Gemini's First Look at the Interstellar Medium in the "Redshift Desert"
November 4, 2003
November 4, 2003
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
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.
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.