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The effect of mergers on galaxy formation and evolution [Elektronische Ressource] / put forward by Rosalind Eugenie Skelton

128 pages
DISSERTATIONSUBMITTED TO THECOMBINED FACULTIES FOR THE NATURAL SCIENCES AND MATHEMATICSOF THE RUPERTO CAROLA UNIVERSITY OF HEIDELBERG, GERMANYFOR THE DEGREE OFDOCTOR OF NATURAL SCIENCESPUT FORWARD BYROSALIND EUGENIE SKELTONBORN IN: JOHANNESBURG, SOUTH AFRICAthORAL EXAMINATION: FEBRUARY25 , 2010THE EFFECT OF MERGERS ON GALAXYFORMATION AND EVOLUTIONREFEREES: PROF. DR. HANS WALTER RIXPROF. DR. ERIC F. BELLTo my granny Pat,a constant source of inspiration and loveSummaryThis thesis explores the effect of galaxy mergers on the evolution of galaxies over the last8 billion years using the merger trees from a semi analytic model (SAM) of galaxy formation.The SAM produces reasonable agreement with the distribution of mass, luminosity and colourat low redshifts, as well as the observed merger fractions. I revisit two apparent contradictionsbetween the standard hierarchical model of galaxy formation and observations of early typegalaxies, using the galaxy merger trees as the basis for further modelling. The observed colour–magnitude relation from the Sloan Digital Sky Survey has a change in slope and smaller scatterat the bright end. A simple toy model shows that dry mergers produce similar characteristics.Contrary to previous claims, the small scatter in the observed CMR thus cannot be used toconstrain the amount of dry merging.
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DISSERTATION
SUBMITTED TO THE
COMBINED FACULTIES FOR THE NATURAL SCIENCES AND MATHEMATICS
OF THE RUPERTO CAROLA UNIVERSITY OF HEIDELBERG, GERMANY
FOR THE DEGREE OF
DOCTOR OF NATURAL SCIENCES
PUT FORWARD BY
ROSALIND EUGENIE SKELTON
BORN IN: JOHANNESBURG, SOUTH AFRICA
thORAL EXAMINATION: FEBRUARY25 , 2010THE EFFECT OF MERGERS ON GALAXY
FORMATION AND EVOLUTION
REFEREES: PROF. DR. HANS WALTER RIX
PROF. DR. ERIC F. BELLTo my granny Pat,
a constant source of inspiration and loveSummary
This thesis explores the effect of galaxy mergers on the evolution of galaxies over the last
8 billion years using the merger trees from a semi analytic model (SAM) of galaxy formation.
The SAM produces reasonable agreement with the distribution of mass, luminosity and colour
at low redshifts, as well as the observed merger fractions. I revisit two apparent contradictions
between the standard hierarchical model of galaxy formation and observations of early type
galaxies, using the galaxy merger trees as the basis for further modelling. The observed colour–
magnitude relation from the Sloan Digital Sky Survey has a change in slope and smaller scatter
at the bright end. A simple toy model shows that dry mergers produce similar characteristics.
Contrary to previous claims, the small scatter in the observed CMR thus cannot be used to
constrain the amount of dry merging. I incorporate stellar population synthesis modelling into
this framework to explore the evolution of early type galaxies sincez = 1. There is strong
evolution in colour and magnitude if no mergers occur after this time. Dry mergers and the
recent addition of younger populations onto the red sequence reduce the evolution, mimicking
that of an ancient passively evolving population. Early type galaxies can therefore appear to
have evolved passively even though significant merging activity continues to recent times.
Zusammenfassung
Diese Arbeit untersucht die Auswirkungen des Verschmelzens von Galaxien auf die Evolu
tion der Galaxien in den vergangenen 8 Milliarden Jahren unter Zuhilfenahme von merger trees
(die baumartige Struktur, die das Verschmelzen innerhalb einer Anzahl Galaxien beschreibt) aus
einem halbanalytischen Modell (semi analytic model, SAM) der Galaxienentstehung. Dieses
Modell stimmt gut mit der Verteilung von Masse, Leuchtkraft und Farbe bei niedriger Rotver-
schiebung, so wie auch mit dem beobachteten Anteil an Verschmelzungen, uberein.¨ Die merger
trees werden im folgenden als Basis fur¨ weitere Modelle benutzt, um zwei scheinbare Wider-
spruche¨ zwischen dem hierarchischen Standardmodell der Galaxienenstehung und den Beobach
tungen fruher¨ Galaxientypen neu zu beleuchten. Die gemessene Farb Helligkeitsbeziehung aus
dem Sloan Digital Sky Survey verandert¨ am hellen Ende ihre Steigung und hat dort eine gerin
gere Streuung. Mit einem einfachen Sandkastenmodell kann ich demonstrieren, dass sogenan
nte dry merger (Verschmelzungen von Galaxien, die kein Gas enthalten) ahnliche¨ Merkmale
im Farb Helligkeitsdiagramm erzeugen. Im Gegensatz zu fruheren¨ Behauptungen, kann die
beobachtete geringe Streuung des Farb Helligkeitsdiagramms nicht benutzt werden, um die
¨Zahl der dry mergers festzulegen. Um die Entwicklung fruher Galaxientypen ab einer Rotver-
schiebung z = 1 zu untersuchen, binde ich Sternpopulationssynthese Modelle in diesen Rah
men ein. Ohne Verschmelzungen spater¨ als z = 1 findet man eine starke Entwicklung der
Galaxien in Farbe und Leuchtkraft. Dry mergers und das kurzliche¨ Hinzufugen¨ jungerer¨ Pop
ulationen zur red sequence verlangsamen die Entwicklung, und ahmen so eine sich passiv en
twickelnde, altere¨ Population nach. Fruhe¨ Typen konnen¨ daher erscheinen, als hatten¨ sie sich
passiv entwickelt, obwohl Verschmelzen bis in die jungste¨ Vergangenheit stattgefunden hat.Abstract
This thesis explores the effect of galaxy mergers on the evolution of galaxies over the last 8
billion years (sincez∼1) using the merger trees from a semi analytic model (SAM) of galaxy
formation. I compare the predictions of the SAM to the distributions of galaxy mass, luminosity
and colour in the local Universe and out toz∼1. The SAM matches the local observations well
but there is too little evolution in the mass function compared to observations, indicating that
the low mass galaxy population builds up too early. I investigate how the merger fraction and
rate vary with redshift for galaxies of different mass and gas content. The fraction of galaxies
involved in mergers increases with mass. Gas poor mergers become increasingly important with
decreasing redshift, particularly for high mass galaxies, as the mass on the red sequence builds
up. I test the predicted merger fraction against the results of two recent observational studies
that use different methods to identify mergers, finding satisfactory agreement. The fraction of
10recent merger remnants withM ≥ 2.5×10 M evolves mildly from 3 – 10% fromz = 0.2? fl
toz = 1. Major mergers contribute 1.5 – 4.5% over this period. The fraction of galaxies with
10M >5×10 M involved in mergers ranges from 2.8 to 3.3% for0.2<z <1.2.? fl
I revisit two apparent contradictions between the standard hierarchical model of galaxy
formation and observations of early type galaxies. I develop a simple toy model that assumes
gas rich major mergers are effective at quenching star formation and moving galaxies onto
the colour–magnitude relation (CMR) of early type galaxies. Subsequent dry mergers build
up mass but do not change galaxy colours. More massive galaxies undergo more dry merging,
resulting in a change in slope and decrease in scatter at the bright end of the relation. The amount
of dry merging predicted by a hierarchical model results in a CMR that matches well with the
observed relation from the Sloan Digital Sky Survey, averaged over all environments. Contrary
to previous claims, the small scatter in the observed CMR cannot be used to constrain the
amount of dry merging. I incorporate stellar population synthesis modelling into this framework
to explore the evolution of early type galaxies. The observed CMR atz =1 can be reproduced
either by the recent formation (z =2) and subsequent passive evolution of a galaxy populationf
or by the recent quenching of star formation in galaxies that formed earlier (z = 4). If nof
mergers occur after z = 1 there is too much evolution in colour and magnitude compared to
observations. Dry mergers and the recent addition of younger populations onto the red sequence
after z = 1 result in bluer colours and a smaller change in magnitude. This slower evolution
mimics that of an ancient passively evolving population. Early type galaxies can therefore
appear to have evolved passively even though significant merging activity continues to recent
times.

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