Edinburgh Research Explorer Triangulum II: A Very Metal-poor and Dynamically Hot Stellar System Citation for published version: Martin, NF, Ibata, RA, Collins, MLM, Rich, RM, Bell, EF, Ferguson, AMN, Laevens, BPM, Rix, H-W, Chapman, SC & Koch, A 2016, 'Triangulum II: A Very Metal-poor and Dynamically Hot Stellar System', Astrophysical Journal, vol. 818, no. 1, pp. 40. https://doi.org/10.3847/0004-637X/818/1/40 Digital Object Identifier (DOI): 10.3847/0004-637X/818/1/40 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Astrophysical Journal General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 30. Sep. 2021 The Astrophysical Journal, 818:40 (7pp), 2016 February 10 doi:10.3847/0004-637X/818/1/40 © 2016. The American Astronomical Society. All rights reserved. TRIANGULUM II: A VERY METAL-POOR AND DYNAMICALLY HOT STELLAR SYSTEM Nicolas F. Martin1,2, Rodrigo A. Ibata1, Michelle L. M. Collins3,4,10, R. Michael Rich5, Eric F. Bell6, Annette M. N. Ferguson7, Benjamin P. M. Laevens1,2, Hans-Walter Rix2, Scott C. Chapman8, and Andreas Koch9 1 Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, F-67000 Strasbourg, France; [email protected] 2 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany 3 Astronomy Department, Yale University, New Haven, CT 06520, USA 4 Department of Physics, University of Surrey, Guildford, GU2 7XH, Surrey, UK 5 Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA 6 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA 7 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 8 Department of Physics and Atmospheric Science, Dalhousie University, Coburg Road, Halifax, NS B3H 1A6, Canada 9 Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, D-69117 Heidelberg, Germany Received 2015 October 15; accepted 2015 December 11; published 2016 February 5 ABSTRACT We present a study of the recently discovered compact stellar system TriangulumII. From observations conducted with the DEIMOS spectrograph on KeckII, we obtained spectra for 13 member stars that follow the CMD features of this very faint stellar system and include two bright red giant branch stars. TriII has a very negative radial ( +3.0 -1) -1 fi velocity ávrñ=-383.7-3.3 km s that translates to ávr,gsrñ- 264 km s and con rms it is a Milky Way satellite. We show that, despite the small data set, there is evidence that TriII has complex internal kinematics. Its +2.8 -1 +5.8 -1 radial velocity dispersion increases from 4.4-2.0 km s in the central 2¢ to 14.1-4.2 km s outwards. The velocity +3.2 -1 dispersion of the full sample is inferred to be svr = 9.9-2.2 km s . From the two bright RGB member stars we measure an average metallicity á[]Fe/ Hñ=- 2.6 0.2, placing TriII among the most metal-poor Milky Way dwarf galaxies. In addition, the spectra of the fainter member stars exhibit differences in their line widths that could be the indication of a metallicity dispersion in the system. All these properties paint a complex picture for TriII, whose nature and current state are largely speculative. The inferred metallicity properties of the system however lead us to favor a scenario in which TriII is a dwarf galaxy that is either disrupting or embedded in a stellar stream. Key words: galaxies: individual (Tri II) – galaxies: kinematics and dynamics – Local Group 1. INTRODUCTION indirect detection of the elusive dark matter particle (e.g., Geringer-Sameth et al. 2015). Their small baryonic component A large number of faint and small stellar systems have been (L ~ 24- L ) fi uncovered over the last decade thanks to wide photometric 10 makes them hard to nd and study but, at the surveys. The harvest of such objects, which started with same time, gives powerful insight into the interplay of physical Willman1 (Willman et al. 2005) andthenSegue1 (Belokurov processes that drive galaxy formation at low masses and in et al. 2007), blossomed through systematic searches of the Sloan shallow potential wells. The characterization of these systems ( ) is, however, made difficult by the potential presence of binary Digital Sky Survey SDSS; Belokurov et al. 2009 and, more fi fl recently, of the Dark Energy Survey (DES; Bechtol et al. 2015; stars that can signi cantly in ate the intrinsic velocity Drlica-Wagner et al. 2015; Koposov et al. 2015) and the dispersion of a system with a velocity dispersion of only a -1 ( ) Panoramic Telescope and Rapid Response System1 (Pan- few km s McConnachie & Côté 2010 . In addition, the STARRS1; Laevens et al. 2015a, 2015b). The photometric usual assumption of dynamical equilibrium can be inappropri- properties of many of these systems ambiguously locate them in a ate for systems that are often found within ~40 kpc of the ( region of parameter space where dwarf galaxies appear to mix Galactic center, further impeding their study e.g., Willman 1; ) fi with globular clusters (Gilmore et al. 2007).Spectroscopic Willman et al. 2011 . Finally, the dif culty to disentangle studies of their stars are therefore unavoidable to show that they member stars from foreground contamination can sometimes are either dynamically cold and display no metallicity dispersion further compound the analysis of these faint objects (e.g., (e.g., Laevens 1; Kirby et al. 2015), as expected for globular Bonnivard et al. 2015). clusters, or that they are dynamically hot (e.g., Segue 1; Geha In this paper, we report a spectroscopic study of the et al. 2009), have a metallicity dispersion (e.g., Segue 2; Kirby TriangulumII (Tri II) stellar system discovered by Laevens et al. 2013a),and/or lie on the luminosity–metallicity relation et al. (2015b) in Pan-STARRS1 and confirmed with deep Large followed by dwarf galaxies (e.g., Hydra II and Draco II; Kirby Binocular Camera (LBC) photometry. TriII is very faint ( ) ( +9 ) et al. 2015; Martin et al. 2015). MV =-1.8 0.5 , fairly compact rh = 34-8 pc , and Although they are expected by the dozen in simulations located at 30 2 kpc from the Sun, or 36 2 kpc from the (Tollerud et al. 2008; Bullock et al. 2010), only a handful of Galactic center. So far as one can infer from the photometry, it these faint and small stellar systems have so far been confirmed appears to contain only old and metal-poor stars. as dwarf galaxies. Any new addition to the list is particularly We present the KeckII/DEIMOS data used for the analysis valuable as these objects are among the most promising for the in Section2 of this paper, the results of the spectroscopic study in Section3, while Section4 is devoted to a discussion on our 10 Hubble Fellow findings. 1 The Astrophysical Journal, 818:40 (7pp), 2016 February 10 Martin et al. 2. OBSERVATIONS AND DATA Table 1 Properties of TriII Two masks targeting TriII potential member stars were a observed during the night of 2015 September 17, with the RA (ICRS) 02:13:17.4 a ( ) + DEep Imaging Multi-Object Spectrograph (DEIMOS) on Dec ICRS 36:10:42.4 Heliocentric distancea ( kpc) 30±2 KeckII (Faber et al. 2003) under reasonably good conditions a Galactocentric distance ( kpc) 36±2 (0 7–1 0 seeing and ∼60% humidity; PI: Rich, program ID: a (′) +1.1 ) rh 3.9-0.9 2015B_U064D . The LBC photometry used by Laevens et al. a ( ) +9 rh pc 34-8 (2015b) to confirm the discovery of the satellite was used to a MV −1.8±0.5 place slits on stars selected in the color–magnitude diagram a 2.6 0.2 LV ( L) 10 (CMD) to follow the system’s main sequence turn off (MSTO), sub-giant branch (SGB), and red giant branch (RGB). The Global kinematics ( -1) +3.0 selection was purposefully tight around the sharp MSTO and áñvr km s -383.7-3.3 ( -1) − loose around the RGB region that cannot be selected out of the áñvr,gsr km s 264 ( -1) +3.2 MW foreground contamination from the photometry alone. svr km s 9.9-2.2 Each mask was integrated for 3600 s, split into three sub- exposures for cosmic-ray removal. The spectrograph was set up Inner kinematics (<¢2 ) ( -1) +2.1 with the 1200 lines/mm grating, which translates to ∼0.33 Å áñvr km s -379.8-2.7 s ( km s-1) 4.4+2.8 per pixel in the calcium II triplet (CaT) region we focus on. The vr -2.0 – Å full spectra cover the range 6600 9400 . Raw frames are Outer kinematics (>2¢) processed through our own pipeline, which we developed over ( -1) +5.7 áñvr km s -387.3-6.3 the years to specifically reduce DEIMOS spectra. We refer the ( -1) +5.8 svr km s 14.1-4.2 reader to Ibata et al.