Kavli Institute Fellow Michele Trenti and his team have been awarded a large allocation of Hubble Space Telescope time to carry out a search for the most distant galaxies that Hubble can see. Trenti's "Brightest of Reionizing Galaxies" Hubble survey, BoRG in short, will look for objects whose light started its journey toward Earth when the Universe was just about 500 Million years old, that is more than 13.2 billion years ago (a redshift factor of z~9). These distant galaxies can be identified by observing the same patch of the sky at different wavelengths, and then by selecting sources that appear only in the images at the longest wavelengths, that is qualitatively with "red" colors. The light from these distant galaxies is observed today in the near infrared, just beyond the range of what the human eye can see, because of a combination of the Universe expansion, which stretches light toward the red, and absorption of the shorter-wavelength visible light by intergalactic hydrogen.
Previous observations with Hubble found a handful of such galaxies, but so far the search has been limited to only a few pointings in the sky, like the Hubble UltraDeep Field. The BoRG survey is instead expected to acquire images at more than one hundred different locations in the sky. Hubble's field of view is tiny, so the total area covered by the BoRG observations in the infrared will "only" be approximately equal to seven tenths of the full moon area. Yet, the survey will take more than one month time to be executed during the next yearly cycle of observations, starting in October. The BoRG survey is the largest program approved this year by the Hubble Time Allocation Committee with an award of 480 orbits, or 32 days at 96 minutes per orbit, of which about 18 days will be spent taking data on target. "I am still amazed by realizing that each time I will think of Hubble next year there will be a 1 in 12 chance that the spacecraft will be busy with the BoRG program." says Trenti. Then he continues: "Hubble is arguably the most iconic and productive telescope ever built, and as such it is highly oversubscribed. Thus, I feel very privileged for the opportunity to make such an extensive use of it". Since Trenti and his team aim at exploring a large number of pointings in the sky, they will be able to share observing time with other programs whose main goal is that of taking spectra of distant stars or active galactic nuclei. In fact, instruments are mounted at different positions in the Hubble's focal plane, making it possible to observe two slightly offset regions of the sky with a camera and a spectrograph at the same time. "It is almost like doubling the efficiency of an already incredibly productive telescope!" explains Trenti.
The rate of galaxy formation seems to evolve dramatically at times earlier than 700 million years after the Big Bang, making this epoch a golden age for the formation of the first generations of galaxies. Therefore, it is crucial to understand if and how this evolution proceeds with look-back time. At 500-600 million years after the Big Bang, the BoRG team expects to find two dozen galaxies with brightness comparable to that of our own Milky Way, but with masses ten to hundred times smaller owing to the higher luminosity of the young and hot stars which are expected in systems formed when the Universe was in its youth. "We are exploring galaxy formation when the Universe was just half a billion years old, and to me it is fascinating to think that the process has been so efficient that galaxies each made of billions of stars were already in place. But we are not really sure of what we will find by pushing these observations to Hubble's final frontier. The definitive answer will come only after we get the images." In addition to this higher risk but higher reward search for the most distant galaxies, the BoRG observations should lead to a sample of around 200 galaxies at 600-900 million years after the Big Bang, or about 13 billion years ago. Finally, the images will capture an additional quarter million galaxies between Earth and the more distant objects the team is hunting for.
"The capabilities of the Hubble Space Telescope continue to amaze anyone working with its cameras and spectrographs." says BoRG Co-Investigator Prof. Mike Shull, currently at the Institute of Astronomy in Cambridge as Sackler Distinguished Visitor, concluding "The opportunity to search the cosmos for some of the earliest, most distant galaxies is fantastic. It gives us just a taste of what James Webb Space Telescope may accomplish at the end of the decade."
The BoRG team, led by Kavli Institute Fellow Michele Trenti at the University of Cambridge, is an international collaboration including astronomers at the Space Telescope Science Institute (Baltimore, MD, USA); the University of California, Santa Barbara and Los Angeles campuses (USA); the University of Colorado, Boulder (USA); Yale University (USA); Leiden University (NL); Eidgenossische Technische Hochschule Zurich (CH); and Pontificia Universidad Catolica (Chile).
Optical and near-IR color composite of one WFC3 Hubble pointing (about 2x2 square arcmin) illustrating a typical view of the distant Universe as seen by Hubble with a few hours of observing time. Credit: NASA, ESA, M. Trenti, L. Bradley, and the BoRG team.