Observational Constraints on the Influence of Active Galactic Nuclei on the Evolution of Galaxies

208 Pages·2016·9.26 MB·English

by  Christopher Mark Harrison (auth.)

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.