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2. The M 81 group

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 tex2html_wrap_inline1550 = 09tex2html_wrap_inline1552176, tex2html_wrap_inline1554 = 7557tex2html_wrap1606) due to its high redshift of 659 tex2html_wrap_inline1496, which was unknown in 1993. We also rectified the identification of Ho IX, erroneously named Ho IV in Binggeli (1993).

 

Ident. R.A. Dec. Type Diam tex2html_wrap_inline1560 tex2html_wrap_inline1562 HI Nançay
(h m s) (tex2html_wrap1608 tex2html_wrap1606 tex2html_wrap1612) (tex2html_wrap1606) (mag) (km/s)
1. HoII 08 13 53.5 70 52 13 Im 7.9 10.2 157tex2html_wrap_inline15721 det
2. Kar 52 08 18 43.0 71 11 25 Im 1.3 14.2 114tex2html_wrap_inline15726 det
3. DDO 53 08 29 33.3 66 21 08 Im 1.5 13.6 19tex2html_wrap_inline157210 det
4. UGC 4483 08 32 07.0 69 57 16 Im 1.1 13.9 156tex2html_wrap_inline15725 det
6. UGC 4998 09 20 52.9 68 35 53 dS0: 1.6 13.9 no det *
7. HoI 09 36 00.9 71 24 55 Im 3.6 12.2 136tex2html_wrap_inline15723 det
P 8. Kar 1N 09 41 00.0 69 37 00 Im 1.2? 15.5 *
9. N 2976 09 43 11.5 68 08 45 Sd 5.9 10.9 3tex2html_wrap_inline15725 det
10. Kar 2N 09 43 42.0 69 30 00 dE? 15.9 no det *
11. Kar 59 09 46 40.0 72 17 41 Im 0.3 17.1 no det *
12. Kar 3N 09 49 42.0 69 12 00 Im 0.5 17.1 -40tex2html_wrap_inline157260 no det *
13. M 81 09 51 27.3 69 18 08 Sb 26.9 7.9 -34tex2html_wrap_inline15724 det
14. M 82 09 51 43.6 69 55 00 Amorph 11.2 9.3 203tex2html_wrap_inline15724 det
15. A952+69 09 53 27.0 69 31 18 Im 14.3 *
16. Kar 61 09 53 01.0 68 49 48 dE,N 1.2 13.8 *
17. Ho IX 09 53 28.0 69 16 53 Im 2.5 13.5 46tex2html_wrap_inline15726 det
18. NGC 3077 09 59 21.9 68 58 33 Amorph 5.4 10.6 14tex2html_wrap_inline15724 det
19. Garland 09 59 54.0 68 55 30 Im 50 *
20. Kar 5N 10 00 42.0 68 30 00 dE: 18.1 no det *
21. DDO 71 10 01 18.0 66 47 53 dE,N 0.9 14.6 no det *
P22. UGC 5423 10 01 25.3 70 36 27 BCD 0.9 13.8 349tex2html_wrap_inline15725 det
23. Kar 64 10 03 07.2 68 04 20 dE,N 1.8 14.9 no det *
24. DDO 78 10 22 48.0 67 54 40 dE 2.0 14.3 no det *
P25. UGC 5658 10 23 52.6 71 29 34 Im 1.0 15.0 no det *
26. IC 2574 10 24 41.3 68 40 18 Sm 13.2 11.0 47tex2html_wrap_inline15723 det
27. DDO 82 10 26 48.0 70 52 33 Sm/BCD: 3.2 11.8 40 no det *
28. Kar 6N 10 31 00.0 66 16 00 dE 16.0 no det *
P29. Anon 1 10 45 30.0 65 02 00 ? 17.4
P30. UGCA 220 10 46 04.0 64 59 00 Im: 1.7 16.9 no det *
P31. DDO 87 10 46 17.0 65 47 40 Im: 2.4 14.9 338tex2html_wrap_inline15725 det
P32. Anon 2 10 46 48.0 65 00 00 ? 16.3
P33. Kar 7N 10 47 06.0 65 22 00 ? 16.2 no det *
P34. Anon 3 10 47 12.0 65 00 00 ? 16.5
P35. Anon 4 10 47 18.0 65 00 00 ? 16.5
36. Kar 73 10 49 30.0 69 48 55 Im 0.6 14.9 115 no det *
P37. Anon 5 10 50 30.0 65 31 00 ? 15.5 *
P38. Anon 6 10 50 54.0 65 17 00 ? 16.1 *
P39. Kar 8N 10 51 06.0 65 28 00 ? 15.4 *
P40. Anon 7 10 51 18.0 65 33 00 ? 15.8 *
41. Kar 74 10 59 05.2 70 32 01 dE/Im: 1.0 15.2 no det *
Table 1: Members and possible members (P) of the M 81 group of galaxies 

Note: Column 1: P denotes a possible group member; Column 10: indicates objects previously observed in HI and status, i.e. detection or no detection; Column 11: * marks dwarfs observed in present survey (see Table 2 (click here)).

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 (tex2html_wrap_inline1604). 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)).

  figure235
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).


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