Herschel Provides Gravitational Lens Bonanza

The image shows the first area of sky viewed as part of the Herschel-ATLAS survey. The five inset show enlarged views of the five distant galaxies whose images are being gravitationally lensed by foreground galaxies (unseen by Herschel). The distant galaxies are not only very bright, but also very red in colour in this image, showing that they are brighter at the longer wavelengths measured by the SPIRE instrument. Image credits: ESA/SPIRE/Herschel-ATLAS/SJ Maddox (top); ESA/NASA/JPL-Caltech/Keck/SMA (bottom).

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One of the predictions of Einstein’s predictions from general relativity was that gravity could distort space itself and potentially, act as a lens. This was spectacularly confirmed in 1919 when, during a solar eclipse, Arthur Eddington observed stars near the Sun were distorted from their predicted positions. In 1979, this effect was discovered at much further distances when astronomers found it to distort the image of a distant quasar, making one appear as two. Several other such cases have been discovered since then, but these instances of gravitational lensing have proven difficult to find. Searches for them have had a low success rate in which less than 10% of candidates are confirmed as gravitational lenses. But a new method using data from Herschel may help astronomers discover many more of these rare occurrences.

The Herschel telescope is one of the many space telescopes currently in use and explores the portion of the spectrum from the far infrared to the submillimeter regime. A portion of its mission is to produce a large survey of the sky resulting in the Herschel ATLAS project which will take deep images of over 550 square degrees of the sky.

While Herschel explores this portion of the electromagnetic spectrum in far greater detail than its predecessors, in many ways, there’s not much to see. Stars emit only very faintly in this range. The most promising targets are warm gas and dust which are better emitters, but also far more diffuse. But it’s this combination of facts that will allow Herschel to potentially discover new lenses with improved efficiency.

The reason is that, although galaxies lack strong emission in this regime in the modern universe, ancient galaxies gave off far more since during the first 4 billion years. During that time, many galaxies were dominated by dust being warmed by star formation. Yet due to their distance, they too should be faint… Unless a gravitational lens gets in the way. Thus, the majority of small, point-like sources in the ALTAS collection are likely to be lensed galaxies. As Dr Mattia Negrello, of the Open University and lead researcher of the study explains, “The big breakthrough is that we have discovered that many of the brightest sources are being magnified by lenses, which means that we no longer have to rely on the rather inefficient methods of finding lenses which are used at visible and radio wavelengths.”

These panels show a zoom of one of the lenses, with high resolution images from Keck (optical light, blue) and the submillimeter Array (sub-millimetre light, red). Image credits: ESA/NASA/JPL-Caltech/Keck/SMA
These panels show a zoom of one of the lenses, with high resolution images from Keck (optical light, blue) and the submillimeter Array (sub-millimetre light, red). Image credits: ESA/NASA/JPL-Caltech/Keck/SMA

Already, this new technique has turned up at least five strong candidates. A paper, to be published in the current issue of Science discusses them. Each of them received followup observations from the Z-Spec spectrometer on the California Institute of Technology Submillimeter Observatory. The furthest of these these objects, labeled as ID81, showed a prominent IR spectral line had a redshift of 3.04, putting it at a distance of 11.5 billion lightyears. Additionally, each system showed the spectral profile of the foreground galaxy, demonstrating that the combined light received was indeed two galaxies and the bright component was a gravitational lens.

This method of using gravitational lenses will allow the Herschel team to probe distant galaxies in detail never before achieved. As with all telescopes, longer wavelengths of observations result in less resolution which means that, even if one of the distant systems were to be broken into distinct portions, Herschel would be unable to resolve them. But the fact that we can see them at all means their spectral signatures of the galaxies as a whole can still be studied. Additionally, as Professor Steve Eales from Cardiff University and the other leader of the survey noted: “We can also use this technique to study the lenses themselves.” This potential to explore the mass of the nearby galaxies may help astronomers to understand and constrain the enigmatic Dark Matter that makes up ~80% of the mass in our universe.

Dr Loretta Dunne of Nottingham University and joint-leader of the Herschel-ATLAS survey adds, “What we’ve seen so far is just the tip of the iceberg. Wide area surveys are essential for finding these rare events and since Herschel has only covered one thirtieth of the entire Herschel-ATLAS area so far, we expect to discover hundreds of lenses once we have all the data. Once found, we can probe the early Universe on the same physical scales as we can in galaxies next door.”

First Quasar Gravitational Lens Discovered (w/video)

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Gravitation lensing – a phenomenon that falls out of Einstein’s theory of general relativity – has been observed numerous times, making for some fantastic images of rings, arcs and crosses composed of massive galaxies light years away. As the light from a background object is bent by gravity around a foreground object, multiple, magnified images of the background object are produced from our vantage point.

For the first time, a quasar (quasi-stellar object) has been shown to gravitationally lens a galaxy behind it. About a hundred instances of gravitational lenses that consist of a foreground galaxy and a background quasar have been found, but this is the very first time where the opposite is the case; that is, a quasar bending the light from a background galaxy around it to create a multiple image of that galaxy.

Quasars are thought to be the result of a supermassive black hole at the center of a galaxy attempting to swallow up all of the matter that surrounds it. As the matter bunches up when it gets closer to the black hole, it heats up due to friction and begins to emit light across the electromagnetic spectrum. The light from a quasar can outshine an entire galaxy of stars, making it difficult to separate the light from a background galaxy from the overwhelming glare of the quasar itself.

To make this initial detection (there are surely many to follow), astronomers from the EPFL’s Laboratory of Astrophysics in cooperation with Caltech used data from the Sloan Digital Sky Survey (SDSS). They analyzed 22,298 quasars from the SDSS Data Release 7 catalog, and looked for images that had a strongly redshifted emission spectra. According to the paper announcing the results, “In these spectra, we look for emission lines redshifted beyond the redshift of the [quasar].”

In other words, a quasar that is lensing a galaxy in the background will exhibit a higher redshift than one that is not lensing a background galaxy, since the light from the galaxy and the quasar are combined in the SDSS data. So, quasars that had an expected redshift were thrown out, and a statistical analysis of quasars with emission lines that might mimic a gravitational lens eliminated many more of the objects. This left about 14 objects of the 22,298 analyzed as potential candidates. Of these 14, the team selected one to perform follow-up observations on, named SDSS J0013+1523.

SDSS J0013+1523 lies about 1.6 billion light years away, and is lensing a galaxy that is about 7.5 billion light years away from Earth. Using the Keck II telescope, they were able to confirm that SDSS J0013+1523 was indeed lensing the light from a galaxy located behind it. Hubble images of the discovery are in the works.

Here’s a video produced by the EPFL describing the results.

What is significant about this discovery – besides the novel aspect of a quasar acting as a lens – is that it will allow researchers to better refine their understanding of quasars. When light is bent around an object, it bends because of gravity, and gravity is a result of mass. So, something that is very massive will act as a stronger lens than something that is tiny, and the mass of the object doing all of the lensing work – in this case, the foreground quasar – can be determined.

Their results were published in a letter to Astronomy & Astrophysics on July 16th. The original paper is available for your perusal here.

Source: Eurekalert here and here, Arxiv paper here

Hubble Confirms Cosmic Acceleration with Weak Lensing

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Need more evidence that the expansion of the Universe is accelerating? Just look to the Hubble Space Telescope. An international team of astronomers has indeed confirmed that the expansion of the universe is accelerating. The team, led by Tim Schrabback of the Leiden Observatory, conducted an intensive study of over 446,000 galaxies within the COSMOS (Cosmological Evolution Survey) field, the result of the largest survey ever conducted with Hubble. In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the same part of the Universe using the Advanced Camera for Surveys (ACS) onboard the orbiting telescope. It took nearly 1,000 hours of observations.

In addition to the Hubble data, researchers used redshift data from ground-based telescopes to assign distances to 194,000 of the galaxies surveyed (out to a redshift of 5). “The sheer number of galaxies included in this type of analysis is unprecedented, but more important is the wealth of information we could obtain about the invisible structures in the Universe from this exceptional dataset,” said co-author Patrick Simon from Edinburgh University.

In particular, the astronomers could “weigh” the large-scale matter distribution in space over large distances. To do this, they made use of the fact that this information is encoded in the distorted shapes of distant galaxies, a phenomenon referred to as weak gravitational lensing. Using complex algorithms, the team led by Schrabback has improved the standard method and obtained galaxy shape measurements to an unprecedented precision. The results of the study will be published in an upcoming issue of Astronomy and Astrophysics.

The meticulousness and scale of this study enables an independent confirmation that the expansion of the Universe is accelerated by an additional, mysterious component named dark energy. A handful of other such independent confirmations exist. Scientists need to know how the formation of clumps of matter evolved in the history of the Universe to determine how the gravitational force, which holds matter together, and dark energy, which pulls it apart by accelerating the expansion of the Universe, have affected them. “Dark energy affects our measurements for two reasons. First, when it is present, galaxy clusters grow more slowly, and secondly, it changes the way the Universe expands, leading to more distant — and more efficiently lensed — galaxies. Our analysis is sensitive to both effects,” says co-author Benjamin Joachimi from the University of Bonn. “Our study also provides an additional confirmation for Einstein’s theory of general relativity, which predicts how the lensing signal depends on redshift,” adds co-investigator Martin Kilbinger from the Institut d’Astrophysique de Paris and the Excellence Cluster Universe.

The large number of galaxies included in this study, along with information on their redshifts is leading to a clearer map of how, exactly, part of the Universe is laid out; it helps us see its galactic inhabitants and how they are distributed. “With more accurate information about the distances to the galaxies, we can measure the distribution of the matter between them and us more accurately,” notes co-investigator Jan Hartlap from the University of Bonn. “Before, most of the studies were done in 2D, like taking a chest X-ray. Our study is more like a 3D reconstruction of the skeleton from a CT scan. On top of that, we are able to watch the skeleton of dark matter mature from the Universe’s youth to the present,” comments William High from Harvard University, another co-author.

The astronomers specifically chose the COSMOS survey because it is thought to be a representative sample of the Universe. With thorough studies such as the one led by Schrabback, astronomers will one day be able to apply their technique to wider areas of the sky, forming a clearer picture of what is truly out there.

Source: EurekAlert

Paper: Schrabback et al., ‘Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS’, Astronomy and Astrophysics, March 2010,

Using Gravitational Lensing to Measure Age and Size of Universe

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Handy little tool, this gravitational lensing! Astronomers have used it to measure the shape of stars, look for exoplanets, and measure dark matter in distant galaxies. Now its being used to measure the size and age of the Universe. Researchers say this new use of gravitation lensing provides a very precise way to measure how rapidly the universe is expanding. The measurement determines a value for the Hubble constant, which indicates the size of the universe, and confirms the age of Universe as 13.75 billion years old, within 170 million years. The results also confirm the strength of dark energy, responsible for accelerating the expansion of the universe.

Gravitational lensing occurs when two galaxies happen to aligned with one another along our line of sight in the sky. The gravitational field of the nearer galaxy distorts the image of the more distant galaxy into multiple arc-shaped images. Sometimes this effect even creates a complete ring, known as an “Einstein Ring.”
Researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) used gravitational lensing to measure the distances light traveled from a bright, active galaxy to the earth along different paths. By understanding the time it took to travel along each path and the effective speeds involved, researchers could infer not just how far away the galaxy lies but also the overall scale of the universe and some details of its expansion.

Distinguishing distances in space is difficult. A bright light far away and a dimmer source lying much closer can look like they are at the same distance. A gravitational lens circumvents this problem by providing multiple clues as to the distance light travels. That extra information allows them to determine the size of the universe, often expressed by astrophysicists in terms of a quantity called Hubble’s constant.

“We’ve known for a long time that lensing is capable of making a physical measurement of Hubble’s constant,” KIPAC’s Phil Marshall said. However, gravitational lensing had never before been used in such a precise way. This measurement provides an equally precise measurement of Hubble’s constant as long-established tools such as observation of supernovae and the cosmic microwave background. “Gravitational lensing has come of age as a competitive tool in the astrophysicist’s toolkit,” Marshall said.

When a large nearby object, such as a galaxy, blocks a distant object, such as another galaxy, the light can detour around the blockage. But instead of taking a single path, light can bend around the object in one of two, or four different routes, thus doubling or quadrupling the amount of information scientists receive. As the brightness of the background galaxy nucleus fluctuates, physicists can measure the ebb and flow of light from the four distinct paths, such as in the B1608+656 system that was the subject of this study. Lead author on the study Sherry Suyu, from the University of Bonn, said, “In our case, there were four copies of the source, which appear as a ring of light around the gravitational lens.”

Though researchers do not know when light left its source, they can still compare arrival times. Marshall likens it to four cars taking four different routes between places on opposite sides of a large city, such as Stanford University to Lick Observatory, through or around San Jose. And like automobiles facing traffic snarls, light can encounter delays, too.

“The traffic density in a big city is like the mass density in a lens galaxy,” Marshall said. “If you take a longer route, it need not lead to a longer delay time. Sometimes the shorter distance is actually slower.”

The gravitational lens equations account for all the variables such as distance and density, and provide a better idea of when light left the background galaxy and how far it traveled.

In the past, this method of distance estimation was plagued by errors, but physicists now believe it is comparable with other measurement methods. With this technique, the researchers have come up with a more accurate lensing-based value for Hubble’s constant, and a better estimation of the uncertainty in that constant. By both reducing and understanding the size of error in calculations, they can achieve better estimations on the structure of the lens and the size of the universe.

There are several factors scientists still need to account for in determining distances with lenses. For example, dust in the lens can skew the results. The Hubble Space Telescope has infra-red filters useful for eliminating dust effects. The images also contain information about the number of galaxies lying around the line of vision; these contribute to the lensing effect at a level that needs to be taken into account.

Marshall says several groups are working on extending this research, both by finding new systems and further examining known lenses. Researchers are already aware of more than twenty other astronomical systems suitable for analysis with gravitational lensing.

These results of this study was published in the March 1 issue of The Astrophysical Journal. The researchers used data collected by the NASA/ESA Hubble Space Telescope, and showed the improved precision they provide in combination with the Wilkinson Microwave Anisotropy Probe (WMAP).

Source: SLAC