Spatially resolved stellar population parameters in the BCGs of two fossil groups
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Date
2014
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Abstract
We report the results of Gemini/GMOS long-slit spectroscopic observations along the major and minor axes of the central galaxies in two fossil groups, SDSS J073422.21+265133.9 and SDSS J075828.11+374711.8 (the NGC 2484 group). Spatially resolved kinematics and stellar population parameters (ages, metallicities and alpha-element abundance ratios) derived using similar to 20 Lick indices are presented. Despite remarkable similarities in their morphologies, photometric properties (luminosity and colour) and kinematics, the two galaxies exhibit significantly different stellar population parameters. SDSS J073422.21+265133.9 exhibits a strong metallicity gradient (Delta [Z/H]Delta R similar to -0.4) all the way into the centre of the galaxy. It also exhibits an age profile that suggest a relatively recent, centrally concentrated burst of star formation superimposed on an older, more spatially extended population. NGC 2484, a well-known X-ray AGN, exhibits a flat core-like structure in its metallicity gradient, but no detectable age gradient. The a-element abundance ratio ([E/Fe]) profiles of the two galaxies are also significantly different. SDSS J073422.21+265133.9 exhibits a slightly positive gradient (Delta[E/H]Delta R similar to 0.1), perhaps again suggesting a more recent central burst of star formation, while NGC 2484 shows a negative gradient (Delta [E/H]/Delta R similar to -0.1), indicating that star formation may have happened 'inside out'. Our analysis of these two galaxies of similar mass, morphology and kinematics therefore suggests two different mechanisms to have been in action during their formation. Consequently, we conclude that the central galaxies of fossil groups cannot be considered a homogeneous group with regard to their formation processes or star formation histories.
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galaxies: elliptical and lenticular, cD, galaxies: groups: general, galaxies: kinematics and dynamics, galaxies: stellar content