The globular cluster system of NGC 4526 Leonie Chevalier Presented in fulfillment of the requirements of the degree of Masters by Research { Faculty of Science and Technology Swinburne University i You couldn't put off the inevitable. Because sooner or later, you reached the place when the inevitable just went and waited - Terry Pratchett, Small Gods ii Abstract With ages up to ∼ 12.8 Gyr, globular clusters (GCs) are some of the oldest objects we observe in galaxies and are thought to preserve their parent galaxy's chemo-dynamical properties at the time of their formation. A single galaxy can have tens of thousands of GCs associated with it. Globular cluster systems have long been thought to be a useful tool for constraining galaxy formation scenarios and even to infer a galaxy's individual formation history. More recently GCs have been used to infer dark matter masses of galaxies. This thesis focuses on the GC system of the lenticular galaxy NGC 4526. We used Subaru imaging to study the system's full extent and infer the total number of GCs as well as the substructure in the GC system. Additionally we used the DEIMOS spectrograph on the Keck telescope to obtain radial velocities for 106 GCs to be used in studying the dark matter content of NGC 4526. The combination of multiband photometric data in combination with the spectroscopic data gained by DEIMOS is a significant improvement on existing data sets that only partially imaged the GC system. To put our findings into a broader context we compared NGC 4526 to the results of 26 other elliptical and lenticular galaxies (and their GC systems) from existing literature. Additionally we investigated the dark matter content of NGC 4526 and how this measurement is dependent on sample selection and model assumptions. We found that this galaxy has an unusually high fraction (80%) of metal-poor GCs and a high dark matter fraction for its mass. In fact it was one of the highest metal poor fractions observed in our sample of 27 galaxies. However, it is still consistent with the general trend we derived from our full sample of galaxies. The GC system itself did not appear overly extended. We propose that this particular galaxy most likely formed through a series of minor mergers. iii iv Acknowledgements I would like to thank the people who have supported me throughout writing this thesis. To Vincent for his love and support throughout, to Manodeep and Ned for the best coding help and high fives and most of all to Caitlin without whom this thesis would never have been finished. v vi Declaration The work presented in this thesis has been carried out in the Centre for Astrophysics & Supercomputing at Swinburne University of Technology between 2016 and 2018. This thesis contains no material that has been accepted for the award of any other degree or diploma. To the best of my knowledge, this thesis contains no material previously published or written by another author, except where due reference is made in the text of the thesis. Leonie Chevalier Melbourne, Victoria, Australia 2018 vii Contents Abstract ii Acknowledgements iii Declaration v List of Figures x List of Tables xi 1 Introduction 1 1.1 Individual globular clusters . 1 1.2 Globular cluster systems . 2 1.3 Globular cluster systems and their host Galaxies . 3 1.4 Large scale studies of globular cluster systems . 5 1.4.1 ACS Virgo and Fornax Cluster Survey . 5 1.4.2 Next Generation Virgo Cluster survey . 5 1.4.3 SLUGGS . 6 1.5 The globular cluster system - dark matter connection . 8 1.6 The lenticular galaxy NGC 4526 . 8 2 Simulations 11 2.1 Simulations of globular cluster system formation and evolution . 11 2.1.1 Globular cluster systems in galaxy formation models . 11 2.1.2 Globular clusters in cosmological simulations . 14 3 Observations 21 3.1 Data Collection and Reduction . 21 4 Photometry 23 4.1 Galaxy light model . 23 4.2 Object detection . 24 4.3 Zeropoint estimates and final magnitudes . 24 ix x Contents 4.4 Globular cluster candidate selection . 25 4.5 Globular cluster colour distribution . 31 4.6 HST sources . 32 4.7 Completeness . 33 5 Analysis 37 5.1 Radial colour gradients . 37 5.2 Surface density profiles . 38 6 Global relations of GC systems 43 6.1 Radial extent of GC system . 43 6.2 Mass and dark matter fraction . 44 6.3 Specific frequency . 49 7 Discussion 53 8 Conclusion and proposed further work 59 8.1 Proposed further work . 60 Bibliography 70 A Appendix A 71 List of Figures 4.1 Colour-magnitude diagram . 26 4.2 Colour-colour diagram . 27 4.3 CLASS STAR selection . 29 4.4 GC candidate positions . 30 4.5 Histogram of GC candidates . 32 4.6 Completeness tests . 34 5.1 Colour Gradient . 38 5.2 Surface density profiles . 40 6.1 Comparison of NGC 4526 to other lenticular and elliptical galaxies . 45 6.2 Comparison of GCs extent to Host galaxy properties . 46 6.3 Radial velocities of NGC 4526 GCs . 48 6.4 Dark matter fraction scaling relation . 50 xi List of Tables 4.1 GMM results . 33 5.1 Sersic fits . 41 6.1 Dark matter fractions . 51 A.1 Full candidate list . 71 xiii 1 Introduction 1.1 Individual globular clusters Globular clusters (GC) are gravitationally bound clusters of ∼ 104 - 106 stars situated in and around galaxies. They are spherical with an average half light radius of 3pc (Masters 4 −3 et al., 2010) and have an average core density of ∼ 8×10 M pc Portegies Zwart et al. (2010). With ages up to ∼ 12:8 Gyr (Forbes et al., 2015) globular clusters are among the oldest objects we observe in galaxies. For a long time astronomers believed that all stars within a globular cluster formed at the same time and therefore are an example of a simple stellar population. This would imply that they have the potential to preserve their parent galaxy's chemo-dynamical properties at the time of their formation, and therefore they make an ideal tracer for galaxy formation and evolution. Conditions that promote the formation of globular clusters are gas rich, high density and high temperature, which makes galaxies with gas-rich clumpy discs or those that are undergoing major mergers almost ideal for GCs formation. It has also been proposed that GCs may form within their own low-mass dark matter (DM) halos (Kimm et al., 2016). Initial issues with this model have been that current observations show no evidence of DM halos associated with GCs (Conroy et al., 2011). Furthermore, based on our current understanding of cosmology and star formation, had GCs indeed formed in a DM halo that was subsequently stripped, we would expect to see a range of stellar populations within the GCs due to the continuous accretion of gas onto the DM halo. 1 2 Chapter 1. Introduction Whether or not globular clusters are single stellar populations has been debated for some time. To determine the presence of multiple populations one can plot the stars in a GC on a Hertzsprung-Russell diagram, which would indicate the main sequence of the individual GCs in the GC system as well as any turnoff points that would indicate the age of the stellar population. Most GCs we observe do not contain any gas that could trigger renewed star formation. The number of GCs with multiple stellar populations (MSP) is growing (Wang et al., 2017) as technology is improving. We now find MSP in most massive GCs. We also have compelling evidence for MSPs in most of the galactic GCs (Bastian & Lardo, 2017). One example of a GC with MSP is ! Centauri, which is the most massive Milky Way GC. The stars in ! Centauri were found to have a large spread of metallicities by Freeman & Rodgers (1975); Freeman & Norris (1981). Additional evidence was found in the form of multiple main sequence turnoffs (Sollima et al., 2005; Piotto, 2006) and multiple red giant branches (Lee et al., 1999; Pancino et al., 2000). All together this is compelling evidence for MSPs which are several Gyrs apart in age. It has been suggested by several studies (Zinnecker et al., 1988; Freeman, 1993; Gnedin et al., 2002) that ! Centauri is not what would classically be called a GC but instead is an ultra-compact dwarf galaxy, the stripped core of a galaxy that had in the past been acquired by the host galaxy (i.e the Milky Way). Regardless of the nature of omega centauri it is apparent that the view of GCs as simple stellar population systems is outdated. This could potentially change on how we use GC systems of tracers of galaxy formation histories. 1.2 Globular cluster systems A single galaxy can have tens of thousands of GCs associated with it. When talking about not a single but a multitude of GCs associated with the same galaxy, we refer to it as a GC system. Globular cluster systems have a long history of being used to constrain galaxy assembly histories (Ashman & Zepf, 1992; West et al., 2004; Brodie, 2009) and have further uses in inferring global properties of galaxies such as their mass. Their attributes such as individual GC radial velocities, spatial distribution and the presence of subpopulations (in the system not in individual GCs) are closely linked to their host galaxy's formation 1.3. Globular cluster systems and their host Galaxies 3 history. Not only are globular clusters observed in galaxies of all morphological types (Brodie & Strader, 2006; Harris et al., 2013), they offer the advantage of extending past the diffuse galaxy light, allowing the galaxy's potential to be probed out to galactic radii of ≥ 10Re (where Re refers to the effective radius of the host galaxy).