The M81 group

[M81 group radio image] 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


    Hartmut Frommert
    Christine Kronberg
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    Last Modification: December 10, 2003