Even after almost three decades of faithful service, the Hubble Space Telescope continues to operate and provide breathtaking images of the cosmos. As one of NASA’s Great Observatories, its observations of distant galaxies, exoplanets, and the expansion of the Universe have had a revolutionary impact on astronomy, astrophysics and cosmology.
Hubble’s latest contribution comes in the form of a deep-sky mosaic image that was constructed using 16 years’ worth of observations. Known as the “Hubble Legacy Field“, this mosaic is being described as the largest and most comprehensive “history book” of galaxies. All told, it contains roughly 265,000 galaxies that date back to just 500 million years after the Big Bang.
Nearly 7,500 individual exposures went into the creation of the Hubble Legacy Field, providing a wide portrait of the distant Universe that looks back to the earliest visible times. In so doing, the image shows how galaxies have changed over time, growing through mergers to become the giant galaxies we see in the Universe today. This effectively means that 13.3 billion years of cosmic evolution have been chronicled in this one image.
This ambitious endeavor comprises the collective work of 31 Hubble programs by different teams of astronomers. It also incorporated observations taken by several Hubble deep-field surveys. These include the Hubble Deep Field in 1995, the Great Observatories Origins Deep Survey (GOODS) of 2003, the Hubble Ultra Deep Field of 2004, and the eXtreme Deep Field (XDF) of 2012, which is the deepest view of the Universe to date.
As Garth Illingworth, Professor Emeritus at UCSC and head of the team that assembled the image, said in a recent NASA press release:
“Now that we have gone wider than in previous surveys, we are harvesting many more distant galaxies in the largest such dataset ever produced. No image will surpass this one until future space telescopes like James Webb are launched.”
In addition to showing galaxies in the visible light, the wavelength range spans from the ultraviolet to the near-infrared part of the spectrum. This is key in modern astronomy and cosmology, in that it allows key features of galaxy assembly to be made apparent. A good example is cosmic dust and gas, which are not always visible unless they illuminated by nearby stars.
“Such exquisite high-resolution measurements of the numerous galaxies in this catalog enable a wide swath of extragalactic study,” said catalog lead researcher Katherine Whitaker of the University of Connecticut, in Storrs. “Often, these kinds of surveys have yielded unanticipated discoveries which have had the greatest impact on our understanding of galaxy evolution.”
About a century ago, Edwin Hubble (for whom the HST is named) described galaxies are the “markers of space”. At the time, he was observing distant galaxies and noted how light coming from the majority of them was shifted towards the red end of the spectrum – aka. “redshifted”, which is an indication that astronomical objects are moving away from us.
These observations confirmed a prediction made by Einstein’s Theory of General Relativity – that the Universe was either in a state of expansion or contraction. Subsequent surveys have used galaxies to measure the rate of cosmic expansion (known as the Hubble Constant), which has also offered clues as to the underlying physics of the cosmos, when chemical elements originated, and how our Solar System and life eventually appeared.
This wider view is especially helpful in that respect since it contains about 30 times as many galaxies as the previous deep fields. The Legacy Field has also revealed several unusual objects, many of which are the remnants of collisions and mergers that took place during the early Universe – what are referred to as galactic “train wrecks”.
As you can imagine, assembling this image was no easy task. As Dan Magee, of the University of California, Santa Cruz, the team’s data processing lead, explained:
“Our goal was to assemble all 16 years of exposures into a legacy image. Previously, most of these exposures had not been put together in a consistent way that can be used by any researcher. Astronomers can select the data in the Legacy Field they want and work with it immediately, as opposed to having to perform a huge amount of data reduction before conducting scientific analysis.”
Despite being the most detailed and expansive image of galaxies ever taken, this new image is merely the first in a series of Hubble Legacy Field images. The team is currently working on another set of images, which total more than 5,200 Hubble exposures, from another area of the sky. Looking ahead, astronomers hope to broaden the multiwavelength range in the legacy images to include even more data on galaxies.
This will include longer-wavelength IR and high-energy X-ray observations from two other NASA Great Observatories – the Spitzer Space Telescope and the Chandra X-ray Observatory. As team member Rychard Bouwens of Leiden University in the Netherlands said in ESA press release:
“One exciting aspect of these new images is the large number of sensitive color channels now available to view distant galaxies, especially in the ultraviolet part of the spectrum. With images at so many frequencies, we can dissect the light from galaxies into the contributions from old and young stars, as well as active galactic nuclei.”
In the meantime, no image of the Universe is expected to surpass the Hubble Legacy Field images one until next-generation space telescopes are launched. These include the James Webb Space Telescope (JWST) and the Wide-Field Infrared Space Telescope (WFIRST), both of which have instruments will that offer improved resolution and sensitivity over Hubble and thus enable more in-depth surveys.
Hubble Legacy Field image. Credit: NASA/ESA/G. Illingworth and D. Magee (UCSC), K. Whitaker (University of Connecticut), R. Bouwens (Leiden University), P. Oesch (University of Geneva)/Hubble Legacy Field team
The vast number of galaxies in the Legacy Field image are also prime targets for future telescopes. As Illingworth said in a HubbleSite press release:
“We’ve put together this mosaic as a tool to be used by us and by other astronomers. The expectation is that this survey will lead to an even more coherent, in-depth and greater understanding of the universe’s evolution in the coming years… This will really set the stage for NASA’s planned Wide-Field Infrared Survey Telescope (WFIRST). The Legacy Field is a pathfinder for WFIRST, which will capture an image that is 100 times larger than a typical Hubble photo. In just three weeks’ worth of observations by WFIRST, astronomers will be able to assemble a field that is much deeper and more than twice as large as the Hubble Legacy Field.”
In addition, the JWST’s imaging capabilities in the IR band (which are beyond the limits of Hubble or Spitzer) will allow astronomers to probe much deeper into the Legacy Field image to reveal more about how infant galaxies grew. The image (along with the individual exposures that went into making it) is available through the Mikulski Archive for Space Telescopes (MAST).