Speaker
Description
Galaxies within a group or a cluster have their radial velocities and positions known to a high degree of accuracy. This information is sufficient to estimate the group mass from the observed deviations from the external Hubble field of velocities by making critical assumptions on eccentricities and equipartition. For a full understanding of the evolution of a galaxy group, we require accurate knowledge of the distances and the three-dimensional velocity vectors. Knowing the 3-D velocity structure within a group requires knowledge of a combination of precise radial and proper motions. We present VLBI derived proper motion measurements of one of our nearest group is the M81 group at a distance of 3.63Mpc. It is a fascinating interacting galaxy system containing galaxies M81, M82 and NGC3077. The derived M81 galaxy proper motion relative to the Milky Way is -11±5.7 μas/yr (-193±100 km/s) towards the East and -15.6±2.1μas/yr (-273±36 km/s) towards the North. We also derive its motion towards the Sun to be 113±6 km/s, such that, the total space velocity of M81 becomes 353±106km/s relative to Milky Way. On the other hand, the proper motion of M82 galaxy relative to M81 is 13.7±4.3 μas/yr (233±73 km/s) towards the East and 8.6±4.1 μas/yr (146±70 km/s) towards the North. With a derived motion of 108±6km/s towards the Sun, we obtain its total space velocity of M82 to be 295±101 km/s relative to M81 galaxy. Since M82 is embedded in the dark matter halo of M81, it can be considered bound, and we therefore derive a lower limit of the mass of M81 to be 3.34x1011M0.
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