ISO observations of the Wolf-Rayet galaxy NGC 7714 and its companion object NGC 7715

Abstract

The interacting system Arp 284 consisting of the Wolf-Rayet galaxy NGC 7714 and its irregular companion NGC 7715 was observed using the Infrared Space Observatory. Deconvolved ISOCAM maps of the galaxies using the 14.3, 7.7 and 15 micron LW3, LW6 and LW9 filters, along with ISOPHOT spectrometry of the nuclear region of NGC 7714 were obtained and are presented. Strong ISOCAM emission was detected from the central source in NGC 7714, along with strong PAH features, the emission line [Ar II], molecular hydrogen at 9.66 microns and a blend of features including [S IV] at 10.6 microns. IR emission was not detected from the companion galaxy NGC 7715, the bridge linking the two galaxies or from the partial stellar ring in NGC 7714 where emission ceases abruptly at the interface between the disk and the ring. The morphology of the system can be well described by an off-centre collision between the two galaxies. The LW3/LW2, where the LW2 flux was synthesized from the PHOT-SL spectrum, LW9/LW6 and LW3/LW6 ratios suggest that the central burst within NGC 7714 is moving towards the post-starburst phase, in agreement with the age of the burst. Diagnostic tools including the ratio of the integrated PAH luminosity to the 40 to 120 microns infrared luminosity and the far-infrared colours reveal that despite the high surface brightness of the nucleus, the properties of NGC 7714 can be explained in terms of a starburst and do not require the prescence an AGN.