Browsing by Author "Olszewski, EW"

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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: I.: Method and a preliminary measurement for Fornax
    (2002) Piatek, S; Pryor, C; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Monet, DG; Morrison, H; Tinney, CG
    This article presents and discusses a method for measuring the proper motions of the Galactic dwarf spheroidal galaxies using images taken with the Hubble Space Telescope. The method involves fitting an effective point-spread function to the image of a star or quasi-stellar object to determine its centroid with an accuracy of about 0.005 pixels (0.25 mas)-an accuracy sufficient to measure the proper motion of a dwarf spheroidal galaxy, using images separated by just a few years. The data consist of images, dithered to reduce the effects of undersampling, taken at multiple epochs with the Space Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor dwarf spheroidal galaxies, and each has at least one quasi-stellar object whose identity has been established by other studies. The rate of change with time of the centroids of the stars of the dwarf spheroidal with respect to the centroid of the quasi-stellar object is the proper motion. Four independent preliminary measurements of the proper motion of Fornax for three fields agree within their uncertainties. The weighted average of these measurements is mu(alpha) = 49 +/- 13 mas century(-1) and mu(delta) = -59 +/- 13 mas century(-1). The Galactocentric velocity derived from the proper motion implies that Fornax is near perigalacticon, may not be bound to the Milky Way, and is not a member of any of the proposed streams of galaxies and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way must have a mass of at least (1.6 +/- 0.8) x 10(12) M.
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: II.: Measurement for Carina
    (2003) Piatek, S; Pryor, C; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Tinney, CG
    This article presents and discusses a measurement of the proper motion for the Carina dwarf spheroidal galaxy (dSph) from images in two distinct fields in the direction of Carina taken with the Hubble Space Telescope, at three epochs. Each field contains a confirmed quasi-stellar object that is the reference point for measuring the proper motion of the dSph. The consecutive epochs are 1-2 yr apart. The components of the measured proper motion for Carina, expressed in the equatorial coordinate system, are mu(alpha)=22+/-9 mas century(-1) and mu(delta)=15+/-9 mas century(-1). The quoted proper motion is a weighted mean of two independent measurements and has not been corrected for the motions of the Sun and of the local standard of rest. Given the proper motion and its uncertainty, integrating the family of possible orbits of Carina in a realistic gravitational potential for the Milky Way indicates that Carina is bound gravitationally to the Milky Way and is close to apogalacticon. The best estimate of, and the 95% confidence interval for, the apogalacticon of the orbit is 102 kpc and (102, 113) kpc, for the perigalacticon is 20 kpc and (3.0, 63) kpc, and for the orbital period is 1.4 Gyr and (1.3, 2.0) Gyr. Carina does not seem to be on a polar orbit. The best estimate of the inclination of the orbit with respect to the Galactic plane is 39degrees, but the 95% confidence interval is so wide, (23degrees, 102degrees), that it includes a polar orbit. We are unable to confirm or to rule out the membership of Carina in a "stream'' of galaxies in the Galactic halo because the difference between the observed and predicted directions of the proper motion is 1.6 times the uncertainty of the difference. Carina must contain dark matter to have survived the tidal interaction with the Milky Way until the present. The triggering of star formation by perigalacticon passages and crossings of the Galactic disk do not explain the history of star formation in Carina.
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: III.: Measurement for Ursa Mminor
    (2005) Piatek, S; Pryor, C; Bristow, P; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Tinney, CG
    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 (mu(alpha); mu(delta)) = ( - 50 +/- 17; 22 +/- 16) mas century(-1). 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: (mu(Grf)(alpha), mu(Grf)(delta)) = (- 8 +/- 17, 38 +/- 16) mas century(-1). The implied space velocity with respect to the Galactic center has a radial component of V-r = - 75 +/- 44 km s(-1) and a tangential component of V-t = 144 +/- 50 km s(-1). 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 degrees (94 degrees, 136 degrees) 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.
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: IV.: Measurement for sculptor
    (2006) Piatek, S; Pryor, C; Bristow, P; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Tinney, CG
    This article presents a measurement of the proper motion of the Sculptor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope using the Space Telescope Imaging Spectrograph in the imaging mode. Each of two distinct fields contains a quasi-stellar object that serves as the "reference point.'' The measured proper motion of Sculptor, expressed in the equatorial coordinate system, is (mu(alpha), mu(delta)) (9 +/- 13; 2 +/- 13) mas century(-1). Removing the contributions from the motion of the Sun and the motion of the local standard of rest produces the proper motion in the Galactic rest frame: (mu(Grf)(alpha), mu(Grf)(delta)) = (-23 +/- 13; 45 +/- 13) mas century(-1). The implied space velocity with respect to the Galactic center has a radial component of V-r = 79 +/- 6 km s(-1) and a tangential component of V-t = 198 +/- 50 km s(-1). Integrating the motion of Sculptor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc, respectively, where the values in the parentheses represent the 95% confidence interval derived from Monte Carlo experiments. The eccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital period is 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the Milky Way: the angle of inclination is 86 degrees (83 degrees, 90 degrees).