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Radio Emission in the - Virgo Cluster and in So Galaxies

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Radio Emission in the - Virgo Cluster and in So Galaxies RADIO EMISSION IN THE - VIRGO CLUSTER AND IN SO GALAXIES C. KOTANYI RADIO EMISSION IN THE VIRGO CLUSTER AND IN SO GALAXIES Front cover; NGC4438, a galaxy in the wind Back cover: Precession in M84 ? RIJKSUNIVERSITEIT TE GRONINGEN RADIO EMISSION IN THE VIRGO CLUSTER AND IN SO GALAXIES PROEFSCHRIFT ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit te Groningen op gezag van de Rector Magnificus Dr. M. R. van Gils in het openbaar te verdedigen op maandag 15 juni 1981 des namiddags te 4.00 uur door CHRISTOPHE KOTANYI geboren te Budapest 1981 Drukkerij Dijkstra Niemeijer bv, Groningen Promotoren: Prof. Dr. R.D. Ekers Prof. Dr. H. van Woerden Coreferent: Dr. R.H. Sanders -3- Dankbetuigingen De wetenschappelijke motivatie voor dit onderzoek was mij door Ron Ekers gegeven, en tijdens het afwerken ervan heb ik van de aandachtvolle begeleiding van Hugo van Woerden genoten. Vele dank aan hen en aan Miller Goss voor al hun aanmoedigingen. De medewerkers van de Stichting Radiostraling van Zon en Melk- weg (SRZM), met name de telescoopgroep te Westerbork en de reductie- groep in Leiden, hebben voor waarnemingen van een hoge kwaliteit gezorgd. De medewerkers van het Kapteyn Laboratorium ben ik dankbaar voor hun bijdragen aan mijn werk; voor gesprekken en kritieken, in het bijzonder Renzo Sancisi en Bob Sanders; en voor de computer- programma's van het reductiesysterii in Groningen. Ik ben dankbaar voor de medewerking van Ko Hunr » (onder anderen bij de waarneming van NGC4410) en van Jacqueline van Gorkom (onder anderen bij de waarnemingen van NGC708 en NGC4410, en voor een kritische bewerking van artikel 4), maar ook en niet minst voor hun vriendschap. Ik dank ook Piet van der Kruit, Tjeerd van Albada, Martin Schwarz- schild en Jay Gallagher voor hun bijdragen aan dit proefschrift. Georges Comello wist mijn kladwerk in mooie figuren te trans- formeren en Gineke uit een manuscript een leesbare tekst te maken. De fotografische illustraties zijn het zorgvuldige werk van de heer W. Haaima (Centrale Fotodienst R.U.G.) en het zetwerk van artikels 7 en 8 dat van de heer R.S.M, van Bolhuis (Reproductiedienst R.U.G.) Tijdens dit onderzoek heb ik van financële steun van een Belgisch- Nederlands Culturele uitwisselingsbeurs en van de Nederlandse Organi- satie voor Zuiver Wetenschappelijk Onderzoek (ZWO) genoten. De SRZM is gefinanciëerd door ZWO. -4- Samenvatting Melkwegstelsels zijn de omgeving waarin sterren zich vormen en zich ontwikkelen, en het onderzoek naar het ontstaan en de ontwikkeling van sterren leidt tot de studie van melkwegstelsels. Sinds de ont- dekking van continuum-radiostraling in melkwegstelsels, die, via het zogenaamde synchroton-mechanisme, afkomstig is van relativis- tische electronen versneld in een magneetveld, is het gebleken dat de electronen en het magneetveld een belangrijke rol spelen in de ontwikkeling van melkwegstelsels. De herkomst van de relativistische electronen en van het magneetveld is echter nog niet volledig bekend. Informatie hierover kan door de studie van de verdeling van continuum- radiostraling in nabije stelsels verkregen worden. Een verkennend onderzoek (survey) van nabije stelsels werd gedaan met de Westerbork Synthese Radio Telescoop (WSRT) in een voorafgaand proefschrift door Hummel. In dit proefschrift worden twee aspecten van de radiostraling in nabije stelsels met de WSRT nader onderzocht: het effect op de radiostraling van de omgeving van de melkwegstelsels in de nabije rijke Virgo-cluster (hoofdstuk II), en de radio-eigen- schappen van de stelsels van het type SO (hoofdstuk III). De Virgo-cluster is de naastbijzijnde rijke cluster van melkwegstel- sels, waar meer gedetailleerde gegevens verkregen kunnen worden dan in voorafgaande studies van clusters op grotere afstand. De cluster omvat meerdere honderden stelsels op vrijwel gelijke afstand, en vrij veel ervan kunnen worden gedetecteerd. Een klein aantal stelsels wordt in detail bestudeerd: 1) M49 = NGC4472 is een van de helderste elliptische stelsels in de cluster. Het vertoont een kleine en zwakke dubbele radiobron met dubbele structuur, dat het zwakste radiostelsel tot dusver gevonden bleek te zijn, en een radiohelderheid heeft die meer typisch is voor spiraalstelsels dan voor reuze elliptische stelsels. Dit be- tekent dat de grote afmeting en energie, die in het algemeen in radiostelsels worden gevonden, niet de oorzaak zijn van de vorming van een dubbele radiobron. 2) Een kaart van het centrum van de cluster toont dat in het bijzondere spiraalstelsel NGC4438 de radiostraling verplaatst is ten opzichte van het optisch-stralende lichaam, waarschijnlijk ten gevolge van de druk van het diffuse intergalactische gas in de cluster. -5- 3) In M87 = NGC4486, het centrale reuze elliptische stelsel in de cluster, wordt de uitgebreide radiohalo in kaart gebracht; hij vertoont dezelfde dubbele structuur als normale radiostelsels. M87 is dus geen 'kern-halo' bron. Een statistische studie wordt gedaan van 200 stelsels in de cluster. De analyse toont dat de radioheiderheid van spiraalstelsels in de cluster statistisch gelijk is aan die buiten de cluster. Er is een afname van de radiohelderheid naar de buitendelen van de cluster gevonden, hetgeen waarschijnlijk verband houdt met een grotere fractie van vroeg-type spiraalstelsels in het centrum van de cluster. Enkele stelsels geven ook aanwijzingen van mogelijke effecten van wissel- werking tussen melkwegstelsels en het diffuse intergalactische gas. De SO-stelsels werden oorspronkelijk als een tussenvorm tussen ellip- tische en spiraalstelsels gezien, hoewel ze tegenwoordig meer als een gastvrije zijtak van de spiraalstelsels beschouwd worden. Hun radio- eigenschappen zouden meer informatie over hun oorsprong kunnen geven, daar elliptische en spiraalstelsels scherp verschillende radio- eigenschappen vertonen. SO-stelsels komen veel voor in clusters, naar wordt vermoed als een gevolg van de hoge waarschijnlijkheid in clusters voor het uitwegen van gas uit de schijf van een spiraal- stelsel, door wisselwerking tussen stelsels of met het intergalactische gas. Daar er te weinig van de SO-stelsels in de Virgo-cluster werden ge- detecteerd, worden ze samengevoegd met nabije SO's buiten de cluster. De resultaten voor 145 SO-stelsels worden geanalyseerd en vergeleken met die voor elliptische en spiraalstelsels. 1) De SO-stelsels hebben een zwakkere continuum-straling dan de beide andere types. 2) De zwakte van de compacte kernbron is vermoedelijk te wijten aan de kleinere massa van de centrale bol vergeleken met de massa van elliptische stelsels. 3) De zwakte van uitgebreide centrale emissie en van schijfemissie is vermoedelijk te wijten aan de kleine hoeveelheid gas in SO-stelsels vergeleken met spiraalstelsels. 4) De aanwezigheid in SO-stelsels van dubbele radiostructuur, die typisch is voor elliptische nevels, wordt uitgebreid besproken. -6- a) De dubbele structuur in NGC3665, een nabije SO, werd in kaart gebracht. Daar dit stelsel geen optische schijf vertoont, is het ver- moedelijk een mis-geclassificeerde elliptische nevel met de bijzonder- heid van een kleine stofband. b) Het SO-stelsel NGC612, dat ook dubbele structuur vertoont maar wel een optische schijf blijkt te bevatten, wordt in detail beschreven. c) De in deze twee stelsels gevonden oriëntatie van de radio-as, bijna loodrecht op de stofband, komt meer algemeen voor, zoals blijkt uit een analyse van 8 radiostelsels met stofbanden. d) Deze eigenschap en de structuur van de radiobron en van de stofband worden gebruikt om een model van radiostelsels als ellipsoïden met drie ongelijke hoofdassen te ontwerpen. e) Een analyse van de asverhoudingen in SO-stelsels met dubbele radioszructuur bevestigt het vermoeden, dat die stelsels meestal mis- geclassificeerde elliptische nevels zijn. f) Er wordt geconcludeerd dat de kans op dubbele radiostructuur in SO-stelsels erg klein is. -7- Contents Page Chapter I - General Introduction 8 1. Radio emission in galaxies 8 2. Radio observations with high sensitivity and resolution 9 3. The Virgo cluster 9 4. The SO galaxies 10 Chapter II - A radio study of the Virgo Cluster 12 1. Introduction 12 2. Observatipns Paper 1; "NGC 4472 - A very weak radio galaxy?" (with R.D. Ekers) 13 Paper 2: "Radio observations in the core of the Virgo Cluster" (with J.H. van Gorkora and R.D. Ekers) 17 Paper 3: "A radio map of Virgo A at 610 MHz" 40 Paper 4: "A radio survey at 1.4 GHz of the galaxies in the Virgo region" 44 3. A radio continuum survey of the Virgo Cluster: Analysis 56 Chapter III - Radio studies of SO galaxies 82 1. Introduction 82 2. Paper 5: "The radio continuum properties of SO galaxies" (with E. Hummel) 83 3. Radio observations of elliptical galaxies with dust lanes Paper 6: "Radio observations of NGC 3665: An elliptical galaxy with a dust lane" 105 Paper 7: "NGC 612 - A radio galaxy with a disk" (with R.D. Ekers, W.M. Goss and J.D. Skellern) 111 Paper 8: "Radio galaxies with dust lanes" (with R.D. Ekers) 115 4. Paper 9: "A model for elliptical radio galaxies with dust lanes" (with T.S. van Albada and M. Schwarzschild) 119 5. The radio continuum properties of SO galaxies: Review and discussion 135 Chapter IV - Summary 154 -8- Chapter I. GENERAL INTRODUCTION. 1. Radio emission in galaxies. Radio emission of galactic origin was discovered by Jansky (1932). Soon after the impetus given to radio astronomy by the war-time developments in radar technology, continuum radio emission was found in external galaxies as well (Hey et al., 1946). The emission has a diffuse character indicating an interstellar origin rather than a background of sun-type stellar sources (Shklovski, 1952). In the rising era of particle accelerators (cyclotrons, synchrotrons) it was soon proposed that the emission originates from electrons accelerated in magnetic fields (Kiepenheuer, 1950). The prediction, confirmed by observations, of a frequency spectrum characterized by a single exponent a - -0.7 associated with the cosmic ray energy spectrum, and the polarization properties, established the synchrotron theory. The magnetic field and cosmic rays play an important role in the dynamical processes of the interstellar medium (Parker, 1966; Kaplan, 1966).
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