The paucity of globular clusters around the field elliptical NGC 7507^⋆

¹ Instituto de Astrofísica de La Plata (CCT La Plata−CONICET−UNLP), Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina
e-mail: jpceda@gmail.com
² Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), C1033AAJ, Ciudad Autónoma de Buenos Aires, Argentina
³ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C Concepción, Chile
⁴ Finnish Centre for Astronomy with ESO, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁵ Department of Physics and Astronomy, San José State University, One Washington Square, San Jos, CA 95192, USA
⁶ University of California Observatories, 1156 High St., Santa Cruz, CA 95064, USA

Received: 2 January 2013
Accepted: 3 May 2013

Abstract

Context. There is strong evidence that globular cluster systems (GCSs) of massive galaxies are largely assembled by infall/accretion processes. Therefore, we expect the GCSs of isolated elliptical galaxies to be poor. Although not completely isolated, NGC 7507 is a massive field elliptical galaxy with an apparently very low dark matter content.

Aims. We determine the richness, the colour distribution, and the structural properties of the GCS of NGC 7507.

Methods. We performed wide-field Washington photometry with data obtained with the MOSAIC II camera at the 4m-Blanco telescope (CTIO, Chile).

Results. The GCS is very poor with S_N ≈ 0.6. We identify three subpopulations with peaks at (C − T1) colours of 1.21, 1.42, and 1.72. The bluest population may represent the old, metal-poor component. This interpretation is supported by its shallow density profile. The red population is more concentrated, resembling the galaxy light. The intermediate-colour population is strongly peaked in colour, and we interpret this population as the signature of a starburst whose age depends on the metallicity, but should be quite old, since no signatures of a merger are identifiable. In addition, we find a main sequence in the stellar foreground population, which we attribute to the Sagittarius dwarf tidal stream.

Conclusions. The extrordinarily poor GCS of NGC 7507, a massive elliptical galaxy, is an illustration of how important the environmental conditions are for producing rich GCSs.