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Astronomers ‘Unscramble’ Einstein Ring to Reveal Most Detailed View Ever of Star Formation in the Distant Universe

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Posted June 10, 2015

ALMA’s Long Baseline Campaign produced spectacular images of the distant, gravitationally lensed galaxy called HATLAS J090311.6+003906, otherwise known as SDP.81. New analyses of these images reveal details never before seen in a galaxy so remote, including phenomenally massive yet concentrated clumps of star-forming material.

ALMA’s Long Baseline Campaign has produced a spectacularly detailed image of a distant galaxy being gravitationally lensed, revealing star-forming regions — something that has never been seen before at this level of detail in a galaxy so remote. The left panel shows the foreground lensing galaxy (observed with Hubble), and the gravitationally lensed galaxy SDP.81, which forms an almost perfect Einstein Ring, is hardly visible. The middle image shows the sharp ALMA image of the Einstein ring, with the foreground lensing galaxy being invisible to ALMA. The resulting reconstructed image of the distant galaxy (right) using sophisticated models of the magnifying gravitational lens, reveals fine structures within the ring that have never been seen before: several dust clouds within the galaxy, which are thought to be giant cold molecular clouds, the birthplaces of stars and planets. Credit:ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)/Mark Swinbank (Durham University)

The left panel shows the foreground lensing galaxy (observed with Hubble), and the gravitationally lensed galaxy SDP.81, which forms an almost perfect Einstein Ring, is hardly visible. The middle image shows the sharp ALMA image of the Einstein ring, with the foreground lensing galaxy being invisible to ALMA. The resulting reconstructed image of the distant galaxy (right) using sophisticated models of the magnifying gravitational lens, reveals fine structures within the ring that have never been seen before: several dust clouds within the galaxy, which are thought to be giant cold molecular clouds, the birthplaces of stars and planets. Credit:ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)/Mark Swinbank (Durham University)

The ALMA observations of SDP.81, made at the end of 2014, were enabled by a cosmic effect known as gravitational lensing. A large galaxy nestled between SDP.81 and ALMA [1] is acting as a lens, magnifying the more distant galaxy’s light and warping it into a near-perfect example of a phenomenon known as an Einstein Ring [2].

In the months following these observations, at least seven groups of scientists [3] have independently analyzed the ALMA data on SDP.81. This flurry of research papers has divulged unprecedented information about the galaxy, including details about its structure, contents, motion, and other physical characteristics.

The image shows the foreground lensing galaxy (observed with Hubble), and the gravitationally lensed galaxy SDP.81, which forms an almost perfect Einstein Ring, is hardly visible. Credit: ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)/Mark Swinbank (Durham University)

The image shows the foreground lensing galaxy (observed with Hubble), and the gravitationally lensed galaxy SDP.81, which forms an almost perfect Einstein Ring, is hardly visible. Credit: ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)/Mark Swinbank (Durham University)

Since ALMA is an interferometer — a single instrument made up of multiple antennas — it can adjust its resolution by repositioning its antennas. During this observing campaign, ALMA’s antennas were at their greatest separation – up to 15 kilometers apart – providing the highest resolution ever achieved by the telescope. As a result, these new images of SDP.81 have a resolution up to six times greater [4] than those taken in the infrared with the NASA/ESA Hubble Space Telescope.

By using sophisticated models to correct for the distortion produced by the magnifying gravitational lens, the astronomers were able to reveal fine, never-before-seen structure within SDP.81 in the form of dusty clouds thought to be giant repositories of cold molecular gas — the birthplaces of stars and planets.

The gravitationally lensed galaxy SDP.81, which appears as an almost perfect Einstein Ring, is seen here. Credit: ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)

The gravitationally lensed galaxy SDP.81, which appears as an almost perfect Einstein Ring, is seen here. Credit: ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)

As a result, the ALMA observations are so sharp that researchers can see clumps of star formation in the galaxy down to a size of about 200 light-years. This is the first time this phenomenon has been seen at such an enormous distance.

“ALMA was designed to be the most powerful telescope of its kind, but by harnessing the magnifying power of this gravitational lens we were able to study a distant and mysterious object in detail that would have been impossible otherwise,” said Todd Hunter, an astronomer at the National Radio Astronomy Observatory and co-author on one of the papers. “This one dataset has spawned an entire series of highly intriguing research, confirming that ALMA offers the astronomical community new avenues to probe the distant Universe.”

The resulting reconstructed image of the distant galaxy using sophisticated models of the magnifying gravitational lens, reveal fine structures within the ring that have never been seen before: several dust clouds within the galaxy, which are thought to be giant cold molecular clouds, the birthplaces of stars and planets. Note that some of the smaller structures visible here might be artifacts caused by the reconstruction method. Credit: ALMA (NRAO/ESO/NAOJ)/Mark Swinbank (Durham University)

The resulting reconstructed image of the distant galaxy using sophisticated models of the magnifying gravitational lens, reveal fine structures within the ring that have never been seen before: several dust clouds within the galaxy, which are thought to be giant cold molecular clouds, the birthplaces of stars and planets. Note that some of the smaller structures visible here might be artifacts caused by the reconstruction method. Credit: ALMA (NRAO/ESO/NAOJ)/Mark Swinbank (Durham University)

“The reconstructed ALMA image of the galaxy is spectacular,” says Rob Ivison, co-author of two of the papers and ESO’s Director for Science. “ALMA’s huge collecting area, the large separation of its antennas, and the stable atmosphere above the Atacama Desert all lead to exquisite detail in both images and spectra. That means that we get very sensitive observations, as well as information about how the different parts of the galaxy are moving. We can study galaxies at the other end of the Universe as they merge and create huge numbers of stars. This is the kind of stuff that gets me up in the morning!”

Using the spectral information gathered by ALMA, astronomers also measured how the distant galaxy rotates and estimated its mass. The data show that the gas in this galaxy is unstable; clumps of it are collapsing inward and will likely turn into new giant star-forming regions in the future.

Notably, the modeling of the lensing effect also indicates the existence of a supermassive black hole at the center of the foreground galaxy lens [5]. The central part of SDP.81 is too faint to be detected, leading to the conclusion that the foreground galaxy holds a supermassive black hole more than 200–300 million times the mass of the Sun.

The number of papers published using this single ALMA dataset demonstrates the excitement generated by the potential of the array’s high resolution and light-gathering power. It also shows how ALMA will enable astronomers to make more discoveries in the years to come, also uncovering yet more questions about the nature of distant galaxies.

Notes

[1] The lensed galaxy is seen at a time when the Universe was only 15 percent of its current age, just 2.4 billion years after Big Bang. The light has taken over twice the age of the Earth to reach us (11.4 billion years), detouring along the way around a massive foreground galaxy that is comparatively close at 4 billion light-years away from us.

[2] Gravitational lenses were predicted by Albert Einstein as part of his theory of general relativity. His theory tells us that objects bend space and time. Any light approaching this curved space-time will itself follow the curvatures created by the object. This enables particularly massive objects — huge galaxies and galaxy clusters — to act as cosmic magnifying glasses. An Einstein Ring is a special type of gravitational lens, in which the Earth, the foreground lensing galaxy, and the background lensed galaxy are in perfect alignment, creating a harmonious distortion in the form of a ring of light. This phenomenon is illustrated in the accompanying video.

[3] The science teams are listed below.

[4] Details down to 0.023 arcseconds, or 23 milliarcseconds, can be measured in these data. Hubble observed this galaxy in the near-infrared, with a resolution of about 0.16 arcseconds. Note, however, that when observing at shorter wavelengths, Hubble can reach finer resolutions, down to 0.022 arcseconds in the near-ultraviolet. ALMA’s resolution can be adjusted depending on the type of observations by moving the antennas further apart or closer together. For these observations, the widest separation was used, resulting in ALMA’s finest resolution possible at these wavelengths.

[5] The high-resolution ALMA image enables researchers to look for the central part of the background galaxy, which is expected to appear at the center of the Einstein ring. If the foreground galaxy has a supermassive black hole at the center, the central image becomes fainter. The faintness of the central image indicates how massive the black hole in the foreground galaxy is.

Source: NRAO

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