Spiral arms and their effects on secular evolution and star formation in disk galaxies [Elektronische Ressource] / put forward by Kelly Foyle

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DISSERTATIONSUBMITTED TO THECOMBINED FACULTIES OF THE NATURAL SCIENCES AND MATHEMATICSOF THE RUPERTO-CAROLA-UNIVERSITY OF HEIDELBERG, GERMANYFOR THE DEGREE OFDOCTOR OF NATURAL SCIENCESPUT FORWARD BYKELLY FOYLEBORN IN: MONTREAL, CANADAthORAL EXAMINATION: OCTOBER13 , 2010SPIRAL ARMS AND THEIR EFFECTS ON SECULAREVOLUTION AND STAR FORMATION IN DISKGALAXIESREFEREES: PROF. DR. HANS-WALTER RIXPROF. DR. RALF S. KLESSEN“The most important reason for going from one place to another is to see what’s in between.”- Norton Juster (The Phantom Tollbooth)AbstractWe investigate how spiral structure affects the observational properties of disk galaxies bothin terms of dynamical secular evolution and of star formation. We derived the first observa-tional estimate of the torque-induced instantaneous angular momentum flow, resulting fromnon-axisymmetric features in the stellar distribution for a sample of 24 galaxies. The strongesttorques were found among barred galaxies. In the inner regions, the average torques are strongenough to redistribute angular momentum on a timescale of4 Gyr with an outward angularmomentum flow. In examining the role of spiral arms in star formation we found that they donot dominate, even in grand-design spiral galaxies as there is a comparable amount of interarmstar formation. Further, we found that the arms show no enhancement in the efficiency of starformation in terms of molecular gas.

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Published 01 January 2010
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DISSERTATION
SUBMITTED TO THE
COMBINED FACULTIES OF THE NATURAL SCIENCES AND MATHEMATICS
OF THE RUPERTO-CAROLA-UNIVERSITY OF HEIDELBERG, GERMANY
FOR THE DEGREE OF
DOCTOR OF NATURAL SCIENCES
PUT FORWARD BY
KELLY FOYLE
BORN IN: MONTREAL, CANADA
thORAL EXAMINATION: OCTOBER13 , 2010SPIRAL ARMS AND THEIR EFFECTS ON SECULAR
EVOLUTION AND STAR FORMATION IN DISK
GALAXIES
REFEREES: PROF. DR. HANS-WALTER RIX
PROF. DR. RALF S. KLESSEN“The most important reason for going from one place to another is to see what’s in between.”
- Norton Juster (The Phantom Tollbooth)Abstract
We investigate how spiral structure affects the observational properties of disk galaxies both
in terms of dynamical secular evolution and of star formation. We derived the first observa-
tional estimate of the torque-induced instantaneous angular momentum flow, resulting from
non-axisymmetric features in the stellar distribution for a sample of 24 galaxies. The strongest
torques were found among barred galaxies. In the inner regions, the average torques are strong
enough to redistribute angular momentum on a timescale of4 Gyr with an outward angular
momentum flow. In examining the role of spiral arms in star formation we found that they do
not dominate, even in grand-design spiral galaxies as there is a comparable amount of interarm
star formation. Further, we found that the arms show no enhancement in the efficiency of star
formation in terms of molecular gas. We searched algorithmically for angular offsets between
star formation tracers and found that there was no systematic spatial ordering of these tracers,
as would be predictable by a shock triggering model of spiral structure. It seems spiral structure
is most likely transient or at least more complex than the simplest models predict. These results
point to a spiral structure that plays a lesser role in shaping a galaxy’s observable properties as
was previously thought. The strength of gravitational torques depends more strongly on bars
than on spiral structure, and spiral arms are not regions of enhanced star formation efficiency.
At best they act to reorganize the interstellar medium and concentrate the gas.
Zusammenfassung
Wir untersuchen, wie dynamische Entwicklung und Sternentstehung von Scheibengalaxien
durch Spiralarmstruktur beeinflusst werden. Es gelang die erste beobachterische Abschatzung¨
des drehmomentinduzierten Drehimpulstransport, der durch eine asymmetrische Sternverteilung
entsteht, basierend auf 24 Galaxien. Es zeigt sich, dass Balkengalaxien die starkste¨ Drehmo-
mente haben. Im Durchschnitt sind die gravitativen Drehmomente in den Zentralbereichen der
Galaxien stark genug, um den Drehimpuls auf Zeitskalen von4 Gyr nach außen zu trans-
portieren. Wir haben die Sternentstehung in den Spiralarmen und zwischen den Spiralarmen
untersucht und gefunden, dass die Sternentstehung in beiden Gebieten vergleichbar stark ist.
Zudem zeigen die Arme keine großere¨ Sternentstehungs-Effizienz in Bezug auf molekularen
Wasserstoff. Daruberhinaus¨ gibt es keine Anzeichen fur¨ eine systematische raumliche¨ Anord-
nung von Sternentstehungs-Indikatoren, wie man etwa in einem Schock-Trigger-Modell der
Spiralstruktur erwarten wurde.¨ Diese Ergebnisse legen nahe, dass Spiralarmstruktur nur eine
untergeordnete Rolle in der Entwicklung der beobachtbaren Galaxien-Eigenschaften spielt.Selbst die gravitativen Drehmomente werden stark¨ er durch Balken als durch Spiralarmstruktur
beeinflusst. Spiralarme haben keine erhohte¨ Sternentstehungseffizienz. Es scheint, dass Spi-
ralarmstruktur relativ kurzlebig ist oder mindestens komplexer als einfachste Modelle erwarten
lassen.Contents
Contents i
List of Figures iii
List of Tables vii
1 Introduction 1
1.1 Island Universes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Disk Galaxy Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Spiral Galaxies, the Interstellar Medium & Star Formation . . . . . . . . . . . 5
1.3.1 Components of the ISM . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3.2 Star Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 Spiral Arms: their Formation, Persistence and Effects . . . . . . . . . . . . . . 10
1.4.1 Basic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.2 Theories of Spiral Structure . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.3 The Effects of Spiral Arms . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Secular Evolution in Spiral Galaxies 19
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Background & Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Determining the Angular Momentum Flow Timescale from Photometric Data . 23
2.4 Testing Torque Estimates using Disk Galaxy Simulations . . . . . . . . . . . . 25
2.4.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4.2 Projected Mass Densities and the 2D Potential . . . . . . . . . . . . . 26
2.4.3 Torques on Stars from Stars versus Torques from the Gas & Dark Matter 28
2.5 The Role of Torques from Stars: a Pilot Study . . . . . . . . . . . . . . . . . . 30
2.5.1 Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.5.2 Stellar Mass Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.5.3 Making Radial Torque and Angular Momentum Profiles . . . . . . . . 37
2.5.4 M100 - Comparison with the analysis Gnedin, Goodman & Frei (1995) 44
2.6 Results & Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
iii CONTENTS
2.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3 Arm & Interarm Star Formation 53
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.2.1 Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.2 Defining Spiral Arms . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.3 Star Formation & Gas Tracers in the Arm and Interarm Regions . . . . . . . . 59
3.4 The Star Formation Efficiency in the Arm and Regions . . . . . . . . 61
3.5 Molecular Gas Fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4 Testing Spiral Structure Theories 71
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.1.1 Long-Lived Spiral Arms & Shock Triggered Star Formation . . . . . . 72
4.1.2 Method of Tamburro et al. 2008 . . . . . . . . . . . . . . . . . . . . . 75
4.1.3 Toy Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.2 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2.1 2D Cross-Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.3 Comparison with Tamburro et al. 2008 . . . . . . . . . . . . . . . . . . . . . . 82
4.4 Fitting Offsets using the 2D Cross-Correlation . . . . . . . . . . . . . . . . . . 85
4.5 Angular Offsets with Other Tracers . . . . . . . . . . . . . . . . . . . . . . . . 89
4.6 Dispersion of Star Formation Tracers . . . . . . . . . . . . . . . . . . . . . . . 93
4.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5 Conclusions & Outlook 103
5.1 Secular Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.2 Star Formation & Spiral Arms . . . . . . . . . . . . . . . . . . . . . . . . . . 104
5.3 Towards an Understanding of Spiral Structure . . . . . . . . . . . . . . . . . . 105
5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Acknowledgments 109
Bibliography 111