Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging. II. Measurement for Carina

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.