Observational Constraints on the Influence of Active Galactic Nuclei on the Evolution of Galaxies PDF
Preview Observational Constraints on the Influence of Active Galactic Nuclei on the Evolution of Galaxies
Springer Theses Recognizing Outstanding Ph.D. Research Christopher Mark Harrison Observational Constraints on the Influence of Active Galactic Nuclei on the Evolution of Galaxies Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. (cid:129) The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Christopher Mark Harrison Observational Constraints fl on the In uence of Active Galactic Nuclei on the Evolution of Galaxies Doctoral Thesis accepted by Durham University, UK 123 Author Supervisor Dr. Christopher Mark Harrison Prof. DavidAlexander Department ofPhysics, Centre Department ofPhysics, Centre forExtragalactic Astronomy forExtragalactic Astronomy Durham University Durham University Durham Durham UK UK ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-319-28453-8 ISBN978-3-319-28454-5 (eBook) DOI 10.1007/978-3-319-28454-5 LibraryofCongressControlNumber:2015959577 ©SpringerInternationalPublishingSwitzerland2016 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor foranyerrorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAGSwitzerland Dedicated to Mum and Dad ’ Supervisor s Foreword One of the key discoveries in the field of astronomy and cosmology over the past twodecadesisthefindingthateverymassivegalaxyinthelocalUniverseharbours a supermassive black hole (BH) at its core. These BHs are truly colossal, with masses between one hundred thousand and ten billion times the mass of the Sun. Remarkably, observations have also shown that the mass of the BH is directly proportionaltothemassofthegalaxybulge,suggestingthatthegrowthoftheBH andthegalaxybulgearesomehowlinked.ThattheBHandthegalaxybulgeshould beconnectedinanywayistrulyastonishingbecausetheBHisjustonebillionththe physical size of the bulge, an equivalent difference in size scale between that of a grape and the Earth! Understanding this linked growth between BHs and galaxies over these vast differences in size scale is one of the main driving forces behind astronomy and cosmology today, and is the focus of Harrison’s thesis. BHs grow through the accretion of material, primarily gas, in a process that is often referred to as “feeding”. When these BHs feed they become bright due to frictionfromthe(doomed)gasinorbitaroundtheBHand,inexceptionalcases,can besobrightthatthey areseenouttotheedges oftheobservable Universe.During thesefeedingstagestheyarereferredtoasanActiveGalacticNucleus,oranAGN. By comparison, galaxies grow due to the formation of stars from the collapse of cold gas clouds. The proportionality between the mass of the BH and the galaxy bulge therefore suggests a connection between AGN activity and star formation. ThekeyquestiontoaddressthereforebecomeshowcanAGNactivity,asmall-scale process, have any influence on the formation of stars in the galaxy, a large-scale process? The leading candidates to forge the connection between BH and galaxy bulge growth are AGN-driven winds, outflows and jets of gas (generically referred to as “outflows”). Detailed supercomputer simulations have shown that if just a fraction oftheenergyproducedduringAGNactivityisinjectedintothegalaxy,intheform of an outflow, it can be sufficient to control the formation of stars in the galaxy. Essentially, this AGN-driven outflow can be thought of as a galaxy sized arm that can shut down or regulate star formation and influence the evolution of galaxies. vii viii Supervisor’sForeword TheideathatAGNcandriveoutflowsisnotneworcontroversial.Observationsof AGN for decades have provided evidence for outflows but the observational sig- natureshaveusuallyonlyprovidedinsightintotheregionsveryclosetotheBH,or the studied objects have been unusual in some way and so are not representative of the overall AGN population. A direct test of the AGN outflow model and the influencethatAGNhaveontheevolutionofgalaxiesrequiresidentifyingoutflows over galaxy sized scales for representative samples of the AGN population. Harrison’s thesis takes a multi-pronged approach in testing the AGN outflow model using state-of-the-art observations from world-leading telescopes. The observations consisted of (1) spatially resolved spectroscopy, which allows for the identification of AGN outflows and measurements of the outflow kinematics over galaxy scales and (2) deep infrared observations, which allow for reliable mea- surements of the amount of ongoing star formation to assess whether any differ- encesareseenintheamountofstarsformingonthebasisofthepoweroftheAGN. Thesignificantdevelopmentofthisthesiswas,forthefirsttime,tobeabletoplace these detailed observations into the context of the overall galaxy population. With these ground-breaking datasets, and utilizing novel data analysis tech- niques, Harrison showed that essentially all powerful AGN drive outflows with sufficient energy to shut down the ongoing star formation, a major advance over previouswork.TheestimatedamountofenergythattheseAGNdriveintogalaxies are in good agreement with that predicted by the models. These observations therefore appear to suggest that the models are correct. However, interestingly, there was no clear evidence that the AGN were having a significant long-term influence on the star formation, a result in apparent contradiction with the funda- mental reason for the construction of the models in the first place! Howcanthemodelsandtheobservationsbereconciled? Harrisonhypothesizes that,dependingonthetimescaleoftheBH“feeding”event,andthedelaybetween theoutflowoccurringandtheshutdownofstarformation,itmaybedifficultseeing thedefinitiveevidenceforthecauseandeffectoftheoutflows.Alternatively,itmay bethattheoutflowsgenuinelydonothaveasignificantimpactontheevolutionof galaxies,inwhichcasethechallengelaiddownbyHarrison’sthesisis:howcanwe explain the survival of galaxies from this onslaught of energy injection from the AGN? Durham, UK Prof. David Alexander December 2016 Preface Just over a century has passed since the first observations of active galactic nuclei (AGN)wererecorded.Theseobjects,atthecentreofgalaxies,turnedouttobethe sitesofgrowingsupermassiveblackholes.IthasbecomeclearthattheseAGNare among the most powerful energy sources in the Universe and are extremely luminous over the whole electromagnetic spectrum. For many decades these energetic phenomena were largely considered rare, yet fascinating objects to study in their own right. However, over the last quarter of a century or so, high-quality observations and theoretical work have moved AGN to the forefront of extra- galactic astronomy research. Observations have now shown us that all massive galaxiesarelikelytohavehostedAGNactivityduringtheirlifetimes.Furthermore, theoretical work now suggests that AGN activity has a profound influence on the evolutionofitshostgalaxyandthelargerscaleenvironmentthatitresidesin.These galaxy evolution models invoke AGN-driven galaxy-wide “feedback” processes (e.g., the expulsion of gas through outflows) in order to reproduce many of the fundamentalpropertiesofgalaxiesandtheintergalacticmedium.However,placing observational constraints on the influence of AGN on the evolution of galaxies remainsanopenareaofresearch,withmanyoutstandingquestions.Themotivation of this thesis was to use a variety of observational techniques to address this limitation and to test theoretical predictions. Inthisthesis Iuseatechnique calledspatially-resolved spectroscopy(described inChap.2),toobservehowtheionizedgasinAGN-host galaxies isbehavingand to search for evidence that gas is being expelled from these galaxies. I observed galaxiesoverawide range ofcosmicepochs;from those relativelyclose bytoour own Milky Way, right out to the distant Universe where the galaxies are being observedastheywereroughly12billionyearsago(Chaps.3–5).Akeyfocusofthe work in this thesis is to carefully select objects to observe those that are more typical of the overall galaxy population than previous work. An extra emphasis is placed onaskingwhat myobservations ofsome objectstellusabout galaxiesasa whole. The model predictions that AGN expel gas from galaxies, appears to be holding up reasonably well. However, some of the other results presented in this ix x Preface thesis challenge key model predictions on the impact that these outflows are expectedtohaveontheformationofstarsinsidethesegalaxies.Using far-infrared data from the Herschel space telescope, I measured the rate at which stars are forming in the host galaxies of some of the most powerful AGN in the Universe (Chap.6).ThesimplepicturethatAGNsuppresstheformationofstarsintheirhost galaxies,turns outtonotbesosimple. Towardstheendofthisthesis Idiscussthe implication of my work (Chap. 7) and the outstanding questions that are being addressed with ongoing observational projects (Chap. 8). The study of growing supermassive black holes, and their role in the formation of galaxies, continues to be an exciting and hugely active area of research. Therearemanypeopletothankwhohavehelped,inavarietyofways,togetmy thesis to this finished state. I consider myself extremely lucky to have had Dave Alexander as my supervisor, with his infectious enthusiasm for extragalactic astronomy. Throughout the four years of my Ph.D. he provided me with excellent advice and created a number of fantastic opportunities for me to progress as a researcher. I am very appreciative of his supervising style that has allowed me to explore my own ideas, whilst drawing upon his own experience and expertise to guide me in the right direction. Therearenumerouspost-docsandstaffwhohaveassistedmeinmytrainingand producing the research that is presented in this thesis. Ian Smail, Alastair Edge, John Stott, Alasdair Thomson, Agnese Del Moro and all of the co-authors of my papers have been particularly helpful. Special thanks to Mark Swinbank and Jim Mullaney who have invested considerable time into teaching me a range of skills, answering my stream of questions and coping with my irritatingly fastidious work ethic. It has been a delight to work surrounded by my office mates, both past and present,andmyotherfellowPh.D.studentswithinthedepartment.Itisnotpossible tonamethemallhere;however,Imustacknowledgemylong-standingoffice-mate Alice Danielson,who isnotonly agreatscientistbutalsohasanunrivalledability toinspirejoy,enthusiasmandaloveforastronomyintoeverybodyshemeets.Any department will be lucky to have her. I am grateful to my whole family, which has always provided me with a huge amount of encouragement. In particular, I will never find sufficient words to fully express my gratitude to my parents, who have given me their unrelenting support throughoutmyentirelife.Finally,IfeeltrulyblessedtohaveLucyasmywife.She hasshownanunbelievableamountofcare,supportandpatienceduringmytimeas a Ph.D. student. Lucy, I love you dearly.
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