Multi-wavelength landscape of the young galaxy cluster RXJ 1257.2+4738 at z = 0.866

II. Morphological properties^⋆

¹ Centro de Astrobiología, INTA-CSIC, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: ipintos@cab.inta-csic.es
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Facultad de Física, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁴ ISDEFE, Madrid, Spain
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), 18071 Granada, Spain
⁶ European Space Astronomy Centre (ESAC), 38205 Villanueva de la Cañada, Madrid, Spain
⁷ Laboratoire AIM Saclay, CEA/IRFU, CNRS/INSU, Université Paris Diderot, 75000 Paris, France
⁸ Centro de Estudios de Física del Cosmos de Aragón, 44001 Teruel, Spain
⁹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK
¹⁰ European Southern Observatory, Garching, Germany
¹¹ Institute of Astro- and Particle Physics, University of Innsbruck, Austria

Received: 15 June 2015
Accepted: 3 May 2016

Abstract

Studying the evolution of the morphological distribution of galaxies in different environments can provide important information about the effects of the environment and the physical mechanisms responsible for the morphological transformations. As part of a complete analysis of the young cluster RXJ 1257+4738 at z ~ 0.9, in this work we study the morphological properties of its galaxies. We used non-parametric methods of morphological classification, as implemented in the galSVM code. The classification with the applied method was possible even using ground-based observations, as the r′-band imaging from OSIRIS/GTC. We defined very conservative probability limits, taking into account the probability errors, to obtain a trustworthy classification. In this way we were able to classify ~30% of all cluster members and to separate between late-type (LT) and early-type (ET) galaxies. Additionally, when analysing the colour-magnitude diagram, we observed a significant population of blue ET galaxies among the classified ones. We discussed possible explanations for finding this population. Moreover, we studied different physical properties of LT, ET, and blue ET galaxies. They turn out to be comparable, with the exception of the stellar mass that shows that the red ET population is more massive. We also analysed the morphology-density and morphology-radius relations observing that, only when considering the morphological separation between ET and LT galaxies, a mild classical behaviour is obtained. RXJ 1257+4738 is a young galaxy cluster, showing a clumpy structure, which is still in the process of formation, and which could explain the lack of some of the standard morphological relations. This makes this cluster a very attractive case for obtaining higher resolution data and for studying the morphological properties of the entire cluster in more detail and their relation to the environment.