Research Note

Is the giant elliptical galaxy NGC 5018 a post-merger remnant?

¹ INAF - Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: buson@pd.astro.it
² Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy
³ Department of Physics & Astronomy, Arizona State University, Tempe, AZ, 85287-1504, USA
⁴ Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands

Received: 16 February 2004
Accepted: 13 June 2004

Abstract

NGC 5018, 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 M 32, which is several magnitudes fainter in luminosity than NGC 5018 and whose stellar population is known 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'' 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 NGC 5018 has, in its inner regions, a rather uniform dominance of a ~3 Gyr-old stellar population, probably a bit different in metallicity from M 32. Combined with the various structures that indicate that NGC 5018 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. NGC 5018 is likely the older brother of NGC 7252, the canonical gE–in-formation merger. As such, NGC 5018 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 NGC 5018 at all radii are worth studying in detail.