Astronomers shed new light on cosmic dawn

University of Edinburgh astronomers have used the NASA/ESA Hubble Space Telescope to reveal a population of primitive galaxies that formed more than 13 billion years ago, when the Universe was less than 4% of its present age. One of these is probably the most distant galaxy found to date (at redshift 12). These new observations shed new light on the earliest years of cosmic history.

A team of scientists, led by Edinburgh astronomers Ross McLure and Jim Dunlop in collaboration with Richard Ellis at Caltech, have used the Hubble Space Telescope to make new observations of a tiny patch of sky called the Hubble Ultra Deep Field (HUDF), as part of a project to improve our understanding of the early years of the Universe. The resulting images offer the deepest ever view of the Universe. Although taken at near-infrared wavelengths, the images capture ultraviolet light emitted by the most distant galaxies, which has been “redshifted” to infrared wavelengths by the expansion of the Universe. Because light takes so long to travel from these remote objects, the astronomers are effectively looking back in time, and see these young galaxies as they appeared less than 600 million years after the Big Bang.

Previously unknown galaxies

The new data have allowed the team to uncover six previously unknown galaxies in this era and to rule out a number of tentative identifications of distant galaxies made by other scientists in previous research. This is the first reliable census of galaxies at such an early time in cosmic history, and shows that the number of galaxies steadily increased with time. This supports the idea that the first galaxies did not form in a sudden burst, but gradually assembled their stars, causing the Universe to slowly emerge from darkness into “cosmic dawn”.

One previously-claimed extreme redshift galaxy in the Hubble Ultra Deep Field has survived the scrutiny of the Edinburgh-USA team. This is UDFj-39546284, for a while claimed to be the most distant known galaxy, at redshift 10. However, the improved and extended dataset has allowed the scientists to show that it either lies at an even greater distance than previously thought (at a redshift of 12), or must be a hitherto undiscovered type of extreme emission-line galaxy at much lower redshift.

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

Studying galaxies in the early years of the Universe is fraught with difficulties. Scientists are working at the very limits of Hubble’s capabilities, and some previous claims of very distant galaxies have had to be revised in the light of improved observations. The scientists thus placed special emphasis on obtaining data of the quality necessary to provide robust identifications of galaxies at redshifts greater than 8. Part of this involves taking additional data through filters which have already been used to take deep images of the Hubble Ultra Deep Field, but the team has also added new imaging through previously unexploited filters. “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 James Dunlop of the Institute for Astronomy, University of Edinburgh.

UDF12 project

The observations, part of a project called UDF12, were made over a period of six weeks during August and September 2012, and the first results are now appearing in a series of scientific papers.The UDF12 team has now released the final images for public use, after preparing them for ease of exploitation by other research groups.

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

“This discovery of a significant population of galaxies at redshifts greater than 8, coupled with our new analysis of the number and properties of galaxies at redshift 7 and 8, support the idea that galaxies assembled progressively over time,” said the project co-PI Dr. Ross McLure from the Institute for Astronomy, University of Edinburgh.

The results from the UDF12 campaign suggest there will be many undiscovered galaxies even deeper in space waiting to be revealed by the James Webb Space Telescope, which will be launched in 2018.

The team’s new distant galaxy census, including the new candidate for the most distant object ever seen, has been accepted for publication in the Astrophysical Journal Letters. The paper is entitled: The abundance of star-forming galaxies in the redshift range 8.5 to 12: new results from the 2012 Hubble Ultra Deep Field campaign.

Images

Figure 1: A new robust sample of the most distant galaxies

This new image of the Hubble Ultra Deep Field (HUDF) reveals seven newly discovered distant galaxies, which are seen as they appeared in a period 350 million to 600 million years after the Big Bang. The galaxy census, the result of a program called `HUDF 2012’, provides the most robust sample of galaxies ever found at these early epochs. One of the galaxies may be a distance record breaker, observed only 380 million years after the birth of our universe. All were detected in near-infrared light using Hubble’s Wide Field Camera 3.

The coloured squares in the main image indicate the locations of these most distant galaxies within the Hubble Ultra Deep Field, while the boxes at the top show zoomed-in black-and-white images centred on each object. The “redshift” of each galaxy is indicated beside each box, denoted by the symbol “z”. Redshift measures how much a galaxy’s ultraviolet and visible light has been stretched to infrared wavelengths by the expansion of the Universe. The larger the redshift, the more distant the galaxy, and hence (due to the finite speed of light) the further we are seeing back in time. The most distant galaxy of these seven is the most remote object discovered to date, at redshift 11.9. This means we view it only 380 million years after the Big Bang, when the Universe was less than 3% of its present age.

The HUDF12 observations were taken in August and September 2012.

Credit: NASA, ESA, R. Ellis (California Institute of Technology, Pasadena, Calif.), J. Dunlop and Ross McLure (Institute for Astronomy, University of Edinburgh), B. Robertson (University of Arizona, Tucson), and A. Koekemoer (Space Telescope Science Institute, Baltimore, Md.).

Figure 2: UDF 12 Cosmic History

A schematic representation of the history of the Universe, highlighting the key epoch of "cosmic dawn" probed by the new 2012 Hubble Space Telescope observations. The Big Bang is on the left, with the present day on the right. Lookback time is indicated on the bottom, with redshift on the top. The space observatories shown in the picture are the Hubble Space Telescope on the right, and its successor the James Webb Space Telescope on the left, which is expected to launched in 2018 and will open up the study of galaxies beyond a redshift of 12.

Figure 3: Discovering the most distant galaxies with Hubble

This graphic illustrates the technique used by astronomers to discover the most distant galaxies with Hubble. The coloured shapes indicate the different wavebands of light that are detected in the images taken by Hubble through different filters. The blue-green filters on the left allow regions of normal optical light to pass through to Hubble’s optical camera (the Advanced Camera for Surveys; ACS), while the redder filters to the right allow infrared light through to be detected by the newer infrared-sensitive camera Wide Field Camera 3 (WFC3) which was installed in Hubble during the last shuttle servicing mission in 2009. By imaging the same patch of sky through these different filters, astronomers can establish the spectral shape of the light from different objects in the field-of-view.

The white curve then shows the distinctive shape anticipated from a very high-redshift galaxy, which shows a sharp step at ultraviolet wavelengths due to foreground absorption by neutral hydrogen gas in the young Universe. This sharp step is redshifted all the way from the ultraviolet to infrared wavelengths by redshifts greater than 7, and here its progressive movement to ever redder wavelengths is shown at redshifts of z = 8, 10 and 12. Objects like this are therefore detected in the red bands but disappear when imaged through the bluer filters. The lowest plot shows why Hubble cannot see objects at all much beyond z ~ 12.

Credit: NASA, ESA.

Papers

Ellis et al. 2012: http://adsabs.harvard.edu/abs/2012arXiv1211.6804E

Dunlop et al. 2012: http://adsabs.harvard.edu/abs/2012arXiv1212.0860D

Koekemoer et al 2012: http://adsabs.harvard.edu/abs/2012arXiv1212.1448K