Kinematics and Metallicity of RGB Stars in the Triangulum (M33) Galaxy This dissertation is submitted for the degree of Master of Science David Trethewey Institute of Astronomy & Robinson College University of Cambridge April 28, 2011 ii Preface Declaration This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration except where specifically indicated in the text. No part of this dissertation has been submitted for any other degree or qualification. This dissertation does not exceed 60,000 words. iii Figure 1: An image of the sky centred on M31 (upper right) and M33 (lower left), taken by the author using a Nikon D50 (at 70mm focal length) piggybacked on the Thorrowgood telescope mount at the Institute of Astronomy, Cambridge. 10 mins exposure, a background flattening routine is applied using MaxIm DL to reduce the visual impact of light pollution. iv Acknowledgements I thank my supervisors, Scott Chapman and Mike Irwin for guiding me in my research during my time at the Institute, and other members of the Institute of Astronomy who have also helped me in my academic life. I thank my office mates in O26 (and the now demolished SPO13) at the Institute of Astronomy; Scott Brown, Tim Staley, Duncan Hanson, Christopher Berry, Jonathan Crass, and other fellow graduate students I have known during my time here. I would also like to thank all of the IoA support staff, for everything from computing to morning coffee at the Institute. I thank the students, fellows and staff of Robinson College for the role they have played in the last nine years of my life, one third of my time on this Earth to date. One of them, Matt Davis as a PhD student in Geophysics always said he was more “down to earth” than me. I would like to thank the editors of Astronomy Now, noting that my interest in astronomy really began at the age of eight, with the January 1993 issue of that magazine. I also thank others who have been important in guiding me through my time as a graduate student. I thank Tony James, Peter Squibb, Mick Harvey and other members of the Cornwall Astronomical Society, through which my interest in astronomy developed. I also thank the people of the IAYC (International Astronomical Youth Camp) who I enjoyed spending time with in the forests of Central Europe. Finally I thank my family, for encouraging me and supporting me at every stage. v vi Abstract Kinematics and Metallicity of RGB Stars in the Triangulum (M33) Galaxy David Trethewey Eleven Keck DEIMOS (DEep Imaging Multi Object Spectrometer) fields around the Triangulum Galaxy (M33) are analysed in this work in combination with CFHT (Canada France Hawaii Telescope) photometry taken as part of the PAndAS (Pan-Andromeda Archaeological Survey) survey (McConnachie et al., 2009), containing a total of 2000 targets. A stellar population of candidate M33 RGB (Red Giant Branch) stars ∼ that is not rotating with the M33 disk is detected in most fields. The relative contribution of this population is estimated using kinematic windowing, both using a method of hard “disk” and “halo” kinematic windows in radial velocity and a Bayesian method taking account of priors for the expected disk and halo radial velocity distributions and relative contributions. The “disk” component has an exponential scale length of 1.7 kpc, agreeing with previous photometric work. If the non-disk population is assumed to be a “true” ∼ smooth halo, and fit by an exponential, it would contribute far more (9% of the disk luminosity integrated at 1% Bayesian halo prior and 17% integrated using simple windowing) than is indicated by photometric surveys (such as the INT-WFC (Isaac Newton Telescope Wide Field Camera) survey as published in Ferguson et al. 2007) and theoretical expectations, which constrain the contribution of a smooth stellar halo to the overall luminosity to less than a few percent. The metallicity properties of both the “disk” and “halo” samples are analysed. Photometric metallicities are derived using the Dartmouth isochrones (Dotter et al., 2008), and spectroscopic metallicities are derived from the equivalent widths of the Ca II triplet of stacked spectra. The “disk” shows a downward metallicity 1 trend with radius on the major axis of a magnitude 0.02-0.04 dex kpc− consistent between the photometric and spectroscopic analyses, which in the photometric analysis is significant at the 2σ level. The “halo” sample 1 shows a downward metallicity trend with radius on the major axis of a magnitude 0.04-0.05 dex kpc− in the photometric analysis, but this is not significant at 2σ level, and can be taken to be consistent with constant [Fe/H]. In the spectroscopic analysis, the metallicity trend of the halo is stronger, being of magnitude 0.11- 1 0.14 dex kpc− and significant at a 2σ level. The overall weighted mean [Fe/H]s of the “halo” sample across all fields, are -1.37 and -1.07 in the spectroscopic (CaT2 only) and photometric analyses respectively, and for the “disk” sample, -0.93 and -0.97. It is likely that the “halo” sample is largely the result of tidal stripping during a past interaction with M31 rather than a “true” primordial halo. vii viii Contents 1 Introduction 1 1.1 Andromeda and Triangulum historically . ...... 1 1.1.1 Distances to galaxies in the Local Group . ....... 4 1.2 Galaxy formation and motivation for the present study . ............. 4 1.2.1 The structure of galaxies . ..... 5 1.3 Near-field cosmology in the Local Group . ....... 7 1.3.1 Review of the study of galaxy formation and near-field cosmology. ........ 7 1.4 Introduction to the Local Group . ....... 8 1.4.1 The Local Group in context . .... 8 1.4.2 Basic parameters of the Milky Way, Andromeda and Triangulum galaxies ....... 10 1.4.3 Satellites of the major Local Group galaxies . ......... 14 1.5 TheTriangulumGalaxy ................................. ...... 17 1.5.1 Context of its relation to Andromeda . ...... 17 1.5.2 Previous work on Triangulum . ..... 17 1.5.3 DistancetoM33 ................................... 21 2 Context - The INT and PAndAS surveys and DEIMOS spectroscopic observations 23 2.1 Photometric studies . ...... 23 2.1.1 TheINTWFCsurvey .................................. 23 2.1.2 PAndAS - the Pan-Andromeda Archaeological Survey . 23 2.2 Spectroscopic work with DEIMOS . ...... 26 2.2.1 Technical detail about the DEIMOS instrument and its operation . ...... 26 2.2.2 The use of DEIMOS to observe Andromeda and Triangulum . 30 2.2.3 Targetselection................................... ..... 30 2.2.4 Accuracy of radial velocity and metallicity measurements. ....... 30 2.2.5 Summary of scientific work using DEIMOS on M31 . 30 2.2.6 TheDEIMOSdatainM33 ............................... 32 3 M33 disk and halo in the DEIMOS data 35 3.1 Processing of the pipelined data . ...... 35 3.1.1 Kinematic summary plots . 35 3.1.2 Diskrotation .................................. ....... 40 ix 3.1.3 Milky Way dwarf contamination . ....... 41 3.1.4 Quality cuts to the data . 46 3.1.5 Kinematic windowing . 47 3.2 Halo and disk profiles . ..... 50 3.2.1 Bayesianselection ................................. ..... 50 3.2.2 Simple kinematic windowing . 52 3.2.3 Halo/diskratios ................................ ....... 52 3.3 The outer two fields - EC2 and 511TrS . ..... 56 3.4 Halo velocity dispersions . ......... 56 3.5 Photometric metallicities - how photometry in various colour bands can provide information onmetallicity........................................ ..... 61 3.5.1 Consequences of the distance assumption . 61 3.5.2 Effect of age and alpha assumptions on photometric metallicity results . ........ 65 3.6 Spectroscopic metallicities . ........ 66 3.6.1 Stacking the spectra . 66 3.6.2 Effect of distance of the [Fe/H] calibration . ....... 77 3.7 Overall trends in metallicity . ........ 78 4 Discussion 83 4.1 Thediskprofile....................................... ..... 83 4.1.1 General properties of the disk . 83 4.1.2 The disk scale length . 83 4.2 Thehaloprofile....................................... ..... 84 4.2.1 Comparing the observed “halo” population to photometric surveys. ........ 84 4.2.2 Theoretical expectations . 87 4.3 Metallicity and age properties . ......... 88 4.3.1 Disk.......................................... 89 4.3.2 Halo.......................................... 92 4.4 Modelling of a possible interaction between M31 and M33 . ......... 93 4.5 The “halo” population - genuine stellar halo or stripped disk material? ............. 97 A Summary of M31 DEIMOS Fields 101 B [Fe/H] results for various combinations of CaT lines 105 C Tables of the individual stars 107 x List of Figures 1 An image of the Andromeda-Triangulum area of the sky . iv 1.1 A drawing of M33 based on Lord Rosse’s observations . ......... 2 1.2 Early photographs of M33 and M31 . ...... 3 1.3 The Hubble “tuning fork” classification of galaxies. .......... 6 1.4 Spatial density of SDSS stars around the north Galactic cap showing the Sagittarius and other streams............................................. .... 9 1.5 Visible light and near-IR (2MASS) all-sky panoramas . ......... 12 1.6 The distribution of Local Group members, as viewed from two orthogonal directions. 15 1.7 Structural properties of galaxies. (from Tolstoy et al. 2009) . ............... 16 1.8 M33 from the 2nd Palomar Digitised Sky Survey . ........ 18 1.9 Positions of fields studied in M33 in the literature . .......... 19 2.1 CoverageoftheINTsurveyofM31. ........ 24 2.2 CoverageoftheINTsurveyofM33. ........ 25 2.3 Coverage of the PAndAS survey. ...... 27 2.4 A diagram showing the mounting of DEIMOS on Keck II. ...... 28 2.5 The optics of the DEIMOS instrument. ..... 29 2.6 Locations of the DEIMOS spectroscopic masks around M31 and M33 shown on an image of the PAndAS survey . 31 2.7 The locations of the M33 DEIMOS masks on a section of an image of the PAndAS survey. 33 3.1 Velocity histogram for the 7 southern DEIMOS masks in M33.