To carry out research in astronomy (or in any ASTRONOMY BEAT. The Discovery of the First Gravitational Lenses. Editor s Introduction

Transcription

1 ASTRONOMY BEAT Number 33 October 5, 2009 Publisher: Astronomical Society of the Pacific Editor: Andrew Fraknoi Designer: Leslie Proudfit , Astronomical Society of the Pacific 390 Ashton Avenue, San Francisco, CA The Discovery of the First Gravitational Lenses Ray Weymann Staff Member Emeritus, Carnegie Observatories Editor s Introduction One of the most intriguing suggestions from Einstein s general theory of relativity is that, as gravity warps space-time, beams of light traveling near a massive object can have their paths bent so that distorted or multiple images are seen by a distant observer. This is similar to how the warping of a trick lens or fun-house mirror can change or split images, showing your little brother much fatter or taller than nature ever made him. Formally calculated by Einstein from his theory in 1936, and elaborated by Fritz Zwicky in 1937, such gravitational lenses were long thought to be so difficult to observe that they were merely theoretical curiosities. Then in 1979, former ASP President Ray Weymann and his colleagues Robert Carswell and Dennis Walsh, found this lensing effect while observing the light of a quasar, a distant galaxy whose center shines especially bright and, from far away, looks like a star. We asked Dr. Weymann to reminisce for our readers about his path-breaking discovery. To carry out research in astronomy (or in any of the sciences, for that matter) successfully requires, in my opinion, the following ingredients: 1) a firm grasp of the basic subject matter and knowledge of the recent developments, 2) some imagination, 3) availability of stimulating colleagues, 4) time and patience in pursuing the research program, 5) access to the tools needed to do the job (e.g. instrumentation or computing facilities) and, in some cases, frankly, 6) just plain luck. Let me illustrate how my An image of , the first gravitational lens known. In this Hubble Space Telescope view, the two quasar images are the ones that have little spikes of light coming from them (it was a long exposure). The lensing galaxy is seen between them. (Image courtesy of George Rhee, William Keel and NASA). colleagues and I used these ingredients in the discovery of the first two known examples of gravitational lenses. While it is popularly supposed that the idea that a ray of light could be bent by gravity originated with Einstein, in fact a German astronomer, Johann Soldner, calculated the effect back in Soldner derived his formula long before Einstein s formulation of General Relativity and gave a value for the deflection half of Einstein s correct value (using General Relativity). Implicit in the notion of the deflection of light is the possibility that rays of light from a distant object can Astronomy Beat No. 33 October 5, 2009 Page 1

2 Johann Soldner [left] and Albert Einstein [right] follow two or more paths around an intervening concentration of matter to reach an observer, so that they appear to come from slightly different directions, thus forming separate images. Though this possibility had been realized for several decades, and theoretical work on the properties of such gravitational lenses had been published by a small group of astronomers, no actual examples of such lenses were known prior to our first two discoveries in Quasars, the highly luminous star-like nuclei of distant galaxies, provide the ideal sources for these multiple images. While today more than 75,000 quasars are known (thanks largely to the Sloan Digital Sky Survey), three decades ago astronomers were slowly enlarging the number of known quasars one by one. Most of the quasars known at that time gave off powerful energy at radio wavelengths, so a common discovery technique involved using approximate positions on the sky provided by radio astronomers and then identifying starlike candidates at optical (visible-light) wavelengths close to these positions. Taking a spectrum of each candidate could then reveal which had the tell-tale signature of a distant quasar, especially if it seemed to have more intense blue light than red compared with a run-of-the-mill star in our own galaxy. My longtime collaborator and friend, Dr. R. F. (Bob) Carswell, has provided the following recollections and background to the first observation of a gravitational lens: In the mid-1970s radio astronomers were starting to use pairs of radio telescopes as interferometers to obtain much more accurate positions of celestial sources of radio waves. A radio survey at a frequency of 966 MHz, made at the Jodrell Bank Observatory in England, provided positions with errors as little as 2 seconds of arc for unresolved sources. Dennis Walsh embarked on a program of optical [visible-light] confirmation of the quasar candidates from this survey with a number of collaborators. Time on the Kitt Peak 84 telescope was awarded so that he and I could do part of this program, so in March 1979 we were doing routine optical spectroscopy of quasar candidates there. Near the radio position of one, designated , there were two blue objects which were candidate quasars, though the radio position did not match either very well. The spectrum of the first of the two we chose to look at (the northern one) confirmed it as a quasar, and we then took a spectrum of the second. Its spectrum appeared to be identical. So much so that we wondered if the telescope operator had again pointed the telescope at the first object by mistake! But repeat observations of both objects confirmed their essentially identical nature. So it When light from a distant quasar passes by a large mass (such as a massive galaxy), its gravity bends light beams so that they appear to come from different directions than the original quasar. In this diagram, from the Chandra telescope, nothing is to scale, and the deflection has been immensely exaggerated for educational purposes. Astronomy Beat No. 33 October 5, 2009 Page 2

3 was clear to Dennis and me that we were on to something exciting even if neither of us knew what it meant at that stage. The next day Bob called me to discuss what they had found. It happened that I was scheduled on the Steward Observatory 90" telescope the night following their discovery, and so the three of us decided that higher resolution spectra should be obtained at the 90" to allow a more detailed comparison of the two quasars. The higherresolution spectra revealed identical sets of absorption lines, which could be very accurately measured, and this strongly confirmed the remarkable similarity between the two. This measurement strongly reinforced the idea that the light was originally coming from one object and that we must be looking at a double image produced by a gravitational lens. The results of these observations and this proposed explanation were soon published in the magazine Nature. For a time some radio astronomers disputed this interpretation, but further investigations confirmed it. The idea of gravitational lenses was not completely out of the blue for us. Both Bob and I had some previous exposure to the notion of gravitational lenses, so we were at least aware of their general characteristics. In Bob s case, this was from having shared an office with Nigel Sanitt a few years earlier; part of Nigel s PhD thesis had been a quasar lens search. In my case the reason was a bit less direct. In the 1960s and 70s, when quasars still seemed a great mystery to scientists, Jeno and Madeline Barnothy, a husband & wife team of astronomers born in Hungary, promoted a novel solution to why these points of light were so bright. Their idea was that quasars were in fact the nuclei of active galaxies, whose brightness was greatly amplified by gravitational lensing. Their suggestion was widely dismissed and to some extent even ridiculed, which I thought was a bit unfair. Along with a graduate student, we examined their proposal, and while we concluded it was not Ray Weymann (left) and Robert Carswell in the mid 1970s (Weymann Photo by A. Fraknoi) correct, it was certainly not crazy; in the process it acquainted me with the theory of gravitational lensing. Not long after our first lens discovery, in one of those coincidences which seem to occur not so infrequently in science, a postdoctoral fellow, a graduate student and I were carrying out another spectroscopic survey of quasars (this time identified on the basis of their colors, not their radio properties), using the same spectrograph on the 90" telescope. Usually the seeing at that telescope is so-so, but toward the end of the night while we were taking spectra of one of these candidates, the seeing suddenly got very good and we were astonished to see the single blob of light resolve itself into three very closely spaced points of light. We immediately stopped our exposure so that we could try and get spectra of each one of the three separately. But the data we obtained that night were of poor quality since the period of good seeing did not last. Fortunately, the new Smithsonian Astrophysical Observatory-University of Arizona Multiple Mirror Telescope (MMT) on Mt. Hopkins was coming into operation with a new spectrograph. With the normally better seeing and larger light-gathering power of the MMT, we were able to get much better quality spectra of each of the components of this object, and again they were identical. This turned out to be the second known example of gravitational lensing, which my colleague, Roger Angel, nicknamed Mickey Mouse (since the early images looked a bit like the famous cartoon character.) It is interesting to note how varied and powerful a tool Astronomy Beat No. 33 October 5, 2009 Page 3

4 for exploring the universe the basic fact of gravitational lensing has since become. It would be an exaggeration to say that these first two examples led to these new ways of probing the cosmos, though they may have raised the threshold of consciousness concerning gravitational lensing a bit in astronomers minds, much the way that the work of the Barnothy s did for me. Among the new tools are: probing the distribution of the mysterious dark matter in the universe by the tiny distortions they produce in images of galaxies; amplifying the brightness of otherwise very faint, distant galaxies so that they are bright enough to be studied; searching for low-mass objects toward the center of our galaxy by means of the micro-lensing they produce in background stars; and even discovering extra-solar planets by the spikes they produce in the brightness of stars undergoing micro-lensing. No doubt there will be other applications, and no doubt the Barnothys would have been pleased. I wish to thank Bob Carswell for sharing his recollections of the first observations of Both of us wish to record our indebtedness to Dennis Walsh for his role in this discovery and our regret at his untimely death. About the Author Ray Weymann received his undergraduate degree from Cal Tech in 1956 and his PhD from Princeton in He has worked in a variety of astronomical areas, both observational and theoretical, including mass loss from red giants, the microwave background radiation, Seyfert galaxy nuclei, intergalactic matter and high Editor s Note: Just as this column was going to press, the newly refurbished Hubble Space Telescope captured an image with remarkable structure visible in the arc of smeared-out light produced by a gravitational lens. We thought it would make a spectacular coda to Dr. Weymann s article. The caption from the Space Telescope Science Institute reads: Hubble Space Telescope's newly repaired Advanced Camera for Surveys (ACS) has peered nearly 5 billion light-years away to resolve intricate details in the galaxy cluster Abell 370. This was one of the very first galaxy clusters where astronomers observed the phenomenon of gravitational lensing, where the warping of space by the cluster's gravitational field distorts the light from galaxies lying far behind it. This is manifested as arcs and streaks in the picture, which are the stretched images of background galaxies. Ground-based telescopic observations in the mid-1980s of the most prominent arc [seen in red and blue in this image] allowed astronomers to deduce that the arc was not a structure of some kind within the cluster, but the gravitationally lensed image of an object two times farther away. Hubble resolves new details in the arc that reveal structure in the lensed background galaxy. Astronomy Beat No. 33 October 5, 2009 Page 4

5 velocity winds from quasars. He played a major role in initiating the Multiple Mirror Telescope project at the University of Arizona. Weymann served as President of the ASP from 1973 to 1975 and is a member of the National Academy of Sciences. Resources for Further Information: Readings: Petersen, C. The Universe through Gravity s Lens in Sky & Telescope, Sept. 2001, p. 32. Good introduction with nice diagrams and photos. Wambsganss, J. Gravity s Kaleidoscope in Scientific American, Nov. 2001, p. 65. On gravitational lenses and micro-lensing. Gates, Evalyn Einstein s Telescope: The Hunt for Dark Matter and Dark Energy in the Universe. 2009, Norton. A book that focuses on some of the ways gravitational lensing is being used to explore the large-scale properties of the universe. Overbye, D. Lenses in the Sky in Discover Magazine, May 1984, p. 30. An early overview. Web Sites: A Brief History of Gravitational Lenses at Einstein On Line: grav_lensing_history/index.html University of Tennessee Tutorial: Hubble Space Telescope Archive of News Releases about Gravitational Lenses: F Astronomy Beat is a service exclusively for members of the Astronomical Society of the Pacific. For more information about becoming a member, visit One copy of this article may be downloaded on any single computer and/or printed for your personal, non-commercial use. No part of any article may be reproduced in any form, sold, or used in commercial products without written permission from the ASP. The Astronomical Society of the Pacific increases the understanding and appreciation of astronomy by engaging scientists, educators, enthusiasts and the public to advance science and science literacy. Astronomy Beat No. 33 October 5, 2009 Page 5