Hubble captured an image of these galaxies after 10 days of exposure. Credit: R. Williams (STScI)/Hubble Deep Field Team/NASA

Any one picture may say a thousand words, but only a few can show us a thousand galaxies. These are the images of the Hubble Deep Field and its subsequent iterations, taken by the Hubble Space Telescope. They show scientists more than 10,000 galaxies.

In 1996, Hubble’s Wide Field and Planetary Camera 2 (WFPC2) first focused on a tiny patch of sky—about the width of a dime if placed about 22 meters away—near the Big Dipper’s handle. Over 10 days and with 300 separate exposures, Hubble’s camera peered deeper into the universe than ever before, unveiling glimpses of more than 1500 galaxies in various stages of evolution.

Hubble’s Deep Field images are often likened to a core sample of the universe.

Hubble’s Deep Field (HDF) images are often likened to a core sample of the universe. Because these galaxies are so far away and light takes time to travel from such far reaches of the universe to Hubble’s cameras, the deep field images also offer a peek back in time. Scientists study them to investigate the formation of young galaxies, which can teach them about our own galaxy, the Milky Way.

“One of the great legacies of the Hubble Telescope will be these deep images of the sky showing galaxies to the faintest possible limits with the greatest possible clarity from here out to the very horizon of the universe,” Harry Ferguson, an astronomer who contributed to the HDF image, said when it was released in 1996.

Multiple Views

In 2004, the Hubble team turned to a patch of sky in the constellation Fornax and released the Ultra Deep Field image, illuminating more than 10,000 galaxies and gazing farther back in time than ever before. These newly captured galaxies coalesced during the “dark ages,” a term astronomers use to describe the universe’s early years after the big bang, when the first stars appeared.

Scientists peer into the “dark ages” of star formation in Hubble’s Ultra Deep Field image. Credit: NASA/ESA/S. Beckwith (STScI)/HUDF Team

Researchers cobbled this image together using data from two different instruments aboard Hubble: the Advanced Camera for Surveys and the Near Infrared Camera and Multi-object Spectrometer. Both are many times more sensitive than WFPC2 and are tuned to catch light that is redshifted, meaning that they find sources of light moving away from Earth.

Because this Ultra Deep Field image captures a time shortly after the big bang, scientists could use it to study galaxies at very early stages of the universe’s evolution.

Although scientists observed the earliest galaxies at the edge of the universe in their infancy using Hubble’s infrared cameras, they were still missing the most recent 9–10 billion years of star formation. To get this view, scientists needed to capture images in ultraviolet light, which is less redshifted; that is, the light is moving away from us less quickly. This reveals the turbulent beginnings of the largest, hottest stars that are closer to Earth than those at the universe’s edge.

Using the ultraviolet spectrum, scientists discovered more about the life cycle of stars. Credit: NASA/ESA/H. Teplitz and M. Rafelski (IPAC/Caltech)/A. Koekemoer (STScI)/R. Windhorst (ASU)/Z. Levay (STScI)

In 2009, astronauts added to Hubble the Wide Field Camera 3, which images in ultraviolet light as well as infrared. Scientists then pointed Hubble to the same patch of sky that revealed the original Ultra Deep Field image and combined 10 years of existing images with new ultraviolet data. In 2014, the Hubble team released the most comprehensive view of the universe ever seen.

Hubble’s UV-Ultra Deep Field image lit the way for scientists to fully study how galaxies around the universe—including our own Milky Way—grew from small clouds of gas to their large, stable structures today, further filling in our cosmic timeline.

For a detailed look at Hubble’s 25 years of breathtaking images and groundbreaking science, read the feature story. For more on how scientists create Hubble images, check out our earlier story.

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2015), Gazing toward the Universe’s edge: Hubble’s deep field legacy, Eos, 96, doi:10.1029/2015EO029053.

Text © 2015. The authors. CC BY-NC 3.0
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