Exploring the Epoch of Galaxy Formation: Gemini's First Look at the Interstellar Medium in the "Redshift Desert"

A group of astronomers has recently studied the chemical make-up of distant galaxies in the "redshift desert," which is a virtually unexplored region of space between eight billion and ten billion light years away. Because the light from these galaxies moves at a finite speed, the galaxies are being seen when the Universe was between 20% and 40% of its present age. Studying the properties of galaxies at this particular time is important because it corresponds to a special age in the history of the Universe - a time during which it is thought to have made most of its stars. These observations were undertaken using the Gemini North telescope and a uniquely capable instrument, the Gemini Multi-Object Spectrograph (GMOS), which is able to target extraordinarily faint galaxies using a newly developed technique known as "nod and shuffle."

This work is reported in "The Gemini Deep Deep Survey: II. Metals in Star-Forming Galaxies at Redshift 1.3<z<2," scheduled to appear in The Astrophysical Journal, vol. 602, no. 1, February 10, 2004, and authored by Sandra Savaglio (Johns Hopkins University) and her colleagues in the Gemini Deep Deep Survey (GDDS). The GDDS is an ambitious observing program executed on Gemini North during the last year. Data from this survey will soon be publicly available (see http://www.ociw.edu/lcirs/gdds.html). The GDDS program required about 100 hours of telescope time on Gemini North, resulting in the most sensitive redshift survey ever conducted. The main goal of the GDDS is to measure the ages and masses of distant galaxies in the redshift desert by combining Gemini visible-light spectra with infrared observations. The GDDS is collaboration between astronomers at the Gemini Observatory, the University of Toronto, the Johns Hopkins University, the Carnegie Institution of Washington, the Herzberg Institute of Astrophysics, the Massachussetts Institute of Technology, the University of Oxford, and the San Francisco State University.

Savaglio and her collaborators determined the heavy element enrichment of the interstellar medium (ISM) in the redshift desert using a sub-sample of 13 galaxies with high star formation activity. These galaxies are very massive (M > 10^10 M sun) and have colors typical of irregular and spiral galaxies (the stellar mass of our galaxy, the Milky Way, is of the order of 5x10^10 M sun). The composite spectrum of the 13 galaxies (with a mean redshift of z=1.6) is shown in Figure 1. This spectrum reveals strong ISM absorption lines due to the presence of singly ionized iron, magnesium and manganese (Mg II, Fe II and MnII) plus neutral magnesium (Mg I). Abundances of these ions are considerably larger than values expected from studies of absorption lines in QSOs at similar redshifts. From the observed iron absorption, the GDDS team has estimated that only 20% of the original light is being seen in the optical, with the rest being hidden by dust obscuration. Such highly obscured objects are usually too faint to be detected in ordinary surveys conducted using visible light observations.

Normalized flux - Rest wavelength
Figure 1. Combined spectrum of 13 GDDS galaxies with ISM absorption lines (lower spectrum). Absorption features are marked with dotted lines. As a reference, a composite spectrum (shown in black) of 14 local starburst dwarf galaxies observed with HST/FOS is shown (top spectrum in red, courtesy of C. Tremonti). The dotted curve at the bottom indicates the level of the noise in the GDDS combined spectrum.

The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai`i (Gemini North) and the Gemini South telescope is located on Cerro Pachón in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Particle Physics and Astronomy Research Council (PPARC), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The Observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.