probing the intergalactic medium with high-redshift quasars alexander peter calverley

190 Pages·2011·9.51 MB·English

UNIVERSITY OF CAMBRIDGE
INSTITUTE OF ASTRONOMY
A THESIS SUBMITTED FOR THE DEGREE
OF DOCTOR OF PHILOSOPHY
[X\
P I M
ROBING THE NTERGALACTIC EDIUM
WITH
H -R Q
IGH EDSHIFT UASARS
\X[
A P C
LEXANDER ETER ALVERLEY
DARWIN COLLEGE
SubmittedtotheBoardofGraduateStudies
19thOctober2011
UNDERTHESUPERVISIONOF
DR. GEORGE D. BECKER

“Scienceiswhatweunderstand wellenoughtoexplaintoacomputer.
Everythingelsewedoisart.”
- DONALD KNUTH

DECLARATION
IherebydeclarethatmythesisentitledProbingtheIntergalacticMediumWithHigh-RedshiftQuasars
is not substantially the same as any that I have submitted for a degree or diploma or other qualifi-
cationatanyotherUniversity. Ifurtherstatethatnopartofmythesishasalreadybeenorisbeing
concurrentlysubmittedforanysuchdegree,diplomaorotherqualification. Thisdissertationisthe
result of my own work and includes nothing which is the outcome of work done in collaboration
exceptwherespecificallyindicatedinthetext.
I note that Chapters 1 and 2 are intended as reviews, and as such contain little, if any, original
work.Theycontainanumberofimagesandplotsextractedfromotherpublishedworks,allofwhich
areclearlycitedintheappropriatecaption.
Thosepartsofthisthesiswhichhavebeenpublishedoracceptedforpublicationareasfollows:
• Much of the work contained in Chapters 4 and 5 formed the basis of the work published as
Calverley,A.C., Becker, G.D., Haehnelt, M.G., andBolton, J.S.(2011)‘Measurementsofthe
ultravioletbackgroundat4.6<z<6.4usingthequasarproximityeffect’,MonthlyNoticesofthe
RoyalAstronomicalSociety412,2543-2562andwascompletedincollaborationwiththenamed
authors.
Thisthesiscontainsfewerthan60,000words.
AlexCalverley
Cambridge,October19,2011
v

ACKNOWLEDGEMENTS
Thisthesiswouldnotexistwithoutthecontinuinghelpandsupportofthosearoundme,andthere
aremanypeoplewhodeserveamention.Thegreatestthanksarereservedformysupervisor,George
Becker, who chose to take me on as his student, and who has been a great source of ideas and in-
spirationduringthesepastthreeyears. IamalsorelativelyrareamongstgradstudentsintheIoA,in
thatIhavehadregularcontactwithmysecondarysupervisor,MartinHaehnelt. Hefirstbecamein-
volvedduringthewritingofmypaperinmysecondyear,andhaskeptacarefuleyeonmyworkever
since. Inmanyways,thefactmythesishasbeencompletedontimeislargelydowntohisconstant
encouragement(andrelentlessdeadlines!).
BeforeIcouldchooseasupervisorthough,IneededaplaceintheIoAfirst.Iwillforeverbegrateful
to Bob Carswell for sending me that email offering me a place, and to the examiners who looked
at my work during each year of my PhD (Max Pettini, Ian McCarthy, Richard McMahon, Anthony
ChallinorandAndrewFox)anddecidedtoletmestay!Similarly,ImustthankPaulHewettforshowing
himselftobethequasarexpert, andforalwaysbeingwillingtoanswermyquestionsonbothbasic
astrophysics,andtopicstangentialtomyresearch.
Ofcourse,theIoAismorethanjustabuildingfullofresearchers.Workingbehindthescenesarea
dedicatedteamofsupportstaffwhoseexcellentworkkeepsthedepartmentworkingwithincredible
efficiency.ParticularmentionsmustgotoSiânOwen,whoisquitesimplythesourceofallknowledge,
andMargaretHarding,whomIhavetothankforallhersageadvicewhenplanningmyobservingand
conferencetrips.ImustalsomentiontheHelpdeskstaff(Hardip,AndyB,AndyDandSue)whohave
alwayscometomyrescuewhenmyinabilitytousethecommand-linehascausedmycomputerto
throwatantrum.
IalsogivethankstoSTFC,whohavefundedmyPhDstudentshipandgivenmetheopportunity
tospendthreeyearsstudyingastronomy. They,togetherwithDarwinCollege,havealsocoveredthe
costsofmyadventuresabroad,forconferencesandobserving.ViathemIhavevisitedbothIndiaand
Hawai’iduringmyPhD,andthesewereexperiencesIwillneverforget.
My time at the IoA has been greatly enhanced by my friendships with other grad students. The
most notable of these are my year group (Adrian, Amy, Becky, Dom, James M, James O, Jon, Ryan,
Stephanie,WarrickandYin-Zhe)withwhomIhavesomegreatsharedmemories,rangingfromgames
of ‘foilball’ to hiding under a particular professor’s desk. A special mention must go to the subset
knownasthe‘West’sCrew’(Becky,JamesMandJon),whoensuredthatIcouldspendmylunchtimes
discussingcurrentaffairsawayfromtheIoA,insteadofeatingaloneatmydesk.Onadaytodaybasis
though,ithasbeenmyofficematesinH27,pastandpresent(Caitlin,Simeon,Julie,Maciek,Adriano,
Kim,StephenandWarrick),whohavekeptmesmiling,evenwhenmycoderefusestowork.Aspecial
thankyougoesouttoCaitlinforteachingmesomeofherIDLskills(plusgivingmeherentireIDL
library),andtoWarrickforputtingupwithmyendlessrequestsforcommand-linehelp,questionson
grammar,andshowingmehowtouseMathematica.
Duringmyfinalyear,Ibecamemuchmoreinvolvedintheoutreachactivitiesthattakeplaceatthe
IoA.TheseareallorganisedbyCarolinCrawford,soIamverygratefultoherforlettingmetakepart.
vii
viii Acknowledgements
Indeed,ifitwasn’tforCarolin,chancesareIwouldneverhaveheardaboutthejobthatIultimately
took,soIwillalwaysbethankfultoherforforwardingonthatemail.
My world has not just revolved around theIoA during my time in Cambridge. I have met some
greatpeopleinmycollege, particularlyinmyfirstyear,whohavebeeninstrumentalinhelpingme
adjust to life in Cambridge. Similarly, student societies have helped me to balance my work with
play. One of my greatest regrets in undergrad was never picking up rowing, so I’m very glad that
DarwinCollegeBoatClubletmemakeamendsforthat!Also,Iamparticularlygratefultotheexcellent
charactersIhavemetontheCambridgemusicscene,andIwillalwaysfeelhonouredtohavemade
musicwithsuchatalentedbandofmusicians.
Therehavebeenseveralgreatscienceteachersduringmylifewhohaveinspiredmetopursuemy
dreamofstudyingastronomy,althoughnonemoresothanmyMScithesissupervisor,AlastairEdge.
MyprojectwithhimhelpedmerealisethatIenjoyeddoingresearch,andwithhisencouragementI
sentinmyapplicationtotheIoA.Ihopethatthroughthisthesis,Ihaveliveduptothepotentialthat
hesawinme.
Noacknowledgementswouldbecompletewithoutmentioningthetirelessloveandsupportofmy
parents. Theyhavealwaysbeenthereforme, andIcannotthankthemenoughforeverythingthey
havedone. Theyalsoformedonehalfofmydiligentteamofthesisproof-readers. Theotherhalfis
someoneveryspecialtome,whohasbeenatmysidethroughitall. Sarah,thankyousomuchforall
thatyouhavebroughttomylife.
This thesis has been typeset in LATEX using TEXnic Center and JabRef. Thanks to all the former
IoAmemberswhohavecontributedtothethesistemplateonwhichthisisbased,andparticularlyto
MarkBoothfromwhomIreceivedthisversion.
AlexCalverley
Cambridge,October19,2011
SUMMARY
ProbingtheIntergalacticMediumwithHigh-RedshiftQuasars
AlexCalverley
Cluesaboutthetimingofreionizationandthenatureoftheionizingsourcesresponsibleareim-
printed in the ionization and thermal state of the IGM. In this thesis, I use high-resolution quasar
spectra in conjunction with state-of-the-art hydrodynamical simulations to probe the IGM at high
redshift,focusingontheionizationandthermalstateofthegas.
After reionization, the ionization state of the IGM is set by the intensity of the ultraviolet back-
ground(UVB),quantifiedbythehydrogenphotoionizationrate,Γ . Athighredshiftsthishasbeen
bkg
estimatedbymeasuringthemeanfluxintheLyαforest,andscalingΓ insimulationssuchthatthe
bkg
simulated mean flux matches the observed value. In Chapter 3 I investigate whether the precision
oftheseestimatescanbeimprovedbyusingtheentirefluxprobabilitydistributionfunction(PDF)
insteadofonlythemeanflux. AlthoughIfinditcannotimprovetheprecisiondirectly,thefluxPDF
canpotentiallybeusedtoconstrainothersourcesoferrorinobservationalestimatesofΓ ,andso
bkg
mayincreasetheprecisionindirectly.
The ionizing output of a quasar will locally dominate over the UVB, and this leads to enhanced
transmissionbluewardsofthequasarLyαline,knownastheproximityeffect. InChapter4Ipresent
thefirstmeasurementsofΓ atz>5fromtheproximityeffect.TheUVBintensitydeclinessmoothly
bkg
withredshiftover4.6<z <6.4,implyingasmoothevolutioninthemeanfreepathofionizingpho-
tons. This suggests that reionization ends at z >6.4. There is a drop in Γ by roughly a factor of
bkg
five,whichcorrespondstoadropintheionizingemissivitybyaboutafactoroftwo. Sucharedshift
evolution in the emissivity cannot continue to much higher redshift without reionization failing to
complete,whichsuggeststhatreionizationcannothaveendedmuchhigherthanz=6.4.
EstimatesofΓ fromtheproximityeffectandthemeanfluxaregenerallydiscrepantatz∼2−4,
bkg
with those from the proximity effect systematically higher. This is generally attributed to effects of
thequasarenvironment. Iinvestigatethesignificanceofseveralenvironmentalbiasesonproximity
effectmeasurementsatz∼5−6inChapter5. Thebiasesarefoundtobesmall,andsotheproximity
effectisexpectedtogiverelativelyunbiasedestimatesofΓ atz>5,incontrasttolowerredshifts.
bkg
PhotoionizationheatsthegasintheIGM,andsothethermalhistoryoftheIGMprovidesimpor-
tantconstraintsonreionization. ThethermalstateoftheIGMisreflectedinthelevelofsmall-scale
structureintheLyαforest. InChapter6Iquantifythesmall-scalestructureusingtwoindependent
statistics,thecurvatureandthepeakiness,andconverttheseintoatemperaturebycomparingwith
simulations. ThesearethefirstmeasurementsofthetemperatureinthegeneralIGMatz>5. Both
statistics show an increase in the temperature by a factor of roughly two from z =4.4 to 5.6. This
rise is sensitive, however, to any smoothing of the gas density distribution due to the thermal his-
toryspanningreionization. Ifindthatthisshouldonlybeasmalleffect,asotherwisethecorrected
ix
x Summary
temperatures at z ∼4−5 are implausibly low. The temperature evolution therefore suggests a late
reionization. The temperatures at z ≥4.8 are well fit by an adiabatic cooling curve, for which rea-
sonablepeaktemperaturesattheendofreionizationarereachedat6.z.7. Thetemperaturesat
z∼4−5areconsistentwithreionizationbeingcarriedoutbyPopIIstars.
In conclusion, the ionization and thermal state of the IGM at z ∼5−6 suggest a late hydrogen
reionization,drivenbystar-forminggalaxiesandendingaround6.5.z.7. Thisisconsistentwith
otherrecentlinesofobservationalevidence,andsupportstheoreticalmodelsthatinferalatereion-
izationfromtheobservedstarformationratehistory.