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EXPLORING HALO SUBSTRUCTURE with GIANT STARS. XV. DISCOVERY of a CONNECTION BETWEEN the MONOCEROS RING and the TRIANGULUM-ANDROMEDA OVERDENSITY?*†‡ Ting S

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EXPLORING HALO SUBSTRUCTURE with GIANT STARS. XV. DISCOVERY of a CONNECTION BETWEEN the MONOCEROS RING and the TRIANGULUM-ANDROMEDA OVERDENSITY?*†‡ Ting S Draft version November 13, 2018 Preprint typeset using LATEX style emulateapj v. 01/23/15 EXPLORING HALO SUBSTRUCTURE WITH GIANT STARS. XV. DISCOVERY OF A CONNECTION BETWEEN THE MONOCEROS RING AND THE TRIANGULUM-ANDROMEDA OVERDENSITY?*yz Ting S. Li1,2,3, Allyson A. Sheffield4, Kathryn V. Johnston5, Jennifer L. Marshall2,3, Steven R. Majewski6, Adrian M. Price-Whelan7, Guillermo J. Damke6,8, Rachael L. Beaton9, Edouard J. Bernard10, Whitney Richardson6, Sanjib Sharma11, Branimir Sesar12 Draft version November 13, 2018 ABSTRACT Thanks to modern sky surveys, over twenty stellar streams and overdensity structures have been discovered in the halo of the Milky Way. In this paper, we present an analysis of spectroscopic observations of individual stars from one such structure, \A13", first identified as an overdensity using the M giant catalog from the Two Micron All-Sky Survey. Our spectroscopic observations show −1 that stars identified with A13 have a velocity dispersion of . 40 km s , implying that it is a genuine coherent structure rather than a chance super-position of random halo stars. From its position on the sky, distance (∼15 kpc heliocentric), and kinematical properties, A13 is likely to be an extension of another low Galactic latitude substructure { the Galactic Anticenter Stellar Structure (also known as the Monoceros Ring) { towards smaller Galactic longitude and farther distance. Furthermore, the kinematics of A13 also connect it with another structure in the southern Galactic hemisphere { the Triangulum-Andromeda overdensity. We discuss these three connected structures within the context of a previously proposed scenario that one or all of these features originate from the disk of the Milky Way. Keywords: Galaxy: Formation, Galaxy: Structure, Galaxy: Halo, Galaxy: Disk, Galaxies: Interac- tions 1. INTRODUCTION Survey (hereafter SDSS; York et al. 2000), have pro- Over the last two decades, large-area digital sky sur- vided deep and global photometric catalogs of stars in veys such as the Two Micron All Sky Survey (hereafter the Milky Way. A variety of substructures in the Galac- 2MASS; Skrutskie et al. 2006) and the Sloan Digital Sky tic halo have been revealed as a result of mapping the Milky Way with these modern surveys using various stel- [email protected] lar tracers. The most prominent structures are the tidal * This paper includes data taken at The McDonald Observa- tails of the Sagittarius dwarf galaxy (Ibata et al. 1994; tory of The University of Texas at Austin. Majewski et al. 2003), which provide dramatic evidence y This work is based on observations obtained at the MDM that the Milky Way is still being shaped by the infall Observatory, operated by Dartmouth College, Columbia Univer- sity, Ohio State University, Ohio University, and the University and merging of neighboring smaller galaxies. These ob- of Michigan. servational results have lent strong support to the hier- z Based on observations at Kitt Peak National Observatory, archical picture of galaxy formation under the ΛCDM National Optical Astronomy Observatory, which is operated model (Bullock et al. 2001; Johnston et al. 2008; Helmi by the Association of Universities for Research in Astronomy et al. 2011). (AURA) under cooperative agreement with the National Science Foundation. While the discovery of local overdensities in stellar sur- 1 Fermi National Accelerator Laboratory, P. O. Box 500, veys by visual inspection has proven successful, the scale Batavia, IL 60510, USA and sophistication of the data provided by current and 2 Department of Physics & Astronomy, Texas A & M Univer- sity, College Station, TX 77840, USA future sky surveys motivate an exploration of methods 3 George P. and Cynthia Woods Mitchell Institute for Fun- that can instead objectively and automatically identify damental Physics and Astronomy, College Station, TX, 77843- structures (Sharma et al. 2011a). Sharma et al.(2011b) 4242, USA 4 developed a density-based hierarchical group finding al- arXiv:1703.05384v2 [astro-ph.GA] 11 Jul 2017 City University of New York, LaGuardia Community Col- lege, Department of Natural Sciences, 31-10 Thomson Ave., Long gorithm Enlink to identify stellar halo substructures and Island City, NY 11101, USA applied it to a catalog of M giant stars selected from 5 Department of Astronomy, Columbia University, Mail Code 2MASS (Sharma et al. 2010, hereafter S10). This algo- 5246, New York, NY 10027, USA rithm uncovered 16 candidate substructures in the Milky 6 Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA Way halo, six of which had not been previously identified. 7 Department of Astrophysical Sciences, Princeton University, This paper presents the moderate-resolution spectro- Princeton, NJ 08544, USA scopic analysis of stars in one of the substructures re- 8 Departamento de F´ısicay Astronom´ıa,Facultad de Ciencias, ported by S10, namely the A13 candidate. The goal of Universidad de La Serena, Cisternas 1200, La Serena, Chile 9 The Carnegie Observatories, 813 Santa Barbara Street, this study is to determine, using their kinematical and Pasadena, CA 91101, USA chemical properties, whether the M giants in A13 are 10 Universit´eC^oted'Azur, OCA, CNRS, Lagrange, France genuinely associated with each other | an important 11 Sydney Institute for Astronomy, School of Physics, Univer- test of the performance of the group finding algorithm for sity of Sydney, NSW 2006, Australia 12 Max Planck Institute for Astronomy, K¨onigstuhl17, D- finding substructure in the Milky Way halo using 2MASS 69117 Heidelberg, Germany; photometry. 2 Li et al. This work also aims to explore associations between parison between all the structures using a single tracer. A13 and other known substructures. Its position on the The origins of both GASS/Mon and TriAnd are still un- sky { close to the Galactic anticenter with Galactic lati- der debate. While many studies argue that GASS/Mon tude b ∼ 30◦ { is suggestive of a possible connection be- and TriAnd could be the remnants of past accretion tween this and two low Galactic latitude structures { the events (see, e.g., Crane et al. 2003; Martin et al. 2004; Monoceros Ring (Mon) and the Triangulum-Andromeda Pe~narrubiaet al. 2005; Sollima et al. 2011; Slater et al. overdensity (TriAnd). 2014; Sheffield et al. 2014), more recent works suggest Mon was first discovered by Newberg et al.(2002) the possibility that GASS/Mon and TriAnd may be the using main-sequence turnoff stars selected from SDSS. result of a strong oscillation in the outer disk that throws Mon appears to be a low Galactic latitude ring-like disk stars to large scale heights (see, e.g., Xu et al. 2015; structure near the Galactic anticenter. It was indepen- Price-Whelan et al. 2015). A clear picture of the origins dently mapped using M giants selected from the 2MASS of these stellar structures may also have more general im- catalog by Crane et al.(2003) and Rocha-Pinto et al. plications for how the Milky Way formed. In particular, (2003) (who refer to it as the Galactic Anticenter Stel- the extent to which the stellar halo has grown from stars lar Structure { GASS { in their work). Crane et al. accreted from other systems relative to stars formed in (2003) report the properties they derive from spectra of our own Galaxy. a group of 53 M giants in the direction of the overden- This paper is organized as follows. Section2 describes sity they identify as GASS/Mon. They find a velocity our sample, the observations and data reductions. In Sec- −1 dispersion of σv ∼ 20 km s and a mean metallicity of tion3, we present the properties of our target stars de- [Fe=H] = −0:4 ± 0:3 and derive heliocentric distances of rived from the spectra, including their kinematics, metal- these stars in the range d ∼ 10 − 12 kpc. licities and distances. In Section4 we present a discus- Soon after, the Triangulum-Andromeda cloud sion of the possible connection of A13 to GASS/Mon and (TriAnd) was discovered as a \cloud-like" spatial over- TriAnd. Section5 summarizes the results. density by Rocha-Pinto et al.(2004) using M giants from the 2MASS catalog, spanning the range 100◦ < l < 160◦ and −50◦ < b < −15◦. Majewski et al.(2004a) also identified a main-sequence turnoff in the foreground 40 of a M31 halo survey that is consistent with the M giant population identified by Rocha-Pinto et al.(2004). 20 Martin et al.(2007) subsequently detected a second main-sequence in the region of TriAnd { referred to as 0 TriAnd2 { using the imaging data from the MegaCam Survey. Sheffield et al.(2014) conducted an expanded b (deg) survey of M giants in the TriAnd region using the −20 A1 GASS 2MASS catalog, and found a Red Giant Branch (RGB) TriA1 sequence at brighter apparent magnitude in addition −40 TriAnd2 to the fainter one discovered by Rocha-Pinto et al. (2004). The brighter sequence (refered to as TriAnd1) 240 220 200 1 1 1 1 1 l (deg) was assessed to be younger (6-10 Gyr) and closer (distance of ∼15-21 kpc), while the fainter sequence Figure 1. Spatial distribution (in Galactic coordinates) of the (TriAnd2) was found to be older (10-12 Gyr) and farther M giants in the known structures GASS (red diamonds), TriAnd1 (green stars), TriAnd2 (purple triangles), and in the new candidate (∼24-32 kpc). The velocity dispersion for each sequence substructure A13 (blue circles). −1 is σv ∼ 25 km s . We note that several low Galactic latitude structures were also discovered near the Milky Way anti-center 2. OBSERVATION AND DATA REDUCTIONS which may be associated with Monoceros Ring, such as 2.1. Target selection the Anticenter Stream (Grillmair 2006; Belokurov et al. 2007) and the Eastern Banded Structure (Grillmair 2006, The spectroscopic targets for this study were selected 2011).
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