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

Abstract

We present the first proper-motion (PM) measurements for the galaxy M31. We obtained new V-band imaging data with the Hubble Space Telescope ACS/WFC and the WFC3/UVIS instruments of three fields: a spheroid field near the minor axis, an outer disk field along the major axis, and a field on the Giant Southern Stream. The data provide five to seven year time baselines with respect to pre-existing deep first-epoch observations of the same fields. We measure the positions of thousands of M31 stars and hundreds of compact background galaxies in each field. High accuracy and robustness is achieved by building and fitting a unique template for each individual object. The average PM for each field is obtained from the average motion of the M31 stars between the epochs with respect to the background galaxies. For the three fields, the observed PMs (μ W , μ N ) are, in units of mas yr −1 , (−0.0458, −0.0376) ± (0.0165, 0.0154), (−0.0533, −0.0104) ± (0.0246, 0.0244), and (−0.0179, −0.0357) ± (0.0278, 0.0272), respectively. The ability to average over large numbers of objects and over the three fields yields a final displacement accuracy of a few thousandths of a pixel, corresponding to only 12 μas yr −1 . This is comparable to what has been achieved for other Local Group galaxies using Very Long Baseline Array observations of water masers. Potential systematic errors are controlled by an analysis strategy that corrects for detector charge transfer inefficiency, spatially and time-dependent geometric distortion, and point-spread function variations. The robustness of the PM measurements and uncertainties are supported by the fact that data from different instruments, taken at different times and with different telescope orientations, as well as measurements of different fields, all yield statistically consistent results. Papers II and III of this series explore the implications of the new measurements for our understanding of the history, future, and mass of the Local Group.

The Local Group in the Cosmic Web

“…The LG is dominated by two big spirals: the Milky Way (MW) and M31, the next most-luminous galaxy is M33 which is ∼ 10 times less massive than M31, followed by several dozen less luminous dwarf galaxies, up to a distance of ∼ 3Mpc. The velocity vector of M31, with a low tangential velocity is consistent with a head-on collision orbit toward the MW (Cox & Loeb 2008;van der Marel et al 2012b;Sohn et al 2012).…”

Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

“…The combination of these three programs provide deep, multi-epoch optical imaging of three fields in M31 (M31 Spheroid, M31 Disk, and M31 Stream). These observations were used to measure the PM of M31 (Sohn et al 2012), and during the course of this study, PM catalogs for individual stars in the three HST fields were created. D13 selected Milky Way halo star candidates in colormagnitude space, using photometry from Brown et al (2009) Figure 1 demonstrates how our initial sample from D13 evolved into the sample used in this analysis.…”

Local Group Proper Motion Dynamics

“…Thus it is necessary to perform careful calibrations of PSF shapes, geometric distortions, charge-transfer efficiency, color effects, time variability in calibrations, etc. (e.g., Anderson & King 2000;Sohn et al 2012). When this is done, the systematics can be controlled to levels below the typical random errors.…”

“…In these models, the Clouds were assumed to have been long-term MW satellites moving on near-circular orbits in a logarithmic potential (e.g., Gardiner & Noguchi 1996). By contrast, orbit calculations based on the HST PMs in a cosmologically motivated MW halo potential (Sohn et al 2012) and the observed M33 PM from water maser observations (Brunthaler et al 2005). The MW and M31 will collide in ∼ 4 Gyr, and they will fully merge in ∼ 6 Gyr.…”