The BoRG survey boosts numbers of earliest galaxies ever known
28 July 2016
Between 100 million to one billion years after the Big Bang, the Universe went through a transition, where neutral hydrogen was ionised, transforming the intergalactic medium from opaque to transparent. This "Epoch of Reionisation" is receiving intense research focus, as there are still many open questions about this period. While future telescopes like the Square Kilometre Array (SKA) will be able to look at the neutral hydrogen, observing the stars and galaxies that were the sources of reionisation is still difficult. One possibility of investigating the formation and evolution of galaxies in the early Universe, along with the progression of reionisation, is to observe the ultraviolet light emitted by these high-redshift sources (stretched to be detected at infrared wavelengths from Earth) with space telescopes, located above the Earth’s atmosphere. Deep legacy surveys with the Hubble Space Telescope (HST) have produced a growing sample of galaxies during this period.
At the highest redshifts, the number of galaxy candidates is still small, with only a handful detected in HST imaging. To increase the sample, CAASTRO PhD student Stephanie Bernard (University of Melbourne), along with international collaborators, used HST/Wide Field Camera 3 (WFC3) imaging from the Brightest of Reionising Galaxies (BoRG) survey to search for the brightest galaxies only 500 million years after the Big Bang. The team found six interesting candidates in archival data. They conclude that half of the sample are strong candidates to be some of the highest-redshift galaxies so far discovered, while the other half likely to be lower-redshift sources.
The researchers also calculated the "number density", that is, the number of high-redshift galaxies in a given volume. They then determined the "luminosity function" or the number of galaxies at a particular brightness. Interestingly, when compared to previous calculations from surveys like the Cosmic Assembly Near-infrared Deep Intergalactic Legacy Survey (CANDELS), which also used WFC3 imaging, Bernard and her collaborators found almost ten times as many bright galaxies than expected. They also noted that three candidates in their sample are located in the same WFC3 pointing, which is an indication of significant clustering, as expected for very bright galaxies at high redshift. Ongoing follow-up observations by the team will shed more light on the nature of these candidate galaxies.