Evolutionary properties of the low-luminosity galaxy population in the NGC5044 Group

Abstract

With this third paper of a series we present Johnson-Gunn B,g,V,r,i,z multicolour photometry for 79 objects, including a significant fraction of the faintest galaxies around NGC5044, assessing group membership on the basis of apparent morphology and low-resolution optical spectroscopy to estimate redshift for 21 objects. Together, dE's and Im's provide the bulk of the galaxy luminosity function, around M(g)\sim-18.0, while the S0 and dSph components dominate, respectively, the bright and faint-end tails of the distribution. This special mix places the NGC 5044 group just "midway" between the high-density cosmic aggregation scale typical of galaxy clusters, and the low-density environment of looser galaxy clumps like our Local Group. The bright mass of the 136 member galaxies with available photometry and morphological classification, amounts to a total of 2.3x10^{12}M_sun while current SFR within the group turns to be about or higher than 23M_sun/yr. In this regard, a drift toward bluer integrated colours is found to be an issue for dE's pointing to a moderate but pervasive star-formation activity even among nominally "quiescent" stellar systems. Through Lick narrow-band index analysis, dwarf ellipticals are found to share a sub-solar metallicity (-1.0 < [Fe/H] <-0.5) with a clear decoupling between Iron and alpha elements, as already established also for high-mass systems. Both dE's and dS0's are consistent with an old age, about one Hubble time, although a possible bias, toward higher values of age, may be induced by the gas emission affecting the Hbeta strength.