4 Nature of the mid-infrared emitting species

The spectra shown in Fig. 1 are strikingly similar to one another. They contain some features also seen in spectra of reflection nebulae, atomic and molecular envelopes of H II regions, atmospheres of C-rich evolved stars as well as the diffuse interstellar medium. We can thus safely assume that the results obtained on these resolved Galactic objects can be readily extrapolated to the emission of galaxies where individual sources are no longer resolved.

The emission between 5 and 16$\mu$m is dominated by the so-called unidentified infrared bands (UIBs) at 6.2, 7.7, 8.6, 11.3 and 12.7$\mu$m. Our spectra also display weak features which have previously been detected as broad features in SWS spectra of starburst objects (Sturm et al. 2000) at e.g. 5.3, 5.7, 10.7, 12.0, 13.6, 14.3 and 15.7$\mu$m. A 7.0$\mu$m feature can tentatively be identified as an [Ar II] line (6.99$\mu$m) or an H₂ rotational line (6.91$\mu$m), but our spectral resolution ( $\Delta \lambda / \lambda \approx 40$) prevents a more definite identification. We note however that the [Ar II] line has been identified in the high-resolution SWS spectra of starburst galaxies (Sturm et al. 2000).

It was originally proposed by Duley & Williams (1981) that UIBs are due to organic functional groups on carbonaceous grains. Léger & Puget (1984) instead favoured vibration modes of C-C and C-H bonds only, in large polycyclic aromatic molecules not in thermal equilibrium with the local radiation field (the so-called PAH model). The constancy of the spectral energy distribution of UIBs, regardless of the radiation field (Sellgren 1984; Uchida et al. 2000), implies an impulsive heating mechanism, where upon absorption of a single UV photon, the carriers undergo a very rapid and large temperature increase and then radiatively cool before the next absorption. Alternative candidates for the UIB carriers are various hydrogenated and oxygenated carbon grains, amorphous but partially ordered at the smallest scale (Borghesi et al. 1987; Sakata et al. 1987; Papoular et al. 1989), much similar to the idea of Duley & Williams (1981). Recent work by Boulanger et al. (1998b) indicates that UIBs are not due to molecules such as PAHs, but more likely to aggregates of several hundred atoms.

In the interstellar medium surrounding the OB association Trapezium (Roche et al. 1989), the Orion bar (Giard et al. 1994) and M17 (Cesarsky et al. 1996a; Tran 1998), these features are detected in the H II region and the molecular cloud front (provided projection effects are minor), but the emission peaks at the photodissociation interface (see also Brooks et al. 2000). UIB carriers are likely destroyed in H II region cores, although the estimation of the critical radiation field necessary to obtain a significant reduction in UIB carrier abundance still remains to be done (compare e.g. Boulanger et al. 1988; Boulanger et al. 1998a; Contursi et al. 2000).

Figure 1: Spectra of central regions (left) and the inner disk (middle). The upper and lower limits are determined from limits on the zodiacal spectrum shown with dotted lines (right), adjusted using the average spectrum of the faintest pixels, also shown with its dispersion. The flux unit for all spectra is mJy arcsec-2.

While the 7$\mu$m flux in spiral galaxies essentially consists of the UIB emission, the 15$\mu$m filter covers the emission from mainly two dust species: the hot tail of a continuum attributed to very small grains (VSGs) of the order of 0.5-10 nm in size and most often impulsively heated like UIB carriers (Désert et al. 1990), and also UIBs. The red wing of the 11.3$\mu$m band contributes little, but the band at 12.7$\mu$m and the emission plateau that connects it to the 11.3$\mu$m band can be important; the smaller UIB features listed above also contribute, although to a lesser extent. When spatial resolution is high enough, the emission from VSGs and UIB carriers can be clearly separated: around M17 and in the reflection nebula NGC7023, the VSG continuum strongly peaks in a layer closer to the excitation sources than the UIBs, inside the ionized region for M17 (Cesarsky et al. 1996a, 1996b). Therefore, the F₁₅/F₇flux ratio decreases with increasing distance from the exciting stars of an H II region.

In the spectra of all five galaxies (Fig. 1), the intensity ratios of UIBs are remarkably stable, which is a common property of a variety of astronomical sources (Cohen et al. 1986; Uchida et al. 2000). The only highly varying feature is the VSG continuum that is best seen longward of 13$\mu$m. It has various amplitudes and spectral slopes in galactic nuclei. It remains very modest compared to that in starburst galaxies (Tran 1998; Sturm et al. 2000), and is hardly present in averaged disks. In Paper II, we show that the integrated mid-infrared luminosity of normal spiral disks is dominated by the contribution from photodissociation regions (where the UIB emission is maximum). From a comparison with H$\alpha$ luminosities, we show that this predominance of the photodissociation region emission results in the fact that, when integrated over the disk, the UIB emission is a good tracer of massive young stars.

Finally, as alluded to earlier, a number of fine-structure lines can be present in the mid-infrared spectral range, although their contribution to the broadband flux is always negligible in spirals. In normal galaxies, the most prominent is the [Ne II] line at 12.81$\mu$m, which at the spectral resolution of ISOCAM is blended with the UIB at 12.7$\mu$m. No lines from high excitation ions such as [Ne III] at 15.56$\mu$m are convincingly detected, and the [Ne II] line at 12.81$\mu$m is weak, since the intensity of the blend with the UIB at 12.7$\mu$m, relative to the isolated UIB at 11.3$\mu$m, is rather stable in different excitation conditions. Some variation however exists. To compare the strength of the [Ne II] line in our galaxies to that observed by Förster-Schreiber et al. (2001) in the starburst galaxies M82, NGC253 and NGC1808, we have measured in a similar way the flux of the blend F_12.75 above the pseudo-continuum drawn as a straight line between 12.31 and 13.23$\mu$m, and the flux of the 11.3$\mu$m UIB F_11.3 with its respective continuum level defined in the same way between 10.84 and 11.79$\mu$m. We find that the energy ratio F_12.75/F_11.3 of circumnuclear regions decreases from 0.67 in NGC1365 to 0.60 in NGC613 and 5236, 0.52 in NGC1097 and 0.47 in NGC5194; in the averaged inner disks of NGC1365, 5236 and 5194, where it is still measurable, it takes the approximate values 0.5, 0.45 and 0.4. These figures are much lower than those observed in cores of starburst galaxies by Förster-Schreiber et al. (2001), where it can reach 1.7, and argue for a generally small contribution of [Ne II] to the spectra. Adopting as the intrinsic F_12.7/F_11.3 UIB energy ratio the minimum value of F_12.75/F_11.3 that we measure in our spectra, i.e. 0.4, we obtain a maximum [Ne II] equivalent width of 0.22$\mu$m in the nucleus of NGC5236. As for the UIBs, their equivalent widths in disks and central regions range respectively between $EW(12.7) = 0.3{-}0.6~\mu$m and $EW(11.3) = 1.2{-}1.9~\mu$m (these numbers do not take into account broad UIB wings that occur if the bands are described by Lorentzians). Our estimates give circumnuclear values for $F_{12.7}/F_{\rm [Ne{\scriptsize II}]}$ between 1.5 and 5.7, that we can compare with the results of Sturm et al. (2000) in the starburst galaxies M82 and NGC253 from their ISOSWS spectra with a high spectral resolution ( $\lambda/\Delta \lambda \approx 1500$, versus $\approx$40 for ISOCAM). They obtain values of 0.96 and 1.32. The contribution from the [Ne II] line to our spectra is thus confirmed to be negligible with respect to starburst galaxies.