What is the temperature structure in the giant HII region NGC 588?

¹ LUTH, Observatoire de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
² Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain e-mail: luc@iaa.es

Received: 22 November 2004
Accepted: 29 July 2005

Abstract

We present the results of an exhaustive study of the ionized gas in NGC 588, a giant Hii region in the nearby spiral galaxy M 33. This analysis uses a high number of diagnostics in the optical and infrared ranges. Four temperature diagnostics obtained with optical lines agree with a gas temperature of 11 000 K, while the [Oiii] λ5007/λ88 μm ratio yields a much lower temperature of 8000 K. This discrepancy suggests the presence of large temperature inhomogeneities in the nebula. We investigated the cause of this discrepancy by constructing photoionization models of increasing complexity. In particular, we used the constraints from the Hα and Hβ surface brightness distributions and state-of-the-art models of the stellar ionizing spectrum. None of the successive attempts was able to reproduce the discrepancy between the temperature diagnostics, so the thermal balance of NGC 588 remains unexplained. We give an estimate of the effect of this failure on the O/H and Ne/O estimates and show that O/H is known to within ±0.2 dex.