Fig. 1a shows an image of the region we observed, in the 11.3 µm aromatic hydrocarbon feature. The 11.3 µm emission feature comes mostly from the northwest corner of the image. Fig. 1b shows an image in the broad LW2 filter (5-8.5 µm). The LW2 image shows some diffuse emission that is spatially associated with the emission in the 11.3 µm feature, as well as several point sources. The similarity between the wide-band image and the 11.3 µm image suggests that they are both tracing the same type of emission.
The diffuse emission in the 11.3 µm image is coincident with the molecular cloud SMC B1#1. This cloud has been mapped in the CO(2-1) emission line, revealing a cloud size of 50"40" (Rubio et al. 1996), similar in size and shape to the 11.3 µm emission we see with ISOCAM. Fig. 2a shows the spectrum of the cloud, averaged over the diffuse emission, comparable to the extent of the CO emission, and excluding the embedded source, which is the peak pixel in the CVF image and a point-like source in the LW2 filter image. It is evident that the diffuse emission is dominated by features at 6.2, 7.7, and 11.3 µm, corresponding to the brightest of the mid-infrared features that appear in similar spectra of Galactic interstellar clouds (e.g. Boulanger et al. 1996). The reasonably good correlation between the diffuse emission in the LW2 image and the 11.3 µm feature image is most likely due to the LW2 filter containing the 6.2 and 7.7 µm features. Thus Fig. 1a shows only emission from one aromatic hydrocarbon feature, while Fig. 1b shows a combination of point-like continuum sources and diffuse emission from aromatic hydrocarbon features.
The Milky Way foreground is faint compared to our target SMC B1#1: no aromatic hydrocarbon features (from the Milky Way or the SMC), are detected in the southeast corner of our CVF image, nor in the reference position outside the SMC (Fig. 2). Thus it is evident that the carrier of these features, widely considered to be aromatic hydrocarbons, also exist in the SMC.
The relatively bright point source in the LW2 image, SMC004532.2-731840, has an extremely red spectrum, shown in Fig. 2b, rising continuously from 5-16 µm. The source spectrum is relatively featureless, so it does not appear as a prominent source in the 11.3 µm feature image. Using the filter images (with pixels), the point-like source has a flux densities of 23 and 5 mJy in the LW3 (12-18 µm) and LW2 (5-8.5 µm) filters, respectively. The source's luminosity in the mid-infrared, integrating over 5-16 µm, is . A possible Galactic analog for this source is an ultracompact H II region: the mid-infrared spectrum is similar to that observed for an ultracompact H II region in M 17 (Cesarsky D. et al. 1996). The source could also be a reflection nebula, but the featureless and extremely red spectrum is completely different from reflection nebulae around Milky Way stars.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000