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  • richardmitnick 12:32 pm on March 13, 2018 Permalink | Reply
    Tags: , , , , Double or Nothing: Astronomers Rethink Quasar Environment, , , Searching for Protoclusters,   

    From NAOJ: “Double or Nothing: Astronomers Rethink Quasar Environment” 

    NAOJ

    NAOJ

    March 12, 2018
    No writer credit

    Using Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope, astronomers have identified nearly 200 “protoclusters,” the progenitors of galaxy clusters, in the early Universe, about 12 billion years ago, about ten times more than previously known.

    NAOJ Subaru Hyper Suprime-Cam

    NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level

    They also found that quasars don’t tend to reside in protoclusters; but if there is one quasar in a protocluster, there is likely a second nearby. This result raises doubts about the relation between protoclusters and quasars.

    In the Universe, galaxies are not distributed uniformly. There are some places, known as clusters, where dozens or hundreds of galaxies are found close together. Other galaxies are isolated. To determine how and why clusters formed, it is critical to investigate not only mature galaxy clusters as seen in the present Universe but also observe protoclusters, galaxy clusters in the process of forming.

    Because the speed of light is finite, observing distant objects allows us to look back in time. For example, the light from an object 1 billion light-years away was actually emitted 1 billion years ago and has spent the time since then traveling through space to reach us. By observing this light, astronomers can see an image of how the Universe looked when that light was emitted.

    Even when observing the distant (early) Universe, protoclusters are rare and difficult to discover. Only about 20 were previously known. Because distant protoclusters are difficult to observe directly, quasars are sometimes used as a proxy. When a large volume of gas falls towards the super massive black hole in the center of a galaxy, it collides with other gas and is heated to extreme temperatures. This hot gas shines brightly and is known as a quasar. The thought was that when many galaxies are close together, a merger, two galaxies colliding and melding together, would create instabilities and cause gas to fall into the super massive black hole in one of the galaxies, creating a quasar. However, this relationship was not confirmed observationally due to the rarity of both quasars and protoclusters.

    In order to understand protoclusters in the distant Universe a larger observational sample was needed. A team including astronomers from the National Astronomical Observatory of Japan, the University of Tokyo, the Graduate University for Advanced Studies, and other institutes is now conducting an unprecedented wide-field systematic survey of protoclusters using the Subaru Telescope’s very wide-field camera, Hyper Suprime-Cam (HSC). By analyzing the data from this survey, the team has already identified nearly 200 regions where galaxies are gathering together to form protoclusters in the early Universe 12 billion years ago.

    1
    Figure 1: Galaxy distribution and close-ups of some protoclusters revealed by HSC. Higher- and lower-density regions are represented by redder and bluer colors, respectively. In the close-ups, white circles indicate the positions of distant galaxies. The red regions are expected to evolve into galaxy clusters. From the close-ups, we can see various morphologies of the overdense regions: some have another neighboring overdense region, or are elongated like a filament, while there are also isolated overdense regions. (Credit: NAOJ)

    The team also addressed the relationship between protoclusters and quasars. The team sampled 151 luminous quasars at the same epoch as the HSC protoclusters and to their surprise found that most of those quasars are not close to the overdense regions of galaxies. In fact, their most luminous quasars even avoid the densest regions of galaxies. These results suggest that quasars are not a good proxy for protoclusters and more importantly, mechanisms other than galactic mergers may be needed to explain quasar activity. Furthermore, since they did not find many galaxies near the brightest quasars, that could mean that hard radiation from a quasar suppresses galaxy formation in its vicinity.

    On the other hand, the team found two “pairs” of quasars residing in protoclusters. Quasars are rare and pairs of them are even rarer. The fact that both pairs were associated with protoclusters suggests that quasar activity is perhaps synchronous in protocluster environments. “We have succeeded in discovering a number of protoclusters in the distant Universe for the first time and have witnessed the diversity of the quasar environments thanks to our wide-and-deep observations with HSC,” says the team’s leader Nobunari Kashikawa (NAOJ).

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    Figure 2: The two quasar pairs and surrounding galaxies. Stars indicate quasars and bright (faint) galaxies at the same epoch are shown as circles (dots). The galaxy overdensity with respect to the average density is shown by the contour. The pair members are associated with high density regions of galaxies. (Credit: NAOJ)

    “HSC observations have enabled us to systematically study protoclusters for the first time.” says Jun Toshikawa, lead author of the a paper reporting the discovery of the HSC protoclusters, “The HSC protoclusters will steadily increase as the survey proceeds. Thousands of protoclusters located 12 billion light-years away will be found by the time the observations finish. With those new observations we will clarify the growth history of protoclusters.”

    These results were published on January 1, 2018 in the HSC special issue of the Publications of the Astronomical Society of Japan (Toshikawa et al. 2018, GOLDRUSH. III. A Systematic Search of Protoclusters at z~4 Based on the >100 deg2 Area, PASJ, 70, S12; Uchiyama et al. 2018, Luminous Quasars Do Not Live in the Most Overdense Regions of Galaxies at z~4, PASJ, 70, S32; Onoue et al. 2018, Enhancement of Galaxy Overdensity around Quasar Pairs at z<3.6 based on the Hyper Suprime-Cam Subaru Strategic Program Survey, PASJ, 70, S31). These projects are supported by Grants-In-Aid JP15H03645, JP15K17617, and JP15J02115.

    See the full article here .

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    The National Astronomical Observatory of Japan (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations – the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude Observatory of Mizusawa, and a part of Research Institute of Atmospherics of Nagoya University.

    In the 2004 reform of national research organizations, NAOJ became a division of the National Institutes of Natural Sciences.

    NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level


    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array
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    Solar Flare Telescope

    Nobeyama Radio Telescope - Copy
    Nobeyama Radio Observatory

    Nobeyama Solar Radio Telescope Array
    Nobeyama Radio Observatory: Solar

    Misuzawa Station Japan
    Mizusawa VERA Observatory

    NAOJ Okayama Astrophysical Observatory Telescope
    Okayama Astrophysical Observatory

    The National Astronomical Observatory of Japan (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations – the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude Observatory of Mizusawa, and a part of Research Institute of Atmospherics of Nagoya University.

     
  • richardmitnick 10:38 am on December 14, 2016 Permalink | Reply
    Tags: , , , , , Searching for Protoclusters   

    From astrobites: “Searching for Protoclusters” 

    Astrobites bloc

    Astrobites

    Dec 14, 2016
    Christopher Lovell

    Title: ANCIENT LIGHT FROM YOUNG COSMIC CITIES: PHYSICAL AND OBSERVATIONAL SIGNATURES OF GALAXY PROTO-CLUSTERS
    Authors: Yi-Kuan Chiang, Roderik Overzier, and Karl Gebhardt
    First authors department: Department of Astronomy, University of Texas at Austin, USA
    Paper status: Published in the Astrophysical Journal December 2013 [open access]

    Galaxy clusters are the most massive objects bound by gravity in the universe. They consist of a ‘halo’ of dark matter typically containing hundreds, sometimes thousands, of galaxies. Understanding how they form and how the properties of their galaxies change during this formation is of interest to both astrophysicists and cosmologists – such massive, extreme objects tend to form extreme galaxies, and can also help us to constrain cosmological models.

    What are protoclusters?

    Clusters are formed when dense regions of matter on large scales collapse due to their mutual gravitation. At the end of this collapse we say the cluster is ‘virialised’, which basically means it has collapsed to a stable point. Before this moment of virialisation the matter that will end up in the cluster tends to be spread out and diffuse. This nebulous entity of things that will end up in a cluster, but has not yet collapsed, is typically called a protocluster.

    1
    The newly discovered protocluster of galaxies located in the Boötes field of the NOAO Deep Wide-field Survey. Green circles identify the confirmed cluster members. Density contours (white lines) emphasise the concentration of member galaxies toward the centre 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 Lyman-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. Image credit: Dr. Rui Xue, Purdue University.

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    Figure 1: The distribution of clusters in the present day universe in the Millennium simulation. The colours distinguish between different mass clusters, labelled by their analogues in the local universe – Coma, Virgo and Fornax, in descending order of mass.

    Today’s Astrobite looks at a paper from 2013 that uses simulations to better understand the properties of protoclusters, and how they can be identified in observations. Simulations offer a unique way of looking at protoclusters since we can identify the clusters at the present time in the simulation, and explicitly follow the constituent galaxies and dark matter back in time to see what it looks like earlier in the simulation. We can then infer what protoclusters should look like in observations of the early universe. This is exactly what Chiang and coauthors do. They use the Millennium Simulation, a large dark matter only simulation coupled with models of how galaxies form and evolve. Figure 1 shows the clusters identified in the Millennium Simulation at the present day.

    Protocluster properties

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    Figure 2: Galaxy overdensity against redshift (equivalent to time, z=5 being further back in time than z=2). The colours distinguish between different descendant masses: Coma, Virgo and Fornax-like cluster masses are in red, green and blue respectively. The grey line shows random regions. Each line is mostly separate, indicating that protoclusters that evolve into different mass clusters can be distinguished by their galaxy overdensity.

    The authors analyse three properties of the protoclusters: their distributions of dark matter density, dark matter halos, and galaxies. All three can be tracked through time in the simulation. They find that protoclusters are overdense in all three of these properties – there is more mass, halos and galaxies in protoclusters than in field regions (regions that don’t turn into clusters). They also find that protoclusters are very spread out at early times, with up to a thousand times greater volume than in their collapsed state.

    Unfortunately, the matter density and dark matter halos are not explicitly observable, only the galaxies are, so we must rely on the galaxy properties to tell us whether a given region is a protocluster or not, and what the mass of the descendant cluster will be. There is a whole zoo of different types of Galaxies, which makes them fascinating to study, but a pain to observe in an unbiased way. The authors account for this by selecting galaxies with a range of different criteria, and test how their results are affected.

    4
    Figure 3: Descendant cluster mass against protocluster galaxy overdensity. There is a strong positive correlation between the two at a range of redshifts.

    In figure 2, an overdensity of zero represents a mean density of galaxies, negative values are underdense regions, and positive values are regions with more galaxies than normal. The grey line shows the distribution for random regions, whereas the coloured lines show protoclusters. Protocluster regions are clearly distinguishable from these random regions, and higher overdensities of galaxies tend to form higher mass (> 1015 Msol) clusters at all redshifts.

    Figure 3 shows the mass of the descendant cluster plotted against the galaxy overdensity in the protocluster. As in figure 2 there is a positive correlation – higher galaxy overdensities turn into bigger clusters. The authors use this relationship to create a fitting function that can be applied to observations – if you see a galaxy overdensity of a particular value, you can make a prediction for the mass of the cluster it will eventually form. Finally, the authors use this relationship and apply it to a few of the most massive protocluster candidates that had been identified at the time (~2013), predicting their present day masses.

    Since this paper was published many more overdensities of galaxies have been discovered in the early universe, and the first catalogues of protoclusters have been compiled. The field is still young, but simulations represent our best tool to predict their subsequent evolution.

    See the full article here .

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    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
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    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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