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Register a new userProper Motions of the LMC and SMC: Reanalysis of Hubble Space Telescope Data
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Kallivayalil et al. have used the textit{Hubble Space Telescope} to measure proper motions of the LMC and SMC using images in 21 and five fields, respectively, all centered on known QSOs. These results are more precise than previous measurements, but have surprising and important physical implications: for example, the LMC and SMC may be approaching the Milky Way for the first time; they might not have been in a binary system; and the origin of the Magellanic Stream needs to be re-examined. Motivated by these implications, we have reanalyzed the original data in order to check the validity of these measurements. Our work has produced a proper motion for the LMC that is in excellent agreement with that of Kallivayalil et al., and for the SMC that is in acceptable agreement. We have detected a dependence between the brightness of stars and their mean measured motion in a majority of the fields in both our reduction and that of Kallivayalil et al. Correcting for this systematic error and for the errors caused by the decreasing charge transfer efficiency of the detector produces better agreement between the measurements from different fields. With our improved reduction, we do not need to exclude any fields from the final averages and, for the first time using proper motions, we are able to detect the rotation of the LMC. The best-fit amplitude of the rotation curve at a radius of 275 arcmin in the disk plane is $120 pm 15$ km s$^{-1}$. This value is larger than the 60--70 km s$^{-1}$ derived from the radial velocities of HI and carbon stars, but in agreement with the value of 107 km s$^{-1}$ derived from the radial velocities of red supergiants.
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In recent years, with new ground-based and HST measurements of proper motions of the Magellanic Clouds being published, a need of a reanalysis of possible orbital history has arisen. As complementary to other studies, we present a partial examination of the parameter space -- aimed at exploring the uncertainties in the proper motions of both Clouds, taking into account the updated values of Galactic constants and Solar motion, which kinematically and dynamically influence the orbits of the satellites. In the chosen setup of the study, none of the binding scenarios of this pair could be neglected.
We report on the proper motions of Balmer-dominated filaments in Keplers supernova remnant using high resolution images obtained with the Hubble Space Telescope at two epochs separated by about 10 years. We use the improved proper motion measurements and revised values of shock velocities to derive a distance to Kepler of 5.1 [+0.8, -0.7] kpc. The main shock around the northern rim of the remnant has a typical speed of 1690 km/s and is encountering material with densities of about 8 cm^-3. We find evidence for the variation of shock properties over small spatial scales, including differences in the driving pressures as the shock wraps around a curved cloud surface. We find that the Balmer filaments ahead of the ejecta knot on the northwest boundary of the remnant are becoming fainter and more diffuse. We also find that the Balmer filaments associated with circumstellar material in the interior regions of the remnant are due to shocks with significantly lower velocities and that the brightness variations among these filaments trace the density distribution of the material, which may have a disk-like geometry.
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.
The measured proper motion of Fornax, expressed in the equatorial coordinate system, is $(mu_{alpha},mu_{delta})=(47.6pm 4.6,-36.0pm 4.1)$ mas century$^{-1}$. This proper motion is a weighted mean of four independent measurements for three distinct fields. Each measurement uses a quasi-stellar object as a reference point. Removing the contribution of the motion of the Sun and of the Local Standard of Rest to the measured proper motion produces a Galactic rest-frame proper motion of $(mu_{alpha}^{mbox{tiny{Grf}}}, mu_{delta}^{mbox{tiny{Grf}}}) = (24.4pm 4.6,-14.3pm 4.1)$ mas century$^{-1}$. The implied space velocity with respect to the Galactic center has a radial component of $V_{r}=-31.8 pm 1.7$ km s$^{-1}$ and a tangential component of $V_{t}=196 pm 29$ km s$^{-1}$. Integrating the motion of Fornax in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 118 (66, 137) kpc and 152 (144, 242) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.13 (0.11, 0.38), and the orbital period is 3.2 (2.5, 4.6) Gyr. The orbit is retrograde and inclined by $101^{circ}$ ($94^{circ}$, $107^{circ}$) to the Galactic plane. Fornax could be a member of a proposed ``stream of galaxies and globular clusters, however the membership of another proposed galaxy in the stream, Sculptor, has been previously ruled out. Fornax is in the Kroupa-Theis-Boily plane that contains eleven of the Galactic satellite galaxies, but its orbit will take it out of that plane.
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.
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