MAUNA KEA, HAWAII — An international team of astronomers has obtained the best view yet of a collision that took place between two galaxies when the universe was only half its current age using the W. M. Keck Observatory and many other telescopes on the ground and in space.
To make this observation, the team also enlisted the help of a gravitational lens, a galaxy-size magnifying glass, to reveal otherwise invisible detail. These new studies of galaxy HATLAS J142935.3-002836 have shown that this complex and distant object looks surprisingly like the comparatively nearby pair of colliding galaxies collectively known as the Antennae.
“While astronomers are often limited by the power of their telescopes, in some cases our ability to see detail is hugely boosted by natural lenses, created by the universe,” explains lead author Hugo Messias of the Universidad de Concepción (Chile) and the Centro de Astronomia e Astrofísica da Universidade de Lisboa (Portugal), “Einstein predicted in his theory of general relativity that, given enough mass, light does not travel in a straight line but will be bent in a similar way to light refracted by a normal lens.”
These cosmic lenses are created by massive structures like galaxies and galaxy clusters, which deflect the light from objects behind them due to their strong gravity — an effect, called gravitational lensing. The magnifying properties of this effect allow astronomers to study objects that would not be visible otherwise and to directly compare local galaxies with much more remote ones, seen when the universe was significantly younger.
But for these gravitational lenses to work, the lensing galaxy, and the one far behind it, need to be very precisely aligned.
“These chance alignments are quite rare and tend to be hard to identify,” Messias said, “but, recent studies have shown that by observing at far-infrared and millimeter wavelengths we can find these cases much more efficiently.”
H-ATLAS J142935.3-002836 (or just H1429-0028 for short) is among the brightest gravitationally lensed objects in the far-infrared regime found so far, even though we are seeing it at a time when the universe was just half its current age.
Probing this object was at the limit of what is possible, so the international team of astronomers started an extensive follow-up campaign using Keck Observatory‘s NIRC2 instrument installed on the 10-meter, Keck II telescope, as well as the NASA/ESA Hubble Space Telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), the Karl Jansky Very Large Array (JVLA), and others. The different telescopes provided different views, which could be combined to get the best insight yet into the nature of this unusual object.
The Keck Observatory and Hubble images revealed a detailed gravitationally-induced ring of light around the foreground galaxy. These high resolution images also showed that the lensing galaxy is an edge-on disc galaxy – similar to our galaxy, the Milky Way – which obscures parts of the background light due to the large dust clouds it contains.
“Keck Observatory’s contribution was actually quite critical to the study.” Messias, said, “The high resolution enabled by the adaptive optics mode on Keck II, allowed us to de-blend the foreground lensing galaxy from the background lensed galaxy in the form of an Einstein ring. With these estimates in H and Ks-bands, together with HST imaging at 1.1 micron, we were then able to derive a much improved estimate of the stellar content of the background galaxy. In the process, we also derived the foreground stellar mass contribution to the lens effect.”
There was a second contribution which is noteworthy, he said. “Although ALMA allowed us to determine the lens model, together with JVLA, and the dynamics of the background source, in reality, ALMA barely sees one of the two colliding galaxies, because it is a lot poorer in gas and dust than the one fully detected. Keck and HST’s imaging thus confirmed the existence of the second galaxy.”
Further characterization of the object was undertaken by ALMA which traced carbon monoxide, allowing for detailed studies of star formation mechanisms in galaxies and for the motion of the material in the galaxy to be measured. This confirmed that the lensed object is indeed an ongoing galactic collision forming hundreds of new stars each year, and that one of the colliding galaxies still shows signs of rotation; an indication that it was a disc galaxy just before this encounter.
The telescopes and surveys that were employed were: the W. M. Keck Observatory’s Keck II telescope, NASA/ESA Hubble Space Telescope, ALMA, APEX, VISTA, the Gemini South telescope, the NASA Spitzer Space Telescope, the Jansky Very Large Array, CARMA, IRAM, and SDSS and WISE.
The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and world-leading laser guide star adaptive optics systems.
NIRC2 (the Near-Infrared Camera, second generation) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.
Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.