The SLUGGS Survey: Outer Triaxiality in the Fast Rotator Elliptical NGC 4473

San Jose State University SJSU ScholarWorks Faculty Publications Physics and Astronomy 1-1-2013 The SLUGGS survey: outer triaxiality in the fast rotator elliptical NGC 4473 C. Foster Australian Astronomical Observatory J. A. Arnold University of California, Santa Cruz D. A. Forbes Swinburne University N. Pastorello Swinburne University Aaron J. Romanowsky San Jose State University, [email protected] See next page for additional authors Follow this and additional works at: https://scholarworks.sjsu.edu/physics_astron_pub Part of the Astrophysics and Astronomy Commons Recommended Citation C. Foster, J. A. Arnold, D. A. Forbes, N. Pastorello, Aaron J. Romanowsky, L. R. Spitler, J. Strader, and J. P. Brodie. "The SLUGGS survey: outer triaxiality in the fast rotator elliptical NGC 4473" Monthly Notices of the Royal Astronomical Society (2013): 3587-3591. https://doi.org/10.1093/mnras/stt1550 This Article is brought to you for free and open access by the Physics and Astronomy at SJSU ScholarWorks. It has been accepted for inclusion in Faculty Publications by an authorized administrator of SJSU ScholarWorks. For more information, please contact [email protected]. Authors C. Foster, J. A. Arnold, D. A. Forbes, N. Pastorello, Aaron J. Romanowsky, L. R. Spitler, J. Strader, and J. P. Brodie This article is available at SJSU ScholarWorks: https://scholarworks.sjsu.edu/physics_astron_pub/25 MNRAS 435, 3587–3591 (2013) doi:10.1093/mnras/stt1550 Advance Access publication 2013 September 11 The SLUGGS survey: outer triaxiality of the fast rotator elliptical NGC 4473 Caroline Foster,1,2‹ Jacob A. Arnold,3 Duncan A. Forbes,4 Nicola Pastorello,4 Aaron J. Romanowsky,5,6 Lee R. Spitler,1,7 Jay Strader8 and Jean P. Brodie6 1Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia 2European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile 3Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA 4Centre for Astrophysics and Supercomputing, Swinburne University, Hawthorn VIC 3122, Australia 5Department of Physics and Astronomy, San Jose´ State University, One Washington Square, San Jose, CA 95192, USA 6University of California Observatories, 1156 High Street, Santa Cruz, CA 95064, USA 7Department of Physics and Astronomy, Faculty of Sciences, Macquarie University, Sydney NSW 2109, Australia 8Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA Downloaded from Accepted 2013 August 15. Received 2013 August 11; in original form 2013 March 25 http://mnras.oxfordjournals.org/ ABSTRACT Systematic surveys of nearby early-type galaxies using integral field unit (IFU) spectrograph data have revealed that galaxies can hide interesting structures only visible through kine matic studies. As part of their pioneering work, the ATLAS3D team have shown that most morphologically elliptical galaxies are centrally kinematically disc like. Hence, while global morphology suggests that ellipticals are ellipsoidal/triaxial in shape, their central kinematics may be consistent with (inclined) oblate systems. Here, we study the fast rotator elliptical galaxy: NGC 4473. Using slitlets, we obtain galaxy at San Jose State University on October 24, 2013 at San Jose State University on October 24, 2013 light kinematics out to unprecedentedly large galactocentric radii (2.5 effective radii). While we confirm the IFU results in the central regions, we find that at large galactocentric radii NGC 4473 exhibits a kinematic transition. In the outskirts, we observe clear minor and major axis rotation, a tell-tale sign of triaxiality, which agrees well with the galaxy’s Hubble type. This outer ‘kinematically distinct halo’ may be expected from simulations of galaxy forma tion, and in this system contains around one-third of the stellar light. While this galaxy may be a special case, it suggests that further investigation of the outskirts of galaxies is needed to confirm the new paradigm of galaxy classification. Key words: galaxies: haloes – galaxies: individual: (NGC 4473) – galaxies: kinematics and dynamics – galaxies: structure. This dichotomy was refined through the exquisite 2D kinematics 1 INTR O DUCTION data of the Spectrographic Areal Unit for Research on Optical Neb- Spiral galaxies are well understood as dominated by thin discs of ulae (SAURON) spectrograph (Bacon et al. 2001) and the ATLAS3D stars and gas on approximately circular orbits. The intrinsic structure (Cappellari et al. 2011a) survey, motivating a proposed paradigm of early-type galaxies (ETGs), on the other hand, has proven more shift in the classification of ETGs (Cappellari et al. 2011b; Emsellem elusive, with the standard picture evolving over time in response to et al. 2011, hereafter E11). Here, the longstanding E and lenticular new waves of kinematic data. The initial discovery of weak stellar (S0) morphology types are deemed ‘misleading’ and displaced by rotation in luminous ellipticals (Es) sparked their interpretation as slow and fast rotators, where the latter are oblate axisymmetric sys triaxial, pressure-dominated systems that formed through violent tems encompassing most Es along with S0s, in a continuous family relaxation (e.g. Illingworth 1977; Binney 1978). However, stronger that may even be more closely related to spirals than to slow rotator rotation was subsequently found at lower luminosities, leading to a Es (also see Krajnovic´ et al. 2013b). recognition of two distinct subclasses of Es (e.g. Davies et al. 1983; While providing remarkable insights, these conclusions should Kormendy & Bender 1996; Emsellem et al. 2007). be tempered by recognition of the limitations of the kinematics data to the central regions of galaxies. The SAURON/ATLAS3D fast and slow rotator classifications are explicitly made within 0.5Re or 1.0Re (effective radii). With wider field spectrographs, examples have * E-mail: [email protected] been found of dramatic transitions in kinematics not far beyond 1Re ©C 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 3588 C. Foster et al. (e.g. Coccato et al. 2009; Proctor et al. 2009, hereafter P09; Arnold 87 SKiMS spectra of the galaxy light out to ∼2.5Re with sufficient et al. 2011; Cortesi et al. 2013). These findings are in agreement with quality for a kinematic analysis. some theoretical predictions (e.g. Novak et al. 2006; Hoffman et al. The stellar light kinematics are obtained using the PPXF routine 2010; Bois et al. 2011; Vera-Ciro et al. 2011) and suggest that ETGs (Cappellari & Emsellem 2004). We measure the first four velocity may be made up of two kinematically distinct components. This is moments (i.e. recession velocity, velocity dispersion, h3 and h4)for reminiscent of the emerging picture of two-phase galaxy formation all SKiMS spectra. Uncertainties on the measured velocity moments (e.g. Oser et al. 2010; Wu et al. 2012). For our purposes, we will call are propagated using the variance at each pixel. these the ‘central’ and ‘halo’ components, corresponding to radii inside and outside of this kinematic transition, respectively. 3AN A LY S IS Such a view is being explored with ongoing, systematic wide- In order to visualize the results of our sparse SKiMS data on field surveys, including the SAGES Legacy Unifying Globulars and NGC 4473, we use the well-established geostatistical Kriging tech GalaxieS1 (SLUGGS; Brodie et al., in preparation) survey, which nique (e.g. Matheron 1963; Cressie 1990; Furrer 2002). Kriging focuses on the chemodynamics of nearby ETGs. One component uses a simple function (polynomial in our case) to fit randomly of SLUGGS is the Spectroscopic Mapping of ETGs to their Outer spaced data of limited sampling without further assumption about Limits (SMEAGOL), which uses a novel mode of mapping two- the underlying distribution (e.g. symmetry/asymmetry). As such, it dimensional stellar kinematics and abundances to large radii via the is not a physically motivated fit. The technique requires that dif Keck/DEep Imaging Multi-object Spectrograph (DEIMOS) spec ferences between individual data points are proportional to their trograph. mutual distance and that there is indeed a ‘smooth’ and ‘stable’ Here, we report interesting results from SLUGGS/SMEAGOL on field to extrapolate. Our tests show that so long as the underlying NGC 4473. This ‘prototypical’ E5 galaxy has a photometric major field is sufficiently well sampled in regions where steep changes are axis position angle PA = 100◦ (de Vaucouleurs et al. 1991; Temi, phot present, the Kriging technique does a remarkable job of estimating Mathews & Brighenti 2005) and resides in the Virgo cluster at a the underlying field (see Pastorello et al., in preparation). In prac distance of 15.2 Mpc (Blakeslee et al. 2009). It has a circularized Re tice, we use the KRIG routine under the R package FIELDS written by of 35.6 arcsec (2.7 kpc) and a luminosity of M =−20.9 (Kormendy V geostatisticians (Furrer 2002). The resulting Kriging velocity maps et al. 2009). Its central kinematics are consistent with that of a fast based on the SKiMS data of NGC 4473 are shown in Fig. 1. We wish rotator (E11), although it is a non-regular rotator that exhibits two to emphasize here that no model of the kinematics is used in order peaks in its velocity dispersion, making it one of the few such to produce these maps, and they are purely uncertainty-weighted galaxies in the ATLAS3D (Krajnovic´ et al. 2011). Cappellari et al. 2D fits of the values. (2007) interpret these ‘2σ ’ peaks as indicating the presence of a There are several salient features visible in Fig. 1. First of all, the counter-rotating stellar disc (30 per cent of the stellar mass). Bois SKiMS recession velocity analysis nicely complements the pub et al. (2011) suggested that such 2σ galaxies may be formed in lished SAURON analysis of the central region as published in binary disc major mergers. Emsellem et al. (2004), including the kinematic transition observed This paper focuses on the kinematics of NGC 4473 at large radii along the major axis near the edge of the SAURON field of view (R ∼ 2.5R ) and is organized as follows: Section 2 presents our e (e.g.

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