Definition

New tree algorithms have led to new lists of groups, based on much more complete catalogues of galaxies with redshifts. One problem with any such algorithm is that we observe only one velocity component out of three, and at the same time have to suspect that the phase space distribution of galaxies in groups is not typically isotropic and is also not really well approximated as being stationary with time: this latter argument is especially important. since the application of the virial theorem to derive masses of groups or clusters critically depends on the assumption that the entire system is stationary - the hypothesis that the evolutionary time scale may be close to the Hubble time for groups or clusters would invalidate many arguments on the mass to light ratio, but would not necessarily lower the implied mass.

1. Dejonghe, H., Merritt, D., Astrophys. J. 391 (1992) 531.
2. Geller, M.J., Huchra, J.P., Astrophys. J. Suppl. 52 (1983) 61.
3. Gourgoulhun, E., Chamaraux, P., Fouque, P., Astron. Astrophys. 255 (1992) 69.
4. Maia, M.A.G., Costa, L.N. da, Latham, D.W., Astrophys. J. Suppl. 69 (1989) 809.
5. Morgan, C.G., Hartwick, F.D.A., Astrophys. J. 328 (1988) 381.
6. Pisani, A., Giuricin, G., Mardirossian, F., Mezzetti, M., Astrophys. J. 389 (1992) 68.
7. Ramella, M., Geller, M.J., Huchra, J.P., Astrophys. J. 344 (1989) 57.
8. Tully, R.B., Astrophys. J. 321 (1987) 280.

One implication is that it becomes difficult to argue what are field galaxies.

1. Binggeli, B., Tarenghi, M., Sandage, A., Astron. Astrophys. 228 (1990) 42.

That coupled with small number statistics implies that any such results can only be trusted when the observational methods have been tested against N-body code calculations describing through some model the true three-dimensional distribution and evolution of galaxies in a group.

Structure

Small groups of galaxies, apparently observable through absorption lines in quasars in their early stages, show evidence of strong interaction between their respective partners leading to feeding of activity - both starburst and active galactic nuclei (AGN) - and merging, so that the dominant galaxy can be optically inconspicuous, but gravitationally totally dominant as seen in extended X-ray emission.

1. Bhavsar, S.P., Barrow, J.D., Mon. Not. R. Astron. Soc. 213 (1985) 857.
2. Driel, W. van, Augarde, R., Bottinelli, L., Gouguenheim, L., Hamabe, M., Maehara, H., Baan, W.A., Goudfrooij, P., Groenewegen, M.A.T., Astron. Astrophys. 259 (1992) 71.
3. Falomo, R., Tanzi, E.G., Treves, A., Astron. Astrophys. 249 (1991) 341.
4. Fricke, K.J., Kollatschny, W., Astron. Astrophys. 213 (1989) 521: Astron. Astrophys. Suppl. 77 (1989) 75.
5. Fried, J.W., Astron. Astrophys. 189 (1988) 42.
6. Giuricin, G., Mardirossian, F., Mezzetti, M., Pisani, A., Ramella, M., Astron. Astrophys. 192 (1988) 95.
7. Gottesman, S.T., Hunter, J.H. Jr., Shostak, G.S., Mon. Not. R. Astron. Soc. 202 (1983) 21P.
8. Haddad, B., Vanderriest, C., Astron. Astrophys. 245 (1991) 423.
9. Hickson, P., Kindl, E., Huchra, J.P., Astrophys. J. 331 (1988) 64.
10. Kibblewhite, E.J., Cawson, M.G.M., Disney, M.J., Phillips, S., Mon. Not. R. Astron. Soc. 213 (1985) 111.
11. Kollatschny, W., Fricke, K.J., Astron. Astrophys. 219 (1989) 34.
12. Oliveira, C.M.de, Hickson, P., Astrophys. J. 380 (1991) 30.
13. Ramella, M., Giuricin, G., Mardirossian, F., Mezzetti, M., Astron. Astrophys. 188 (1987) 1.
14. Schmutzler, T., Biermann, P., Astron. Astrophys. 149 (1985) 383.

The evolution also can lead to merging and growth of a dominant galaxy with a large halo extending throughout the group. The N5846 group is one well studied example for this.

1. Barnes, J., Mon. Not. R. Astron. Soc. 208 (1984) 873.
2. Barnes, J., Mon. Not. R. Astron. Soc. 215 (1985) 517.
3. Bernardeau, F., Schaeffer, R., Astron. Astrophys. 250 (1991) 23.
4. Biermann, P.L., Kronberg, P.P., Schmutzler, T., Astron. Astrophys. 208 (1989) 22.
5. Bower, R.G., Mon. Not. R. Astron. Soc. 248 (1991) 332.
6. Chernin, A.D., Mikkola, S., Mon. Not. R. Astron. Soc. 253 (1991) 153.
7. Haynes, M.P., Giovanelli, R., Astron. J. 102 (1991) 841.
8. Nulsen, P.E.J., Stewart, G.C., Fabian, A.C., Mon. Not. R. Astron. Soc. 208 (1984) 185.
9. Zepf, S.E., Whitmore, B.C., Astrophys. J. 383 (1991) 542.
10. Zepf, S.E., Whitmore, B.C., Levison, H.F., Astrophys. J. 383 (1991) 524.

This interaction between group partners and the resulting feeding of activity may also explain the observation that radio sources and AGN in their clustering properties are between groups and clusters.

1. Balick, B., Heckman, T.M., Annu. Rev. Astron. Astrophys. 20 (1982) 431.
2. Bahcall, N.A., Chokshi, A., Astrophys. J. 385 (1992) L33.
3. Clowes, R.G., Campusano, L.E., Mon. Not. R. Astron. Soc. 249 (1991) 218.
4. Malumyan, V.G., Astrophysics 26 (1987) 190 (orig. Astrofizika 26 (1987) 311).
5. Malumyan, V.G., Astrophysics 30 (1990) 370 (orig. Astrofizika 30 (1989) 595).

The Local Group

Still new members of the Local Group are occasionally discovered, while important new data about known Local Group members are assembled.

1. Costa, G.S. da, Hatzidimitriou, D., Irwin, M.J., McMahon, R.G., Mon. Not. R. Astron. Soc. 249 (1991) 473.
2. Lavery, R.J., Mighell, K.J., Astron. J. 103 (1992) 81.
3. Schmidt, K.-H.. Boller, T., Astron. Nachr. 313 (1992) 189.
4. Wang, Q., Astrophys. J. 377 (1991) L85.

Also, the two large Local Group galaxies, M31 and our Galaxy, both show evidence of a central black hole; also in the tidally truncated galaxy M32 there is such evidence.

1. Kormendy, J., in Stenger, V.J., Learned, J.G., Pakvasa, S., Tata, X. (eds.): High Energy Neutrino Astrophysics, Singapore: World Scientific (1992)., p. 196.

The Local Group also provides evidence of the presence of dark matter in dwarf galaxies, where single stars can be studied.

1. Aaronson, M., Olszewski, E., in Kormendy, J., Knapp, G.R. (eds.): Dark matter in the universe, IAU Symp. No. 117, Dordrecht: Reidel Publ. Comp. (1987)., p. 153.
2. Seitzer, P., Frogel, J.A., Astron. J. 90 (1985) 1796.