A&A 629, A44 (2019) Astronomy https://doi.org/10.1051/0004-6361/201833954 & c ESO 2019 Astrophysics Nuclear angular momentum of early-type galaxies hosting nuclear star clusters? Mariya Lyubenova1 and Athanassia Tsatsi2 1 ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: [email protected] 2 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany Received 25 July 2018 / Accepted 22 March 2019 ABSTRACT Context. Nucleation is a common phenomenon in all types of galaxies and at least 70% of them host nuclear star clusters (NSCs) in their centres. Many of the NSCs co-habit with supermassive black holes and follow similar scaling relations with host galaxy properties. Unlike black holes, NSCs, preserve the signature of their evolutionary path imprinted onto their kinematics and stellar populations. Thus their study provides us with important information about the formation of galactic nuclei. Aims. In this paper we explored the angular momentum of the nuclei of six intermediate mass (9:7 > log(Mdyn=M ) > 10:6) early-type galaxies that host NSCs and are located in the Fornax cluster. Our goal was to derive a link between the nuclear angular momentum and the proposed formation scenarios of NSCs. Methods. We used adaptive optics assisted IFU observations with VLT/SINFONI to derive the spatially resolved stellar kinematics of the galaxy nuclei. We measured their specific stellar angular momenta λRe, and compared these with Milky Way globular clusters (GCs) and N-body simulations of NSC formation. Results. We found that all studied nuclei exhibit varied stellar kinematics. Their λRe and ellipticities are similar to Milky Way GCs. Five out of six galaxy nuclei are consistent with the λRe − e of simulated NSCs embedded in a contaminating nuclear bulge that have formed via the in-spiralling and merging of GCs. Conclusion. It has previously been suggested that the NSCs in higher mass galaxies, such as those studied in this paper, form via dissipational sinking of gas onto the galactic nuclei with hints that some might also involve the merger of GCs. In this work we show that we cannot exclude the pure GC merging scenario as a viable path for the formation of NSCs. Key words. galaxies: elliptical and lenticular, cD – galaxies: nuclei – galaxies: kinematics and dynamics 1. Introduction Two main scenarios for NSC formation have been proposed. The first involves the dry merging of globular clusters (GCs) Observations made over the last few decades have shown that that migrated towards the centre of the galaxy as a consequence nucleation is a common phenomenon, with at least 70% of galax- of dynamical friction (e.g. Tremaine et al. 1975). In the second ies over a broad mass range hosting a nuclear star cluster (NSC) scenario NSCs form in situ via dissipational sinking of gas onto in their photometric and kinematic centres (e.g. Böker et al. the galactic nucleus and subsequent star formation (e.g. Mihos 2002; Côté et al. 2006; Neumayer et al. 2011; Turner et al. & Hernquist 1994). Various studies showed that it is difficult to 2012; den Brok et al. 2014; Georgiev & Böker 2014). Often reconcile unambiguously the proposed theoretical models with these NSCs co-exist with supermassive black holes (SMBH; e.g. the plethora of observational phenomena that NSCs exhibit. For Graham & Spitler 2009; Georgiev et al. 2016). The relationship example, Turner et al.(2012) studied the photometric structural between the mass of these central massive objects (CMOs) and parameters of a large sample of NSCs in the Fornax and Virgo the properties of their host galaxies is likely to be fundamental clusters and suggested that nuclei in high-mass early-type galax- as it connects quantities that differ by several orders of magni- ies (ETGs) most likely grow through gas accretion triggered by tude (e.g. Ferrarese et al. 2006). It is still debated whether this wet mergers. At lower masses they suggested that the dominant relationship is physical through feedback from the SMBH on the mechanism is probably the infall and merging of star clusters host galaxy (e.g. Silk & Rees 1998), or statistical through many with a possible hybrid population where both mechanisms work subsequent mergers of galaxies and their black holes (Jahnke & simultaneously. Spengler et al.(2017) investigated the structural Macciò 2011). The answer might well come from the study of parameters of nuclei in the Virgo cluster and found that the most the formation and evolution of NSCs, which, unlike black holes, massive host galaxies tend to have flatter nuclei, suggesting that preserve their evolutionary history imprinted onto their stellar they may be formed predominantly through dissipative processes populations and kinematics. that can induce flattening and rotation. When looked at up close spectroscopically and with adap- tive optics (AO), galactic nuclei display even larger variety: ? Based on observation collected at the ESO Paranal La Silla Obser- many NSCs contain multiple stellar populations and some are vatory, Chile, Prog. ID 092.B-0892, PI Lyubenova and ID 380.B-0530, embedded in stellar or gaseous discs (e.g. Walcher et al. 2005; PI Infante. Rossa et al. 2006; Seth et al. 2006, 2008; Barth et al. 2009; Article published by EDP Sciences A44, page 1 of8 A&A 629, A44 (2019) Table 1. Basic properties of the 6 galaxies studied in this paper. FCC ID Name Type Distance log(Mdyn) Reff σ BT 00 −1 Mpc M km s mag (1) (2) (3) (4) (5) (6) (7) (8) FCC47 NGC 1336 E4 18.3 10.4 30.0 96.0 13.3 FCC148 NGC 1375 S0 19.9 10 26.9 56.0 13.5 FCC170 NGC 1381 S0 21.9 10.6 12.9 153.0 12.9 FCC177 NGC 1380A S0 20.9 9.7 12.6 55.0 13.6 FCC277 NGC 1428 E5 20.7 9.9 10.1 81.7 14.1 FCC310 NGC 1460 SB0 19.9 10 25.8 60.4 13.7 Notes. Column (1) lists the Ferguson(1989) catalogue designation and (2) the galaxies common names. Column (3) give the galaxies morphologi- cal type as listed in Ferguson(1989). The distances (4) are taken from Blakeslee et al.(2009) and are based on surface brightness fluctuations. The 2 dynamical masses (5) are estimated using Mdyn = 5:0 Reff σeff =G (Cappellari et al. 2006). Reff (6) is taken from Ferguson(1989). The central veloc- ity dispersions (7) are taken from Kuntschner(2000), except for FCC277 and FCC310, which they are taken from Wegner et al.(2003). In order to use these central velocity dispersions as mass estimators in (5) we corrected these using the aperture corrections presented in Falcón-Barroso et al.(2017). Column (8) lists the total B-band magnitude extracted from HyperLeda. Lyubenova et al. 2013). These complex properties have been 2. Observations and data reduction explained with the help of numerical simulations involving both the merging of star clusters and gas dissipation (e.g. Hartmann 2.1. Sample selection et al. 2011; Guillard et al. 2016). However, Tsatsi et al.(2017) In this paper we analysed the nuclear stellar kinematics of six showed that pure globular cluster merging simulations (Antonini ETGs in the Fornax cluster. Our pilot study of the nucleus of et al. 2012; Perets & Mastrobuono-Battisti 2014) provide a good the galaxy FCC 277, presented in Lyubenova et al.(2013), made explanation of the observed stellar kinematics substructures of use of natural guide star assisted AO observations; therefore the the closest NSC, that in the Milky way (Feldmeier et al. 2014). main selection criteria for that study was the availability of a suit- In Lyubenova et al.(2013) we showed that even though able bright star nearby the nucleus. We compiled our sample of j j the NSC in an early-type galaxy can have V /σ < 1, detailed five new targets based on two main selection criteria: the bright- dynamical modelling revealed that a significant stellar angular ness of the NSCs and their contrast with respect to the main body momentum is still preserved in the nucleus. Our first orbit-based of the galaxy, as derived by Turner et al.(2012). These crite- dynamical model of a NSC revealed that co- and counter-rotating ria ensured the collection of high-quality laser guide star (LSG) orbits are simultaneously needed to reproduce the observed stel- AO assisted IFU data within reasonable amounts of observing lar kinematics of the nuclear region in FCC 277. This counter time. In Table1 we listed the basic properties of the selected rotation is indicative of some merger event with orbital angular host galaxies and in Table2 some basic properties of the NSCs momentum opposite to the host galaxy. can be found. The increasing amount of excellent quality AO assisted integral-field spectroscopic data allows us to look at galactic nuclei in a way that was earlier only possible for the large-scale 2.2. Observations structure of galaxies. Although photometrically regular, the 2D stellar kinematics maps of ETGs often exhibit complex struc- The observations were carried out in service mode between 21 tures, with mis-aligned and counter-rotating components (e.g. October and 29 November 2013 with the VLT/SINFONI (pro- Krajnovic´ et al. 2011). Based on their specific stellar angular gramme ID 092.B-0892, PI M. Lyubenova). In order to deter- momentum and ellipticity ETGs separate in two subgroups: slow mine the spatially resolved properties of the NSCs we used the and fast rotators (Emsellem et al. 2007, 2011). Slow-rotator AO system in LGS mode. Owing to the brightness and point- ETGs are usually more massive and mildly triaxial, while fast like nature of our NSCs, we used them as tip-tilt stars for the rotators are less massive, fainter, and oblate axisymmetric in LGS system.