NGC 1808

   
4 Origin and spatial distribution of the PAH and continuum emission

The ubiquity of PAH features - seen towards sources as diverse as circumstellar regions, diffuse atomic clouds, H II regions, molecular clouds, normal galaxies, starbursts, and ULIRGs - and the close association of the MIR VSG continuum emission with Galactic and extragalactic actively star-forming sites now seem well established; the ISO mission played a major role in this recognition (see Geballe 1997; Tokunaga 1997; Tielens et al. 1999; Cesarsky & Sauvage 1999; Genzel & Cesarsky 2000 for reviews). Spatial mapping of Galactic sources has revealed in more detail the origin of the main MIR emission components in star-forming regions: PAH features arise predominantly in the photodissociation regions (PDRs) at the interface between H II regions and molecular clouds while the VSG $\lambda \ga 11~{\rm\mu m}$ continuum is more intense and steeper closer to the exciting source (e.g. Verstraete et al. 1996; Cesarsky et al. 1996a,1996b; Crété et al. 1999). By extension, in star-forming galaxies, the emission components are believed to trace PDRs and H II regions, respectively. This is supported by spectral decomposition of starburst galaxies (Tran 1998; Laurent et al. 2000), and by photometric and spectrophotometric imaging of spiral galaxies and starburst systems (e.g. Mirabel et al. 1998; Roussel et al. 2001a). Low excitation fine-structure line emission such as [Ar II] 6.99$~\mu$m, [Ar III] 8.99$~\mu$m, [Ne II] 12.81$~\mu$m, and [Ne III] 15.56$~\mu$m usually accompanies the long-wavelength VSG continuum and originates primarily in H II regions (e.g. Sturm et al. 2000).

The interpretation of the ISOCAM maps of M 82, NGC 253, and NGC 1808 within the above framework is not straightforward. Figure 9 complements the various maps with light profiles of the 6- $9~{\rm\mu m}$, 13.5- $15~{\rm\mu m}$, and [Ar II] 6.99$~\mu$m emission taken along the major and minor axes of the galaxies. The profiles clearly support the extension along the major axis indicated by the maps, and show that the emission in these tracers along the minor axis is but marginally resolved and essentially indistinguishable within each galaxy.

Somewhat surprisingly given their common origin in H II regions, a close spatial correlation between the VSG continuum and the [Ar II] 6.99$~\mu$m line emission is not observed in our sample galaxies. The exception may be NGC 253 but this probably results from the intrinsically small MIR source size and resolution limitations (Sect. 3.2.2). Although the extension to the northeast is real, the differences between the various profiles even along the major axis are smaller than the PSF FWHM. We conclude for NGC 253 that on scales comparable to and larger than the resolution of our ISOCAM maps (5.2 $^{\prime\prime}$, or $\approx$ $60~{\rm pc}$), no significant differences are observed in the spatial distribution of the PAH, VSG, and [Ar II] 6.99$~\mu$m emission within the central 400 pc.

For M 82, the [Ar II] 6.99$~\mu$m emission along the galactic plane peaks at a different position and is more compact than the VSG and PAH emission. The different distributions of the VSG and [Ar II] 6.99$~\mu$m emission could be interpreted in terms of differences in average energy of the exciting photons: the [Ar II] 6.99$~\mu$m line traces the $\geq$ $16~{\rm eV}$ radiation field while the VSGs may be heated by UV photons at lower energies as well (Désert et al. 1990; Siebenmorgen & Krügel 1992; Boulanger et al. 1994; Dwek et al. 1997; Jones & d'Hendecourt 2000). The brighter PAH emission around the nucleus could be originating predominantly in the PDRs associated with the H II regions within the most active starburst sites. Beyond a radius of $\approx$ $300~{\rm pc}$, the PAH profile is resolved out from the VSG profile and more extended. These differences could be attributed to excitation of aromatic band carriers by yet softer photons or to an increased filling factor for the PDRs compared to the H II regions in less intense starburst regions, and perhaps to a contribution from diffuse cirrus clouds as observed in the Milky Way and in some spiral galaxies (e.g. Ristorcelli et al. 1994; Onaka et al. 1996; Mattila et al. 1999; Roussel et al. 2001a; Li & Draine 2002). Alternative possibilities could include destruction by photodissociation or depletion of the ISM in PAHs and VSGs in the most intense starburst regions, flattening the spatial distribution around the peak (e.g. Boulanger et al. 1988; Carral et al. 1994; Normand et al. 1995).

In NGC 1808, the bulk of the [Ar II] 6.99$~\mu$m emission is clearly displaced to the southeast compared to the emission in the PAH and VSG bands defined above, which have virtually identical distributions. As discussed in Sect. 3.2.3, the [Ar II] 6.99$~\mu$m distribution agrees well with the off-nucleus location of the main star-forming sites. The spectrum integrated over the starburst core and MIR peak in NGC 1808 is also much flatter than observed towards the corresponding regions in M 82 and NGC 253 and resembles more the spectrum of their outer disks (Fig. 3). The long-wavelength continuum in NGC 1808 may in fact not be dominated by VSG emission from pure H II regions but rather produced primarily in PDRs by small particles akin to the carriers of the main UIB bands. This is reminiscent of the situation in disks of spiral galaxies. There, the 15$~\mu$m emission correlates with the 7$~\mu$m PAH-dominated emission and lies in a distinct regime compared to more active regions including circumnuclear regions in spiral (especially barred) galaxies and starbursts, characterized by an excess in the 15$~\mu$m/7$~\mu$m colour (e.g. Roussel et al. 2001a; Dale et al. 2001; Förster Schreiber, Roussel, & Sauvage, in prep.). From radiative transfer modeling of NGC 1808's $\sim$3-200 $\mu$ m spectrum, Siebenmorgen et al. (2001) concluded that the MIR range is dominated by PAHs and large dust grains (radii between 100 and 2400 Å) with negligible contribution by very small graphites and silicates (radii of 5-80 Å). The key feature of their models is the inclusion of hot spots where large grains are heated locally by massive stars to higher temperatures giving rise to the 25$~\mu$m emission and gradually contributing less towards shorter wavelengths where PAHs take over as main emitters.

Figure 9: Major and minor axis light profiles for M 82, NGC 253, and NGC 1808. The curves represent the variations of the integrated flux per unit surface area as a function of projected distance from the nucleus for the [Ar  II]  $6.99~{\rm \mu m}$ line (grey lines and open circles), the 13.5- $15.0~{\rm\mu m}$ VSG-dominated continuum (black lines and filled circles), and the 6.0- $9.0~{\rm\mu m}$PAH-dominated region (black lines and open circles). The width of the synthetic slits along each axis is 3 pixels for M 82 and NGC 1808 and 6 pixels for NGC 253 (corresponding to 144, 477, and 108 pc, respectively). The curves are normalized to the maximum values which are for M 82, NGC 253, and NGC 1808, respectively: 0.26, 0.36, and 0.0036 ${\rm mJy~pc^{-2}}$ for the "PAH'' major axis profiles, 0.28, 0.38, and 0.0035 ${\rm mJy~pc^{-2}}$ for the "PAH'' minor axis profiles, 0.53, 1.20, and 0.0044 ${\rm mJy~pc^{-2}}$ for the "VSG'' major axis profiles, 0.57, 1.27, and 0.0044 ${\rm mJy~pc^{-2}}$ for the "VSG'' minor axis profiles, 6.32, 7.87, and $0.035 \times 10^{-19}~{\rm W~m^{-2}~pc^{-2}}$ for the [Ar  II] major axis profiles, and 5.77, 8.30, and $0.024\times 10^{-19}~{\rm W~m^{-2}~pc^{-2}}$ for the [Ar  II] minor axis profiles. The PSF profiles for the spatially-smoothed data cubes are shown as well (simple black lines; see Sect. 2).

We note that the morphology of the [Ar II] 6.99$~\mu$m emission for M 82 and NGC 1808, especially the off-nucleus peak position, reflects the spatial distribution of the H II regions and not variations in the excitation state of the ionized gas whereby, e.g., [Ar III] emission would become important at the expense of [Ar II]. As discussed in Sect. 3.2.1, the H II regions in M 82 are concentrated in a circumnuclear ring-like structure at radius of $\approx$ $85~{\rm pc}$. Direct tracers such as near-infrared to radio H recombination lines, MIR Ne, Ar, and S fine-structure lines, the He I 2.06$~\mu$m recombination line, and radio thermal continuum all show a similar distribution characterized by peaks flanking the nucleus, with the southwestern complex being the most intense, and little emission at the nucleus (e.g. Satyapal et al. 1995; Achtermann & Lacy 1995; Seaquist et al. 1996; Förster Schreiber et al. 2001). In particular, the [Ar III] 8.99$~\mu$m map of Achtermann & Lacy (1995) tracing the $\geq$ $28~{\rm eV}$ radiation field clearly shows this bilobal distribution and relative lack of emission at the nucleus. Less information is available for NGC 1808 but Br$\gamma$ imaging clearly reveals the H II region complexes to lie predominantly to the east and southeast of the nucleus, thus indicating a circumnuclear location for the starburst (Krabbe et al. 1994; Kotilainen et al. 1996).

Extinction effects could partly account for differences in the PAH, VSG, and [Ar II] 6.99$~\mu$m emission on small spatial scales (comparable to the resolution elements in our ISOCAM data) but are not likely to affect the relative concentrations. The highest levels of obscuration are measured in the near surroundings of the MIR peaks in all three galaxies and generally decrease at larger distances (e.g. Larkin et al. 1994; Engelbracht et al. 1998; Krabbe et al. 1994). Depending on the extinction law applicable in the 3- $10~{\rm\mu m}$ region (Sect. 5.1 below), such a radial gradient in extinction would either affect very little the relative global distributions of the tracers discussed here, or suppress more severely the shorter wavelength emission in the central regions, which is inconsistent with the overall trends for the 15$~\mu$m continuum and [Ar II] 6.99$~\mu$m line.

The nature of the short-wavelength continuum in normal and pure starburst galaxies is still debated. It is much weaker than in systems hosting an active galactic nucleus (AGN) where it is attributed to hot dust (150- $1700~{\rm K}$) close to the AGN (Genzel & Cesarsky 2000 and references therein). In non-AGN galaxies, the existence of 3- $5~{\rm\mu m}$ featureless continuum from small transiently heated dust grains underlying the PAH features has been proposed (e.g. Helou et al. 2000). On the other hand, the PAH features are best fitted with Lorentzian profiles whose wide wings can account for the apparent continuum pedestal (e.g. Boulanger et al. 1998b). Our narrow-band 5.5$~\mu$m continuum map of M 82 provides only tentative evidence of a more extended distribution than for the 15$~\mu$m continuum emission which would suggest a closer association of the emitting particles with PAHs than VSGs (see also Mattila et al. 1999).

NGC 1808