Proper Motions of Dwarf Spheroidal Galaxies fromHubble Space TelescopeImaging. III. Measurement for Ursa Minor

Abstract

This article presents a measurement of the proper motion of the Ursa Minor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope in two distinct fields. Each field contains a quasi-stellar object that serves as the ''reference point.'' The measured proper motion for Ursa Minor, expressed in the equatorial coordinate system, is ( ; ) ¼ (À50 AE 17; 22 AE 16) mas century À1 . Removing the contributions of the solar motion and the motion of the local standard of rest yields the proper motion in the Galactic rest frame: ( Grf ; Grf ) ¼ (À8 AE 17; 38 AE 16) mas century À1 . The implied space velocity with respect to the Galactic center has a radial component of V r ¼ À75 AE 44 km s À1 and a tangential component of V t ¼ 144 AE 50 km s À1 . Integrating the motion of Ursa Minor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 40 (10, 76) and 89 (78, 160) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.39 (0.09, 0.79), and the orbital period is 1.5 (1.1, 2.7) Gyr. The orbit is retrograde and inclined by 124 (94 , 136 ) to the Galactic plane. Ursa Minor is not a likely member of a proposed stream of galaxies on similar orbits around the Milky Way, nor is the plane of its orbit coincident with a recently proposed planar alignment of galaxies around the Milky Way. Comparing the orbits of Ursa Minor and Carina shows no reason for the different star formation histories of these two galaxies. Ursa Minor must contain dark matter to have a high probability of having survived disruption by the Galactic tidal force until the present.

THE M31 VELOCITY VECTOR. I.HUBBLE SPACE TELESCOPEPROPER-MOTION MEASUREMENTS

“…Such a strategy has been used in several studies for measuring absolute PMs of Milky Way satellite galaxies (e.g., Piatek et al 2002Piatek et al , 2003Piatek et al , 2005Piatek et al , 2006Piatek et al , 2007Kallivayalil et al 2006aKallivayalil et al , 2006bPiatek et al 2008). However, the accuracies thus achieved are not sufficient at the distance of M31.…”

The proper motion of the Local Group galaxy IC 10

“…Presently, measurements of extragalactic proper motions by optical telescopes are limited to the most nearby companions of the Milky Way, i.e. the LMC (Jones et al 1994;Kallivayalil et al 2006a;Pedreros et al 2006), the SMC (Kallivayalil et al 2006b), the Sculptor dwarf spheroidal galaxy (dShp) (Schweitzer et al 1995;Piatek et al 2006), the Canis Major dwarf galaxy (Dinescu et al 2005b), the Ursa Minor dSph (Piatek et al 2005), the Sagittarius dSph (Dinescu et al 2005a), the Fornax dSph (Piatek et al 2002;Dinescu et al 2004), and the Carina dSph (Piatek et al 2003). These galaxies are all closer than 150 kpc and show motions between 0.2 and a few milliarcseconds (mas) per year.…”

Local Group Proper Motion Dynamics

“…The PMs of almost all of the MWs classical dSph satellites have been measured using HST (Piatek et al 2003(Piatek et al , 2005(Piatek et al , 2006(Piatek et al , 2007Lepine et al 2011;Pryor et al 2014), and these are generally the most accurate measurements available. Knowledge of the PMs has been used to determine the dSph orbits.…”