The faint end of the luminosity function in clusters of galaxies




De Propris, Roberto

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We have determined luminosity functions, surface density distributions and color distributions for galaxies in eight clusters of galaxies at moderate redshift (z ~ 0.02) observed at the CFHT. 1) In the inner 2' of four cD clusters (Abell 2052, 2107, 2199 and 2666) we find very steep LFs, with α ~ -2.2 ± 0.2, where [special characters omitted](L) α Lα, where L is luminosity 2) For Abell 262, we find no significant contribution from cluster dwarfs in the field around NGC708 (the central cluster elliptical), although there is a small excess over background counts in the inner 3' of this field. For galaxies near UGC1308, we derive a LF with α ~ -1.4 ± 0.1. Galaxies in this field are concentrated towards UGC1308, so that the LF may be due to a population of satellites. 3) For the field centred on NGC1275 in Abell 426 we find a very steep LF, with α ~ -1.9 ± 0.1. We find that galaxies are not centrally concentrated towards NGC1275, except for a central ‘spike’ in their surface density distribution. We see a weak sequence of galaxies in the V vs V - I plot. There is a small color gradient in the sense of bluer galaxies near NGC1275. 4) For the field centred on NGC1265, we find again a very steep LF. Galaxies in this field tend to avoid the neighbourhood of NGC1265 and there is a mild red color gradient towards NGC1265. The LF within 100 kpc of NGC1265 is somewhat flatter than outside of this region, although the significance of this result is marginal. 5) For the field centred on UGC3274 in A539 we find a LF with α ~ -1.4 ± 0.1. We see that the galaxy distribution as a function of distance from UGC3274 is flat, except for a central spike. The LF appears to steepen towards UGC3274. There is a tight sequence of cluster galaxies in the V vs. V - I plot. There also appears to be a blueing trend towards UGC3274. 6) For Hercules (A2151) we derive a LF with α ~ -1.5 and M* = 12.2, where M* is a characteristic luminosity here converted to magnitudes. We interpret these results as supporting the conclusions by Biviano et al. (1995b) in Coma, namely that a population of dwarf galaxies with a steep LF constitutes the main body of the cluster, into which brighter giants fall later, thereby flattening the LF. We also find evidence that steep LFs are correlated with high gas densities. The blueing trend towards NGC1275 and UGC3274 is also consistent with recent star formation in dwarfs due to accretion of intracluster gas.



Galaxies, Clusters