Environmental dependence of polycyclic aromatic hydrocarbon emission at z ~ 0.8

Investigation by observing RX J0152.7-1357 with AKARI

¹ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, 229-8510, Kanagawa, Japan
e-mail: murata@ir.isas.jaxa.jp
² National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, 181-8588 Tokyo, Japan

Received: 8 April 2015
Accepted: 27 June 2015

Abstract

We study the environmental dependence of the strength of polycyclic aromatic hydrocarbon (PAH) emission using AKARI observations of RX J0152.7-1357, a galaxy cluster at z = 0.84. PAH emission reflects the physical conditions of galaxies and dominates 8 μm luminosity (L8), which can be directly measured with the L15 band of AKARI. The ratio of L8 to infrared luminosity (LIR) is used as a tracer of the PAH strength. Both photometric and spectroscopic redshifts are applied to identify the cluster members. The galaxies detected with the L15 band tend to reside in the outskirts of the cluster and have an optically green colour, R − z′ ~ 1.2. We find no clear difference between the L8/LIR behaviour of galaxies in field and cluster environments. The L8/LIR of cluster galaxies decreases with specific star formation rate divided by that of main-sequence galaxies, and with LIR, consistent with the results for field galaxies. The relation between L8/LIR and LIR is between those at z = 0 and z = 2 in the literature. Our data also shows that starburst galaxies, which have lower L8/LIR than main-sequence galaxies, are located only in the outskirts of the cluster. All these findings extend previous studies, indicating that environment affects only the fraction of galaxy types and does not affect the L8/LIR behaviour of star-forming galaxies.