The M 81 group of galaxies is the most nearby rich concentration of dwarf
galaxies beyond the Local Group.
For our study of possible dwarf members of the M 81 group of galaxies, we
used the optically selected catalog of 41 members and possible members
(dwarfs or otherwise) compiled by Binggeli (1993).
These data represent a compilation of various publications, databases and
private communications. Binggeli's catalog is strongly based on the
2 m Tautenberg Schmidt survey by Börngen & Karachentseva
(1982), the photometric work by Börngen et al.
(1982), and the photographic atlas of Karachentseva et al.
(1985a) obtained with the 6 m SAO telescope. Note that we rejected
object No. 5 (Kar 54 = UGC 5954, at = 09
176,
= 7557
) due to its high redshift of 659
, which was unknown
in 1993. We also rectified the identification of Ho IX, erroneously
named Ho IV in Binggeli (1993).
The basic optical data of these objects are listed in Table 1 (click here).
Binggeli's numbering was retained throughout this paper.
The coordinates were taken from NED and are for the epoch 1950.0.
All velocities are heliocentric and calculated according to the
conventional optical definition ().
Heliocentric velocities are from de Vaucouleurs et al. (1990,
RC3), except for the following objects: No. 1 HolmbergII (Strauss
et al. 1992), No. 12 Kar 3N (Tikhonov & Karachentsev
1993), No. 19 Garland (Karachentseva et al. 1985b), No.
22 UGC 5423 (Schneider et al. 1992) and No. 36 Kar 73
(Tikhonov & Karachentsev 1993); note that the heliocentric
velocity of 180 km/s listed for No. 27, DDO 82, in the RC3 is incorrect (see
Sect. 4.1 (click here)).
Figure 1: Distribution on the plane of the sky of all 40 members and
possible members of the M 81 group of galaxies listed in Table 1 (click here),
within the boundaries of the Schmidt survey of Börngen & Karachentseva
(1982). The morphological type of each galaxy has been indicated.
Uncertain group members are put in parentheses, the latter also include the
clustering of low-surface brightness objects of unknown nature in the SE
corner. The names of
galaxies previously detected in HI have been underlined. The linear
scale bar of 0.5 Mpc is based on the assumed distance of 4 Mpc.
Also shown is the HPBW of the Nançay radio telescope
The distribution on the sky of all 40 members and possible members of the group listed in Table 1 (click here) is shown in Fig. 1 (click here), where the morphological type of each object has been indicated. We have adopted a distance of 4 Mpc for the M 81 group. This is significantly smaller than the value of 5.5 Mpc given in the RSA (Sandage & Tammann 1987), but is in good accord with more recent work (Karachentsev 1996, cf. his Table 1 (click here)).
The M 81 group, like others, shows a clear morphological segregation: most early-type dwarfs are found in a dense core around M 81, while the dwarf irregulars are spread out over the entire survey area.
The M 81 group is more compact than the Local Group, and the core galaxies are known to be strongly interacting, as shown by the large, dynamically complex HI cloud embedding M 81, M 82, NGC 3077, and NGC 2976 (van der Hulst 1977; Appleton et al. 1981, recent VLA results in Yun et al. 1994, and also the dynamical analysis by Karachentsev 1996). Because of this, and the lack of radial velocities, it is impossible to determine which dwarf belongs to which specific large galaxy.
The M 81 group may well be more dynamically evolved than the Local Group, having already released its formerly bound dwarfs through dynamical friction. On the other hand, the Garland system is a knotty dwarf irregular that apparently formed only recently from a tidal tail, a system that is likely to have been born free. A peculiar case is that of the clustering of low-surface brightness objects (Kar 7N, Kar 8N, and Anon 1 to 7 in Table 1 (click here)) noted by Börngen et al. (1984). It is not clear whether these are of an extragalactic nature; they may be Galactic cirrus clouds; the M 81 group is an area of the sky rife with potential for confusion between Galactic and extragalactic objects (cirrus, HVCs, tidal tails).