The hidden universe revealed

The discovery of a new population of distant star forming galaxies obscured by dust

15 July 1998

A team of American and Japanese astronomers has detected a population of distant, dusty galaxies which are radiating roughly the same amount of stellar energy as the entire optical Universe. The astronomers Amy Barger, Lennox Cowie, David Sanders, Eliza Fulton (University of Hawaii), Yoshi Taniguchi (Tohoku University, Japan), Yasu Sato, Haruyuki Okuda (Institute of Space and Astronautical Science, Japan), and Kimiaki Kawara (University of Tokyo, Japan) announced their discovery in the July 16 issue of the internationally renowned journal Nature. A parallel study of a smaller area around the well known Hubble Deep Field performed by a British group led by David Hughes (University of Edinburgh) appears in the same issue of Nature and reaches broadly similar conclusions.

These results are important because they suggest that much of the star formation occurring in the distant Universe may be hidden to visual observations from ground-based observatories and the Hubble Space Telescope.

"The recent submillimeter observations have opened an exciting new era in cosmological exploration comparable to that which occurred with the restoration of image quality with the Hubble Space Telescope," said Richard Ellis, Director of the Institute of Astronomy at the University of Cambridge in the UK. "The pioneering deep exposures conducted by groups in Hawaii, the UK, and Canada have shown the importance of studying galaxies at large look-back times at wavelengths other than simply the traditional optical and infrared regions. Understanding this new population is essential in order to obtain a comprehensive picture of cosmic galaxy formation."

Dust in galaxies absorbs starlight emitted at visible wavelengths by hot young stars and reradiates it at much longer wavelengths. In very dusty galaxies most of the light emitted by stars in the visible may be reradiated into the far-infrared. For galaxies at large distances this light is further "redshifted" by the expansion of the Universe to wavelengths slightly less than a millimeter. For comparison, the wavelength of visible light is about 1000 times shorter than one millimeter. Thus, dust-enshrouded galaxies that may be obscured or even invisible in the optical can be detected in the submillimeter.

The astronomers performed a deep survey of two blank regions of sky using a revolutionary new instrument on the 15-meter diameter James Clerk Maxwell Telescope (JCMT) atop the dormant volcano Mauna Kea on the Big Island of Hawaii. The JCMT is jointly owned and operated by the United Kingdom, Canada, and the Netherlands and is the largest telescope in the world that can observe submillimeter radiation. The instrument is a camera called SCUBA (Submillimeter Common User Bolometer Array), built by the Royal Observatory in Edinburgh (now the UK Astronomical Technology Centre). The supercooled detectors used in SCUBA measure heat emission from small dust particles, enabling astronomers to map a region of sky at submillimeter wavelengths. The Hawaii-Japanese observation of the heavily studied field "SSA13" is the longest exposure (51 hours) which has yet been made with this instrument.

"SCUBA has produced a true revolution in submillimeter astronomy, and it is just fabulous to see the new fields that are opening up from planets around nearby stars to the tremendously exciting cosmological studies that are really opening up our view of the early Universe," said Ian Robson, Director of the Joint Astronomy Centre, the operational headquarters for the JCMT and the United Kingdom Infrared Telescope.

The dusty galaxies discovered by the research team are forming stars at rates that are extremely high, a factor of 10 to 100 times higher than the rates of star formation in most optical sources. The detected submillimeter sources are less numerous than optically-observed sources, but in total radiate as much or more energy.

et al.)"); ?>

The only objects in the local Universe which have characteristics similar to those of the distant submillimeter sources are the "ultraluminous infrared galaxies" which were one of the major discoveries of the IRAS satellite. The infrared light emitted by these objects is produced by reradiation from dust, which has been heated by stars formed in an intense starburst and by the active galactic nuclei in these galaxies. The ultraluminous infrared galaxies are often formed by a strong merger between two gas-rich galaxies, and it is possible that the submillimeter sources at high redshift may be galaxies in the process of formation through the merger of smaller pieces.

The discovery of the submillimeter sources may require a major revision in deductions about the epoch of peak star formation activity. Optical surveys have concluded that star formation in the Universe peaked at a time when the Universe was already about three-quarters of its present age. However, it now appears that these surveys have missed an entire population of rapidly star forming galaxies, which may be located at greater distances and hence earlier times. A combination of both optical and submillimeter observations will be necessary to accurately trace the global star formation rate back to the time when primordial galaxies first assembled. The detection of galaxies in this submillimeter survey has opened up a new frontier for the exploration of the distant Universe.

The SCUBA maps and related information can be found at http://www.ifa.hawaii.edu/~cowie/scuba/scuba_int.html

The Barger et al. paper is available at http://xxx.lanl.gov/abs/astro-ph/9806317, and the Hughes et al. paper is available at http://xxx.lanl.gov/abs/astro-ph/9806297.

Contact details:

Amy Barger
University of Hawaii
barger@ifa.hawaii.edu
+1 808 956-8306

Lennox Cowie
University of Hawaii
cowie@ifa.hawaii.edu
+1 808 956-8134

Dave Sanders
University of Hawaii
sanders@ifa.hawaii.edu
+1 808 956-5055