Abstract
We collected multiband imaging and spectroscopy for two fossil groups (RX J1119.7+2126 and 1RXS J235814.4+150524) and one normal group (NGC 6034). We computed photometric redshifts in the central zones of each group, combining previous data with the SDSS five-band data. For each group we investigated the red sequence (RS) of the color-magnitude relation and computed the luminosity functions, stellar population ages and distributions of the group members. Spectroscopy allowed us to investigate the large-scale surroundings of these groups and the substructure levels in 1RXS J235814.4+150524 and NGC 6034. The large-scale environment of 1RXS J235814.4+150524 is poor, though its galaxy density map shows a clear signature of the surrounding cosmic web. RX J1119.7+2126 appears to be very isolated, while the cosmic environment of NGC 6034 is very rich. At the group scale, 1RXS J235814.4+150524 shows no substructure. Galaxies with recent stellar populations seem preferentially located in the group outskirts. A RS is discernable for all three groups in a color-magnitude diagram. The luminosity functions based on photometric redshift selection and on statistical background subtraction have comparable shapes, and agree with the few points obtained from spectroscopic redshifts. These luminosity functions show the expected dip between first and second brightest galaxies for the fossil groups only. Their shape is also regular and relatively flat at faint magnitudes down to the completeness level for RX J1119.7+2126 and NGC 6034, while there is a clear lack of faint galaxies for 1RXS J235814.4+150524. RX J1119.7+2126 is definitely classified as a fossil group; 1RXS J235814.4+150524 also has properties very close to those of a fossil group, while we confirm that NGC 6034 is a normal group.
Highlights
The galaxy population of a fossil group is dominated by a giant elliptical (D) galaxy and includes in addition only galaxies at least two magnitudes fainter than D following the definition of Jones et al (2003; see Proctor et al 2011, for a less stringent definition)
The luminosity functions based on photometric redshift selection and on statistical background subtraction have comparable shapes, and agree with the few points obtained from spectroscopic redshifts
We have shown in Adami et al (2007b) that given the field luminosity function at low redshift, the probability is low to have a galaxy brighter than the dominant fossil group galaxy in the surrounding cosmic web portions unless a massive galaxy structure is present in the region
Summary
The galaxy population of a fossil group is dominated by a giant elliptical (D) galaxy and includes in addition only galaxies at least two magnitudes fainter than D following the definition of Jones et al (2003; see Proctor et al 2011, for a less stringent definition) These structures have been discovered only quite recently in the late 90 s, concomitantly with the arrival of large-scale X-ray surveys It was originally presented as a FG by Yoshioka et al (2004), but Lopes de Oliveira et al (2010) showed that this group was a normal group of galaxies, and we confirm this in the present work This structure is very nearby, so we can sample its galaxy populations down to relatively deep absolute magnitudes even with small telescopes. We collected PI observations completed by archive data as described in the following
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