Proper Motions of Stellar Streams Discovered in the Dark Energy Survey
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Draft version July 24, 2019 Typeset using LATEX twocolumn style in AASTeX62 Proper Motions of Stellar Streams Discovered in the Dark Energy Survey N. Shipp,1, 2, 3, ∗ T. S. Li,3, 2, 4, 5, y A. B. Pace,6 D. Erkal,7 A. Drlica-Wagner,3, 1, 2 B. Yanny,3 V. Belokurov,8 W. Wester,3 S. E. Koposov,9, 8 G. F. Lewis,10 J. D. Simpson,11 Z. Wan,10 D. B. Zucker,12, 13 S. L. Martell,11, 14 and M. Y. Wang15 (S5 Collaboration) 1Department of Astronomy and Astrophysics, University of Chicago, Chicago IL 60637, USA 2Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA 3Fermi National Accelerator Laboratory, PO Box 500, Batavia, IL 60510, USA 4Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA 5Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 6George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA 7Department of Physics, 12 BC 03, University of Surrey, GU2 7XH, UK 8Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 9McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, USA 10Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, NSW, 2006, Australia 11School of Physics, UNSW, Sydney, NSW 2052, Australia 12Department of Physics & Astronomy, Macquarie University, Sydney, NSW 2109, Australia 13Macquarie University Research Centre for Astronomy, Astrophysics & Astrophotonics, Sydney, NSW 2109, Australia 14Centre of Excellence for All-Sky Astrophysics in Three Dimensions (ASTRO 3D), Australia 15Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15312, USA ABSTRACT We cross-match high-precision astrometric data from Gaia DR2 with accurate multi-band photom- etry from the Dark Energy Survey (DES) DR1 to confidently measure proper motions for nine stellar streams in the DES footprint: Aliqa Uma, ATLAS, Chenab, Elqui, Indus, Jhelum, Phoenix, Tucana III, and Turranburra. We determine low-confidence proper motion measurements for four additional stellar streams: Ravi, Wambelong, Willka Yaku, and Turbio. We find evidence for a misalignment between stream tracks and the systemic proper motion of streams that may suggest a systematic gravitational influence from the Large Magellanic Cloud. These proper motions, when combined with radial velocity measurements, will allow for detailed orbit modeling which can be used to constrain properties of the LMC and its affect on nearby streams, as well as global properties of the Milky Way's gravitational potential. Keywords: Stars: kinematics and dynamics { Galaxy: structure { Galaxy: halo { Local Group 1. INTRODUCTION surface brightness makes them historically difficult to detect. The advent of large sky surveys has rapidly in- arXiv:1907.09488v1 [astro-ph.GA] 22 Jul 2019 Stellar streams, the tidal remnants of accreted glob- ular clusters and dwarf galaxies, are powerful tools for creased the number of known streams around the Milky studying the distribution of matter and the accretion Way (e.g., Mateu et al. 2018, and references therein) history of our Galaxy (Johnston 1998; Bullock & John- and other nearby galaxies (e.g., Zucker et al. 2004; ston 2005). Stellar streams arise naturally in hierar- Mart´ınez-Delgado et al. 2010). This explosion in the chical models of galaxy formation; however, their low known population of stellar streams promises to enable detailed statistical modeling of the stream population in the near future (e.g., Bonaca & Hogg 2018). [email protected] Stellar streams are excellent dynamical tools for mea- ∗ LSSTC Data Science Fellow suring the properties of the Milky Way, including the to- y Hubble Fellow tal enclosed mass within their orbits (e.g., Gibbons et al. 2 2014; Bowden et al. 2015; Bovy et al. 2016; Bonaca & accuracy. Furthermore, the location of these streams in Hogg 2018) and the shape and radial profile of the gravi- the Southern Hemisphere makes them sensitive probes tational field (Law & Majewski 2010; Erkal et al. 2016a). of the joint potential of the Milky Way and LMC. An individual stream can probe the Milky Way's poten- While deep photometric surveys are excellent for de- tial across tens of kiloparsecs (Law & Majewski 2010; tecting faint stellar structures at large distances, addi- Koposov et al. 2010), while a population of a dozen stel- tional phase space information is necessary for compre- lar streams with full kinematic information is expected hensive dynamical modeling (e.g. Eyre & Binney 2009; to constrain the gravitational potential of the Milky Way Bowden et al. 2015; Law & Majewski 2010; Bovy 2014; at sub-percent levels (Bonaca & Hogg 2018). Bovy et al. 2016; Erkal et al. 2019). The 3D kinemat- Stellar streams are also sensitive tracers of perturba- ics of faint stream stars can be obtained via a combina- tions in the Milky Way's gravitational field. Streams can tion of proper motion measurements from high-precision be used to detect perturbations in the gravitational field astrometric surveys and radial velocity measurements of the halo from known satellites (e.g. Vera-Ciro & Helmi from deep spectroscopic observations. The second data 2013; G´omezet al. 2015; Erkal et al. 2018, 2019) and release from the Gaia satellite (Gaia DR2; Gaia Collab- smaller dark matter substructure (e.g. Ibata et al. 2002; oration et al. 2018) provides unprecedented proper mo- Johnston et al. 2002; Yoon et al. 2011; Carlberg 2009, tion measurements for more that 1 billion stars. Gaia 2012; Erkal & Belokurov 2015). The Milky Way's largest DR2 has enabled proper motion measurements for stel- satellite, the Large Magellanic Cloud (LMC), resides in lar streams at distances of tens of kiloparsecs (e.g., Price- a dark matter halo that may be 10% as massive as that Whelan & Bonaca 2018; Koposov et al. 2019; Fardal of the Milky Way (Busha et al. 2011; Boylan-Kolchin et al. 2019), as well as joint photometric and astromet- et al. 2012). Direct measurements of the LMC mass ex- ric discovery of previously unknown streams (Malhan & 10 ist only within 9 kpc yielding values of 2 10 M Ibata 2018; Malhan et al. 2018). ∼ ∼ × (e.g., Schommer et al. 1992; van der Marel & Kallivayalil In addition to providing kinematic information, the 2014); however, cosmological arguments predict that the systemic proper motions of stellar streams can also mass of the LMC is nearly an order of magnitude larger greatly improve the efficiency of target selection for spec- (Busha et al. 2011; Boylan-Kolchin et al. 2012). Such troscopic follow-up surveys. Proper motions can be used a large gravitational perturber located at a distance of to discriminate likely stream members from foreground only 50 kpc would have an appreciable affect on mea- Milky Way stars and other halo stars. For example, the surements of the gravitational field in the halo of the Southern Stellar Stream Spectroscopic Survey (S5; Li et Milky Way. Stellar streams, particularly those in spa- al., submitted), an on-going program to map the line- tial proximity to the LMC, offer a sensitive mechanism of-sight velocities and metallicities of the DES streams to independently measure the mass of the LMC at large using the 3.9-m Anglo-Australian Telescope's 2-degree- radii (e.g., Erkal et al. 2018, 2019). Field (2dF) fibre position and AAOmega spectrograph, Large-area, ground-based, digital photometric sur- efficiently selects targets following the techniques de- veys like the Sloan Digital Sky Survey (SDSS; York scribed here. et al. 2000), Pan-STARRS (Chambers et al. 2016), VST In this paper, we cross-matched data from DES DR1 ATLAS (Shanks et al. 2015), and the Dark Energy and Gaia DR2 to measure proper motions for stellar Survey (DES; DES Collaboration 2016) have rapidly streams in the DES footprint.1 We performed two dis- increased the number of known stellar streams (e.g., tinct analyses that each combined precise photometry Odenkirchen et al. 2001; Grillmair 2006; Grillmair & from DES DR1 with precise astrometry from Gaia DR2. Dionatos 2006; Belokurov et al. 2006; Grillmair 2009; First, we performed a simple \by-eye" analysis to visi- Bonaca et al. 2012; Koposov et al. 2014; Grillmair 2014; bly highlight the proper motion signal of stellar streams Drlica-Wagner et al. 2015; Balbinot et al. 2016; Bernard by removing the majority of the Milky Way foreground et al. 2016; Grillmair 2017a,b; Myeong et al. 2017; Shipp contamination with physically motivated cuts. Next, we et al. 2018; Jethwa et al. 2018). The population of stel- performed a more statistically rigorous Gaussian mix- lar streams discovered in DES is of particular interest for ture model (GMM) fit, in which we applied a less-strict constraining the gravitational field in the Milky Way's data selection, and then fit a two-component Gaussian outer halo (Shipp et al. 2018). The DES streams consti- tute some of the faintest and most distant streams dis- 1 We excluded the Palca stream from this analysis due to its covered around the Milky Way and, due to the excellent large width and extent on the sky, which make it difficult to char- photometry provided by DES, they can be distinguished acterize. from foreground stellar populations with unprecedented 3 model in proper motion space to account for both the 2018). We found that this cut removes very few objects Milky Way foreground and the stream signal. from our final catalog, and a looser selection on the DES We detected and confirmed proper motion signatures star/galaxy separation (i.e., EXTENDED COADD 1) had ≤ for nine of the 14 streams (Aliqa Uma, ATLAS, Chenab, no effect on the results of this analysis. Elqui, Indus, Jhelum, Phoenix, Tucana III, Turran- We also removed objects with bad astrometric fits in burra), including the most distant DES stream, Elqui, Gaia DR2.