Hubble Provides First Census of Galaxies Near Cosmic Dawn

 

Hubble Ultra Deep Field 2012 (z=8.6 Candidate)

Credit: NASA, ESA, R. Ellis (Caltech), and the UDF 2012 Team

Astronomers study the distant universe in near-infrared light because the expansion of space stretches ultraviolet and visible light from galaxies into infrared wavelengths, a phenomenon called “redshift.” The more distant a galaxy, the higher its redshift.

A major goal of the new program was to determine how rapidly the number of galaxies increases over time in the early universe. This measure is the key evidence for how quickly galaxies build up their constituent stars.

“Our study has taken the subject forward in two ways,” Ellis explained. “First, we have used Hubble to make longer exposures than previously. The added depth is essential to reliably probe the early period of cosmic history. Second, we have used Hubble’s available color filters very effectively to more precisely measure galaxy distances.”

 

Hubble Ultra Deep Field 2012 (z=11.9 Candidate)

Credit: NASA, ESA, R. Ellis (Caltech), and the UDF 2012 Team

The team estimated the galaxy distances by studying their colors through a carefully chosen set of four filters at specific near-infrared wavelengths. “We added an additional filter, and undertook much deeper exposures in some filters than in earlier work in order to convincingly reject the possibility that some of our galaxies might be foreground objects,” said team member James Dunlop of the Institute for Astronomy, University of Edinburgh.

For galaxies whose light has been shifted to infrared wavelengths, Dunlop said, the intervening hydrogen will have absorbed all of the light that was originally emitted as visible light and most of the light initially released at near-infrared wavelengths. Therefore, these galaxies will not be detected in most of Hubble’s filters. They will only be seen in Hubble’s longer-wavelength infrared filters, which hold the key to discovering the earliest galaxies.

 

 

This illustration depicts cosmic history, from the Big Bang approximately 13.7 billion years ago (starting from the left) until the local universe today (far right). A detailed description of the illustration follows after the break.

 

Immediately after Recombination, there were no stars or galaxies in the universe. Without sources of light the universe existed in what we call the “Cosmic Dark Ages”. However, overtime the dark matter and gas in the universe could form bound objects through gravity. As the gas in these objects became dense, they could form the first stars. We think these first stars formed at or around redshift z~20, or about 100-200 million years after the Big Bang. As more of these stars formed into populations, the first true galaxies came into being. The first galaxies form probably soon after the first stars.

The first stars and galaxies are likely so faint that only the James Webb Space Telescope can observe them. However, with the Hubble Space Telescope the UDF12 team has been able to peer back in time to observe early galaxies in the range redshift z~8.5-12 (marked as Hubble 2012 in this illustration, about 380-650 million years after the Big Bang). We have found the earliest known population of galaxies, and they appear to be already well-developed and likely not the first galaxies forming in the universe. This exciting discovery demonstrates that there will be a rich hunting ground of galaxies at yet higher redshifts for the James Webb Space Telescope.

The UDF12 data builds on previous observations taken with Hubble in 2009 that reached only up to redshift z~8. We have been able to extend the depth probed by the Hubble Space Telescope by another factor of 2, roughly doubling the infrared observations in the Hubble Ultra Deep Field.