Colours, line strengths and stellar kinematics of NGC 2663 and 5018

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Abstract
Results of the analysis of broad-band optical images and long-slit spectra of the two elliptical galaxies NGC 2663 and 5018 are presented. Line-strength profiles of Mg2, Fe5270 and Fe5335 have been derived for both galaxies, together with rotational velocity and velocity dispersion curves at three and four position angles (PAs) for NGC 2663 and 5018, respectively. The measurements extend to about 1.8 effective radii for NGC 2663, and to about 3 effective radii for NGC 5018. Axisymmetric dynamical models with distribution function f ≡ f(E, Jz) (i.e. depending only on the energy E and the angular momentum along the symmetry axis Jz) have been fitted to the kinematic profiles of both galaxies, to derive information about the intrinsic shapes of the luminous galaxies and the sizes and shapes of possible dark haloes. In agreement with what has been found for other ellipticals: (i) 〈Fe〉 versus Mg2 shows, within both galaxies, a steeper slope than that shown by galactic nuclei, and (ii) the slope is approximately equal in the two galaxies and constant within each galaxy (over the whole radial range, to ≈ 2 Re). A shift is observed between the two galaxies, implying different [Mg/Fe] abundance ratios. In NGC 2663, the derived [Mg/Fe] abundance ratio is (as is normally found in ellipticals) larger than solar; its colour [d(B – R)/d log r = – 0.08] and Mg2 (dMg2/d log r = – 0.06) gradients are consistent with each other and with a change in metallicity only. The galaxy shows a hint of counter-rotation within the innermost 20 arcsec and minor axis rotation. Any oblate or prolate model fitting its kinematics along any two position angles fails to fit the third position angle available, the discrepancy with the data being a systematic shift. Most probably, this galaxy is a triaxial object (observed Vrot/σ ≈ 0.15 with ε ≈ 0.3, anisotropy parameter δ ≥ 0.2–0.3); alternatively, it might be prolate but always with an anisotropic velocity dispersion tensor. In NGC 5018, an almost flat velocity dispersion profile is observed out to the last measured point. Assuming that the galaxy is an axisymmetric rotator, such a flat σ profile can be reproduced at a confidence level ≳ 5 sigma only if a very massive dark matter halo surrounds the luminous component (MD ≈ 6 Mlum). A rather strong (B – R) colour gradient is observed in NGC 5018 [d(B – R)/dlog r = – 0.13], despite its shallow Mg2 gradient (dMg2/dlog r= – 0.04). Although the whole galaxy is blue, its central (B – R) colour and the observed patchy structure in the (B – R) colour map suggest a considerable amount of extinction in its central regions. If the dust follows the Galactic extinction law, at 1550 Å one could even expect ≈ 2 mag of extinction. This might hide a younger stellar population diluting the Mg2 line strength, and might explain the lack of UV flux reported by other authors. The dilution of Mg2 would also explain the solar [Mg/Fe] abundance ratio.

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