The first analysis of observations made of the galaxy cluster Abell 2744 with NASA’s Hubble and Spitzer space telescopes has revealed the presence of a candidate galaxy located approximately 13 billion light-years away, making it one of the brightest and most distant cosmic objects discovered to date.
The results come from a new observing campaign called Frontier Fields. As reported in a previous AmericaSpace article, the Frontier Fields utilises NASA’s Great Observatories—the Hubble, Spitzer, and Chandra space telescopes—to create six new Deep Fields centered around specific galaxy clusters that could allow astronomers to delve even deeper into the Universe, using gravitational lensing as a helping tool.
Gravitational lensing can be described as the phenomenon of the bending of light of distant, faraway cosmic sources from the gravity of massive objects that lie in between. The gravity of these intermediate objects bends and refocuses the light of the more distant sources, acting like a lens, allowing us to observe distant parts of the Universe that would otherwise be beyond our view. Gravitational lensing, as an effect of the curvature of space-time by gravity, was first described by Einstein’s theory of General Relativity in the early 20th century and was later observed by astronomers in their studies of distant galaxies and galactic clusters.
The first galaxy cluster to be observed as part of the Frontier Fields program, Abell 2744 (also known as the Pandora Cluster), is believed to be the result of previous collisions between four smaller galaxy clusters and lies approximately 4 billion light-years away at the direction of the southern constellation Sculptor. Most of the cluster’s mass is believed to be composed of invisible dark matter, making it a perfect choice as a gravitational lensing tool for bringing previously unobservable distant objects into clear focus.
Astronomers started their observations of Abell 2744 in infrared wavelengths in September 2013 using Spitzer’s Infrared Array Camera, or IRAC, and in visible and near-infrared wavelengths in October, using Hubble’s Advanced Camera for Surveys, or ACS, and Wide Field Camera 3, or WFC3, respectively. These initial set of observations will be followed up with additional ones through July of this year. The additional observations are part of the Frontier Fields observing strategy. Six months after Hubble has started observing a cluster, when the Earth will be at the opposite side of the sky in its orbit, the space telescope will have an 180-degree opposite orientation. As an effect, its two cameras will “swap” places, each camera now observing the other’s previous spot on the same cluster, thus allowing for more detailed and overlapping set of observations.
During the short amount of time since the start of the Frontier Fields, the program has already showed its potential, allowing astronomers to discover thousands of previously unseen and distant galaxies with the help of Abell 2744’s gravitational lensing effects. Now, an international team of astronomers led by Nicolas Laporte from the Instituto de Astrofísica de Canarias in Santa Cruz de Tenerife, Spain, has published a new study in the February 2014 issue of the journal Astronomy & Astrophysics, detailing the discovery of a very distant candidate galaxy with a measured redshift of eight.
Astronomers use redshift as a measure of distance to describe how far away from Earth an object is in the Universe. Any cosmic object that is moving away from us will result in its light being shifted toward the red part of the spectrum—its wavelength will be increased due to the expansion of the Universe. The higher the redshift of an object, the higher it is moving away from us and the more shifted toward longer wavelengths its light will be, ultimately making it visible only in the infrared and far-infrared parts of the electromagnetic spectrum.
The discovery of the object detailed in the study, named Abell2744_Y1, was made with the use of the “Lyman-break technique,” which identifies galaxies using a set of color filters in the ultraviolet part of the spectrum by looking for the presence of certain emission lines of hydrogen, called the Lyman series. Although these emission lines are normally present in ultraviolet wavelengths, the expansion of the Universe would make these spectral lines emitted by a very distant receding object to be shifted toward longer wavelengths, appearing in the optical or infrared part of the spectrum. That would indicate that the object emitting the light is indeed many billions of light-years away. Laporte’s team utilised all seven color filters on Hubble’s Advanced Camera for Surveys for the Abell 2744 observations and combined them with previous observations of the cluster made by Hubble in optical wavelengths and with the recent ones made by Spitzer in the infrared. This combined dataset allowed them to increase their total coverage of wavelength observations, allowing for more precise measurements.
Their results show that Abell2744_Y1 lies at a distance of approximately 13.2 billion light-years away, observed at a time when the Universe was 650 million light-years old. “Just a handful of galaxies at these great distances are known,” says Jason Surace, an astronomer at the California Institute of Technology and Co-Investigator at NASA’s Spitzer Science Center in Pasadena. “The Frontier Fields program is already working to find more of these distant, faint galaxies. This is a preview of what’s to come.”
Indeed, this discovery comes to add itself to the long list of discoveries of very distant cosmic objects made through the years, some of which have been observed to exhibit redshifts as high as 12. Still, most of them await spectroscopic confirmation in order to be verified. Such is the case with Abell2744_Y1 as well, which was only made visible through the magnifying effects of gravitational lensing. “We expected to find very distant galaxies close to the cluster core, where the light amplification is maximum,” notes Laporte. “However, this galaxy is very close to the edge of the Hubble image where the light is not strongly amplified. We are really lucky that we could find it in the small field of view of Hubble.”
Yet the recent Hubble observations show Abell2744_Y1 to be of a clumpy, irregular shape with a size about 30 times smaller than that of our Milky Way galaxy, and a star-formation rate 10 times more intense. These characteristics are in agreement with what is predicted by current models of the evolution of galaxies in the very early Universe, strengthening the case for Abell2744_Y1 to really being a protogalaxy in its early stages of development. “It’s morphology is exactly what is expected for a high-redshift object,” says Laporte. “The next step is to confirm this object by spectroscopy. In 15 hours [of observing time] we hope to be able to confirm this object, using the X-Shooter and K-band Multi-Object Spectrograph [installed on the Very Large Telescope on Cerro Paranal in Chile].”
NASA’s Frontier Fields program is already providing us with tantalising hints of what is to come in the future, in our efforts to probe even deeper into the infinite reaches of the Cosmos. The objects that the program will bring into view, like Abell2744_Y1, will be prime candidates for follow-up studies by the next generation of ground and space-based observatories, like the planned 39-m European Extremely Large Telescope in Chile and the joint NASA/ESA James Webb Space Telescope. We can only speculate as to the kind of wonders that this next generation of instruments will reveal.