Empty Sky? Not To Hubble’s Eye

Peering deep into the early universe, this picturesque parallel field observation from the NASA/ESA Hubble Space Telescope reveals thousands of colorful galaxies swimming in the inky blackness of space. A few foreground stars from our own galaxy, the Milky Way, are also visible.
Peering deep into the early universe, this picturesque parallel field observation from the Hubble Space Telescope reveals thousands of colorful galaxies swimming in the inky blackness of space. A few foreground stars from our own galaxy, the Milky Way, are also visible. Credit: NASA, ESA and the HST Frontier Fields team (STScI)

Rarely has so much been accomplished by staring into empty space. But that’s exactly what astronomers have been doing with the Hubble Space Telescope in the Frontier Fields program since 2012. It works like this: while Hubble keeps one eye  (camera) on a distant galaxy cluster, it aims another at an adjacent but empty field of view. In the first, the massive cluster acts as a “gravitational lens”. Its immense gravity warps and magnifies the light of far more distant galaxies directly behind the cluster, coaxing objects into view what would otherwise be invisible.

This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada
This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends and brightens the light of a distant galaxy as well as creating multiple images of it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada

Gravitational lensing was predicted by Einstein in his General Theory of Relativity Hubble 101 years ago, but it took big telescopes and sensitive detectors to finally prove Einstein’s assertion. The first gravitational lens was discovered only in 1979. Now, thanks to lensing, Hubble can capture photos of galaxies so far away, they fired up not long after the universe made its debut in the Big Bang.

At the same time as the cluster and lensed galaxies are being recorded, a second Hubble camera soaks up light in a “parallel field”, an empty bit of sky next door to the cluster. Much like trick o’ treat on Halloween, you never know what might fall into Hubble’s bag in these adjacent views. Astronomers want to find out if the dense carpeting of galaxies seen in earlier deep images varies across the sky. Just look at the wealth of galaxies in that bit of sky! Nearly every disk and speck you see is a galaxy, not a foreground Milky Way star.

The massive galaxy cluster Abell 2744, nicknamed Pandora's Cluster, takes on a ghostly look in this NASA/ESA Hubble Space Telescope view. In this image the total starlight from the cluster has been artificially coloured blue. This reveals that not all the starlight is contained within the cities of stars — the galaxies — which appear as bright blue-white blobs. A fraction of the starlight is also dispersed throughout the cluster, as seen in the darker blue regions. This light comes from dead galaxies. The galaxies were torn apart long ago by the cluster's gravitational forces, and their stars were scattered into what is known as intracluster space — the space between the galaxies. These orphaned stars roam the cluster, without being gravitationally tethered to any single galaxy. Because these extremely faint stars are brightest at near-infrared wavelengths of light, this type of observation could only be accomplished with Hubble’s infrared sensitivity to extraordinarily dim light. Credit: NASA, ESA, M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)
The massive galaxy cluster Abell 2744, nicknamed Pandora’s Cluster, looks ghostly in this Hubble view. The blue haze is artificially-colored starlight, and the galaxies appear as bright, blue-white blobs. The blue haze represents light from billions of stars from dead galaxies that were torn apart long ago by the cluster’s gravity. These orphaned stars roam the cluster, without being gravitationally tethered to any single galaxy. The odd streaks and arcs are lensed galaxies from much deeper in space — about 12 billion light years in the distance.  Credit: NASA, ESA, M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Hubble has probed these cosmic lenses before, but this concentrated effort has devoted 560 orbits around the Earth — 630 hours of camera time — to just six clusters of galaxies and their parallel fields. They were chosen based on features like the strength of their lensing, their ability to support complementary observations by telescopes like the Spitzer Space Telescope, the upcoming James Webb Space Telescope, and the avoidance of bright stars whose light would obstruct the faint light from distant galaxies. The photo featured at the top of this article was made at the same time as the f camera focused in the galaxy cluster Abell 2744, nicknamed Pandora’s Cluster, in the southern constellation of Sculptor and located some 3.5 billion light years from Earth.

Every time Hubble takes a step backward in time, we take a step forward in our understanding of the early universe.