The optical properties of galaxies in the Ophiuchus cluster

TL;DR

This study analyzes the optical properties of the Ophiuchus galaxy cluster, comparing its galaxy luminosity function and structure to the more disturbed Coma cluster, revealing insights into their formation histories.

Contribution

It provides the first detailed optical analysis of Ophiuchus, including galaxy luminosity functions and density maps, and compares its properties to those of the Coma cluster.

Findings

Ophiuchus has a strong excess of bright galaxies compared to Coma.

The GLF fits improve with a Gaussian component, indicating a bright galaxy excess.

Ophiuchus appears more relaxed and formed earlier than the dynamically active Coma.

Abstract

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 disturbed dynamically. 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 pollute the image, due to the low Galactic latitude of the cluster, and 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 studied by Adami et al. (2007). 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 deg^2 respectively centered on the cluster (C), northwest (NW) and southeast (SE) of the cluster, in which we compute the GLFs. 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. 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 has built up long ago, as confirmed by its relaxed state (see paper I) while Coma is still in the process of forming.