CORE PROPERTIES OF ELLIPTIC GALAXIES .1. A NORTHERN-HEMISPHERE SAMPLEAT HIGH-RESOLUTION

We introduce an ongoing survey of core properties of elliptical galaxi es and present the first results from high-resolution direct imaging o f a subsample. The data were obtained in the V, R and I bands at the N OT telescope, with median seeings of 0.69, 0.65 and 0.62 arcsec FWHM, respectively. We determine the 'classical' core parameters, and we int roduce a new parameter, pi(c), which is a measure of the deviation of the observed brightness profile from an isothermal profile. pi(c) meas ures the intensity of a central brightness concentration in excess of an isothermal model, relative to the intensity of the core. Measuring pi(c), for the objects presented here, we find that only five of 22 ob jects are consistent with pi(c) = 0 at the 1 sigma level, and that 14 of 22 are inconsistent with pi(c) at the 3 sigma level. We see no evid ence for significant colour difference(in V, R and I) between the cent ral spike and the rest of the core, except for M87 and NGC 4278. For t he 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.