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Signatures of Tidal Disruption in Ultra-Faint Dwarf Galaxies: a Combined HST, Gaia, and MMT/Hectochelle Study of Leo V

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Signatures of Tidal Disruption in Ultra-Faint Dwarf Galaxies: a Combined HST, Gaia, and MMT/Hectochelle Study of Leo V The Astrophysical Journal, 885:53 (15pp), 2019 November 1 https://doi.org/10.3847/1538-4357/ab45ec © 2019. The American Astronomical Society. All rights reserved. Signatures of Tidal Disruption in Ultra-faint Dwarf Galaxies: A Combined HST, Gaia, and MMT/Hectochelle Study of Leo V Burçin Mutlu-Pakdil1 , David J. Sand1 , Matthew G. Walker2 , Nelson Caldwell3 , Jeffrey L. Carlin4 , Michelle L. Collins5 , Denija Crnojević6, Mario Mateo7, Edward W. Olszewski1, Anil C. Seth8, Jay Strader9 , Beth Willman10 , and Dennis Zaritsky1 1 Department of Astronomy/Steward Observatory, 933 North Cherry Avenue, Rm. N204, Tucson, AZ 85721-0065, USA; [email protected] 2 Center for Cosmology, Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA 3 Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA 4 LSST, 950 North Cherry Avenue, Tucson, AZ 85719, USA 5 Department of Physics, University of Surrey, Guildford GU2 7XH, UK 6 University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA 7 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA 8 University of Utah, 115 South 1400 East, Salt Lake City, UT 84112-0830, USA 9 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA 10 LSST and Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA Received 2019 July 16; revised 2019 August 31; accepted 2019 September 17; published 2019 October 30 Abstract The ultra-faint dwarf galaxy LeoV has shown both photometric overdensities and kinematic members at large radii, along with a tentative kinematic gradient, suggesting that it may have undergone a close encounter with the Milky Way. We investigate these signs of disruption through a combination of (i) high precision photometry obtained with the Hubble Space Telescope (HST), (ii) two epochs of stellar spectra obtained with the Hectochelle Spectrograph on the MMT, and (iii) measurements from the Gaia mission. Using the HST data, we examine one of the reported stream-like overdensities at large radii, and conclude that it is not a true stellar stream, but instead a clump of foreground stars and background galaxies. Our spectroscopic analysis shows that one known member star is likely a binary, and challenges the membership status of three others, including two distant candidates that had formerly provided evidence for overall stellar mass loss. We also find evidence that the proposed kinematic gradient across LeoV might be due to small number statistics. We update the systemic proper motion of LeoV, fi − ± −1 nding (mdmadcos ,)(= 0.009 0.560, 0.777 0.314) mas yr , which is consistent with its reported orbit that did not put LeoV at risk of being disturbed by the Milky Way. These findings remove most of the observational clues that suggested LeoV was disrupting; however, we also find new plausible member stars, two of which are located >5 half-light radii from the main body. These stars require further investigation. Therefore, the nature of LeoV still remains an open question. Unified Astronomy Thesaurus concepts: Dwarf galaxies (416); Stellar populations (1622); Tidal disruption (1696); Galactic and extragalactic astronomy (563); HST photometry (756); Spectroscopy (1558) Supporting material: machine-readable tables 1. Introduction systems (e.g., Triangulum II: Kirby et al. 2015, 2017; Martin Ultra-faint dwarf (UFD) galaxies are the oldest, smallest, et al. 2016; Carlin et al. 2017; Ji et al. 2019, Tucana III: Simon most dark-matter dominated, and least chemically evolved et al. 2017; Li et al. 2018, Tucana V: Conn et al. 2018, stellar systems known. The study of these system has Sagittarius II: Mutlu-Pakdil et al. 2018; Longeard et al. 2019). significant implications from the faint-end of the galaxy The existing dynamical analyses rely heavily on the assump- luminosity function (Koposov et al. 2009) to the validity of tions of dynamical equilibrium. Yet, it has frequently been cosmological models (e.g., Bullock & Boylan-Kolchin 2017; suggested that several UFDs have been affected by Galactic Kim et al. 2017; Pawlowski et al. 2017; Tulin & Yu 2018), tides (e.g., Belokurov et al. 2006, 2009; Zucker et al. 2006; including the nature of dark matter (e.g., Geringer-Sameth et al. Niederste-Ostholt et al. 2009; Sand et al. 2009, 2012; Muñoz 2015; Calabrese & Spergel 2016; Errani et al. 2018; Jethwa et al. 2010; Kirby et al. 2013; Roderick et al. 2015; Collins et al. 2018; Bozek et al. 2019; Robles et al. 2019a, 2019b) and et al. 2017; Simon et al. 2017; Garling et al. 2018). Thanks to the formation of the first galaxies (e.g., Bovill & Ricotti 2011; Gaia, the orbits of the UFDs are now constrained (e.g., Fritz Wheeler et al. 2015). et al. 2018; Simon 2018), and TucanaIII is unambiguously However, there are several observational challenges in suffering substantial stripping (Erkal et al. 2018; Li et al. 2018; understanding such faint systems. Given the presence of only Shipp et al. 2018), with its orbital pericenter of only ∼3kpc a handful of bright stars, it has been very difficult to study them (Fritz et al. 2018; Simon 2018). Several UFDs are found to spectroscopically. Even under the assumption of minimal have orbits that might bring them close to the inner regions of contamination by binary and foreground stars, they are largely the Milky Way, making them likely to be tidally disturbed: doomed to suffer from small number statistics. The presence or BoötesIII (Carlin & Sand 2018), Willman1, Segue1, absence of dark matter, and thus whether they are indeed TriangulumII (Fritz et al. 2018), CraterII, Hercules (Fritz galaxies, remains unclear in some of the recently discovered et al. 2018; Fu et al. 2019), and DracoII (Fritz et al. 2018; 1 The Astrophysical Journal, 885:53 (15pp), 2019 November 1 Mutlu-Pakdil et al. Longeard et al. 2018). On the other hand, LeoV shows signs of tidal disturbance but does not have an orbit that seems to put it at risk of being tidally disturbed by the Milky Way. In this paper, we focus on LeoV (discovered in Belokurov et al. 2008) and critically assess previously observed signatures of tidal influence in combination with new observations. Ground-based observations suggest LeoV has an extended morphology, with a highly elongated shape (ellipticity ∼0.5) and stellar overdensities outside of several half-light radii (Sand et al. 2012). Walker et al. (2009) found that the likely blue horizontal branch member distribution was more extended than the bulk of red giant branch stars. LeoV also displays a tentative velocity gradient, which may indicate it is in an advanced stage of dissolution (Collins et al. 2017). These ubiquitous hints for tidal effects among distant dwarfs like LeoV are particularly surprising because they can only experience tidal stripping if their orbits are extremely eccentric, bringing them within 10–20kpc of the Galactic center (Simon 2019). Based on Gaia proper motions of five central LeoV member candidates reported in Walker et al. (2009), Fritz et al. (2018) found it unlikely for LeoV to have such an orbit. The +14 +12 authors estimated an orbital pericenter of 165-126 (168-104) kpc for LeoV, assuming a Milky Way dark matter halo with virial 12 12 mass 1.6×10 Me (0.8 × 10 Me). Therefore, it is crucial to verify whether the photometric tidal features observed around Figure 1. Our observing strategy: the smoothed matched-filter map of LeoV LeoV are true tidal material—stripped stars—or whether from the ground-based imaging of Sand et al. (2012) with half-light radii clustered background galaxies are masquerading as stellar marked in orange (for Leo V, this value is comparable to the smoothing size). debris and, by extension, whether the kinematic gradients are Dashed boxes, labeled as Field1, represent the archived HST/ACS and WFC3 real or due to small number statistics. The confirmation of even imaging which we utilized for our analysis. The solid boxes, labeled as Field 2, are our HST/ACS (red) pointing along the putative streams (far from the main a small number of tidally stripped stars would imply much body of Leo V) and WFC3 (blue) parallel. The black dashed ellipse is the more global mass loss, in both dark matter and stars. approximate isodensity contour at the radius of our data (r ∼ 10×half-light Here we investigate the signs of tidal disruption in LeoV radii), and the arrow shows the direction toward the Galactic center. An through a combination of high-precision photometry obtained additional arrow is shown to highlight the direction to LeoIV. with the Hubble Space Telescope (HST)/ACS and WFC3, two epochs of stellar spectra obtained with the MMT/Hectochelle Table 1 Spectrograph, and proper motion measurements from GaiaDR2. Observation Log and Field Completeness of LeoV-related fields (Fields 1-2, We describe our observations and data reduction in Section 2.We see Figure 1) and Leo IV revisit the distance to LeoV and present its color–magnitude Field Name Camera Filter Exp 50% 90% diagram (CMD) in comparison to those of LeoIV and M92 in (s)(mag)(mag) Section 3.UsingHST data, we investigate the true nature of the Leo V Field 1 ACS F606W 4557 27.55 26.48 tentative candidate debris stream of Leo V in Section 4.In (Main Body) ACS F814W 4565 27.29 26.67 Section 5, we present our new spectroscopic results, along with WFC3 F606W 4596 27.24 26.29 the relevant GaiaDR2 data, to search for any signs of tidal WFC3 F814W 4605 27.07 26.66 disturbance, while updating the proper motion measurement of LeoV. Finally, we summarize our key results in Section 6. Leo V Field 2 ACS F606W 5174 27.59 26.50 (Stream Candidate) ACS F814W 5174 27.30 26.67 WFC3 F606W 5264 27.34 26.32 WFC3 F814W 5264 27.14 26.58 2.
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