EDP Sciences
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Volume 416, Number 1, March II 2004
Page(s) 243 - 250
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20031712

A&A 416, 243-250 (2004)
DOI: 10.1051/0004-6361:20031712

An unusual high-ionization nebula in NGC 55

R. Tüllmann1 and M. R. Rosa2

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

(Received 26 August 2003 / Accepted 1 December 2003 )

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 $\ion{He}{ii}$ 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.

Key words: stars: Wolf-Rayet -- ISM: abundances -- ISM: individual: TR 001507.7-391206 -- galaxies: individual: NGC 55 -- X-rays: ISM

Offprint request: R. Tüllmann, tullmann@astro.rub.de

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