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  • richardmitnick 9:33 am on October 9, 2017 Permalink | Reply
    Tags: , , Baryons, , , , , , Sunyaev-Zel’dovich effect   

    From New Scientist: “Half the universe’s missing matter has just been finally found” 


    New Scientist

    9 October 2017
    Leah Crane

    Discoveries seem to back up many of our ideas about how the universe got its large-scale structure
    Andrey Kravtsov (The University of Chicago) and Anatoly Klypin (New Mexico State University). Visualisation by Andrey Kravtsov

    The missing links between galaxies have finally been found. This is the first detection of the roughly half of the normal matter in our universe – protons, neutrons and electrons – unaccounted for by previous observations of stars, galaxies and other bright objects in space.

    Two separate teams found the missing matter – made of particles called baryons rather than dark matter – linking galaxies together through filaments of hot, diffuse gas.

    “The missing baryon problem is solved,” says Hideki Tanimura at the Institute of Space Astrophysics in Orsay, France, leader of one of the groups. The other team was led by Anna de Graaff at the University of Edinburgh, UK.

    Because the gas is so tenuous and not quite hot enough for X-ray telescopes to pick up, nobody had been able to see it before.

    “There’s no sweet spot – no sweet instrument that we’ve invented yet that can directly observe this gas,” says Richard Ellis at University College London. “It’s been purely speculation until now.”

    So the two groups had to find another way to definitively show that these threads of gas are really there.

    Both teams took advantage of a phenomenon called the Sunyaev-Zel’dovich effect that occurs when light left over from the big bang passes through hot gas. As the light travels, some of it scatters off the electrons in the gas, leaving a dim patch in the cosmic microwave background [CMB] – our snapshot of the remnants from the birth of the cosmos.

    CMB per ESA/Planck


    Stack ‘em up

    In 2015, the Planck satellite created a map of this effect throughout the observable universe. Because the tendrils of gas between galaxies are so diffuse, the dim blotches they cause are far too slight to be seen directly on Planck’s map.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Both teams selected pairs of galaxies from the Sloan Digital Sky Survey that were expected to be connected by a strand of baryons. They stacked the Planck signals for the areas between the galaxies, making the individually faint strands detectable en masse.

    Tanimura’s team stacked data on 260,000 pairs of galaxies, and de Graaff’s group used over a million pairs. Both teams found definitive evidence of gas filaments between the galaxies. Tanimura’s group found they were almost three times denser than the mean for normal matter in the universe, and de Graaf’s group found they were six times denser – confirmation that the gas in these areas is dense enough to form filaments.

    “We expect some differences because we are looking at filaments at different distances,” says Tanimura. “If this factor is included, our findings are very consistent with the other group.”

    Finally finding the extra baryons that have been predicted by decades of simulations validates some of our assumptions about the universe.

    “Everybody sort of knows that it has to be there, but this is the first time that somebody – two different groups, no less – has come up with a definitive detection,” says Ralph Kraft at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

    “This goes a long way toward showing that many of our ideas of how galaxies form and how structures form over the history of the universe are pretty much correct,” he says.

    Journal references: arXiv, 1709.05024
    A Search for Warm/Hot Gas Filaments Between Pairs of SDSS Luminous Red Galaxies

    and 1709.10378v1
    Missing baryons in the cosmic web revealed by the Sunyaev-Zel’dovich effect

    See the full article here .

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  • richardmitnick 3:17 pm on March 9, 2017 Permalink | Reply
    Tags: , , , , , , Intra-cluster medium (ICM), Same ol’ same ol’? Galaxy Clusters across Cosmic Time, Sunyaev-Zel’dovich effect   

    From astrobites: “Same ol’ same ol’? Galaxy Clusters across Cosmic Time” 

    Astrobites bloc


    Mar 9, 2017
    Gourav Khullar

    Title: The Remarkable Similarity of Massive Galaxy Clusters from z~0 to z~1.9
    Authors: Michael McDonald, Steve W. Allen, Matt Bayliss et al.
    First Author’s Institution: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, USA
    Status: Submitted to The Astrophysical Journal [open access]

    Introducing…Galaxy Clusters and X-rays!

    We have come a long way since the 1930s, when the words ‘galaxy cluster‘ were posited for the first time by Fritz Zwicky, in relation with the presence of dark matter in the Coma cluster.
    [Please do not forget the contributions of Vera Rubin.]

    Coma Cluster via Hubble

    Developments in multi-wavelength astrophysics have allowed us to probe different components of a cluster with different telescopes. For example, star-forming galaxies of galaxy clusters are observed using optical telescopes because starlight in these galaxies loves emitting photons with the roughly the same energy that we see from the sun. Some of these galaxies are super-red, have no star-formation and a ton of dust, that is best seen from infra-red and radio telescopes. Today’s story takes us to the intermittent space between different galaxies inside a cluster, called the intra-cluster medium (ICM) and its emissions.

    The ICM of a galaxy cluster is filled with gas or plasma that comprise free electrons and protons. This medium reaches temperatures of the order of 10^7 to 10^8 K, that emits light in the form of X-rays. This happens due to a phenomena called free-free emission of electrons, or Bremsstrahlung. X-ray observations of galaxy clusters are a crucial element in understanding how the cluster gas evolves with time, and how they influence the formation and evolution of massive galaxies in clusters. Moreover, the effect of active galactic nuclei (AGN) that heat up cluster environments after firing up from individual member galaxies can also be analyzed through X-ray studies, using telescopes like XMM-Newton and Chandra.



    Looking for Distant Galaxy Clusters

    Fig 1. Picture of an SPT Cosmic Microwave Background [CMB] map (created by Bradford Benson at Fermilab and University of Chicago, USA). This is a small patch of 50 sq. degrees with CMB anisotropies seen clearly. Small bright point sources are dusty galaxies that come out in these maps. Similarly, the dark spots are shadows on the CMB caused by inverse-comptonization of CMB photons by galaxy clusters.

    This is easier said than done. We know a lot about close-by galaxy clusters by pointing an X-ray telescope to the sky, but finding X-ray emitting clusters that are extremely far-away is a tough job. This was made easy by the advent of sub-mm (or Cosmic microwave background (CMB)) telescopes, like the South Pole Telescope (SPT) or Planck.

    CMB per Planck

    South Pole Telescope (SPT)


    These telescopes discover galaxy clusters that cast a shadow on the background CMB, by a phenomena called the Sunyaev-Z’eldovich effect (look at this bite for details!) This makes cluster detection in CMB telescopes a distance (or redshift) independent activity, that gives us a better look at far away clusters.

    Let’s make this slightly easier on us. If I was to summarize my chain of thought in the last two paragraphs, I would do so with the following steps:

    Step 1: Study the CMB and look for shadows in the maps. These shadows are galaxy clusters that are distorting the background CMB light.

    Step 2: Use an X-ray telescope to point to these shadows – you will see the X-ray ICM gas of these clusters.

    Step 3: Make a list of these clusters, and study the X-ray ICM gas as a function of their distance, or redshift.

    Step 4: Party.

    Today’s paper is exactly that!
    Evolution of the ICM

    Fig 2. Plotted here is Mass of Cluster vs redshift for the clusters considered in today’s paper. The orange background is an evolution map, incorporating the physics of galaxy cluster evolution. This implies that clusters at high redshift (the black stars) could very well be the ancestors of nearby clusters (the blue squares) which are much more massive and fall within the orange band. No image credit.

    McDonald et al. present the first ever X-ray analysis of 8 galaxy clusters (with masses of ~2 to 4 x 10^14 solar masses) at redshifts greater than 1.2 that were detected with the SPT telescope, that add to the thermodynamic studies done by the same collaboration for low-redshift (or nearby) clusters. This allows them to discuss the evolution of cluster ICM from redshift 1.9 to redshift 0 i.e. from a time when the universe was 3 billion years old, to now! What they are looking for are signs of similarity between far away and nearby clusters. Not just looking alike, but whether distant clusters are younger versions of the nearby clusters. We call this property self-similarity – young less massive clusters accrete matter, cool down and evolve into massive clusters.

    They find that centres of clusters, called cool cores, show no significant evolution in the density of the ICM gas when comparing far away clusters with close ones. As we go further out – about 20% of the ‘defined’ cluster radii – we see that far away and nearby clusters have self-similar densities. Based on their analysis, they propose a scenario where the cool cores formed at redshifts > 1.5 and their size, mass and density roughly remained constant. The rest of the cluster around them merrily continued accreting matter, and grew in their size and mass. This is possible if there is a gigantic AGN at the centre of these clusters, that is reheating all the cool gas that would have fallen to the centre of these clusters. This cooling and reheating seems to be tightly regulated, just like a thermostat on a fixed temperature. This explains the preservation of density around the cool cores, but not the rest of the massive cluster.

    Fig 3. (a) Absolute gas density as a function of radius for the 8 new galaxy clusters studied in today’s paper. At low radii i.e. near centre of clusters, there is considerable difference in the density profiles, with a big scatter. As one goes outwards, the outskirts of the clusters look remarkably self-similar. (b) A similar result is seen when comparing clusters across different redshifts (or epochs).


    This is huge. The work in today’s paper indicates that far-away clusters could very well be progenitors to nearby clusters, if given enough time to evolve into massive structures. The cluster centres seem to stand the test of time, unfazed by the chaos around them. This is irrespective of how disturbed or relaxed the shapes of these clusters are, or how many galaxies are merging into these clusters.

    Fig 4. Photon assymetry (A, a tracer of disturbance in the clusters) vs electron density in galaxy clusters considered in today’s paper. The black stars are the 8 new clusters added to the analysis sample. This plot shows that there is no bias in the new sample, and the clusters span the typical range of these numbers, distributed uniformly.

    A study like this makes us reach regimes where we can connect the physics of central cluster environments to its macro-surroundings, that we find so hard to replicate in hydrodynamical simulations at the moment. With the advent of new X-ray, CMB and optical telescopes, the precision with which we can make these claims only gets better!

    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.
    Why read Astrobites?

    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.

    • MIKE EYE 3:33 pm on March 9, 2017 Permalink | Reply

      Great inspiration for Sci-Fi novelists, thank you! And the findings seem to flow with the way Eye see it. Starlight pumps outward in waves right, and clusters can become entangled regardless of how many light years away they are from one another, is that generally correct on a basic layman’s level? Or am I totally off?


    • richardmitnick 3:42 pm on March 9, 2017 Permalink | Reply

      Sci-Fi and Science constantly feed each other. I am sure that the incandescent bulb and the cell phone were once science fiction.

      What you propose works if Quantum Mechanics works. The problem is that Quantum Mechanics is short on peer reviewed evidence.


  • richardmitnick 10:16 am on September 4, 2016 Permalink | Reply
    Tags: , , , Sunyaev-Zel’dovich effect   

    From particlebites: “The CMB sheds light on galaxy clusters: Observing the kSZ signal with ACT and BOSS” 

    particlebites bloc


    August 17, 2016 [Just brought to social media by astrobites.]
    Eve Vavagiakis

    Article: Detection of the pairwise kinematic Sunyaev-Zel’dovich effect with BOSS DR11 and the Atacama Cosmology Telescope
    Authors: F. De Bernardis, S. Aiola, E. M. Vavagiakis, M. D. Niemack, N. Battaglia, and the ACT Collaboration
    Reference: arXiv:1607.02139

    Editor’s note: this post is written by one of the students involved in the published result.

    Like X-rays shining through your body can inform you about your health, the cosmic microwave background (CMB) shining through galaxy clusters can tell us about the universe we live in.

    Cosmic Microwave Background per ESA/Planck
    CMB per ESA/Planck

    When light from the CMB is distorted by the high energy electrons present in galaxy clusters, it’s called the Sunyaev-Zel’dovich effect. A new 4.1σ measurement of the kinematic Sunyaev-Zel’dovich (kSZ) signal has been made from the most recent Atacama Cosmology Telescope (ACT) cosmic microwave background (CMB) maps and galaxy data from the Baryon Oscillation Spectroscopic Survey (BOSS).

    Princeton ACT
    Princeton ACT

    With steps forward like this one, the kinematic Sunyaev-Zel’dovich signal could become a probe of cosmology, astrophysics and particle physics alike.

    The Kinematic Sunyaev-Zel’dovich Effect

    It rolls right off the tongue, but what exactly is the kinematic Sunyaev-Zel’dovich signal? Galaxy clusters distort the cosmic microwave background before it reaches Earth, so we can learn about these clusters by looking at these CMB distortions. In our X-ray metaphor, the map of the CMB is the image of the X-ray of your arm, and the galaxy clusters are the bones. Galaxy clusters are the largest gravitationally bound structures we can observe, so they serve as important tools to learn more about our universe. In its essence, the Sunyaev-Zel’dovich effect is inverse-Compton scattering of cosmic microwave background photons off of the gas in these galaxy clusters, whereby the photons gain a “kick” in energy by interacting with the high energy electrons present in the clusters.

    The Sunyaev-Zel’dovich effect can be divided up into two categories: thermal and kinematic. The thermal Sunyaev-Zel’dovich (tSZ) effect is the spectral distortion of the cosmic microwave background in a characteristic manner due to the photons gaining, on average, energy from the hot (~107 – 108 K) gas of the galaxy clusters. The kinematic (or kinetic) Sunyaev-Zel’dovich (kSZ) effect is a second-order effect—about a factor of 10 smaller than the tSZ effect—that is caused by the motion of galaxy clusters with respect to the cosmic microwave background rest frame. If the CMB photons pass through galaxy clusters that are moving, they are Doppler shifted due to the cluster’s peculiar velocity (the velocity that cannot be explained by Hubble’s law, which states that objects recede from us at a speed proportional to their distance). The kinematic Sunyaev-Zel’dovich effect is the only known way to directly measure the peculiar velocities of objects at cosmological distances, and is thus a valuable source of information for cosmology. It allows us to probe megaparsec and gigaparsec scales – that’s around 30,000 times the diameter of the Milky Way!

    A schematic of the Sunyaev-Zel’dovich effect resulting in higher energy (or blue shifted) photons of the cosmic microwave background (CMB) when viewed through the hot gas present in galaxy clusters. Source: UChicago Astronomy.

    Measuring the kSZ Effect

    To make the measurement of the kinematic Sunyaev-Zel’dovich signal, the Atacama Cosmology Telescope (ACT) collaboration used a combination of cosmic microwave background maps from two years of observations by ACT. The CMB map used for the analysis overlapped with ~68000 galaxy sources from the Large Scale Structure (LSS) DR11 catalog of the Baryon Oscillation Spectroscopic Survey (BOSS). The catalog lists the coordinate positions of galaxies along with some of their properties. The most luminous of these galaxies were assumed to be located at the centers of galaxy clusters, so temperature signals from the CMB map were taken at the coordinates of these galaxy sources in order to extract the Sunyaev-Zel’dovich signal.

    While the smallness of the kSZ signal with respect to the tSZ signal and the noise level in current CMB maps poses an analysis challenge, there exist several approaches to extracting the kSZ signal. To make their measurement, the ACT collaboration employed a pairwise statistic. “Pairwise” refers to the momentum between pairs of galaxy clusters, and “statistic” indicates that a large sample is used to rule out the influence of unwanted effects.

    Here’s the approach: nearby galaxy clusters move towards each other on average, due to gravity. We can’t easily measure the three-dimensional momentum of clusters, but the average pairwise momentum can be estimated by using the line of sight component of the momentum, along with other information such as redshift and angular separations between clusters. The line of sight momentum is directly proportional to the measured kSZ signal: the microwave temperature fluctuation which is measured from the CMB map. We want to know if we’re measuring the kSZ signal when we look in the direction of galaxy clusters in the CMB map. Using the observed CMB temperature to find the line of sight momenta of galaxy clusters, we can estimate the mean pairwise momentum as a function of cluster separation distance, and check to see if we find that nearby galaxies are indeed falling towards each other. If so, we know that we’re observing the kSZ effect in action in the CMB map.

    For the measurement quoted in their paper, the ACT collaboration finds the average pairwise momentum as a function of galaxy cluster separation, and explores a variety of error determinations and sources of systematic error. The most conservative errors based on simulations give signal-to-noise estimates that vary between 3.6 and 4.1.

    The mean pairwise momentum estimator and best fit model for a selection of 20000 objects from the DR11 Large Scale Structure catalog, plotted as a function of comoving separation. The dashed line is the linear model, and the solid line is the model prediction including nonlinear redshift space corrections. The best fit provides a 4.1σ evidence of the kSZ signal in the ACTPol-ACT CMB map. Source: arXiv:1607.02139.

    The ACT and BOSS results are an improvement on the 2012 ACT detection, and are comparable with results from the South Pole Telescope (SPT) collaboration that use galaxies from the Dark Energy Survey. The ACT and BOSS measurement represents a step forward towards improved extraction of kSZ signals from CMB maps. Future surveys such as Advanced ACTPol, SPT-3G, the Simons Observatory, and next-generation CMB experiments will be able to apply the methods discussed here to improved CMB maps in order to achieve strong detections of the kSZ effect. With new data that will enable better measurements of galaxy cluster peculiar velocities, the pairwise kSZ signal will become a powerful probe of our universe in the years to come.

    Implications and Future Experiments

    One interesting consequence for particle physics will be more stringent constraints on the sum of the neutrino masses from the pairwise kinematic Sunyaev-Zel’dovich effect. Upper bounds on the neutrino mass sum from cosmological measurements of large scale structure and the CMB have the potential to determine the neutrino mass hierarchy, one of the next major unknowns of the Standard Model to be resolved, if the mass hierarchy is indeed a “normal hierarchy” with ν3 being the heaviest mass state. If the upper bound of the neutrino mass sum is measured to be less than 0.1 eV, the inverted hierarchy scenario would be ruled out, due to there being a lower limit on the mass sum of ~0.095 eV for an inverted hierarchy and ~0.056 eV for a normal hierarchy.

    Forecasts for kSZ measurements in combination with input from Planck predict possible constraints on the neutrino mass sum with a precision of 0.29 eV, 0.22 eV and 0.096 eV for Stage II (ACTPol + BOSS), Stage III (Advanced ACTPol + BOSS) and Stage IV (next generation CMB experiment + DESI) surveys respectively, with the possibility of much improved constraints with optimal conditions. As cosmic microwave background maps are improved and Sunyaev-Zel’dovich analysis methods are developed, we have a lot to look forward to.

    See the full article here .

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