SPACE AND GROUND

Course: Understanding the high-redshift Universe using quasar absorption lines

Prof. Jill Bechtold
University of Arizona
USA

It has been said about the Hubble Deep Field, that never in the history of astronomy has so much effort been put into a completely blank piece of the sky. HDF (North and South) results soon showed tha it was worth the effort. The next step is the Next Generation Space Telescope (NGST), which is expected to complete the most significant galaxy surveys. Jill Bechtold, from the University of Arizona and member of the NGST Working Science Group, comments on the expectations sorround this new space telescope.

The observations with the Hubble Space Telescope of the Hubble Deep Fields (North and South) and the follow-up spectroscopic studies with large ground-based telescopes are a major step forward in our understanding of the young Universe. What is the next step in these studies? Do we have to wait for the Next Generation Space Telescope (NGST) to achieve significant new results?

"Certainly I expect many advances in our understanding of galaxy formation and evolution before the launch of NGST. In fact, it’s funny that you ask this, since the NGST project is a bit defensive the other way: they spend a lot of effort to articulate the reasons why NGST will still be useful in 10 years or so, particularly for deep galaxy surveys. Between now and the NGST launch, there will have been thousands of nights of observing on the new large ground-based telescopes with multiobject near-IR spectrographs and IR imagers. Significant optical galaxy surveys, such as the Sloan Digital Sky Survey and the DEEP Keck survey will be completed. SIRTF will be launched and will carry out deep galaxy surveys in the mid- and far-IR. Wide-field imaging in the IR will be possible for the first time with the arrival of large HgCdTe focal plane arrays. Several projects are planned to build telescopes optimized for wide field imaging and spectroscopy. I’d be astonished if these new data sets didn’t change our understanding of the young Universe before the launch of NGST."

What discoveries do you expect from the NGST? Are the near- and mid-IR the most important spectral ranges for studying high-redshift galaxies?

"Near- and mid-IR are important spectral regions to study high-redshift galaxies for a number of reasons. The observed optical for a high redshift object is the rest-frame far-UV, and hence is produced by the very youngest massive stars. Deriving quantities of interest such as the star-formation rate from optical data alone can be very uncertain, due to the large correction for dust reddening in the UV, and the fact that the optical surveys select for galaxies which have just undergone a large burst of star-formation. The rest-frame optical and near-IR spectral energy distribution on the other hand reveals older stellar populations, which may dominate the visible mass, and allows the study of very dusty, obscured galaxies. In addition, there are many spectral features with well understood diagnostics which are shifted into the mid-IR at high redshift, such as the H-alpha and other nebular emission lines from HII regions.

NGST will "beat" any ground-based telescope in sensitivity by a very large factor, particularly longward of 2 microns, because of the poor atmospheric transmission in the mid-IR and the high thermal background in ground-based telescopes. NGST can also in principle provide diffraction limited imaging over a much wider field than any ground-based telescope, even with adaptive optics. How far into the mid-IR NGST will go is still under debate. Our group at Arizona has argued for various technical reasons that the logical cut-off wavelength should be 40 microns. Longward of 40 microns, the thermal background of the NGST telescope will be a problem.

Other spectral regions are also important, of course. The millimeter and sub-mm are an obvious example. The current and planned large interferometers have the spatial resolution and sensitivity to study the rich molecular spectrum of high redshift star-forming regions. Redshifted 21-cm studies will also provide a crucial complement to IR and UV studies. What’s clear is that the broader the spectral region you study, the more comprehensive your understanding of the galaxies will be."

What are the main contributions of the new large ground-based telescopes to the understanding of the interstellar/intergalactic medium at high redshift?

"High spectral resolution optical spectroscopy from the ground probes the intergalactic medium through Lyman alpha absorption for redshifts greater than about 2. The UV absorption lines of molecular hydrogen and the other rest frame UV absorption lines are also accessible from the ground. The ground-based data have shown that the Lyman alpha forest consists of large, highly ionized sheet-like or filimentary structures. The density of structures decreases rapidly with redshift, presumably because the gas is collapsing into galaxies. However, the IGM appears to be somewhat metal-enriched early on; how exactly this happened is of great interest, and quite uncertain. The interstellar lines allow the precise measurement of metal abundances and in principle give strong constraints on the nucleosynthetic history of the gas. The interpretation of the abundance patterns and the implications for galaxy formation are still a bit controversial in my opinion however."

The quality of the night sky at the Observatorio del Roque de los Muchachos in La Palma is protected by law to avoid light polution. Are the observatories in Arizona also protected?

"In the early 1970’s, Kitt Peak astronomer Arthur Hoag began working with Tucson and Pima County officials to limit light pollution. In 1972 Tucson adopted one of the first light control ordinances, which required that outdoor lighting be shielded to direct the light to the ground. Over 10 years ago, Kitt Peak astronomer David Crawford and Tucsonan Tim Hunter founded the International Dark-Sky Association, which is headquartered in Tucson. Due to their tireless efforts, in 1991 it became illegal to operate mercury vapor lamps in Tucson and Pima County. Fifty similar ordinances have been adopted throughout Arizona, including all counties and most incorporated cities. Crawford has been very successful at persuading the public that the lighting ordinances which benefit astronomers, provide enough light for public safety, and have a beneficial or at least neutral economical impact.

Unfortunately, however, Tucson’s dark skies are still under attack. Real estate development is booming in southern Arizona, and the environmental impact of development is one of the most contentious local political issues. The most important environmental issue is the dwindling water supply, but sometimes light pollution is also an issue. Earlier this year, developers wanted to rezone the area next to the Multiple Mirror Telescope to allow a large shopping mall complex, a golf course and the construction of about 8000 houses at the base of Mount Hopkins. Opponents included archeologists who had found numerous important historical sites in the area, and local residents who like the rural life style of the Santa Cruz valley. Astronomers of the Smithsonian Institution and Multiple Mirror Telescope also spoke out in opposition to the rezoning of course because of the potential light pollution. When the developer threated to sue the Smithsonian astronomers for 900 million dollars if they didn’t back off, public opinion shifted overwhelmingly in favor of the astronomers and against the developers who appeared to be bullies, and the proposed rezoning was defeated. Unfortunately, I’m sure the developers will be back with a revised rezoning plan next year."

PROFILE

JILL BECHTOLD studied Astronomy at the California Institute of Technology and was awarded her PhD in Astronomy in 1985 (University of Arizona).

Since 1990 she has been a Professor at the Astronomy Department of the UA.

Her research activities centre around observations using a wide variety of ground-based and space-based observatories, including X-ray, UV and IR imaging and spectroscopy from space, and, from the ground, optical and IR spectroscopy and imaging, sub-mm and mm wave spectroscopy and bolometry.

Her interests include quasar emission line regions, continuum spectral energy distributions of active galactic nuclei and quasars, quasar absorption lines, the intergalactic medium, galaxy formation and evolution, and clusters of galaxies.

Bechtold is Principal investigator of the optical echelle spectrograph funded by the NSF for the upgraded 6.5m Multi Mirror Telescope (MMT), as well as of the University of Arizona/Lockheed Martin pre-Phase A design study for the NGST integrated science instrument module funded by NASA.

She has received several awards for her work in astrophysics: the Chretien International Research Grant, AAS (1990); the Presidential Young Investigator NSF award (1990); the Ernest F. Fullam Award, The Dudley Observatory (1989) and Robert J. Trumpler Award of the Astronomical Society of the Pacific (1988).

Jill Bechtold is or has been part of a number of important committees, such as the Next Generation Space Telescope (NGST) Ad-Hoc Science Working Group, the Constellation-X Science Team and the ROSAT Users Group. She also belonged to the Review Panels of HST, AXAF, ROSAT, MIDEX, Hubble Fellows, and AXAF Fellows.