Near-perfect Einstein Ring reveals most detailed view ever of star formation in distant galaxy
In a series of papers, scientists have described how they have used ALMA observations of SDP.81 – a gravitationally lensed galaxy – to reveal unprecedented information about the galaxy, including details about its structure, contents, motion, and other physical characteristics.
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.
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 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.
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 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.
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 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. 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.