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The Age of the Milky Way Halo Stars : Implications for Galaxy Formation

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The Age of the Milky Way Halo Stars : Implications for Galaxy Formation The Age of the Milky Way Halo Stars – Implications for Galaxy Formation – Paula Jofr´ePfeil M¨unchen 2010 The Age of the Milky Way Halo Stars – Implications for Galaxy Formation – Paula Jofr´ePfeil Dissertation an der Physik Fakult¨at der Ludwig–Maximilians–Universit¨at M¨unchen vorgelegt von Paula Jofr´ePfeil aus Santiago de Chile M¨unchen, den 30 September 2010 Erstgutachter: Dr. Achim Weiss Zweitgutachter: Dr. Markus Kissler-Patig Tag der m¨undlichen Pr¨ufung: 10. Dezember 2010 Contents Zusammenfassung ix Abstract xi 1 Introduction 1 2 MAχ 7 2.1 Overview and Motivation for MAχ ...................... 8 2.2 History of the MAχ data-compressionmethod. 9 2.3 Description of the MAχ method ....................... 10 2.3.1 Maximum likelihood description . .. 10 2.3.2 Karhunen-Lo`eve data compression . .. 11 2.3.3 Goodnessoffit ............................. 14 2.3.4 Retaining the information after the compression . ....... 15 2.4 MAχ Cookbook ................................ 16 2.4.1 Configurationofspectra . 16 2.4.2 Compressionprocedure. 18 2.4.3 Parameterrecovery ........................... 19 2.5 Summary .................................... 19 3 MAχ for Low-Resolution Spectra 21 3.1 Introduction................................... 22 3.2 Data:SDSS/SEGUE .............................. 22 3.3 Gridofmodels ................................. 24 3.4 Matchingmodelstodata. .. .. 25 3.5 ApplicationtoSDSSspectra . .. 26 3.5.1 Whole wavelength domain vs. spectral windows . .... 27 3.5.2 Effect of radial velocity uncertainties . ..... 30 3.5.3 Comparison with the SEGUE Stellar Parameter Pipeline ...... 31 3.6 Systematiceffects................................ 33 3.6.1 Choiceoffiducialmodel . .. .. 33 3.6.2 Choiceofgridofmodels . .. .. 36 3.7 Summaryandconclusions . 40 vi CONTENTS 4 MAχ for high-resolution spectra 43 4.1 Motivation.................................... 44 4.2 Data....................................... 44 4.3 Gridofhigh-resolutionmodels . .... 44 4.4 Fittinghigh-resolutionspectra . ...... 47 4.5 Results of the high-resolution analysis using MAχ .............. 47 4.5.1 Results of a “classical” analysis . ... 47 4.5.2 “Restricted” parameter recovery . ... 50 4.6 Summaryandconclusions . 51 5 Main-Sequence Turn-Off 53 5.1 Theturn-offtemperature. 54 5.2 Turn-Offdetectionmethods . 56 5.2.1 CubicFit ................................ 57 5.2.2 SobelKerneledge-detection . 58 5.2.3 Uncertainties in the turn-off detection . ..... 59 5.2.4 SobelKernelv/sCubicFit. 61 5.3 Metallicity-Temperaturediagram . .... 62 5.4 The Turn-Off for different stellar samples . ...... 64 5.4.1 SDSS/SEGUE Target selections . 65 5.4.2 SkySelection .............................. 70 5.4.3 Dependency and errors in the edge-determination . ...... 73 5.5 SummaryandConclusions . 77 6 Age determination 81 6.1 Historicpreludeandmotivation . .... 82 6.2 AtomicDiffusion ................................ 84 6.2.1 DiffusionwithGARSTECmodels . 85 6.2.2 UsingGARSTECisochrones. 86 6.3 [Fe/H] - Teff diagramfortheagedetermination . 89 6.3.1 Agereduction.............................. 89 6.3.2 Theageasafunctionofmetallicity . .. 93 6.4 [Fe/H] - Teff forGalaxyevolution. 97 6.4.1 Onedominatinghalopopulation. 97 6.4.2 Diskstars ................................ 98 6.4.3 Bluemetal-poorstars. .. .. 99 6.5 Discussionandconclusion . .. 99 7 Globular Clusters 103 7.1 Motivation.................................... 104 7.2 Globularclusterssample . 105 7.3 Observationalevidence . 108 7.3.1 Turn-offcolor .............................. 108 Contents vii 7.3.2 Colordistribution............................ 112 7.3.3 Implications............................... 117 7.4 Globularclustersages ............................ 119 7.4.1 An09scale................................ 120 7.4.2 SW02scale ............................... 122 7.4.3 D10scale ................................ 122 7.4.4 Discussion................................ 124 7.5 SummaryandConclusions . 125 8 Concluding Remarks 127 A The MAχ code 131 A.0.1 Input................................... 131 A.0.2 Preparation ............................... 133 A.0.3 Fitting.................................. 135 A.0.4 Optionalroutinesandflags. 135 B High-resolution tables 137 C MSTO temperature for field halo stars 139 D Ages for the Galactic halo stars 141 Acknowledgments 153 viii Zusammenfassung Zusammenfassung Das Studium der Verteilung von Sternen in der Milchstraße anhand der Daten des Sloan Digital Sky Survey (SDSS) liefert nicht nur Hinweise zu ihrer Bildung, sondern beant- wortet auch Fragen zur Sternstruktur. Ziel dieser Arbeit ist die Bestimmung des Alter der Sterne, die sich im Halo der Milchstraße befinden. Dazu werden Hauptreihensterne in der N¨ahe des Abknickpunkts (engl. turn-off) im Hertzsprung-Russell Diagramm analysiert. Die Betrachtung von Sternen nahe des Abknickpunktes ist weitverbreitet in Altersbestim- mungen von Kugelsternhaufen, weil Alter und auch Metallizit¨at an diesem Punkt sensitiv zu Anderungen¨ von der Leuchtkraft und der effektiven Temperatur sind. Um die effektive Temperatur und Metallizit¨at aus etwa einhunderttausend Sternspektren des SDSS in einem angemessenen Zeitrahmen zu bestimmen, ist ein effizienter Algorithmus erforderlich. Daf¨ur wurde eine Datenskompressionsmethode angewendet, welche die effektive Temperatur und Metallizit¨at dieser Sterne mit einer Genauigkeit von 120 K bzw. 0.25 dex bestimmt. Der Abknickpunkt f¨ur die j¨ungste Sternpopulation konnte mit hoher Genaugkeit als Funktion der Metallizit¨at abgeleitet werden. Dabei wurden die heißesten Sterne der Stern- population mit einer k¨unstlichen Sternverteilung gleicher Metallizit¨at verglichen. Mit Hilfe von Isochronen aus der Theorie der Sternentwicklung wurde mit den SDSS Daten ein in- nerer Galaktischer Halo mit einer dominanten Sternpopulation gefunden, welcher keinen Gradienten in seinem Alter bez¨uglich der Metallizit¨at aufweist. Daraus kann man schließen, dass die dominante Sternpolulation sich sehr schnell, wahrscheinlich w¨ahrend eines Kol- lapses von proto-galaktischem Gas, gem¨aß des Szenarios von Eggen (1962) gebildet haben k¨onnte. Weiterhin wurde eine signifikante Anzahl von heißen Sternen am Abknickpunkt gefunden, welche als junge Sterne aus Zwerggalaxien interpretiert werden, die sp¨ater von der Milchstraße akkretiert worden sind. Diese Interpretation st¨utzt das hierachische Galax- ienbildungsmodell von Searle & Zinn (1978). Motiviert von der gegenw¨artigen Debatte in der Theorie der Sternstruckturbildung ¨uber das gravitative Absenken von Helium in alten sonnen¨ahnlichen Sternen (atomare Dif- fusion), wurden f¨ur die Altersbestimmung der Halosterne Isochrone mit und ohne atomarer Diffusion verwendet. Diffuse Isochrone lieferten ein Alter der Halosterne von 10 bis 11 Gi- gajahren. Dieses Alter stimmt mit dem absoluten Alter der Kugelsternhaufen ¨uberein. Kanonische Isochrone, ohne Diffusion, lieferten ein Alter vom 14 bis 16 Gigajahren, was im Konflikt mit dem Alter des Universums von 13.7 Gigajahren steht. Dies widerspricht der Annahme, dass atomare Diffusion in alten Halosternen unterdr¨uckt ist. x Abstract Abstract This thesis is about the study of stellar populations of the Milky Way Galaxy using the Sloan Digital Sky Survey (SDSS). The aim was to determine the age of the stellar content of the Galactic halo by considering main-sequence turn-off stars. The turn-off point in the Hertzsprung-Russell diagram is a broadly-used age-indicator in globular clusters, because the color and luminosity at that point are mainly sensitive to age and metallicity. From the immense number of halo stars provided by SDSS, we could accurately detect the turn-off as a function of metallicity, which was done by analyzing the hottest (bluest) stars of a population with the same metallicity. In order to analyze a sample of 100,000 SDSS low resolution stellar spectra an efficient tool was required, which had to be automatic and quick, in order to retain metallicities and temperatures in a timely manner. For this pur- pose, we employed a data-compression method and estimated metallicities and effective temperatures with accuracies of 0.25 dex and 120 K, respectively. We looked for the isochrones with turn-off temperature and metallicity of the field stars and found that our stellar sample consisted of one dominating population, with no age gradient as a function of metallicity. This means that the dominating population of the Galactic halo formed rapidly, probably during the collapse of the proto-Galactic gas, ac- cording to the Milky Way formation scenario proposed by Eggen et al. (1962). Moreover, we could find a significant number of stars with hotter temperatures than the turn-off, which might be explained as young stars formed in dwarf galaxies and accreted later on to our Milky Way. These stars may be an evidence for the current hierarchical galaxy formation scenarios, as proposed by Searle & Zinn (1978). Motivated from the current debate about the efficiency of gravitational settling (atomic diffusion) in the interior of old solar-type stars, we used isochrones with and without set- tling to determine the ages. When using canonical isochrones we obtained ages of 14-16 Gyr. This result is a strong argument against fully-inhibited diffusion in old halo field stars, because if the efficiency of diffusion would be completely inhibited, then the age of these stars would be in serious conflict with the age of the Universe of 13.7 Gyr. The age obtained using diffusive isochrones were of 11-12 Gyr, which is in agreement with the absolute age of the old globular clusters in the inner halo. xii 0. Abstract Chapter
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