SIMBAD references

We present B- and R-band luminosity functions (LFs) for a sample of 25 nearby groups of galaxies. We find that the LFs of the groups with low X-ray luminosity (L_X< 10^41.7 erg/s) are significantly different from those of the X-ray brighter groups, showing a prominent dip around M_B=-18. While both categories show lack of late-type galaxies in their central regions, X-ray dim groups also show a more marked concentration of optical luminosity towards the centre. A toy simulation shows that in the low velocity dispersion environment, as in the X-ray dim group, dynamical friction would facilitate more rapid merging, thus depleting intermediate-luminosity galaxies to form a few giant central galaxies, resulting in the prominent dip seen in our LFs. We suggest that X-ray dim (or low velocity dispersion) groups are the present sites of rapid dynamical evolution rather than their X-ray bright counterparts, and may be the modern precursors of fossil groups. We predict that these groups of low velocity dispersion would harbour younger stellar populations than groups or clusters with higher velocity dispersion.