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Astron. Astrophys. 336, 662-666 (1998)

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1. Introduction

N 4 (Henize 1956), also designated as DEM 8 (Davies et al. 1976) is a complex of H II   regions in the north-west part of the Large Magellanic Cloud (LMC). These H II   regions and the associated molecular complex have been comprehensively studied by Heydari-Malayeri & Lecavelier des Etangs (1994). They make a relatively isolated, simple structure of about [FORMULA] in size, well suited for mapping with the mid-infrared camera ISOCAM on board the Infrared Space Observatory (ISO). In this paper, we present maps of the whole region obtained with [FORMULA] pixels in two broad band filters LW2 and LW3, covering respectively the spectral ranges 5.5-8.5 and 12.0-17.0 µm. At least in our Galaxy, the emission in the LW2 filter is dominated by the Unidentified Infrared Bands (UIBs) at 6.2, 7.7 and 8.6 µm. That in the LW3 filter is partly continuum from the same carriers as the UIBs (often called Polycyclic Aromatic Hydrocarbons or PAHs although their exact nature is not known). If the radiation field is sufficiently large there is also a contribution from Very Small Grains (VSGs), perhaps made of graphite, heated closer to thermal equilibrium than the UIB carriers which are heated only transiently by single photons (Désert et al. 1990; Dwek et al. 1997). There are also some contributions of the UIB at 12.7 µm and possibly of the fine-structure lines [Ne II ] 12.8 µm and [Ne III ] 15.5 µm if there is ionized gas along the line of sight, but these contributions seem small in general. For a discussion of these properties, see Cesarsky et al. (1996a) and Cesarsky et al. (1996b). Consequently, observations with the LW2 and LW3 filters provide the opportunity to investigate the distribution of PAH and VSG emission in molecular-H II   region complexes such as those of N 4. The main interest of a study of a LMC complex is that the properties of interstellar grains are known to differ from those in our Galaxy, as shown by observations with IRAS (Sauvage et al. 1990), probably due to differences in heavy-element abundances or/and in the ultraviolet radiation field.

In Sect. 2, we describe the observations and the reductions; Sect. 3 discusses the filter maps and the LW3/LW2 color map; Sect. 4 contains the conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998