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1. Introduction 2. Data THE ASTRONOMICAL JOURNAL, 118:337È345, 1999 July ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE SOLAR MOTION RELATIVE TO THE LOCAL GROUP STE PHANE COURTEAU AND SIDNEY VAN DEN BERGH Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, BC V8X 4M6, Canada Received 1999 March 4; accepted 1999 March 17 ABSTRACT New data on the membership of the Local Group (LG) are used, in conjunction with new and improved radial velocity data, to reÐne the derivation of the motion of the Sun relative to the LG. The Sun is found to be moving with a velocity of V \ 306 ^ 18 km s~1 toward an apex at l \ 99¡ ^ 5¡ and b \[4¡ ^ 4¡. This Ðnding agrees very well with previous analyses, but we discuss the possibility of a bias if the phase-space distribution of LG galaxies is bimodal. The LG radial velocity dispersion is 61 ^ 8kms~1. We use various mass estimators to compute the mass of the LG and the Andromeda subgroup. We ÐndM \ (2.3 ^ 0.6) ] 1012 M , from whichM/L \ 44 ^ 12 (in solar units). For an LG^ _ V \ ^ assumed LG age of 14 2 Gyr, the radius of an idealized LG zero-velocity surface is r0 1.18 0.15 Mpc. The LG is found to have 35 likely members. Only three of these have (uncertain) distances Z1.0 Mpc from the LG barycenter. Barring new discoveries of low surface brightness dwarfs, this suggests that the LG is more compact and more isolated from its surroundings than previously believed. Key words: galaxies: clusters: general È galaxies: kinematics and dynamics È galaxies: spiral È Local Group INTRODUCTION 1. VSun?LG, to assess Group membership, and to compute a new value for the mass of the LG. Three criteria are usually The motion of the Sun relative to the other members of invoked to assess the probability that a galaxy is associated the Local Group (LG) has been studied for many decades, with the LG: (1) The distance to that galaxy from the LG with investigations by Humason, Mayall, & Sandage (1956), barycenter should be less than (or comparable to) the radius Yahil, Tammann, & Sandage (1977, hereafter YTS77), at the zero-velocity surface (Lynden-Bell 1981; Sandage Sandage (1986), Karachentsev & Makarov (1996), and 1986), (2) the galaxy should lie close to the ridgeline solution Rauzy & Gurzadyan (1998 and references therein; hereafter between radial velocity and the cosine of the angle from the RG98). YTS77 and many others have stressed the impor- solar apex relative to well-established LG members, and (3) tance of understanding the solar motion relative to the LG it should not appear to be associated with any more distant in the context of the large-scale motions of galaxies. The group of galaxies centered well beyond the limits of the LG. reliability of measurements of peculiar motions in the uni- We examine these criteria below. verse, or residual motion from the uniform Hubble expan- This paper is organized as follows: First, in ° 3, we sion, depends in part on accurate knowledge of the motion compute a new solution for the motion of the Sun relative of the solar system relative to any standard inertial frame. to LG members. Then in ° 4 we estimate the radius of the This inertial rest frame is usually taken as the centroid of zero-velocity surface, and we assess LG membership in ° 5, the Local Group of galaxies or the reference frame in which based on the three criteria listed above. We conclude in ° 6 the dipole of the cosmic microwave background (CMB) with a brief discussion and summary and a digression on vanishes (Kogut et al. 1993). The motion of the Sun relative the detectability of small groups like the LG using X-ray to the CMB can be decomposed into a sum of local and telescopes. external components:1 Recent studies of the membership in the LG also include \ ] ] ] VSun?CMB VSun?LSR VLSR?GSR VGSR?LG VLG?CMB . van den Bergh (1994a, 1994b), Grebel (1997), and Mateo (1998). The reader is referred to van den Bergh (2000, here- (1) after vdB2000) for a comprehensive review of the nature of the LG and the question of membership in it. VSun?LSR is the motion of the Sun relative to the nearby stars that deÐne a local standard of rest (LSR), and the 2. DATA motionVLSR?GSR is the circular rotation of the LSR about the Galactic center, which is directed toward l \ 90¡ and A listing of information on the 32 probable (and three \ possible) members of the LG that were isolated using the b 0¡. Externally,VGSR?LG is the motion of the Galactic center (or Galactic standard of rest) relative to the LG cen- criteria discussed above is given in Table 1. Columns (1)È(3) troid, which is caused by nonlinear dynamics within the LG give the names and David Dunlap Observatory morpho- logical types (van den Bergh 1966, 1994b) for each LG (mostly infall of the Galaxy toward M31). Finally, VLG?CMB is the peculiar velocity of the LG in the CMB rest frame, member. Equatorial (J2000.0) and Galactic coordinates are induced by gravitational perturbations in the universe. listed in columns (4)È(7). Various photometric parameters Recent discoveries of new candidate members of the LG, taken from vdB2000 (visual color excess, absolute visual and the deletion of former candidates, have allowed us to magnitude, and true distance modulus) are listed in revise the solution for the solar motion relative to the LG, columns (8)È(10). Column (11) gives the heliocentric radial velocity of each galaxy in kilometers per second, and ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ column (12) lists the cosine of the angle between each galaxy 1 Our notation is analogous to that of RG98. and the solar motion apex in the rest frame of the LG. 337 TABLE 1 OBSERVATIONAL DATA ON LOCAL GROUP MEMBERS lb Vr ddLG [ ~1 Name Alias DDO Type R.A. Decl. (deg) (deg) E(B V ) MV DM (km s )cosh (Mpc) (Mpc) Ref. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) WLM ............ DDO221 IrIVÈV 00 01 57.8 [15 27 51 75.852 [73.626 0.02 [14.4 24.85 [120 0.317 0.93 0.79 1 IC10............. UGC192 IrIV: 0020 24.5 59 17 30 118.973 [3.341 0.85: [16.3 24.10 [344 0.940 0.66 0.27 2 NGC147 ........ UGC326 Sph 0033 11.6 48 30 28 119.816 [14.253 0.17 [15.1 24.10 [193 0.919 0.66 0.22 3 AndIII........... A0032]36 dSph 00 35 17.0 36 30 30 119.313 [26.250 0.05 [10.2 24.40 . 0.866 0.76 0.31 NGC185 ........ UGC396 Sph 0038 58.0 48 20 18 120.792 [14.482 0.19 [15.6 24.10 [202 0.912 0.66 0.22 3 NGC205 ........ M110 Sph 00 40 22.5 41 41 11 120.718 [21.138 0.04 [16.4 24.40 [244 0.887 0.76 0.31 4 M32 .............. NGC221 E2 0042 41.9 40 51 55 121.151 [21.976 0.06 [16.5 24.40 [205 0.880 0.76 0.31 2 M31 .............. NGC224 SbIÈII 00 42 44.2 41 16 09 121.174 [21.573 0.06 [21.2 24.40 [301 0.882 0.76 0.30 5 AndI............. A0043]37 dSph 00 45 43.0 38 00 24 121.690 [24.851 0.04 [11.8 24.55 . 0.861 0.81 0.36 SMC ............. Ir IV/IVÈV 00 52 36.0 [72 48 00 302.812 [44.328 0.06 [17.1 18.85 148 [0.611 0.06 0.48 6 Sculptor .......... dSph 01 00 04.3 [33 42 51 287.686 [83.157 0.00 [9.8 19.70 110 [0.057 0.09 0.44 7 Pisces ............. LGS3 dIr/dSph 01 03 56.5 21 53 41 126.770 [40.884 0.03 [10.4 24.55 [286 0.707 0.81 0.42 8 IC 1613 .......... IrV 0104 47.3 02 08 14 129.730 [60.558 0.03 [15.3 24.30 [232 0.474 0.72 0.47 9 AndV............ dSph 01 10 17.1 47 37 41 126.220 [15.123 0.16 [10.2 24.55 . 0.873 0.81 0.37 AndII............ dSph 01 16 29.8 33 25 42 128.918 [29.152 0.08 [11.8 24.22 . 0.785 0.70 0.26 M33 .............. NGC598 ScIIÈIII 01 33 50.9 30 29 37 133.648 [31.494 0.07 [18.9 24.50 [181 0.732 0.79 0.37 10 338 Phoenix .......... dIr/dSph 01 51 03.3 [44 27 11 272.193 [68.949 0.02 [9.8 23.00 . [0.299 0.40 0.59 Fornax ........... dSph 02 39 53.1 [34 30 16 237.245 [65.663 0.03 [13.1 20.70 53 [0.252 0.14 0.45 11 LMC ............. IrIIIÈIV 05 19 36.0 [69 27 06 280.187 [33.287 0.13 [18.5 18.50 275 [0.801 0.05 0.48 12 Carina ............ dSph 06 41 36.7 [50 57 58 260.112 [22.223 0.05 [9.4 20.00 223 [0.851 0.10 0.51 13 LeoA ............ DDO69 IrV 0959 23.0 30 44 44 196.903 52.410 0.02 [11.5 24.20 24 [0.132 0.69 0.88 14 LeoI ............
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