Core properties of elliptical galaxies – I. A northern hemisphere sample at high resolution

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Abstract
We introduce an ongoing survey of core properties of elliptical galaxies and present the first results from high-resolution direct imaging of a subsample. The data were obtained in the V, R and I bands at the NOT telescope, with median seeings of 0.69, 0.65 and 0.62 arcsec FWHM, respectively. We determine the ‘classical’ core parameters, and we introduce a new parameter, πc, which is a measure of the deviation of the observed brightness profile from an isothermal profile, πc measures the intensity of a central brightness concentration in excess of an isothermal model, relative to the intensity of the core. Measuring πc for the objects presented here, we find that only five of 22 objects are consistent with πc = 0 at the 1 σ level, and that 14 of 22 are inconsistent with πc at the 3 σ level. We see no evidence for significant colour difference (in V, R and I) between the central spike and the rest of the core, except for M87 and NGC 4278. For the remaining galaxies we find that the colours of the spike must match those of the core to within 0.4 mag. We conclude that the majority of all ellipticals seem to have central intensity concentrations in the range 1–10 per cent of the core flux.

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