Is the Giant Elliptical Galaxy NGC5018 a Post-Merger Remnant?

Abstract

NGC5018, one of the weakest UV emitters among giant ellipticals (gE) observed with IUE, appears to consist of an optical stellar population very similar to that of the compact, dwarf elliptical M32, which is several magnitudes fainter in luminosity than NGC5018 and whose stellar population is know to be ~3 Gyr old. Here we show that the mid-UV spectra of these two galaxies are also very similar down to an angular scale hundreds times smaller than the IUE large aperture (as probed by HST/FOS UV spectra obtained through 0.86 arcsec apertures). This implies a reasonably close match of the populations dominating their mid-UV light (namely, their main-sequence turnoff stars). These data indicate that NGC5018 has, in its inner regions, a rather uniform dominance of a ~3 Gyr-old stellar population, probably a bit different in metallicity from M32. Combined with the various structures that indicate that NGC5018 is the result of a recent major merger, it appears that almost all of stars we see in its center regions were formed about 3 Gyr ago, in that merger event. NGC5018 is likely the older brother of NGC7252, the canonical gE-in-formation merger. As such, NGC5018 is perhaps the best galaxy which can tell us how a merger works, after the fireworks subside, to form a gE galaxy today. For this reason alone, the stellar populations in NGC5018 at all radii are worth studying in detail.