An unusual high-ionization nebula in NGC 55 ^*

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany
² Space Telescope European Coordinating Facility, c/o European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Corresponding author: R. Tüllmann, tullmann@astro.rub.de

Received: 26 August 2003
Accepted: 1 December 2003

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 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.