4. The mid-IR emission of the discrete peaks
The CVF and filter observations show strong emission peaks which we discuss here. They are ordered by increasing right ascension and named as shown in Fig. 5. This figure shows the LW2 (6.75 µm) contours superimposed to the Digital Sky Survey image of N66. The isolated stars are identified by numbers given in Massey et al. (1989). In Fig. 6 we present the CVF spectra of these peaks. Most of the spectra represent an average of two pixels: spectra of peaks C and E have been obtained averaging four pixels (1 pixel1.2 pc for the assumed SMC distance). In general the spectra show emission bands and fine structure line on top of a continuum. The wavelengths of the emission bands correspond to those of the Unidentified Infrared Bands already observed before ISO at 6.2, 7.7, 8.6, 11.3 and 12.7 µm (Gillett, Forrest and Merrill 1973, Russell, Soifer and Merrill 1977a, Russell, Soifer and Willner 1977b, Cohen, Tielens and Allamandola 1985, Cohen and Kevin 1989, Jourdain de Muizon et al. 1986, Phillips, Airken and Roche 1984, Roche, Aitken and Smith 1989). They are an universal signature of the ISM in our (Roelfsema et al. 1996, Verstraete et al., 1996, Cesarky et al. 1996a, 1996b, Boulanger et al. 1996, Mattila et al. 1996, Uchida, Sellgren and Werner 1998) and in external galaxies (Boulade et al. 1996, Vigroux et al. 1996, Acosata-Pulido et al. 1996, Metcalfe et al. 1996, Helou et al. 2000). The exact chemical species from which these bands originate have not been yet identified. The best candidates are the Polycyclic Aromatic Hydrocarbons (PAH) (Puget and Lèger 1989), i.e. planar macro-molecules (few hundred atoms) transiently heated by single photon absorption. However, whatever is the exact nature of these carriers, the bands are certainly due to aromatic compounds. For this reason hereafter we will call them Aromatic Infrared Bands (AIBs) carriers. Fig. 6 show the following characteristics:
The CVF spectra of all these emission peaks show [Ne III ] 15.6 µm and [S IV ] 10.5 µm line emission (Fig. 6).
Even if emission bands are observed at the typical wavelengths of the most intense AIBs (6.2, 7.7, 8.6, 11.3 and 12.8 µm), these are very different in their shape and relative intensities from the AIBs observed in the galactic reflection nebulae, to which hereafter we will refer as the "classical" AIBs.
Peak A shows a broad AIB at 7.7 µm, a 11.3 µm AIB not very intense and faint 12.7 (possibly blended with a [Ne II ] line at 12.8 µm), 13.5 and 14.5 µm bands.
Peak B shows very faint AIBs, if any, and a broad silicate emission at 10 µm. Note that there are a few faint hot stars in Peak A (N 346-320 and 325), as well as in Peak B (N 346-347, 352, 353 and 357: Massey et al. 1989).
Peak C, in the direction of the center of the young star cluster, has a spectrum very similar to that of Peak B but with a stronger continuum. It exhibits only faint AIBs and a broad 10 µm silicate band is clearly seen in emission. The spectrum of Peak C is discussed in more detail by Contursi et al. (2000).
The spectrum of peak D is characterized by broad emission near 8 µm where the usual AIBs are partly merged. Note the short-wavelength continuum, also seen towards Peaks C and E. This region contains at least 3 hot stars (N 346-466, 469 and 478) the brightest of which is the evolved or reddened N 346-466 (V=15.91, U-B=-0.54:, B-V=0.27, Massey et al. 1989)
Peak E contains the relatively bright, reddened O8V star N 346-549 with V=15.26, U-B=-0.96, B-V=0.22 (Massey et al. 1989). The continuum near 5 µm is the strongest in the whole map (see Fig. 2). It is too strong to be the photospheric emission of the star, but it can be due at least in part to circumstellar dust or to a red companion. The most conspicuous feature in the spectrum of Peak E is a very broad emission feature centered near 7.7 µm in which the usual AIBs are even less identifiable than in the spectrum of peak D. Both the continuum at 5 µm and the presence of the broad band at 7.7 µm are characteristics of AGN spectra like that of Centaurus A (Mirabel et al. 1999). The origin of the 7.7 µm broad feature has not yet been established: it may be due to coal-like grains. However, it is not clear whether these types of grains normally exist in the ISM of galaxies and become visible only when destruction of classic AIBs carriers occurs, or if they form through hard UV photons processing on the classical AIB carriers. The 6.2 and 11.3 µm bands are surprisingly weak. The peculiar appearance of the 7.7 µm brad feature and the faintness of the 11.3 µm band might be due to some amount of silicate absorption, but the [S IV ] line at 10.5 µm, which should also be affected, does not seem particularly weak. Moreover, the presence of a certain amount of silicate absorption cannot explain the weakness of the 6.2 µm AIB. Note also the features at 13.5 and 14.5 µm which can arise from the out-of-plane C-H bending vibrations on aromatic rings with 3 and 4 contiguous H atoms (trio and quarto ).
The spectrum of Peak F (N 66A) shows probable silicate emission and weak AIBs. Peak F contains at least 7 hot stars, the brightest of which is the O5.5V star N 346-593 with V=14.96, U-B=-1.01, B-V=-0.16 (Massey et al. 1989).
Peak G coincides with two hot stars, N 346-628 and 635 (Massey et al. 1989). This peak is on the molecular cloud not associated with the main HII region (Fig. 12). Its spectrum is the closest to the typical Galactic AIB spectra, e.g. those of NGC 7023 (Cesarsky et al. 1996a).
Peak H has faint bands and peak I displays intense AIB bands. Both show a classical AIB spectrum. They contain a few faint hot stars, respectively N 346-640, 641, 648, 654 and N 346-696 and 697 and in fact it has a steep continuum rising toward long wavelength. Moreover, Peak I contains the bright late O or early B star N 346-690 with V=15.70, U-B=-0.75, B-V=0.00 (Massey et al. 1989) and it has the brightest emission in both CO(1-0) and H2 among the MIR peaks (Rubio et al. 2000). The column density in this peak, relative to the others region, is thus sufficiently high to explain the strength of AIBs.
As the AIBs are believed to be excited mainly by far-UV photons in the hard radiation field of N 66, we have built a rough map of the radiation density at 160 nm using the stellar photometry from Massey et al. (1989) (Fig. 7). Details about how we built this map are given in Appendix A. There are two sources of uncertainties in this calculation. 1) Extinction has not been taken into account (except for determining the intrinsic stellar UV flux). Extinction in N 66 is known to be very small for stars (E(B-V)=0.14, Massey et al. 1989) and the Balmer decrement value of 3.05 0.15 (Ye et al. 1991) is close to the unreddened value of 2.86. If dust is mixed with the ionized gas, our values for the UV fluxes are upper limits and may be too high by 1 mag. (a factor 2.5). If dust is outside the ionized gas regions our values are unaffected. 2) The other uncertainty is due to errors in the assignment of the stellar spectral types. However, changing the luminosity class in the most ambiguous cases changes the radiation density by only 30.
The average values of the ISRF at 1600 Å normalized to the local ISRF (LISRF) at the same wavelength (Gondhalekar et al. 1980) are indicated in Fig. 6 and they range from 2 to 9 105 the LISRF. They correspond to the values obtained per DSS pixel (=1.7") averaged over a circular area of 2.8 pc radius (= 5.6 DSS pix with an assumed distance for SMC=61 kpc). This is the approximate resolution of the ISO data, thus the same aperture was used to obtained the LW3, LW2 and the 160 nm fluxes reported later in Fig. 14. Note that if dust is mixed with gas inside the HII region, the UV flux values still remain very high, ranging from 5.3 104 (peaks A and I) to 2.5 105 (peak C) times that of the solar neighborhood. In Fig. 6 we have not labeled the ISRF average value of peak G because the new CO(2-1) data show that this cloud and probably the "spur" visible as diffuse emission (see Sect. 5) are not associated with the N 66 bar (Rubio et al. 2000).
From the collection of CVF spectra that we have just discussed, several conclusions can be derived:
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100