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Tag Archives: gravitational lensing

Dark Energy Survey First Light!

Last month the Dark Energy Survey project achieved first light from its remote location in Chile’s Atacama Desert. The term first light is used by astronomers to refer to the first observation by a new instrument.

And what an instrument this is! It is in fact the world’s most powerful digital camera. This Dark Energy Camera, or DECam, is a 570 Megapixel optical survey camera with a very wide field of view. The field of view is over 2 degrees, which is rather unusual in optical astronomy. And the camera requires special CCDs that are sensitive in the red and infrared parts of the spectrum. This is because distant galaxies have their light shifted toward the red and the infrared by the cosmological expansion. If the galaxy redshift is one,  the light travels for about 8 billion years and the wavelength of light that the DECam detects is doubled, relative to what it was when it was originally emitted.

Dark Energy Camera

Image: DECam, near center of image, is deployed at the focus of the 4-meter Victor M. Blanco optical telescope in Chile (Credit: Dark Energy Survey Collaboration)

The DECam has been deployed to further our understanding of dark energy through not just one experimental method, but in fact four different methods. That’s how you solve tough problems – by attacking them on multiple fronts.

It’s taken 8 years to get to this point, and there have been some delays, as normal for large projects. But now this new instrument is mounted at the focal plane of the existing 4-meter telescope of the National Science Foundation’s Cerro Tololo Inter-American observatory in Chile. It will begin its program of planned measurements of several hundred million galaxies starting in December after several weeks of testing and calibration. Each image from the camera-telescope combination can capture up to 100,000 galaxies out to distances of up to 8 billion light years. This is over halfway back to the origin of the universe almost 14 billion years ago.

In a previous blog entry I talked about the DES and the 4 methods in some detail. In brief they are based on observations of:

  1. Type 1a supernova (the method used to first detect dark energy)
  2. Very large scale spatial correlations of galaxies separated by 500 million light-years (this experiment is known as Baryon Acoustic Oscillations since the galaxy separations reflect the imprint of sound waves in the very early universe, prior to galaxy formation)
  3. The number of clusters of galaxies as a function of redshift (age of the universe)
  4. Gravitational lensing, i.e. distortion of background images by gravitational effects of foreground clusters in accordance with general relativity

NGC 1365

Image: NGC 1365, a barred spiral galaxy located in the Fornax cluster located 60 million light years from Earth (Credit: Dark Energy Survey Collaboration)

What does the Dark Energy Survey team, which has over 120 members from over 20 countries, hope to learn about dark energy? We already have a good handle on its magnitude, at around 73% presently of the universe’s total mass-energy density.

The big issue is does it behave as a cosmological constant or as something more complex? In other words, how does the dark energy vary over time and is there possibly some spatial variation as well? And what is its equation of state, or relationship between its pressure and density?

With a cosmological constant explanation the relationship is Pressure = – Energy_density, a negative pressure, which is necessary in any model of the dark energy, in order for it to drive the accelerated expansion seen for the universe. Current observations from other experiments, especially those measuring the cosmic microwave background, support an equation of state parameter within around 5% of the value -1, as represented in the equation in the previous sentence. This is consistent with the interpretation as a pressure resulting from the vacuum. Dark energy appears also to have a constant or nearly constant density per unit volume of space. It is unlike ordinary matter and dark matter, that both drop in mass density (and thus energy density) as the volume of the universe grows. Thus dark energy becomes ever more dominant over dark matter and ordinary matter as the universe continues to expand.

We can’t wait to see the first publication of results from research into the nature of dark energy using the DECam.

References:

http://www.noao.edu/news/2012/pr1204.php – Press release from National Optical Astronomical Observatory on DECam first light

www.darkenergysurvey.org

http://www.ctio.noao.edu/noao/ – Cerro Tololo Inter-American Observatory page

http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf – WMAP 7 year results on cosmic microwave background

https://darkmatterdarkenergy.com/2011/03/08/dark-energy-survey/

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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