Gemini/GMOS imaging of globular cluster systems in five early-type galaxies

This paper presents deep high quality photometry of globular cluster (GC) systems belonging to five early-type galaxies covering a range of mass and en vironment. Photometric data were obtained with the Gemini North and Gemini South telescopes in the filter passbands g ′ , r ′ , and i ′ . The combination of these filters with good seeing condition s allows an excellent separation between GC candidates and unresolved field o bjects. In fact, our previously published spectroscopic data indicates a contamination level of only �10 percent in our sample of GC candidates. Bimodal GC colour distributions are found in all five galaxies. Most of the GC systems appear bimodal even in the (g ′ -r ′ ) vs (r ′ -i ′ ) plane. A population of resolved/marginally resolved GC and Ultra Compact Dwarf candidates was found in all the galaxies. A search for the so-called “blue tilt” in the colou r-magnitude diagrams reveals that NGC 4649 clearly shows that phenomenon although no conclusive evidence was found for the other galaxies in the sample. This “blue tilt” translate s into a mass-metallicity relation given by Z / M 0.28±0.03 . This dependence was found using a new empirical (g ′ -i ′ ) vs [Z/H] relation which relies on an homogeneous sample of GC colours and metallicities. This paper also explores the radial trends in both colour and surface density for the blue (metal-poor) and red (metal-rich) GC subpopulations. As usual, the red GCs show a steeper radial distribution than the blue ones. Evidence of galactocentric colour gradients is found in some of the GC systems, being more significant for the two S0 galaxies in the sample. Red GC subpopulations show similar colours and gradients to the galaxy halo stars in their inner region. A GC mean colour-galaxy luminosity relation, consistent with [Z/H]/ LB 0.26±0.08 , is present for the red GCs. An estimate of the total GC populations and specific freq uency SN values is presented for NGC 3115, NGC 3923 and NGC 4649.

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