19 - Properties of the Local Group

Summary

Introduction

A summary of derived properties for the probable members of the Local Group is given in Table 19.1. In the following sections these data will be used to derive various global properties of Local Group galaxies.

The motions of Local Group galaxies

Table 19.1 contains information on 35 probable Local Group members. Giving equal weight to each object, one may use these data to derive both the size of the solar motion relative to the centroid of the Local Group and the position of the apex toward which the Sun is moving. Originally Humason, Mayall & Sandage (1956) adopted a solar motion of V⊙ = 300 km s^-1. More recently Yahil, Tammann & Sandage (1977) derived V⊙ = 308 km s^-1 toward l = 105°, b = –7°. Their study included NGC 404, NGC 6949, IC 342, IC 5152, Maffei 1, Maffei 2, and DDO 187 (Aparicio, García-Pelayo & Moles 1988), all of which are presently considered to be nonmembers of the Local Group, and NGC 3109, Sextans A, and Sextans B, which are probable nonmembers (see Chapter 16). An unweighted least-squares solution for the 27 Local Group galaxies with known radial velocities that are listed in Table 2.1 yields a solar motion of 306±18 km s^-1 toward an apex at l = 99° ± 5° and b = –4° ± 4°, in which the quoted mean errors were determined using a “bootstrap” technique (Courteau & van den Bergh 1999).