The Astrophysical Journal, 839:72 (15pp), 2017 April 20 https://doi.org/10.3847/1538-4357/aa6972 © 2017. The American Astronomical Society. All rights reserved. Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies Christopher P. Ahn1, Anil C. Seth1, Mark den Brok2, Jay Strader3, Holger Baumgardt4, Remco van den Bosch5, Igor Chilingarian6,7, Matthias Frank8, Michael Hilker9, Richard McDermid10, Steffen Mieske9, Aaron J. Romanowsky11,12, Lee Spitler10,13,14, Jean Brodie12,15, Nadine Neumayer5, and Jonelle L. Walsh16 1 University of Utah,Department of Physics & Astronomy, 115 South 1400 East, Salt Lake City, UT 84105, USA; [email protected] 2 ETH Zurich, Switzerland 3 Michigan State University, USA 4 University of Queensland, Australia 5 Max-Planck-Institut für Astronomie, Germany 6 Smithsonian Astrophysical Observatory, 60 Garden StreetMS09, Cambridge, MA 02138, USA 7 Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, 13 Universitetsky prospect, 119992 Moscow, Russia 8 Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königsstuhl 12, D-69117 Heidelberg, Germany 9 European Southern Observatory, Garching, Germany 10 Australian Astronomical Observatory, P.O. Box 915 North Ryde NSW 1670, Australia 11 San Jose State University, USA 12 University of California Observatories/Lick Observatory, USA 13 Research Centre for Astronomy, Astrophysics & Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia 14 Department of Physics & Astronomy, Macquarie University, Sydney, NSW 2109, Australia 15 University of California Santa Cruz, USA 16 George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA Received 2016 November 29; revised 2017 March 8; accepted 2017 March 24; published 2017 April 17 Abstract We present the detection of supermassive black holes (BHs) in two Virgo ultracompact dwarf galaxies (UCDs), VUCD3 and M59cO. We use adaptive optics assisted data from the Gemini/NIFS instrument to derive radial velocity dispersion profiles for both objects. Mass models for the two UCDs are created using multi-band Hubble Space Telescope imaging, including the modeling of mild color gradients seen in both objects. We then find a best- fit stellar mass-to-light ratio (M/L) and BH mass by combining the kinematic data and the deprojected stellar mass profile using Jeans Anisotropic Models. Assuming axisymmetric isotropic Jeans models, we detect BHs in both +2.5 6 +2.5 6 ( σ ) objects with masses of 4.4-3.0 ´ 10 Me in VUCD3 and 5.8-2.8 ´ 10 Me in M59cO 3 uncertainties . The BH mass is degenerate with the anisotropy parameter, bz; for the data to be consistent with no BH requires bz = 0.4 and bz = 0.6 for VUCD3 and M59cO, respectively. Comparing these values with nuclear star clusters shows that, while it is possible that these UCDs are highly radially anisotropic, it seems unlikely. These detections constitute the second and third UCDs known to host supermassive BHs. They both have a high fraction of their total mass in their BH; ∼13% for VUCD3 and ∼18% for M59cO. They also have low best-fit stellar M/Ls, supporting the proposed scenario that most massive UCDs host high-mass fraction BHs. The properties of the BHs and UCDs are 9 consistent with both objects being the tidally stripped remnants of ~10 M galaxies. Key words: galaxies: clusters: general – galaxies: dwarf – galaxies: evolution – galaxies: formation – galaxies: kinematics and dynamics 7 1. Introduction be elevated by∼50% for almost all UCDs above 10 M when ( ) compared to the mass attributed to stars alone (e.g., Haşegan Ultracompact dwarf galaxies UCDs are stellar systems ) discovered in the late 1990s through spectroscopic surveys of et al. 2005; Mieske et al. 2013 . These dynamical mass the Fornax cluster (Hilker et al. 1999; Drinkwater et al. 2000). estimates have been made combining structural information ( ) With masses ranging from a few million to a hundred million from Hubble Space Telescope HST imaging with ground- solar masses and sizes 100 pc, UCDs are among the densest based, global velocity dispersion measurements. These models stellar systems in the universe. In the luminosity-size plane, assume that mass traces light, stars are on isotropic orbits and ( UCDs occupy the region between globular clusters (GCs) and are formed from a Kroupa-like initial mass function IMF; ş ) compact ellipticals (cEs; e.g., Brodie et al. 2011; Misgeld & Ha egan et al. 2005; Mieske et al. 2008, 2013 . Possible Hilker 2011; Norris et al. 2014; Janz et al. 2016). The smooth explanations for this unique phenomenon have included transition between these three classes of objects has led to ongoing tidal stripping scenarios (Forbes et al. 2014; Janz significant debate as to how UCDs were formed. Explanations et al. 2015), and central massive black holes (BHs) making up have ranged from UCDs being the most massive GCs (e.g., ∼10%–15% of the total mass (Mieske et al. 2013). Alter- Fellhauer & Kroupa 2002, 2005; Mieske et al. 2002; Kissler- natively, the elevated dynamical-to-stellar mass ratios can be Patig et al. 2006), to UCDs being the tidally stripped nuclear explained by a change in the stellar IMF in these dense remnants of dwarf galaxies (Bekki et al. 2001, 2003; Pfeffer & environments. For example, a bottom-heavy IMF would imply Baumgardt 2013; Strader et al. 2013; Forbes et al. 2014). an overabundance of low-mass stars that contribute massbut Recently, analyses of the integrated dispersions of UCDs very little light (Mieske & Kroupa 2008),and a top-heavy IMF revealed an interesting property; the dynamical mass appears to would allow for an overabundance of stellar remnants 1 The Astrophysical Journal, 839:72 (15pp), 2017 April 20 Ahn et al. Figure 1. Two galaxy-UCD systems discussed in this paper. The left panel shows the M87-VUCD3 system, and the right panel shows the M59-M59cO system. The main images show 2MASSLGA images of both galaxies (Jarrett et al. 2003). VUCD3 and M59cO are the point-like images outlined in the yellow boxes. The inset images are zoom-in HST archival images of each UCD. The red line connecting the UCD to the host galaxy shows the projected distance assuming thateach object is at a distance of 16.5 Mpc. contributing mass but virtually no light. The former case has been Images of VUCD3, M59cO, and their host galaxies are shown suggested in giant ellipticals (e.g., van Dokkum & Conroy 2010; in Figure 1. VUCD3 is located 14kpc in projection from the Conroy & van Dokkum 2012), while Dabringhausen et al. (2012) center of M87 and has MV =-12.75 (Mieske et al. 2013).The argued that the relative abundance of X-ray binaries in UCDs metallicity of VUCD3 has been estimated to be between −0.28 favored a top-heavy IMF, though an increased X-ray luminosity and 0.35 in several studies (Evstigneeva et al. 2007; Firth in UCDs was not found in subsequent work (Phillipps et al. 2013; et al. 2009; Francis et al. 2012), and it has an [α/Fe]∼0.5 Pandya et al. 2016). (Francis et al. 2012). M59cO is located 10kpc in projection In the context of the tidal stripping scenario, the elevated from the center of M59 and has MV =-13.26 (Mieske dynamical-to-stellar mass ratios could potentially be explained et al. 2013). Its metallicity has been measured in several studies if UCDs still reside within progenitor dark matter halos. with [Z/H] between 0.0 and 0.2, with [α/Fe]∼0.2 (Chilingarian However, to have a measurable effect on the kinematics of &Mamon2008; Sandoval et al. 2015;Janzetal.2016).We compact objects such as UCDs, the central density of the dark assume a distance of 16.5 Mpc for both objects. All magnitudes matter halo would need to be orders of magnitude higher than are reported in the AB magnitude system unless otherwise noted. expected for dark matter halos of the stripped galaxies All magnitudes and colors have been corrected for extinction; in (Tollerud et al. 2011; Seth et al. 2014). In addition, the search VUCD3, we use AFW606 = 0.061 and AFW814 = 0.034, while for an extended dark matter halo in Fornax UCD3, based on its for M59cO we used AFW475 = 0.107 and AFLP850 = 0.041 ( fl ) velocity dispersion profile, yielded a non-detection (Frank Schla y&Finkbeiner2011 . et al. 2011). This paper is organized as follows. In Section 2, we discuss In this paper, we follow-up on the idea that the elevated the data used for analysis and how the kinematics were values of the dynamical-to-stellar mass ratios, which we denote modeled. In Section 3, we present our methods for determining fi in this paper as Γ (º()()ML ML ), can be explained by the density pro le of our UCDs. We present our dynamical dyn * fi the presence of a supermassive BH (Mieske et al. 2013). This modeling methods in Section 4. Our results for the best- tBH scenario was confirmed in one case, M60-UCD1, which hosts a are presented in Section 5, and we conclude in Section 6. BH that makes up 15% of the total dynamical mass of the system and a best-fit Γ of 0.7±0.2 (Seth et al. 2014). 2. Observations and Kinematics Because M60-UCD1 is one of the highest density UCDs, its In this section, we discuss the data and reduction techniques low stellar mass-to-light ratio (M/L) suggests that a systematic / used for our analysis. Section 2.1 discusses the HST archival variation of the IMF with density is not the cause for high M L images and Section 2.2 explains the reduction of our Gemini estimates found in most massive UCDs and strengthens the NIFS integral field spectroscopy.