Enigmatic diffuse ionized gas structures in a cluster of galaxies near cosmic noon

¹ University of Vienna, Department of Astrophysics, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
² Astronomical Institute, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan

^⋆ Corresponding author; christian.maier@univie.ac.at

Received: 20 February 2025
Accepted: 30 March 2025

Abstract

We explore the massive cluster XMMXCS J2215.9-1738 at z ∼ 1.46 with MUSE and KMOS integral field spectroscopy. Using MUSE spectroscopy, we traced the kinematics of the ionized gas using [O II] λλ 3726, 3729 in the central 500 × 500 kpc² area of the cluster, which contains 28 spectroscopically identified cluster galaxies. We detected [O II] λλ 3726, 3729 emission lines in the integrated spectra of 21 galaxies. The remaining seven are passive galaxies. Six of these passive galaxies lie in the central part of the cluster, which has a diameter of 200 kpc and contains no star-forming objects. In this place, star formation in galaxies is quenched. An interesting discovery in this central area of the cluster are three diffuse ionized [O II] λλ 3726, 3729 gas structures, which we refer to as [OII] blobs. They extend over areas of some hundred kpc². The ionization source of one of the gaseous structures that displays two prominent filamentary patterns indicating outflow of gas is an active galactic nucleus (AGN). The KMOS data enabled us to use the BPT diagram to identify this object as a type 2 AGN. The other two diffuse ionized oxygen gaseous structures are more enigmatic. They are located between the stellar components of passive cluster galaxies. One of these blobs lacks a stellar counterpart in the HST optical and near-infrared data, and the other blob has only a very faint counterpart. Ram-pressure stripping of photoionized gas or AGN feedback might be an explanation. Additionally, the galaxy velocity distribution in this high-redshift cluster is bimodal, which indicates that the cluster is probably not fully virialized and that recent and ongoing merging events that produced shocks might provide photoionization sources for the two enigmatic [OII] blobs.