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Gaia DR2 Proper Motions of Seven Ultra-Faint Dwarf Galaxies? D

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Gaia DR2 Proper Motions of Seven Ultra-Faint Dwarf Galaxies? D A&A 620, A155 (2018) Astronomy https://doi.org/10.1051/0004-6361/201833367 & c ESO 2018 Astrophysics With and without spectroscopy: Gaia DR2 proper motions of seven ultra-faint dwarf galaxies? D. Massari and A. Helmi Kapteyn Astronomical Institute, University of Groningen, 9747 AD Groningen, The Netherlands e-mail: [email protected] Received 4 May 2018 / Accepted 15 October 2018 ABSTRACT Aims. We present mean absolute proper motion measurements for seven ultra-faint dwarf galaxies orbiting the Milky Way, namely Boötes III, Carina II, Grus II, Reticulum II, Sagittarius II, Segue 2, and Tucana IV. For four of these dwarfs our proper motion estimate is the first ever provided. Methods. The adopted astrometric data come from the second data release of the Gaia mission. We determine the mean proper motion for each galaxy starting from an initial guess of likely members, based either on radial velocity measurements or using stars on the horizontal branch identified in the Gaia (GBP − GRP, G) colour-magnitude diagram in the field of view towards the UFD. We then refine their membership iteratively using both astrometry and photometry. We take into account the full covariance matrix among the astrometric parameters when deriving the mean proper motions for these systems. Results. Our procedure provides mean proper motions with typical uncertainties of ∼0:1 mas yr−1, even for galaxies without prior spectroscopic information. In the case of Segue 2 we find that using radial velocity members only leads to biased results, presumably because of the small number of stars with measured radial velocities. Conclusions. Our procedure allows the number of member stars per galaxy to be maximized regardless of the existence of prior spectroscopic information, and can therefore be applied to any faint or distant stellar system within reach of Gaia. Key words. Galaxy: kinematics and dynamics – galaxies: dwarf – Local Group – astrometry – proper motions 1. Introduction The advent of the Gaia mission (Gaia Collaboration 2016a,b) has resulted in a quantum leap in our ability to measure The orbits of the satellite galaxies of the Milky Way con- proper motions for distant stellar systems. Already with its first stitute a powerful tool to answer several important questions data release, thanks to the combination with pre-existing datasets of modern astrophysics. They provide valuable constraints on such as Tycho, HST or the Sloan Digital Sky Survey, Gaia the mass and shape of the Milky Way halo (Little & Tremaine has enabled measurements of the proper motions of Galactic 1987; Wilkinson & Evans 1999), as well as on the evolution globular clusters (Massari et al. 2017; Watkins & van der Marel of the satellites themselves and their resilience to tidal forces 2017), stellar streams (Helmi et al. 2017; Deason et al. 2018), (Peñarrubia et al. 2008; Łokas et al. 2012). Orbits also allow and stars in dwarf spheroidal galaxies (Massari et al. 2018). us to investigate whether their dynamical evolution and star Yet, it is with the second data release (DR2; Gaia Collaboration formation histories are connected (Grebel 2001; Tolstoy et al. 2018a), that Gaia becomes transformational. The Gaia DR2 2004; Battaglia et al. 2008). Furthermore, they reveal how our catalogue contains absolute proper motions for more than one Galaxy has assembled its population of dwarf galaxy satellites billion stars, and this has permitted for the first time direct (Kroupa et al. 2005; Li & Helmi 2008), and how this process measurement of proper motions over the full sky and without relates to the large-scale environment in which the Galaxy is the need for external objects for absolute calibration. This has embedded (Libeskind et al. 2005; Buck et al. 2016). resulted in spectacularly precise proper motion estimates for 75 To determine the orbits of Milky Way satellites, their posi- Galactic globular clusters, the Large and the Small Magellanic tion on the sky, distance, line of sight velocity, and proper Clouds, the nine classical dwarf spheroidal galaxies, and one motions are required. Historically, the last two observables of ultra-faint dwarf galaxy (UFD; Gaia Collaboration 2018b). this six-dimensional phase space have been the most difficult to Ultra-faint galaxies are very difficult objects to detect measure. Only recently and mostly thanks to the exquisite astro- because of their low luminosity, but the number of known UFDs metric capabilities of the Hubble Space Telescope (HST), have is continuously increasing thanks to wide-field deep surveys (e.g. absolute proper motions been provided for many of the bright- Drlica-Wagner et al. 2015; Laevens et al. 2015; Torrealba et al. est dwarf galaxies orbiting our Galaxy (e.g. Piatek et al. 2003; 2018). Ultra-faint galaxies might well constitute one of the best Massari et al. 2013; Kallivayalil et al. 2013; Sohn et al. 2013, solutions to the missing-satellites problem (Moore et al. 1999; 2017). Klypin et al. 1999). Despite their cosmological importance, very little is known about their origin, and knowledge of their orbits ? Full Table 2 is only available at the CDS via anonymous ftp around the Milky Way is a powerful way to find clues. Until very to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. recently, only two UFDs had an absolute proper motion mea- u-strasbg.fr/viz-bin/qcat?J/A+A/620/A155 surement (Fritz et al. 2018a; Gaia Collaboration 2018b). Around Article published by EDP Sciences A155, page 1 of6 A&A 620, A155 (2018) Table 1. Results for the seven UFDs analysed in this paper. Name Initial hµα cos δi σµα hµδi σµδ cov(µα cos δ, µδ) Nmembers guess mas yr−1 mas yr−1 mas yr−1 mas yr−1 Carina II RV 1.81 0.08 0.14 0.08 0.01 39 Reticulum II RV 2.34 0.12 −1:31 0.13 0.04 54 Segue 2 RV 1.27 0.11 −0:10 0.15 0.15 22 Boötes III HB −1:21 0.13 −0:92 0.17 0.23 34 Grus II HB 0.22 0.12 −1:41 0.11 −0:02 32 Sagittarius II HB −1:18 0.14 −1:14 0.11 0.24 78 Tucana IV HB 0.72 0.09 −1:71 0.10 0.00 23 Notes. We note that an additional systematic error of 0:035 mas yr−1 should be added to the quoted uncertainties, as explained in Gaia Collaboration (2018b). the time of submission of this paper, several new investigations 4. We perform a further selection on i) the colour-magnitude reported absolute proper motions for UFDs using stars defined diagram (see e.g. the blue box in Fig.3), ii) projected dis- as members based on radial velocity measurements (Simon tance from the centre of the UFD, and iii) we exclude stars 2018; Fritz et al. 2018b; Kallivayalil et al. 2018; Carlin & Sand with 0 < σ$=$ < 0:2 to remove obvious very nearby fore- 2018). However, relying on spectroscopic information only is ground contaminants. not entirely advisable. It limits the number of UFDs whose 5. The stars that survive these selection criteria are defined as proper motions can be determined, as not all UFDs have been new likely members, and their mean astrometric parameters followed up spectroscopically (see also Pace & Li 2018). Fur- and associated uncertainty (defined as the root mean square thermore, as we show here, it might lead to biased results around the mean) are determined. because of small sample sizes and therefore poor statistical sig- 6. We repeat steps 3, 4, and 5 until convergence is reached. nificance. Convergence is defined as the point where the change In this paper we present the absolute proper motion for seven between the mean astrometric parameters in two subsequent UFDs, namely Boötes III, Carina II, Grus II, Reticulum II, Sagit- iterations is smaller than 0:5 times the previously determined tarius II, Segue 2, and Tucana IV. All of these are located within root mean square (rms) error. 70 kpc of the earth and have an integrated absolute V-band mag- The convergence of the whole procedure, especially for the cases nitude brighter than MV = −2:5. Four out of these seven UFDs of poorly populated UFDs or when the contamination of the sur- do not have publicly available spectroscopic information. We rounding field is strong, depends on the adopted projected dis- develop a method to reliably select likely members even in such tance cut. The choice of the projected distance cut results from cases, and show that this method also fares better on UFDs with a balance between avoiding excessive contamination and includ- spectroscopic information, because it allows us to infer a more ing the largest possible number of members. We find that the best complete sample of members that is not limited to stars with solution is different for each single UFD, and we applied cuts measured radial velocities. ranging from ∼3:50 for the most crowded case (Sagittarius II) to ∼200 for the most diffuse UFDs (see the red circles in Figs.2 and5). The mean proper motions obtained in the last step are 2. Stellar membership and mean proper motion adopted as the final solution. determination We underline that the mean proper motions of the UFDs have been computed by taking into account the correlations among the In a recent paper, Simon(2018) determined the mean proper astrometric parameter uncertainties for each of the stars follow- motion for 17 UFDs by using all the stars defined as members ing the method developed by van Leeuwen(2009) and described according to their radial velocities. However, this kind of infor- in detail in Appendix A.1 of Gaia Collaboration(2017) 1.
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