Free Astronomy Magazine May-June 2015

SPACE CHRONICLES missioned on the VLT in 2014 was a long hard look at the Hubble Deep Field South (HDF-S). The results ex- ceeded expectations. “After just a few hours of observa- tions at the telescope, we had a quick look at the data and found many galaxies — it was very encour- aging. And when we got back to Europe we started exploring the data in more detail. It was like fish- ing in deep water and each new catch generated a lot of excitement and discussion of the species we were finding,” explained Roland Bacon (Centre de Recherche Astro- physique de Lyon, France, CNRS) principal investigator of the MUSE instrument and leader of the com- missioning team. For every part of the MUSE view of HDF-S there is not just a pixel in an image, but also a spectrum reveal- ing the intensity of the light’s dif- ferent component colours at that point — about 90,000 spectra in total (each spectrum covers a range of wavelengths from the blue part of the spectrum into the near-infra- red (475 ‒ 930 nanometres). These can reveal the distance, composi- tion and internal motions of hun- dreds of distant galaxies — as well as catching a small number of very faint stars in the Milky Way. Even though the total exposure time was much shorter than for the Hub- ble images, the HDF-S MUSE data revealed more than twenty very faint objects in this small patch of the sky that Hubble did not record at all. (MUSE is particularly sensitive to objects that emit most of their energy at a few particular wave- lengths as these show up as bright spots in the data. Galaxies in the early Universe typically have such spectra, as they contain hydrogen gas glowing under the ultraviolet radiation from hot young stars.) “The greatest excitement came when we found very distant galaxies that T his ESOcast explains what makes the new MUSE observations so signi- ficant and shows how astronomers interpret three-dimensional datacu- bes of the distant Universe. [Editing: Herbert Zodet. Web and technical support: Mathias André and Raquel Yumi Shida. Written by: Christopher Marshall, Richard Hook and Herbert Zodet. Narration: Sara Mendes da Costa. Music: Johan B. Monell ( www.johanmonell.com ). Footage and pho- tos: ESO, MUSE Consortium/R. Bacon, Robert Williams and the Hubble Deep Field Team (STScI), the HDF-S Team, F. Summers (STScI), NASA/ESA/Hubble, L. Calçada, M. Kornmesser, B. Tafreshi (twanight.org), C. Malin (christo- phmalin.com), Mario Nonino, Piero Rosati and the ESO GOODS Team. Direc- ted by: Herbert Zodet. Executive producer: Lars Lindberg Christensen.] were not even visible in the deepest Hubble image. After so many years of hard work on the instrument, it was a powerful experience for me to see our dreams becoming reali- ty,” adds Bacon. By looking carefully at all the spec- tra in the MUSE observations of the HDF-S, the team measured the dis- tances to 189 galaxies. They ranged from some that were relatively close, right out to some that were seen when the Universe was less than one billion years old. This is more than ten times the number of measurements of distance than had existed before for this area of sky. For the closer galaxies, MUSE can do far more and look at the differ- ent properties of different parts of the same galaxy. This reveals how the galaxy is rotating and how other properties vary from place to place. This is a powerful way of un- derstanding how galaxies evolve through cosmic time. “Now that we have demonstrated MUSE’s unique capabilities for ex- ploring the deep Universe, we are going to look at other deep fields, such as the Hubble Ultra Deep field. We will be able to study thousands of galaxies and to disco- ver new extremely faint and distant galaxies. These small infant gal- axies, seen as they were more than 10 billion years in the past, grad- ually grew up to become galaxies like the Milky Way that we see today,” concludes Bacon. n