NGC-5018 - A METAL-POOR GIANT ELLIPTIC

NGC 5018 is a luminous elliptical galaxy with colors and optical absorption-line spectrum similar to those of M32, which is 40 times fainter in luminosity. This giant elliptical is one of the more extreme examples of a minority of elliptical galaxies (perhaps as many as 20%) whose colors are significantly bluer, and Mg2 indices significantly weaker, than the nominal color-luminosity and Mg2-luminoisty relationships for ellipticals. Both SWP and LWP IUE spectra have been obtained of the central regions of NGC 5018, in an effort to detect evidence of ongoing star formation in this galaxy. High S/N IUE spectra for M32 already exist. NGC 5018 has essentially the same mid-UV (1900-3300 angstrom) spectrum as does M32, both in terms of relative flux to the V passband and form of energy distribution. NGC 5018 has somewhat higher far-UV (1300-1900 angstrom) flux than M32. From a differential comparison among the UV-V spectral energy distributions of M32, NGC 205, NGC 2681, and NGC 5018, it is inferred that a star-forming stellar population as old as 3 Gyr can contribute less-than-or-equal-to 5% of the V band flux in NGC 5018 within the IUE aperture. We conclude that ongoing star formation cannot produce the weak-lined spectrum of NGC 5018. Rather, NGC 5018 appears to be a giant elliptical galaxy with the mean metallicity of a dwarf elliptical, thus representing the prototype of a class of ellipticals which strongly deviate from the log sigma-Mg2 relationship due to intrinsic differences in their old stellar populations.