An unusual high-ionization nebula in NGC 55

Abstract

We report on the detection of a previously unknown extended low-density nebula of very high temperature and excitation, TR 001507.7-391206, located above the midplane of NGC 55. The nebular emission line spectrum, in which He II is present, is consistent with photoionization by about 3 very hot massive O3 or WR-type stars. There are no indications for shock ionization. The faint blue optical continuum and XMM-Newton EPIC-pn and OM-data also support our assumption that this nebula is not a supernova remnant. Galactic nebulae harboring massive Wolf-Rayet (WR) stars may be appropriate examples, in particular the highly ionized nebula G2.4+1.4 around WR 102 (aka Sk 4 or LSS 4368). The relatively large diameter of the inhomogeneously expanding nebula in NGC 55 of about 58 pc, compared to 11 pc of G2.4+1.4, would also be consistent with the number of ionizing stars.

Based on observations collected at the European Southern Observatory, La Silla & Cerro Paranal (Chile); Proposal No.: 64.N-0399(A,B), 66.B-0551(A). Based upon data obtained from the ESA XMM-Newton data archive.