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Register a new userOrbits of LMC/SMC with recent ground-based proper motions
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Kate\\v{r}ina Barto\\v{s}kov\\'a
Publication date
2011
fields
Physics
and research's language is
English
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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.
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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.
We present the first entirely ground-based astrometric determination of the proper motion for the Fornax Local Group Dwarf Spheroidal satellite galaxy of the Milky Way, using CCD data acquired with the ESO 3.5 m NTT telescope at La Silla Observatory in Chile. Our unweighted mean from five Quasar fields in the background of Fornax, used as fiducial reference points, leads to $mu_alpha cos delta=0.62 pm 0.16$ $masy$, and $mu_delta=-0.53 pm 0.15$ $masy$. A detailed comparison with all previous measurements of this quantity seems to imply that there is still no convincing convergence to a single value, perhaps indicating the existence of unnacounted systematic effects in (some of) these measurements. From all available proper motion and radial velocity measurements for Fornax, we compute Fornaxs orbital parameters and their uncertainty using a realistic Galactic potential and a Monte Carlo simulation. Properties of the derived orbits are then compared to main star formation episodes in the history of Fornax. All published proper motion values imply that Fornax has recently (200-300 Myr ago) approached perigalacticon at a distance of $sim$150 kpc. However, the derived period exhibits a large scatter, as does the apogalacticon. Our orbit, being the most energetic, implies a very large apogalactic distance of $sim 950$ kpc. If this were the case, then Fornax would be a representative of an hypervelocity MW satellite in late infall.
Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf spheroidal galaxies (dSphs) of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve, with total mass ranging from $2.8times10^{11}$ $M_{odot}$ to $15times10^{11}$ $M_{odot}$. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dSphs are highly concentrated close to their pericenter, rather than to their apocenter as expected from Keplers law. This may challenge the fact that most dSphs are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dSphs lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dSphs.
The second data release from the Gaia mission (DR2) provides a comprehensive and unprecedented picture of the motions of astronomical sources in the plane of the sky, extending from the solar neighborhood to the outer reaches of the Milky Way. I present proper motion measurements based on Gaia DR2 for 17 ultra-faint dwarf galaxies within 100 kpc of the Milky Way. I compile the spectroscopically-confirmed member stars in each dwarf bright enough for Gaia astrometry from the literature, producing member samples ranging from 2 stars in Triangulum II to 68 stars in Bootes I. From the spectroscopic member catalogs I estimate the proper motion of each system. I find good agreement with the proper motions derived by the Gaia collaboration for Bootes I and Leo I. The tangential velocities for 14 of the 17 dwarfs are determined to better than 50 km/s, more than doubling the sample of such measurements for Milky Way satellite galaxies. The orbital pericenters are well-constrained, with a median value of 38 kpc. Only one satellite, Tucana III, is on an orbit passing within 15 kpc of the Galactic center, suggesting that the remaining ultra-faint dwarfs are unlikely to have experienced severe tidal stripping. As a group, the ultra-faint dwarfs are on high-velocity, eccentric, retrograde trajectories, with nearly all of them having space motions exceeding 370 km/s. In a low-mass (M_vir = 0.8 x 10^12 M_sun) Milky Way potential, eight out of the 17 galaxies lack well-defined apocenters and appear likely to be on their first infall, indicating that the Milky Way mass may be larger than previously estimated or that many of the ultra-faint dwarfs are associated with the Magellanic Clouds. The median eccentricity of the ultra-faint dwarf orbits is 0.79, similar to the values seen in numerical simulations, but distinct from the rounder orbits of the more luminous dwarf spheroidals.
We present the first detailed kinematic analysis of the proper motions (PMs) of stars in the Magellanic Bridge, from both the textit{Gaia} Data Release 2 catalog and from textit{Hubble Space Telescope} Advanced Camera for Surveys data. For the textit{Gaia} data, we identify and select two populations of stars in the Bridge region, young main sequence (MS) and red giant stars. The spatial locations of the stars are compared against the known H {small I} gas structure, finding a correlation between the MS stars and the H {small I} gas. In the textit{Hubble Space Telescope} fields our signal comes mainly from an older MS and turn-off population, and the proper motion baselines range between $sim 4$ and 13 years. The PMs of these different populations are found to be consistent with each other, as well as across the two telescopes. When the absolute motion of the Small Magellanic Cloud is subtracted out, the residual Bridge motions display a general pattern of pointing away from the Small Magellanic Cloud towards the Large Magellanic Cloud. We compare in detail the kinematics of the stellar samples against numerical simulations of the interactions between the Small and Large Magellanic Clouds, and find general agreement between the kinematics of the observed populations and a simulation in which the Clouds have undergone a recent direct collision.
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