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A SLUGGS and Gemini/GMOS Combined Study of the Elliptical Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 8 October 2018 (MN LATEX style file v2.2) A SLUGGS and Gemini/GMOS combined study of the elliptical galaxy M60: wide-field photometry and kinematics of the globular cluster system Vincenzo Pota1, Jean P. Brodie1, Terry Bridges2, Jay Strader3, Aaron J. Romanowsky1,4, Alexa Villaume1, Zach Jennings1, Favio R. Faifer5,6, Nicola Pastorello7, Duncan A. Forbes7 Ainsley Campbell2, Christopher Usher7, Caroline Foster8, Lee R. Spitler8,9, Nelson Caldwell10 Juan C. Forte5, Mark A. Norris11, Stephen E. Zepf3, Michael A. Beasley12,13, Karl Gebhardt14 David A. Hanes2, Ray M. Sharples15, Jacob A. Arnold1 1 University of California Observatories, 1156 High Street, Santa Cruz, CA 95064, USA 2 Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, ON K7L 3N6, Canada 3 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA 4 Department of Physics and Astronomy, San Jos´eState University, One Washington Square, San Jose, CA 95192, USA 5 Consejo Nacional de Investigaciones Cientficas y T´ecnicas (CONICET)-Planetario Galileo Galilei, CABA, Rep. Argentina 6 Instituto de Astrof´ısica de La Plata (CCT La Plata - CONICET - UNLP) 7 Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn VIC 3122, Australia 8 Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia 9 Department of Physics & Astronomy, Macquarie University, Sydney, NSW 2109, Australia 10 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA 11 Max Planck Institut f¨ur Astronomie, K¨onigstuhl 17, D-69117 Heidelberg, Germany 12 University of La Laguna. Avda. Astrof´ısico Fco. S´anchez, s/n. E38206, La Laguna, Tenerife, Canary Islands, Spain. 13 Instituto de Astrof´ısica de Canarias. Calle V´ıa L´actea s/n. E38200 - La Laguna, Tenerife, Canary Islands, Spain. 14 Astronomy Department, University of Texas, Austin, TX 78712, USA 15 Department of Physics, University of Durham, South Road, Durham DH1 3LE March 2015 ABSTRACT We present new wide-field photometry and spectroscopy of the globular clusters (GCs) around NGC 4649 (M60), the third brightest galaxy in the Virgo cluster. Imaging of NGC 4649 was assembled from a recently-obtained HST/ACS mosaic, and new Subaru/Suprime-Cam and archival CFHT/MegaCam data. About 1200 sources were followed up spectroscopically using combined observations from three multi-object spectrographs: Keck/DEIMOS, Gem- ini/GMOS and MMT/Hectospec. We confirm 431 unique GCs belonging to NGC 4649, a factor of 3.5 larger than previous datasets and with a factor of 3 improvement in velocity precision. We confirm significant GC colour bimodality and find that the red GCs are more arXiv:1503.07520v1 [astro-ph.GA] 25 Mar 2015 centrally concentrated, while the blue GCs are more spatially extended. We infer negative GC colour gradients in the innermost 20 kpc and flat gradients out to large radii. Rotation is detected along the galaxy major axis for all tracers: blue GCs, red GCs, galaxy stars and planetary nebulae. We compare the observed properties of NGC 4649 with galaxy formation models. We find that formation via a major merger between two gas-poor galaxies, followed by satellite accretion, can consistently reproduce the observations of NGC 4649 at different radii. We find no strong evidence to support an interaction between NGC 4649 and the neigh- bouring spiral galaxy NGC 4647. We identify interesting GC kinematic features in our data, such as counter-rotating subgroups and bumpy kinematic profiles, which encode more clues about the formation history of NGC 4649. 1 INTRODUCTION mation, but they are not identical to each other, implying that for- mation processes vary slightly from galaxy to galaxy (Lintott et al. Galaxies are the building blocks of the visible Universe and the 2008). While high redshift galaxy surveys can be used to infer “av- best tools to study its structure. Galaxies are alike in many ways, erage” formation mechanisms for a sample of galaxies, studying suggesting that similar underlying mechanisms regulate their for- nearby galaxies in detail can allow us to reconstruct their particular c 0000 RAS 2 Pota et al. and unique formation histories. Moreover, observations of galaxies In addition to recent space-based observations, the GC sys- at z = 0 provide the end product that computer simulations attempt tem of NGC 4649 has been studied with ground-based telescopes to reproduce (e.g., Hoffman et al. 2010; Wu et al. 2014; Naab et al. (Bridges et al. 2006; Pierce et al. 2006; Lee et al. 2008; Faifer et al. 2014). 2011; Chies-Santos et al. 2011). A spectroscopic follow up of In the standard galaxy formation theory, small structures col- 121 GCs (Hwang et al. 2008) revealed that the GC system of lapse first (White & Rees 1978; Searle & Zinn 1978; Zolotov et al. NGC 4649 has a large rotation amplitude. This is a rare feature 2009), and then grow hierarchically into larger structures via in large early-type galaxies, whose GC systems are generally pres- galaxy mergers and/or via accretion of satellite galaxies (e.g., sure supported with negligible or weak rotation (e.g., Cˆot´eet al. L´opez-Sanjuan et al. 2010; Font et al. 2011; van Dokkum et al. 2003; Bergond et al. 2006; Schuberth et al. 2010b; Strader et al. 2014; Tasca et al. 2014). The latter is responsible for the building- 2011; Norris et al. 2012; Pota et al. 2013; Richtler et al. 2014), up of galaxy outskirts (which we will refer to as stellar haloes) from although exceptions exist (e.g., Puzia et al. 2004; Arnold et al. high (z 2) to low redshift (Tal & van Dokkum 2011; Oser et al. 2011; Schuberth et al. 2012; Blom et al. 2012). On the other hand, 2010; Khochfar≈ et al. 2011). The various processes which shaped Bridges et al. (2006) obtained radial velocities for 38 GCs in galaxies with time can be simulated and compared with obser- NGC 4649 and found no rotation, probably because of their rel- vations of galaxies at a particular redshift. For example, minor atively small sample size. mergers can create stellar shells and tidal streams observable in In this paper we present the largest spectro-photometric cata- deep imaging (Tal et al. 2009; Ebrova 2013; Atkinson et al. 2013; logue of GCs around NGC 4649. We construct a wide-field pho- Duc et al. 2015), whereas the secular accretion of satellite galax- tometric GC catalogue by combining literature (HST), archival ies can be studied with photometry of stacked galaxies (e.g., (CFHT) and new (Subaru) observations. We follow-up with spec- Oser et al. 2010; Tal & van Dokkum 2011; D’Souza et al. 2014) or troscopy of hundreds of GC candidates using joint observations by studying the chemistry and kinematics of stellar haloes (e.g., from three multi-object spectrographs mounted on the Keck, MMT Forbes et al. 2011; Romanowsky et al. 2012; Coccato et al. 2013). and Gemini telescopes. Our new spectroscopic catalogue is a fac- Galaxy haloes are difficult to study because they are opti- tor of 3.5 larger than the current literature dataset (Lee et al. 2008) cally faint. Globular clusters (GCs), on the other hand, are much and has a factor of 3 greater velocity accuracy. This study exploits more observationally convenient for studying galaxy haloes. They datasets from two complementary surveys of extragalactic GCs: the are relatively easy to detect because of their high surface bright- SLUGGS survey (Brodie et al. 2014), and an ongoing survey car- ness and they generally populate haloes in large numbers (e.g., ried out with Gemini/GMOS (Bridges et al., in preparation). These Brodie & Strader 2006). The outermost GCs can be used to map two surveys combined, contribute 90 per cent of the final sample out halo properties (e.g., kinematics and metallicity) out to tens of confirmed GCs. We use the photometric and kinematic proper- of effective radii (e.g., Ostrov et al. 1993; Schuberth et al. 2010a; ties of the NGC 4649 GC system (including the blue and red GC Usher et al. 2012; Pota et al. 2013; Forbes et al. 2011). Their old subpopulations) to study the formation history of NGC 4649. A ages and their direct connection with the star forming episodes follow-up paper (Gebhardt et al., in preparation) will make use of in a galaxy’s history (Strader et al. 2005; Puzia et al. 2005) mean the dataset presented in this work to model the mass content of that they are ideal tracers of the assembly processes that shaped NGC 4649. the host galaxy (e.g., Romanowsky et al. 2012; Leaman et al. 2013; We adopt a distance of 16.5 Mpc (Mei et al. 2007), an effec- Foster et al. 2014; Veljanoski et al. 2014). tive radius Re = 66 arcsec = 5.3 kpc, an axis ratio q = 0.84 In this paper we study the GC system of the E2 early-type and a position angle PA = 93 deg (Brodie et al. 2014). Galac- galaxy NGC 4649 (M60), the third brightest galaxy in the Virgo tocentric distances R are expressed though the circularized radius cluster. NGC 4649 is itself at the centre of a small group of galax- R = X2q + Y 2/q, where X and Y are the Cartesian coordi- ies and is the dominant member of the galaxy-pair Arp 116 (Arp nates of an object with NGC 4649 at the origin. The absolute mag- p 1966). The proximity to the disturbed spiral galaxy NGC 4647 (13 nitude of NGC 4649 is MB = 21.47 mag or MV = 22.38 − − kpc in projection) potentially makes NGC 4649 an example of a mag. pre major-merger between two massive galaxies in the local Uni- This paper is structured as follows.
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