Tag Archives: galaxy clusters

TAIPAN: A Million Galaxy Survey

Taipan is an ambitious survey planned for southern hemisphere galaxies, with the goal of mapping and measuring as many as one million galaxies in our Milky Way’s neighborhood. This will provide a deeper understanding of cosmology and galaxy evolution in the relatively nearby region of our universe.

There are more than a hundred billion galaxies in our visible universe. In order to refine our understanding of galaxies, their distribution and evolution, and of the overall cosmological properties of the universe, we want to sample a very large number of galaxies.

It is naturally easier to detect galaxies that are relatively nearby, and those that are more luminous.

Since the universe is expanding in an isotropic and homogeneous manner, galaxies are in general receding away from one another – in accordance with the Hubble relation below. The Taipan survey will explore our local neighborhood, with redshifts up to about 0.3.

For nearby galaxies,

V = cz = H*d

where V is the recession velocity, c is the speed of light, z is the redshift, H is the Hubble constant, and d is the galaxy’s distance. If we evaluate for z = 0.3 and the best estimate of the Hubble constant of 68 kilometers/second/Megaparsec, this implies a survey depth of 1300 Megaparsecs, or over 4 billion light-years.

The Taipan galaxy survey will begin next year and run for four years, using the UK Schmidt telescope, which is actually in Australia at the Siding Springs Observatory. Up to 150 galaxies in the field of view will be observed simultaneously with a fibre optic array. Of course the positions of galaxies is different in each field to be observed, so the fibers are robotically placed in the the proper positions. Many thousands of galaxies can thus be observed each night.

Short video of a Starbug fiber robot

One expected result will be refinement of the value of the Hubble constant, now uncertain to a few percent, reducing its uncertainty to only 1%.

The Taipan galaxy survey will also provide a better constraint on the growth rate of structure in the universe, decreasing the uncertainty down to about 5% for the low-redshift data points. This is a factor of 3 improvement and will provide a stricter test on general relativity.

The Taipan survey will also look at galaxies’ peculiar velocities, which are the deviations away from the general Hubble flow described in the equation above. These peculiar velocities reflect the details of the gravitational field – that is dominated by the distribution of dark matter primarily, and ordinary matter secondarily. On average galaxies are moving according to the Hubble equation, but in regions where the density of matter (dark and ordinary both) is higher than average they are pulled away from the Hubble flow toward any concentrations of matter. Bound galaxy groups and clusters form in such regions.

427249main_hicksoncompact31-labeled

The mapping of peculiar velocities and the details of local variations in the gravitation field will enable fundamental tests of gravity on large scales.

Another of the important areas that Taipan will explore is how galaxies evolve from young active star-forming blue galaxies to older reddish, less active galaxies. Ordinary matter cycles through stars and the interstellar medium of a given galaxy. As stars die they shed matter which ends up in molecular clouds that are the sites of new star formation. Taipan will help to increase our understanding of this cycle, and of galaxy aging in general. Star formation slows down as more and more gas is tied up in lower mass, longer-lived stars, and the recycling rate drops. It also can be quenched by active galactic nuclei events (AGN are powered by supermassive black holes found at galactic centers).

Taipan will be the definitive survey of galaxies in the southern hemisphere, and is expected to significantly add to our understanding of galaxy evolution and cosmology. We look forward to their early results beginning in 2016.

Reference:

taipan-survey.org

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Super Colliders in Space: Dark Matter not Colliding

What’s bigger and more powerful than the Large Hadron Collider at CERN? Why colliding galaxy clusters of course.

A cluster of galaxies consists of hundreds or even thousands of galaxies bound together by their mutual gravitation. Both dark matter and ordinary matter in and between galaxies is responsible for the gravitational field of a cluster. And typically there is about 5 times as much dark matter as ordinary matter. The main component of ordinary matter is hot intracluster gas; only a small percentage of the mass is locked up in stars.

One stunning example of dark matter detection is the Bullet Cluster. This is the canonical example found revealing dark matter separation from ordinary matter in a pair of clusters colliding and merging. The dark matter just passes right through, apparently unaffected by the collision. The hot gas (ordinary matter) is seen through its X-ray emission, since the gas is heated by collisions to of order 100 million degrees. The Chandra X-ray Observatory (satellite) provided these measurements.

Image courtesy of Chandra X-ray Observatory

Bullet Cluster. The blue color shows the distribution of dark matter, which passed through the collision without slowing down. The purple color shows the hot X-ray emitting gas. Image courtesy of Chandra X-ray Observatory

The distribution of matter overall in the Bullet Cluster or other clusters is traced by gravitational lensing effects; general relativity tells us that  background galaxies will have their images displaced, distorted, and magnified as their light passes through a cluster on its way to Earth. The magnitude of these effects can be used to “weigh” the dark matter. These measurements are made with the Hubble Space Telescope.

In the Bullet Cluster the dark matter is displaced from the ordinary matter. The interpretation is that the ordinary matter from the two clusters, principally in the form of hot gas, is slowed by frictional, collisional processes as the clusters interact and form a larger single cluster of galaxies. Another six or so examples of galaxy clusters showing the displacement between the dark matter and the ordinary matter in gas and stars have been found to date.

Now, a team of astrophysicists based in the U.K. and Switzerland have examined 30 additional galaxy clusters with data from both Chandra and Hubble, and with redshifts typically 0.2 to 0.6. In aggregate there are 72 collisions in the 30 systems, since some have more than two subclusters. The offsets between the gas and dark matter are quite substantial, and in aggregate indicate the existence of dark matter in these clusters with over 7 standard deviations of statistical significance (probability of the null hypothesis of no dark matter is 1 in 30 trillion).

They then look at the possible drag force on the dark matter due to dark matter particles colliding with other dark matter particles. There are already much more severe constraints on ordinary matter – dark matter interactions from Earth-based laboratory measurements. But the dark matter mutual collision cross section could potentially be large enough to result in a drag. They measure the relative positions of hot gas, galaxies, and dark matter for all of the 72 subclusters.

From paper "The non-gravitational interactions of dark matter in colliding galaxy clusters"

From paper “The non-gravitational interactions of dark matter in colliding galaxy clusters” D. Harvey et al. 2015

The gas should and does lag the most, relative to the direction of the galaxies in a collision. If there is a dark matter drag, then dark matter should lag behind the positions of the stars. They find no lag of the dark matter average position, which allows them to place a new, tighter constraint on the mutual interaction cross-section for dark matter.

Their constraint is σ(DM)/m < 0.47 cm^2/g at 95% confidence level, where σ (sigma) is the cross-section and m is the mass of a single dark matter particle. This limit is over twice as tight as that previously obtained from the Bullet Cluster. And some dark matter models predict a cross section per unit mass of 0.6 cm^2/g, so these models are potentially ruled out by these new measurements.

In summary, using Nature’s massive particle colliders, the authors have found further highly significant evidence for the existence of dark matter in clusters of galaxies, and they have placed useful constraints on the dark matter self-interaction cross-section. Dark matter continues to be highly elusive.

Reference:

D. Harvey et al. 2015 “The non-gravitational interactions of dark matter in colliding galaxy clusters” http://arxiv.org/pdf/1503.07675v1.pdf


Dark Matter Bridge Discovered

A team of astronomers claims to have detected an enormous bridge or filament of dark matter, with a mass estimated to be of order 100 trillion solar masses, and connecting two clusters of galaxies. The two clusters, known as Abell 222 and Abell 223, are about 2.8 billion light-years away and separated from one another by 400 million light-years. Each cluster has around 150 galaxies; actually one of the pair is itself a double cluster.

Clusters of galaxies are gravitationally bound collections of hundreds to a thousand or more galaxies. Often a cluster will be found in the vicinity of other clusters to which it is also gravitationally bound. The universe as a whole is gravitationally unbound – the matter, including the dark matter – is insufficient to stop the continued expansion, which is driven to acceleration in fact, by dark energy.

Dark matter bridge

Figure: Subaru telescope optical photo with mass density shown in blue and statistical significance contours superimposed. In the filament area found near the center of the image, the contours indicate four standard deviations of significance in the detection of dark matter. The cluster Abell 222 is in the south, and Abell 223 is the double cluster in the north of the image. The distance between the two clusters is about 14 arc-minutes, or about ½ the apparent size of the Moon.

Dark matter was originally called “missing matter”, and was first posited by Fritz Zwicky (http://en.wikipedia.org/wiki/Fritz_Zwicky) in the 1930s because of his studies of the kinematics of galaxies and galaxy clusters. He measured the velocities of galaxies moving around inside a cluster and found they were significantly greater than expected from the amount of ordinary matter seen in the galaxies themselves. This implied there was more matter than seen in galaxies because the velocities of the galaxies would be determined by the total gravitational field in a cluster, and the questions have been where is, and what is, the “missing matter” inferred by the gravitational effects. X-ray emission has been detected from most clusters of galaxies, and this is due to an additional component of matter outside of galaxies, namely hot gas between galaxies. But it is still insufficient to explain the total mass of clusters as revealed by both the galaxy velocities and the temperature of the hot gas itself, since both are a reflection of the gravitational field in the cluster.

Dark matter is ubiquitous, found on all scales and is generally less clumped than ordinary matter, so it is not surprising that significant dark matter would be found between two associated galaxy clusters. In fact the researchers in this study point out that “It is a firm prediction of the concordance Cold Dark Matter cosmological model that galaxy clusters live at the intersection of large-scale structure filaments.”

The technique used to map the dark matter is gravitational lensing, which is a result of general relativity. The gravitational lensing effect is well established; it has been seen in many clusters of galaxies to date. In gravitational lensing, light is deflected away from a straight-line path by matter in its vicinity.

In this case the gravitational field of the dark matter filament and the galaxy clusters deflect light passing nearby. The image of a background galaxy located behind the cluster will be distorted as the light moves through or nearby the foreground cluster. The amount of distortion depends on the mass of the cluster (or dark matter bridge) and how near the line of sight passes to the cluster center.

There is also a well-detected bridge of ordinary matter in the form of hot X-ray emitting gas connecting the two clusters and in the same location as the newly discovered dark matter bridge.  The scientists used observations from the XMM-Newton satellite to map the X-ray emission from the two clusters Abell 222 and Abell 223 and the hot gas bridge connecting them. Because of the strong gravitational fields of galaxy clusters, the gas interior to galaxy clusters (but exterior to individual galaxies within the cluster) is heated to very high temperatures by frictional processes, resulting in thermal X-ray emission from the clusters.

The research team, led by Jörg Dietrich at the University of Michigan, then performed a gravitational lensing analysis, focusing on the location of the bridge as determined from the X-ray observations. The gravitational lensing work is based on optical observations obtained from the Subaru telescope (operated by the Japanese government, but located on the Big Island of Hawaii) to map the total matter density profile around and between the two clusters. This method detects the sum of dark matter and ordinary matter.

They analyzed the detailed orientations and shapes of over forty thousand background galaxies observable behind the two clusters and the bridge. This work allowed them to determine the contours of the dark matter distribution. They state a 98% confidence in the existence of a bridge or filament dominated by dark matter.

The amount of dark matter is shown to be much larger than that of ordinary matter, representing over 90% of the total in the filament region, so the gravitational lensing effects are primarily due to the dark matter. Less than 9% of the mass in the filament is in the form of hot gas (ordinary matter). The estimated total mass in the filament is about 1/3 of the mass of either of the galaxy clusters, each of which is also dominated by dark matter.

Observations of galaxy distributions show that galaxies are found in groups, clusters, and filaments connecting regions of galaxy concentration. Cosmological simulations of the evolution of the universe on supercomputers indicate that the distribution of dark matter should have a filamentary structure as well. So although the result is in many ways not surprising, it represents the first detection of such a structure to date.

 

References:

http://ns.umich.edu/new/releases/20623-dark-matter-scaffolding-of-universe-detected-for-the-first-time – press release from the University of Michigan

http://www.gizmag.com/dark-matter-filaments-found/23281/ “Dark matter filaments detected for the first time”

J. Dietrich et al. 2012 http://arxiv.org/abs/1207.0809 “A filament of dark matter between two clusters of galaxies”


Dark Energy Survey

DES logo

Dark Energy Survey logo

The Dark Energy Survey (DES) is a ground-based cosmology experiment led by astronomers from the US, Brazil and Europe. It has begun its trip to Chile where it is scheduled to begin observations in November, 2011 using the 4 meter Victor M. Blanco telescope in the Atacama desert. It uses a new highly sensitive camera design called DECam, with resolution totaling 570 Megapixels and employing very large pixels, and it emphasizes sensitivity in the red and infrared portions of the spectrum, in order to measure galaxies out to redshifts of 1 and beyond. Galaxies in the early universe are far away from us and have high redshift values. Light which they would have originally emitted in the blue or yellow portions of the optical spectrum has shifted toward the red or infrared, thus the emphasis on detection of infrared photons for this work.

The DES uses a 4-pronged attack to improve the measurement of the dark energy and other cosmological parameters. These 4 tests are:

  1. Supernovae – Type 1a supernovae are thought to occur when a white dwarf in a binary stellar system accretes mass from its companion. Once enough mass is accreted, the white dwarf is pushed over the Chandrasekhar limit of 1.4 solar masses, the gravity of the star’s mass overwhelms the pressure support from its ‘degenerate electron’ matter, and the white dwarf undergoes core collapse and becomes a supernova. It is temporarily as bright as an entire galaxy. Such a supernova can be detected at large distances (high redshifts) and very importantly, since the mass of the supernova is always the same, the absolute brightness of this type of supernova is essentially expected to be the same as well. This allows us to use them as standard candles for distance measurement and thus for cosmological tests.
  2. Baryon acoustic oscillations – This test looks at the statistics of galaxy separations at very large scales. In the early universe, sound waves were established in the hot dense plasma, reflecting pressure generated by the interaction of photons and ordinary matter. Dark matter does not participate except gravitationally. A “sound horizon” is expected with a present size of about 500 million light years, and this acts as a standard ruler as the universe expands. A bump in the correlation function, which measures the probability of one galaxy being near another, is expected at this characteristic distance.
  3. Galaxy cluster counts – This test of how many galaxy clusters are detectable versus redshift was apparently first proposed by myself in 1980, in the context of X-ray emission from the very hot diffuse gas found between galaxies in galaxy clusters. This approach offers certain advantages in comparison to simple galaxy counts versus redshift. In this case it will be performed in the infrared and red, observing the galaxies themselves. Galaxy clusters contain up to 1000 or more galaxies within a single cluster. The number of clusters that can be seen at a given redshift is dependent on the cosmological model and the mass of the cluster, since dark matter promotes cluster formation through gravitational attraction. Dark energy inhibits cluster formation, so this helps to measure the relative strength of dark energy at earlier times. The team expects to detect over 100,000 galaxy clusters, out to redshifts of 1.5.
  4. Weak lensing – This refers to gravitational lensing. This occurs when a source galaxy is behind an intervening galaxy cluster and the gravity of the cluster bends the light from the source galaxy in accordance with general relativity. By surveying a very large number of galaxies, a strong statistical measure of this bending, also known as cosmic shear, can be taken. The amount of shear will be measured as a function of redshift (distance). This shear is sensitive to both the shape of the universe and the way in which structure develops over time.

More info: http://www.quantumdiaries.org/2011/02/11/des-first-light-countdown-9-months-to-go-decam-on-telescope-simulator/

http://en.wikipedia.org/wiki/Dark_Energy_Survey

http://news.medill.northwestern.edu/chicago/news.aspx?id=182835