SMHASH: anatomy of the Orphan Stream using RR Lyrae stars
Stellar tidal streams provide an opportunity to study the motion and structure of the disrupting galaxy as well as the gravitational potential of its host. Streams around the Milky Way are especially promising as phase space positions of individual stars will be measured by ongoing or upcoming surveys. Nevertheless, it remains a challenge to accurately assess distances to stars farther than 10 kpc from the Sun, where we have the poorest knowledge of the Galaxy's mass distribution. To address this, we present observations of 32 candidate RR Lyrae stars in the Orphan tidal stream taken as part of the Spitzer Merger History and Shape of the Galactic Halo (SMHASH) program. The extremely tight correlation between the periods, luminosities, and metallicities of RR Lyrae variable stars in the Spitzer IRAC 3.6μm band allows the determination of precise distances to individual stars; the median statistical relative distance uncertainty to each RR Lyrae star is 2.5 per cent. By fitting orbits in an example potential, we obtain an upper limit on the mass of the Milky Way interior to 60 kpc of 5.6^(+1.2)_(−1.1)×10^(11) M⊙, bringing estimates based on the Orphan Stream in line with those using other tracers. The SMHASH data also resolve the stream in line-of-sight depth, allowing a new perspective on the internal structure of the disrupted dwarf galaxy. Comparing with N–body models, we find that the progenitor had an initial dark halo mass of approximately 3.2 × 109 M⊙, placing the Orphan Stream's progenitor amongst the classical dwarf spheroidals.
Additional Information© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices). Accepted 2018 May 25. Received 2018 May 23; in original form 2017 November. Published: 05 June 2018. DH thanks Peter Stetson, Sheila Kannappan, and Vasily Belokurov for helpful discussions and the anonymous referee for a thoughtful and constructive report. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration (NASA). DH and KVJ acknowledge support on various aspects of this project from NASA through subcontract JPL 1558281 and ATP grant NNX15AK78G, as well as from the National Science Foundation (NSF) through the grant AST-1614743. The Space Telescope Science Institute (STScI) coauthors acknowledge NASA support through a grant for HST program GO-13443 from STScI, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. Support for this work was provided by NASA through Hubble Fellowship grant #51386.01 awarded to RLB by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. M-RC acknowledges support from the European Research Council (ERC) under European Union's Horizon 2020 research and innovation programme (grant agreement No. 652115). GC and AG acknowledge support from the Istituto Nazionale di Astrofisica (INAF) through PRIN-INAF2014, "EXCALIBUR'S" (P.I. G. Clementini) and by the Ministero dell'Istruzione, dell'Universita e della Ricerca (MIUR), through Premiale 2015, 'MITiC' (P.I. B. Garilli). DH acknowledges the use of the Shared Research Computing Facility at Columbia University. This work made use of Matplotlib (Hunter 2007), SciPy (Jones et al. 2001), Astropy (Astropy Collaboration et al. 2013), and the Astropy-affiliated Gala package (Price-Whelan 2017). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
Published - sty1455.pdf
Submitted - 1711.04663.pdf
Supplemental Material - sty1455_supplemental_files.zip