toward the upper left of the protocluster. A large-scale structure may begin to form in this early Universe. Only the Subaru Telescope could find this large feature.