Studies of dwarf spheroidal ( dSph ) galaxies with statistically significant sample sizes are still rare beyond the Local Group , since these low surface brightness objects can only be identified with deep imaging data .
In galaxy clusters , where they constitute the dominant population in terms of number , they represent the faint end slope of the galaxy luminosity function and provide important insight on the interplay between galaxy mass and environment .
In this study we investigate the optical photometric properties of early-type galaxies ( dwarf ellipticals ( dEs ) and dSphs ) in the Virgo cluster core region , by analysing their location on the colour magnitude relation ( CMR ) and the structural scaling relations down to faint magnitudes , and by constructing the luminosity function to compare it with theoretical expectations .
Our work is based on deep CFHT V- and I-band data covering several square degrees of the Virgo cluster core that were obtained in 1999 using the CFH12K instrument . We visually select potential cluster members based on morphology and angular size , excluding spiral galaxies .
A photometric analysis has been carried out for 295 galaxies , using surface brightness profile shape and colour as further criteria to identify probable background contaminants .
216 galaxies are considered to be certain or probable Virgo cluster members .
Our study reveals 77 galaxies not catalogued in the VCC ( with 13 of them already found in previous studies ) that are very likely Virgo cluster members because they follow the Virgo CMR and exhibit low Sérsic indices .
Those galaxies reach M _ { V } = -8.7 mag . The CMR shows a clear change in slope from dEs to dSphs , while the scatter of the CMR in the dSph regime does not increase significantly .
Our sample might , however , be somewhat biased towards redder colours .
The scaling relations given by the dEs appear to be continued by the dSphs indicating a similar origin .
The observed change in the CMR slope may mark the point at which gas loss prevented significant metal enrichment .
The almost constant scatter around the CMR possibly indicates a short formation period , resulting in similar stellar populations . The luminosity function shows a Schechter function ’ s faint end slope of \alpha = -1.50 \pm 0.17 , implying a lack of galaxies related to the expected number of low-mass dark matter haloes from theoretical models .
Our findings could be explained by suppressed star formation in low-mass dark matter halos or by tidal disruption of dwarfs in the dense core region of the cluster .