The Nature of Dusty Star-Forming Galaxies

221 Pages·2017·8.842 MB·English

by  William Cowley (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
William Cowley

The Nature
of Dusty Star-
Forming Galaxies
Springer Theses
Recognizing Outstanding Ph.D. Research
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William Cowley
The Nature of Dusty
Star-Forming Galaxies
Doctoral Thesis accepted by
Durham University, UK
123
Author Supervisors
Dr. William Cowley Prof. CedricLacey
Kapteyn Astronomical Institute Department ofPhysics, Institute
Groningen forComputational Cosmology
TheNetherlands Durham University
Durham
UK
Prof. Carlton Baugh
Department ofPhysics, Institute
forComputational Cosmology
Durham University
Durham
UK
Prof. ShaunCole
Department ofPhysics, Institute
forComputational Cosmology
Durham University
Durham
UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-66747-8 ISBN978-3-319-66748-5 (eBook)
DOI 10.1007/978-3-319-66748-5
LibraryofCongressControlNumber:2017950282
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When I heard the learn’d astronomer;
When the proofs, the figures, were ranged in
columns before me;
When I was shown the charts and diagrams,
to add, divide, and measure them;
When I, sitting, heard the astronomer where
they lectured with much applause in the
lecture-room,
How soon, unaccountable, I became tired
and sick;
Till rising and gliding out, I wander’d off by
myself,
In the mystical moist night-air, and from time
to time,
Look’d up in perfect silence at the stars.
Walt Whitman
To mum and dad, for everything
’
Supervisors Foreword
Will Cowley’s Ph.D. thesis focuses on understanding the evolution of galaxies at
far-infrared and sub-millimetre wavelengths within the framework of theoretical
modelsforstructureformationintheUniverse.Theemissionfromgalaxiesatthese
wavelengths comes predominantly from interstellar dust grains that have been
heatedbyabsorbinglightfromstars.Observationsbythefirstspace-basedinfrared
telescopesinthe1970sand1980sshowedthatgalaxiesinthelocalUniverseemita
significant fraction of their radiation in this way, but the full importance of the
far-infraredemissionfromgalaxiesonlybecameapparentwiththediscoveryinthe
1990s of the cosmic infrared background. Measurements of this background
showed that, over the history of the Universe, roughly as much radiation has been
emitted by galaxies at infrared wavelengths by dust grains as at optical and ultra-
violet wavelengths, where we are mainly seeing the direct emission from stars,
unobscuredbydust.Itwasalsorealisedthatmostofthestarlightthatisabsorbedby
dust and reprocessed into infrared radiation is from young stars, since these emit
mostoftheirradiationatshorterwavelengthsthataremoreeffectivelyabsorbedby
dust grains. The light from young stars is used as one of the main observational
tracersofstarformationactivityingalaxies.Sincemostofthislightisabsorbedby
dustandreprocessedintoinfraredradiation,understandingthefar-infraredemission
from galaxies over the history of the Universe is crucial for understanding the
cosmic history of star formation, which describes how the stellar mass of galaxies
has built up over cosmic time.
ThesignificanceofCowley’sworkisthatitseekstounderstandobservationsof
galaxy evolution at far-infrared and sub-millimetre wavelengths in terms of phys-
ical models of galaxy formation that are implemented within hierarchical models
for structure formation in the Universe. Galaxy formation is a complex process.
Structureformsinthedarkmatterbygravitationalgrowthofinitiallysmalldensity
fluctuations, leading to gravitationally bound dark matter halos, and gas then falls
into the gravitational potential wells of these halos. The gas in halos undergoes
heatingandcoolingprocesses,withsomeofitcoolingandcollapsingtothecentres
of halos, where it can form stars and also supermassive black holes. However, the
efficiency of converting baryons in halos into stars is strongly limited by energy
ix
x Supervisors’Foreword
injectionbybothyoungstarsandblackholes,aprocesscalledfeedback.However,
thedetailedoperationofthesefeedbackprocessesisstillpoorlyunderstood,which
is why galaxy formation remains only a partly-solved problem.
Cowley’sworkusesasemi-analyticalmodelofgalaxyformation,GALFORM,that
has been developed over the past two decades by researchers at the Institute for
Computational Cosmology, which is part of Durham University. The GALFORM
model incorporates modelling of all of the processes mentioned above, starting
from dark matter halos predicted by a cosmological dark matter simulation. It also
includes the evolution of stellar populations, the production and ejection of heavy
elements by stars, their conversion into interstellar dust, and the reprocessing of
starlight by this interstellar dust. The treatment of feedback processes is a key
element of the model, which introduces parameters that must be calibrated using
observationsofrealgalaxies.Cowley’sworkusedaversionoftheGALFORM model
that had previously been carefully calibrated on an extensive set of observational
data, resulting in a fully predictive ab initio theoretical model of galaxy formation
that is able to predict galaxy luminosities from both stars and dust.
Inhis thesis, Cowley usedthe GALFORM model tofurther ourunderstandingofa
numberof importantaspectsof galaxy evolutionatfar-infrared andsub-millimetre
wavelengths. First, he constructed simulated images of the sky at sub-millimetre
wavelengths including the effects of galaxy clustering as well as the angular res-
olution of the telescope and instrumental noise, and extracted sources from these
images in the same way as is done when analysing observational data. He found
thatfortherelativelypoorangularresolutionofthesingledish telescopes usedfor
mostsurveysatthesewavelengthsuptonow,blendingofsourcesbythetelescope
beam has a significant effect on the measured number counts of sources, in
agreement with observational results using interferometers.
Additionally, Cowley discovered a new and previously unsuspected effect in
galaxyclusteringcalled‘blendingbias’,wherebytheblendingofmultiplegalaxies
into a single source by the telescope beam causes an artificial boost in the angular
clustering of the observed sources compared to that of the real galaxies. Since the
observedclusteringofgalaxiesisusedtoinferthetypicalmassesofthedarkmatter
haloscontainingthesegalaxies,thisblendingbiascouldresultinhalomassesbeing
greatly overestimated.
He then used the GALFORM model to predict the contribution to the far-infrared
emissionfrom galaxies inhalos ofdifferent masses,finding that overawiderange
of redshifts, the far-infrared emissivity should be dominated by halos of mass
around1012M(cid:1),asaresultofthecompetingeffectsofgascoolingandfeedbackon
the efficiency with which baryons in halos are converted into stars. He shows that,
as a consequence, the model predicts a power spectrum for angular fluctuations in
thecosmicinfraredbackground inremarkablygood agreementwith that measured
by the Herschel Space Telescope. This agreement provides important support for
current theoretical ideas about galaxy formation.
Finally, Cowley coupled GALFORM with a more detailed radiative transfer model
for stellar and dust emission from galaxies to make predictions for what
Supervisors’Foreword xi
high-redshiftgalaxiesshouldbeseeninfuturedeepimagingsurveyswiththeJames
Webb Space Telescope. This type of theoretical modelling will be crucial for
interpreting observations from future telescopes in terms of physical processes in
galaxy formation.
Durham, UK Cedric Lacey
July 2017 Carlton Baugh
Shaun Cole