Hubble’s Deepest Look

At the ripe old age of 25, the Hubble Space Telescope is breaking new ground, peering deeper than ever before. How so? By harnessing gravitationally-warped space-time to beat its old records.

To mark the 25th birthday of the Hubble Space Telescope the Conversation asked a group of astronomers to pick their favorite Hubble images. I picked this one. Here’s why.

First, this is what Hubble is all about. It’s a single, awe-inspiring view that unmasks so many things about our Universe: its distant past, its ongoing assembly, and even the fundamental physical laws that tie it all together.

We’re peering through the heart of a swarming cluster of galaxies called Abell 2744 (also called Pandora’s Cluster), the deepest such image of a galaxy cluster ever obtained. It’s part of a new path forward for Hubble called the Frontier Fields program that teams Hubble with NASA’s other Great Observatories — the Spitzer Space Telescope and the Chandra X-ray Observatory — to probe not just galaxy clusters but also deep behind them, revealing the faintest galaxies yet detected, surpassing even the famous Hubble Ultra Deep Field.

The Hubble Space Telescope’s Ultra Deep Field. Prior to the Fronier Fields program, this was the deepest look into the Universe available to human kind. Credit: NASA, ESA, H. Teplitz, M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

Let’s look again at the Frontier Fields image of Pandora’s Cluster so that we can begin to appreciate the rich science that’s there.

Hubble Frontier Fields image of Pandora’s Cluster. Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Those glowing white balls are giant galaxies that dominate the cluster center. Look closely and you’ll see diffuse shreds of white light being ripped off of them! We can play with the exposure a little bit to really bring that shredded light out and it looks like this.

Credit: NASA, ESA, M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Stars that belong to the cluster have been artificially colored blue. Everything else is either in front or behind. What’s happening here is that the cluster is acting like a gravitational blender, churning many individual galaxies into a single cloud of stars. We think that this kind of process happens whenever galaxies get together, but in giant galaxy clusters like this one, this kind of “ghostly” intra-cluster light can actually contain more stars than the stars in the galaxies themselves!

But the Pandora Cluster is just the first chapter in the cosmic story being revealed by the Frontier Fields image. See those faint blue rings and arcs? Those are the distorted images of other galaxies that sit far in the distance.

The immense gravity of the cluster causes the space-time around it to warp. As light from distant galaxies passes by, it’s forced to bend into weird shapes, like a warped magnifying glass would distort and brighten our view of a faint candle. Leveraging our understanding of Einstein’s General Relativity, Hubble is using the cluster as a gravitational telescope, allowing us to see farther and fainter than ever before possible. We are looking far back in time to see galaxies as they were more than 13 billion years ago!

Gravitational lensing map for Pandora’s Cluster. Overlay of magnification (red) and mass models (blue) on the full-band HST imaging (green) made by Merten, Zitrin et al. Source:

In order to understand exactly what we’re seeing behind the cluster we need to understand how the background light is being warped and magnified by the gravity of the cluster. The map above shows a model of this effect. The blue colors show the mass of the cluster (which is mostly dark matter) and the red shows the areas of highest magnification. Galaxies we see near those red lines have been brightened a lot by the magnifying power of the distorted space-time around the cluster.

Credit: NASA, ESA, A. Zitrin (California Institute of Technology), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

One of the amazing things about gravitational lensing is it allows us to see the same galaxy in multiple places at once! Light can be deflected along several paths and still converge on a single spot. The small white boxes in the picture above (labeled a, b, c) are in fact three images from the same background galaxy. This galaxy is in fact one of the most distant and faintest galaxies ever seen. We are seeing this tiny galaxy as it was before the Earth existed, even before many of the atoms in our bodies existed!

The inset boxes show zoom-in views of the distant galaxy, one for each of the three images. Because of the magnification of the cluster, each galaxy appear as much as 10 times larger and brighter than it would have had we not had Pandora’s Cluster in the way to boost the signal.

As a theorist, I want to understand the full life cycle of galaxies — how they are born (small, blue, bursting with new stars), how they grow, and eventually how they die (big, red, fading with the light of ancient stars). Hubble allows us to connect these stages. Some of the faintest, most distant galaxies far behind Pandora’s Cluster (like the one shown three times in the picture above) are destined to become monster galaxies like those glowing white in the foreground.

Hubble at 25 is allowing us to see the distant past and the present in a single glorious picture. Here’s to another 25 years of awesome space telescopes.

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