The Westerbork Observatory, Continuing Adventure in Radio Astronomy Astrophysics and Space Science Library

THE WESTERBORK OBSERVATORY, CONTINUING ADVENTURE IN RADIO ASTRONOMY ASTROPHYSICS AND SPACE SCIENCE LIBRARY

VOLUME 207

Executive Committee W. B. BURTON, Sterrewacht, Leiden, The Netherlands J. M. E. KUIJPERS, Faculty of Science, Nijmegen, The Netherlands E. P. J. VAN DEN HEUVEL, Astronomical Institute, University ofAmsterdam, The Netherlands H. VAN DER LAAN, Astronomical Institute, University of Utrecht, The Netherlands

Editorial Board I. APPENZELLER, Landessternwarte Heidelberg-Konigstuhl, Germany J. N. BAHCALL, The Institute for Advanced Study, Princeton, U.S.A. F. BERTOLA, Universita di Padova, Italy W. B. BURTON, Sterrewacht, Leiden, The Netherlands J. P. CASSINELLI, University ofWisconsin, Madison, U.SA. C. J. CESARSKY, Centre d' Etudes de Saclay, Gif-sur-Yvette Cedex, France J. M. E. KUIJPERS, Faculty of Science, Nijmegen, The Netherlands R. McCRAY, University of Colorado, JILA, Boulder, U.SA. P. G. MURDIN, Royal Greenwich Observatory, Cambridge, U.K. F. PACINI, Istituto Astronomia Arcetri, Firenze, Italy V RADHAKRISHNAN Paman Research Institute, Bangalore, India F. H. SHU, University of California, Berkeley, U.SA. B. V. SOMOV, Astronomical Institute, Moscow State University, Russia R. A. SUNY AEV, Space Research Institute, Moscow, Russia S. TREMAINE, CITA, University of Toronto, Canada Y. TANAKA, Institute of Space & Astronautical Science, Kanagawa, Japan E. P. J. VAN DEN HEUVEL, Astronomical Institute, University ofAmsterdam, The Netherlands H. VAN DER LAAN, Astronomical Institute, University ofUtrecht, The Netherlands N. O. WEISS, University of Cambridge, U.K. THE WESTERBORK OBSERVATORY, CONTINUING ADVENTURE IN RADIO ASTRONOMY

Edited by

ERNST RAIMOND and RENEGENEE Netherlands Foundation for Research in Astronomy (NFRA). Dwingeloo. The Netherlands

KLUWER ACADEMIC PUBLISHERS DORDRECHT / BOSTON / LONDON Library of Congress Cataloging-in-Publication Data

The Westerbork Observatory. continuing adventure ln radio astronomy edited by Ernst Raimond and Rene Genee. p. cm. -- (Astrophysics and space science llbrary ; v. 207) Inc 1udes index. 1. Westerbork Observatory. 2. Radio telescopes--Netherlands- -Westerbork--History. I. Raimond. Ernst. II. Genee. Rene. III. Ser leS. QB479.N42W478 1996 522' .682' 0949215--dc20 96-28460

ISBN-13: 978-94-010-7276-2 e-ISBN-13: 978-94-009-1734-7 DOl: 10.1007/978-94-009-1734-7

Published by Kluwer Academic Publishers, P.O. Box 17,3300 AA Dordrecht, The Netherlands.

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In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, The Netherlands.

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All Rights Reserved © 1996 Kluwer Academic Publishers Softcover reprint of the hardcover 1st edition 1996 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. CONTENTS

PREFACE vii

OORT'S DREAM (1961) Quoted introduction by Jan H. Oort to the Symposium on Large Radio Telescopes held in Paris, December 1961, followed by Oort's thoughts on the Scientific Programmes that would become possible with the large radio telescope to be built.

HISTORICAL NOTES: FOUR DECADES OF DUTCH RADIO ASTRONOMY, TWENTY-AVE YEARS WESTERBORK TELESCOPE, Ernst Raimond 11 Developments in radio astronomy after World War II. Brief history of the Benelux Cross Antenna Project, the precursor of the Westerbork Telescope. Construction and commissioning of the WSRT. Modifica- tions, extensions, improvements and upgrades in the years 1970 to 1995.

THE SYNTHESIS RADIO TELESCOPE; PRINCIPLES OF OPERATION, DEVELOPMENT OF DATA PROCESSING, Willem N. Brouw 53 Earth Rotation Aperture Synthesis explained. An account of the processing of WSRT data over the years, starting with simple batch processing in 1970 to quite advanced techniques in the nineties.

A QUARTER CENTURY OF LiSTENING PLEASURE - 25 YEARS' RESEARCH WITH THE WSRT, Richard G. Strom 75 Highlights of the scientific results obtained with the Westerbork tele• scope between 1970 and 1995. The early discoveries; structure and dynamics of galaxies; radio galaxies; jets and cosmic rays; Milky Way emission, supernovae, stars, the solar system and high-velocity clouds; the local group of galaxies.

THE WESTERBORK SYNTHESIS RADIO TELESCOPE, A SECOND LEASE ON LIFE, A.G. de Bruyn 109 Review of 25 years of WSRT improvements from an astronomers' perspective. Major upgrades due to be completed in 1997/98 will im• prove the overall performance drastically. Comparison of the up• graded WSRT with the VLA, MERLIN, AT and GMRT; inventory of the impact of the upgrades on astrophysical themes.

THE WESTERBORK TELESCOPE AND VERY LONG BASELINE INTERFEROMETRY, R.T. Schilizzi and L.I. Gurvits 127 The WSRT's significant role in European an global VLBI. Brief de• scription of two space VLBI missions in progress. Scientific impact of VLBI. Astrometric potential of multiple-element interferometry be• tween the VLA and the WSRT. vi

DARK MAITER AND NEUTRAL HYDROGEN IN SPIRAL GALAXIES, Renzo Sancisi 143 An account of past, current and future work with the WSRT on dark halos, stars, gas and dark material. Structure and dynamics of galactic disks and a review of the observations on mutual interaction and ac• cretion of galaxies.

POWERFUL RADIO SOURCES: WESTERBORK AND BEYOND, George Miley 155 Reminiscences of an early WSRT user. Past and current work on very large radio sources, production and transport of their energy and in• teraction with their environment. Optimum properties of a next generation radio telescope for observing distant galaxies discussed.

THE SQUARE KILOMETER AARA Y INTERFEROMETER, Robert Braun 167 Ambitious plans for a new generation decimetric radio telescope. Scientific drivers, technical specifications and instrumental capabilities for different areas of research summarised. A number of con- cepts for realisation reviewed.

PROSPECTS FOR PULSAR RESEARCH, E.P.l. van den Heuvel 187 A review of the present status of observational and theoretical pulsar research and a detailed account of the expected benefits of a new, very powerful radio telescope like the square kilometer array. Revolution• ary improvement in understanding a number of key issues in astro• physics is foreseen.

GAS IN THE EARLY UNIVERSE, F.H. Briggs 205 Discussions of the smooth gaseous origin of the universe, the history of ordinary galaxies and intergalactic gas clouds and the evolution of the neutral gas content of the universe. A new, very powerful radio telescope will improve the knowledge about the early universe, but whether all important questions can be answered is doubtful.

NEW CHALLENGES FOR COSMOLOGY, Rien van de Weygaert and 225 Tjeerd S. van Albada A detailed account of the ways in which a square kilometer array could further cosmological research. Observational and theoretical studies of the large scale structure and morphology of the local uni• verse are reviewed against the potential capabilities of a new genera• tion telescope. Cosmological requirements for such a telescope are discussed.

INDEX OF NAMES 261 PREFACE

A small country builds a world-class telescope in its backyard and lives happily ever after (or at least for a quarter century). That in a nutshell is the story told in this collection of essays. The country of course is the Netherlands, and the telescope is the Westerbork Synthesis Radio Tele• scope (WSRT), brainchild of Jan Oort. Living happily in this context is a continuing record of discovery and as such also a continuing basis for se• curing observing time on facilities in other countries and operating at other frequencies. As our community celebrates the Silver Anniversary of the radio tele• scope at Westerbork, it is fitting that we pause to take account of the scientific discoveries and insights it made possible. Initially the instrument represented the very significant step away from university-run, specialist facilities to a well-supported, common-user radio imager also having spec• tral and polarization capabilities. It pioneered the mode of operation now common for satellite observatories, in which data is taken and calibrated by technicians and provided to researchers ready for analysis. It has been a major source of discovery in, among other areas, research on neutral hy• drogen and studies of dark matter in galaxies. Its story we present here in the form of essays by senior members of our community. The collection can be read partly as a broad-brush history of the telescope, of its main achievements and of the role it has played in making Dutch astronomy what it is today. But we are also busy upgrading and modernizing the facility and we have begun to look to the future both near and distant. Where is the science of radio astronomy going and how can we playa role in its further development? This is a su bject receiving llIuch attention and it is fitting that it also be addressed ill these essays. The collection begins with a reprinted paper by Oort, giving his vision , before the telescope was constructed, of what it should do w hen finally ready. What it actually did during its first 25 years can be found in essays by Ernst Raimond and by Richard Strom. viii

It is an understatement to say that how a synthesis radio telescope makes pictures of the sky is not intuitively obvious, so we asked the practical genius behind the telescope's operational and data processing software, Wim Brouw, to set out for the non-expert the basics of that operation. In contributions by Ger de Bruyn and by Richard Schilizzi and Leonid Gurvits are sketched current ideas and plans for the use and development of the facility in the near future. And while research using radio techniques will inevitably remain important to our community, operation of the WSRT will eventually no longer be cost-effective. With this in mind, we have already begun to look to the more distant future and the so-called Square Kilometer Array Interferometer (SKAI). As Robert Braun relates in his essay, SKAI is the natural next step in radio astronomy, at least at the frequencies at which the WSRT operates. NFRA has been asked by the community to start laying the technical groundwork for making this new telescope possible as soon as is practicable after the turn of the century. Essential to any technical studies for SKAI, of course, is a vision of the scientific potential of radio observations in the future. Our community has begun to gather its thoughts in this regard, and we also present here several essays on the current state and future development of those areas of research of particular interest to us now. Ed van de Heuvel writes of the study of pulsars, George Miley of radio galaxies, Frank Briggs of gas in the early universe, Renzo Sancisi of unseen matter, and finally Tjeerd van Albada and Rien van de Weigaert consider large scale structure in the UIllverse. The essays have not been homogenized to any significant extent. Readers will find that each author writes from his own perspective and based on his own experiences. Occasionally there is repetition and a different emphasis when relating the same period of history, and there are differing views on the important technical specifications of SKAI. But this unevenness reflects the normal diversity among scientists and I hope it will be seen as adding a richness to the collection. Let me conclude by thanking the authors and the editors for making time in their busy schedules to contribute. And on behalf of our whole com• munity I also thank the Netherlands Organisation for Scientific Research (NWO) for its steady support of the WSRT and of its user community through the years.

Harvey R. Butcher Director, Netherlands Foundation for Research in Astronomy Jan Hendrik Oort, 1900 - 1992 Photograph taken during the General Assembly of the International Astronomical Union in Brighton, 1970