Galaxy groups in the 2dF galaxy redshift survey: A Compactness Analysis   of Groups
  A comprehensive study on compactness has been carried out on the 2dF Galaxy Group Catalogue constructed by Merch\'an & Zandivarez. The compactness indexes defined in this work take into account different geometrical constraints in order to explore a wide range of possibilities. Our results show that there is no clear distinction between groups with high and low level of compactness when considering particular properties as the radial velocity dispersion, the relative fraction of galaxies per spectral type and luminosity functions of their galaxy members.   Studying the trend of the fraction of galaxies per spectral type as a function of the dimensionless crossing time some signs of dynamical evolution are observed. From the comparison with previous works on compactness we realize that special care should be taken into account for some compactness criteria definitions in order to avoid possible biases in the identification. 
A. Zandivarez, M.J.L. Dominguez, C.J. Ragone, H. Muriel, H.J. Martinez   (Grupo de Investigaciones en Astronomia Teorica y E...