The optical properties of galaxies in the Ophiuchus cluster★
Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris,
98bis Bd Arago,
2 Faculty of Engineering, Gifu University, 1–1 Yanagido, Gifu 501-1193, Japan
3 LAM, OAMP, Pôle de l’Etoile Site Château-Gombert, 38 rue Frédéric Joliot–Curie, 13388 Marseille Cedex 13, France
4 Department of Astrophysics, Kagoshima University, Nagoya, Japan
5 Universidade Federal de Sergipe, Departamento de Física, Av. Marechal Rondon, S/N, 49000-000 São Cristóvão, SE, Brazil
6 CAPES Foundation, Ministry of Education of Brazil, Brasilia â DF, 70.040-020, Brazil
Accepted: 24 January 2018
Context. Ophiuchus is one of the most massive clusters known, but due to its low Galactic latitude its optical properties remain poorly known.
Aims. We investigate the optical properties of Ophiuchus to obtain clues on the formation epoch of this cluster, and compare them to those of the Coma cluster, which is comparable in mass to Ophiuchus but much more dynamically disturbed.
Methods. Based on a deep image of the Ophiuchus cluster in the r′ band obtained at the Canada France Hawaii Telescope with the MegaCam camera, we have applied an iterative process to subtract the contribution of the numerous stars that, due to the low Galactic latitude of the cluster, pollute the image, and have obtained a photometric catalogue of 2818 galaxies fully complete at r′ = 20.5 mag and still 91% complete at r′ = 21.5 mag. We use this catalogue to derive the cluster Galaxy Luminosity Function (GLF) for the overall image and for a region (hereafter the “rectangle” region) covering exactly the same physical size as the region in which the GLF of the Coma cluster was previously studied. We then compute density maps based on an adaptive kernel technique, for different magnitude limits, and define three circular regions covering 0.08, 0.08, and 0.06 deg2, respectively, centred on the cluster (C), on northwest (NW) of the cluster, and southeast (SE) of the cluster, in which we compute the GLFs.
Results. The GLF fits are much better when a Gaussian is added to the usual Schechter function, to account for the excess of very bright galaxies. Compared to Coma, Ophiuchus shows a strong excess of bright galaxies.
Conclusions. The properties of the two nearby very massive clusters Ophiuchus and Coma are quite comparable, though they seem embedded in different large-scale environments. Our interpretation is that Ophiuchus was built up long ago, as confirmed by its relaxed state (see paper I) while Coma is still in the process of forming.
Key words: galaxies: clusters: individual: Ophiuchus / galaxies: luminosity function, mass function / galaxies: photometry
The photometric catalogue of Ophiuchus (full Table B.1) is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/613/A20
© ESO 2018