Ray Weymann

Much of Ray Weymann's time during the past year was devoted to organizing, convening, and editing the proceedings of a workshop entitled "Photometric Redshifts and High Redshift Galaxies." The event was held in the William T. Golden Auditorium at the Observatories in Pasadena during April 28-30, 1999. Stimulated by research using the Space Telescope NICMOS camera (described below), the workshop was attended by about 65 astronomers from the U.S. and abroad. Photometric redshifts use direct images of galaxies taken in several bandpasses to estimate the redshift, or distance, of galaxies. The virtues of this technique are that it can produce redshifts of huge numbers of galaxies much more efficiently (in terms of telescope time), and it can view the bodies to much fainter brightness levels than can the more accurate method of measuring galaxy spectra with large telescopes. A closely related topic that was addressed involves the search for extremely distant galaxies to trace the history of star and galaxy formation in the very early phase of the expanding universe. The proceedings of the workshop are in volume 191 of the Astronomical Society of the Pacific Conference Series published in late November. Lisa Storrie- Lombardi, formerly of the Observatories and now at Caltech, and Robert Brunner and Marcin Sawicki (also of Caltech) collaborated with Weymann to organize the workshop.

The impetus for the workshop came from research using observations with the infrared NICMOS camera on the Hubble Space Telescope. The observations, combined with previous visible light observations in the Hubble Deep Field, provided a set of images ideally suited for both the determination of photometric redshifts and the search for a very high redshift galaxy. This search yielded one galaxy that was confirmed to have a redshift of z = 5.6 based on spectroscopic observations made with the Keck telescope. The images that identified this as a likely candidate for a very high redshift galaxy are shown in Fig. 11. The main goal of the current research is to see whether, and how well, astronomers can estimate both the redshift of the galaxies using the photometric redshift technique and simultaneously estimate the internal dust absorption of the ultraviolet light associated with stars being formed in the galaxy or protogalaxy. It has recently become apparent that correction for this absorption is crucial to obtaining a true picture of the rate at which stars have been forming in the history of the universe. This is far more difficult than estimating only the redshift since the colors of a galaxy undergoing a vigorous burst of star formation, coupled with moderately heavy dust absorption, closely mimics the colors of a galaxy with only moderate star-formation rates and little or no dust absorption.

In a different field of extragalactic astronomy, Weymann is analyzing data on a bright quasar taken with the new STIS spectrograph on the Hubble Space Telescope. He will compare the properties of intergalactic hydrogen gas clouds and their association with galaxies with the predictions from computer simulations of the evolving universe. These data are being augmented by spectroscopic redshifts of galaxies close to the quasar (in angle, not distance) observed with the 2.5-m du Pont telescope.

Fig. 11. These are Hubble Space Telescope images of a galaxy at a redshift of 5.6. The images labeled F606W and F814W were taken with a visible light CCD camera, and those labeled F110W and F160W were taken with the infrared NICMOS camera. The galaxy, labeled HDF 4-473.0, is relatively bright in the two infrared images, much fainter in the F814W image, and invisible in the shortest-wavelength image, F606W. The brightness ratio of these images has a distinctive signature indicative of a high-redshift star-forming protogalaxy and illustrates the use of the photometric redshift technique. The redshift of this very distant object was confirmed using the Keck telescope on Mauna Kea after several hours of exposure.