ALMA’s observation of Einstein ring reveals extraordinary detail

ALMA’s Long Baseline Campaign has produced a spectacularly detailed image of a distant galaxy being gravitationally lensed. The image shows a magnified view of the galaxy’s star-forming regions, the likes of which have never been seen before at this level of detail in a galaxy so remote. The new observations are far more detailed than those made using the NASA/ESA Hubble Space Telescope and reveal star-forming clumps in the galaxy equivalent to giant versions of the Orion Nebula.

ALMA’s Long Baseline Campaign has produced some amazing observations and gathered unprecedentedly detailed information about the inhabitants of the near and distant universe. Observations made at the end of 2014 as part of the campaign targeted a distant galaxy called HATLAS J090311.6+003906, otherwise known as SDP.81. The light from this galaxy is a victim of a cosmic effect known as gravitational lensing. A large galaxy sitting between SDP.81 and ALMA is acting as a lens, warping the more distant galaxy’s light and creating a near-perfect example of a phenomenon known as an Einstein ring.

At least seven groups of scientists have independently analyzed the ALMA data on SDP.81. Their flurry of research papers has divulged unprecedented information about the galaxy, revealing details about its structure, contents, motion, and other physical characteristics.

ALMA acts as an interferometer. Simply speaking, the array’s multiple antennas work in perfect synchrony to collect light as an enormous virtual telescope. As a result, these new images of SDP.81 have a resolution up to six times higher than those taken in the infrared with the Hubble Space Telescope.

The astronomers’ sophisticated models 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. These models were able to correct for the distortion produced by the magnifying gravitational lens.

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, equivalent to observing giant versions of the Orion Nebula producing thousands of times more new stars at the far side of the universe. This is the first time this phenomenon has been seen at such an enormous distance.

“The reconstructed ALMA image of the galaxy is spectacular,” said Rob Ivison from ESO. “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 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 showed that the gas in this galaxy is unstable; clumps of it are collapsing inwards 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 with 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.