Free Astronomy Magazine March-April 2016

SPACE CHRONICLES A rtists impression of the power of background galaxies to mea- sure the size of gas clouds as com- pared to the conventional method of using quasars. The plane to the far right shows the background gal- axy and overlaid in the center of the galaxy is a bright white light re- presenting a quasar. The DLA gas cloud is shown at the center of the plane in between the galaxy and Earth. The blue/white narrow beam indicates the small area of the DLA gas cloud probed by quasars, the wider red cone of light indicates the large area of the DLA probed by galaxies, which is a 100 million-fold increase in area. [Adrian Malec (Swinburne University) and Marie Martig (Max Planck Institute For Astronomy, Heidelberg)] would enable measurements of their size by determining how much of the galaxy they cover. “Our newmethod first identifies gal- axies that are more likely to have in- tervening DLA gas clouds and then searches for them using long, deep exposures on the powerful Keck Ob- servatory 10m telescopes on Mauna Kea and deep data from the VLT 8m telescopes in Chile,” Cooke said. “The technique is timely as the next gener- ation of giant 30m telescopes will be online in several years and are ideal to take advantage of this method to routinely gather large numbers of DLAs for study.” DLA clouds contain most of the cool gas in the Universe and are predicted to contain enough gas to form most of the stars we see in galaxies around us today, like the Milky Way. However, this prediction has yet to be confirmed. DLAs current- ly have little ongoing star formation, making them too dim to observe di- rectly from their emitted light alone. Instead, they are detected when they happen to fall in the line of sight to a more distant bright object and leave an unmistakeable absorption signature in the background object’s light. Previously, researchers used quasars as the background objects to search for DLAs. Although quasars can be very bright, they are rare and are comparatively small, only a frac- tion of a light year across, whereas galaxies are quite common and pro- vide a 100 million-fold increase in area to probe DLAs. “Using the galaxy technique, DLAs can be studied in large numbers to provide a 3-D tomographic picture of distribution of gas clouds in the early Universe and help complete our understanding of how galaxies formed and evolved over cosmic time,” O’Meara concluded. n