The globular cluster system of an “ old ” merger : NGC 5018 ⋆

We study the properties of the globular cluster system of the “old” merger NGC 5018. We detect a relatively poor globular cluster system, that can be divided into at least two populations based on colors: a small population of blue clusters, aged between several hundred Myr to 6 Gyr, the formation of which was probably induced by the last interaction in the galaxy group; and one (possibly two) population(s) of old(er) clusters that can be associated with the original galaxy(ies). This composite globular cluster system is as elliptical (ǫ = 0.5± 0.3) as the best oblate model for the luminous component (about E6), but as diffuse as the dark halo implied by the same model. In terms of a power law slope, we find a fall-off of −1.3 ± 0.4 for the globular cluster density profile, while the intensity of the stellar light falls off with −2.2± 0.2. We sample the globular cluster luminosity function down to V = 24.4 mag, and do not reach the turnover. Fits to our data with Gaussians and t5 functions can only constrain the turn-over to VTO = 26.3 ± 0.6 mag. The total number of globular clusters derived from the luminosity function and the surface density profile of the globular clusters lies then between 800± 270 and 1700± 750. The amount of young globular clusters is estimated to be less than 10% of the total, evidence against a significant increase of globular clusters by the merger event in this case. The specific frequency, for which uncertainties in distance and total number cancel, is S = 1.0±0.7, lower than expected for a normal elliptical galaxy. However, if the reported young luminous stellar population would brighten NGC 5018 by one magnitude compared with normal ellipticals, the corrected S value would be about 3, comparable with values found for ellipticals in small groups. Send offprint requests to: M. Hilker ⋆ Based on data collected at the European Southern Observatory, La Silla, Chile Finally, the study confirms that NGC 5018 contains a large amount of dust. The globular clusters might even reveal a dust concentration.