The M81 group

This group of galaxies is one of the nearest to our Local Group, being only some 12 million light-years distant, according to 1993 measurements of the Hubble Space Telescope under the direction of Wendy Freedman of the Carnegie Institution of Washington by measuring the periods of 32 Cepheid variables in M81 with the pre-repair Wide Field/Planetary Camera (WFPC I), and corrected for Hipparcos results.

The group contains two Messier objects: M81 and M82 which form an appealing interacting pair visible in the same field of view at moderate magnification. Other members include NGC 3077 and NGC 2976. As radio images such as ours have revealed, NGC 3077 is in considerable interaction with M81 and M82, plus perhaps some very small galaxies, as there is a common gaseous envelope. In our image, M81 is the spiral right and below center, M82 the not in the upper right center, and NGC 3077 is lower left; at least two dwarf galaxies appear to show up closely left of M81.

Radio image of M81/82/NGC 3077 (NRAO) compared to the visible light POSS image

Further, more outlying members of the group include the irregular galaxy NGC 2366 and the fine Sc spiral NGC 2403, both lying in the constellation Camelopardalis, IC 2574, "Coddington's Nebula", which is quite irregular but shows some "faint indications of spiral structure" (Burnham), and the irregulars Holmberg I (UGC 5139), Holmberg II (UGC 4305) and Holmberg IX (UGC 5423).

A full list of the group, as given by R. Brent Tully in his Nearby Galaxies Catalog, is given in the table below (added to this is Holmberg IX, and some recently discovered dwarf galaxies):

Galaxy    Name II   RA (2000.0) Dec type m_v       dim      RV  RV_hel

NGC 2366  H III.748 07:28.9 +69:13  Ir+  10.91   7.3x3.5   252   100
NGC 2403  H V.44    07:36.9 +65:36  Sc    8.39  17.8x11.0  259   131
M81 dw A  PGC 23521 08:23.9 +71:02  I?   16.5:   1.3x0.7   262   113
Holm. II  UGC 4305  08:18.9 +70:43  Ir+  10.75   7.6x6.2   305   157
DDO 53    UGC 4459  08:34.1 +66:10  Ir+  14.38p  1.7x1.3   144    19
Holm. I   UGC 5139  09:40.5 +71:11  Ir+  12.94   3.5x3.0   287   143
NGC 2976  H I.285   09:47.3 +67:55  Scp  10.15   4.9x2.5   175     3
M81       NGC 3031  09:55.6 +69:04  Sb    6.93  25.7x14.1   95  - 34
M82       NGC 3034  09:55.8 +69:41  Pec   8.41  11.2x4.6   388   203
Holm. IX  UGC 5336  09:57.5 +69:03  Im   15.5    1.2x0.5   175    46
NGC 3077  H I.286   10:03.3 +68:44  E2p   9.85   4.6x3.6   148    50
M81 dw B  UGC 5423  10:05.5 +70:22  Ir+  14.94p  1.2x0.9   479   350
IC 2574   Codding.  10:28.4 +68:25  S+   10.56  12.3x5.9   185    57

A0952+69  Arp Loop  09:57.5 +69:17  Im   16.08
K52
K73
BK3N      PGC 28529 09:53.8 +68:59  Im   17.1      0.5          - 40   
Garland   PGC 29167 10:03.9 +68:40  Im                            50
DDO 82    UGC 5692  10:30.6 +70:37  Sm:  13.47   3.2x1.8   347   180

The following galaxies, around NGC 4236, lie nearby and at roughly the same distance, so may either be outlying members of the M81 group, or form a small nearby group of their own:

UGC 6456  VII Zw403 11:28.0 +79:00  Pec  13.7    1.5x0.9
NGC 4236  H V.51    12:16.7 +69:28  SB+   9.66  18.6x6.9   160
DDO 165   UGC 8201  13:06.3 +67:42  Ir+  12.68p  3.2x2.2   201

Key: Galaxy, Name II: Some catalog number or name for primary and secondary identification; RA/Dec: Position 2000.0; type: Hubble type; m_v: apparent visual magnitude; dim: dimension in arc minutes; RV: radial velocity (all data from the Sky Catalog 2000.0 when available).

References

I.D. Karachentsev, A.E. Dolphin, D. Geisler, E.K. Grebel, P. Guhathakurta, P.W. Hodge, V.E. Karachentseva, A. Sarajedini, P. Seitzer, and M.E. Sharina, 2002. The M81 group of galaxies: New distances, kinematics and structure. Astronomy and Astrophysics, Vol. 383, p. 125-136 (January 2002)

Hartmut Frommert
Christine Kronberg
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Last Modification: December 10, 2003