Astronomy & Astrophysics

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$\begin{figure} \par\includegraphics[width=17.7cm,clip]{H4645F1.ps}\end{figure}$ Figure 1: The 2-3000 $\mu$ m spectrum of Arp 220. Filled circles represent the IRAS fluxes. Spectra shown are: the smoothed 2.4-4.9 $\mu$ m ISO-PHT-S spectrum (Spoon et al. 2002); the 3.2-3.8 $\mu$ m CGS4 spectrum obtained in 1.2 $\hbox {$^{\prime \prime }$ }$ slit (Imanishi & Dudley 2000); the 5.0-16 $\mu$ m ISO-CAM-CVF spectrum (Tran et al. 2001); the 17-22 $\mu$ m UCL spectrum (Smith et al. 1989); the smoothed 45-200 $\mu$ m ISO-LWS spectrum (Fischer et al. 1997). Other spectral data included in the plot are ISO-PHT photometry (Klaas et al. 1997; M. Haas, priv. comm.); ISO-SWS background subtracted continuum measurements (E. Sturm, priv. comm.); UKIRT and SCUBA far-IR photometry (Eales et al. 1989; Dunne et al. 2000, 2001) and mm-observations (Anantharamaiah et al. 2000). The dash-dotted line is one choice for the local continuum in the 5-25 $\mu$ m region (see also Fig. 5). The inset shows a comparison of the 5.8-11.7 $\mu$ m ISO-PHT-S (Spoon et al. 2002) and the 5.0-16 $\mu$ m ISO-CAM-CVF spectra (shown as grey and black lines respectively) with the Keck-MIRLIN photometry ( filled circles) of Soifer et al. (1999).
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In the text

$\begin{figure} \par\includegraphics[width=8.7cm,clip]{H4645F2.ps} \end{figure}$ Figure 2: A comparison of the ISO-PHT-S spectra of M 83, Arp 220 and NGC 4418. While the spectrum of M 83 is dominated by PAH emission bands, the spectrum of NGC 4418 is dominated by absorption bands of ices and silicates. The spectrum of Arp 220 shows characteristics of both. The spectra of M 83 and NGC 4418 have been scaled and offset.
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In the text

$\begin{figure} \par\includegraphics[width=8.7cm,clip]{H4645F3.ps} \end{figure}$ Figure 3: Comparison of the spectra of Arp 220 ( thick grey line) and the central region of starburst galaxy M 83 ( black line). The M 83 spectrum shows strong PAH emission features at 6.2, 7.7, 8.6 and 11.2 $\mu$ m. In contrast, in Arp 220 the 6.2, 8.6 and 11.2 $\mu$ m features are weak. The black dashed line illustrates the effect of strong extinction (A(V)= 105; A(7.7 $\mu$ m) = 1.2) on the M 83 spectrum. Both versions of the M 83 spectrum have been scaled to match the 7.7 $\mu$ m peak in the spectrum of Arp 220.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{H4645F4.ps}\end{figure}$ Figure 4: The line of sight to the embedded protostar Mon R2:IRS 1 passes through the ultra compact $H {\sc ii}$ region Mon R2:IRS 2. The ISO-SWS spectrum of Mon R2:IRS 1+2 ( black) hence shows features of both type of environments: PAH emission from the ultra compact $H {\sc ii}$ region and a dust and ice absorbed continuum from the embedded protostar. Here we show a simple 2-component fit to the Mon R2:IRS 1+2 spectrum, using the ISO-SWS spectra of the protostar NGC 7538:IRS 9 ( light grey area) and the reflection nebula NGC 7023 ( dark grey area) as fitting templates.
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In the text

$\begin{figure} \par\includegraphics[width=17.5cm,clip]{H4645F5.ps}\end{figure}$ Figure 5: Comparison of the mid-IR spectra of Arp 220 (multiplied by 90) and NGC 4418. For Arp 220 the following spectra are plotted: in black the 5-16 $\mu$ m ISO-CAM-CVF spectrum, in light grey the 5.6-11.4 $\mu$ m ISO-PHT-S spectrum and in dark grey the 17-22 $\mu$ m UCL spectrum (Smith et al. 1989). ISO-SWS continuum points are marked by open circles, ISO-PHT photometry by filled circles and the IRAS 25 $\mu$ m flux by a cross. For NGC 4418 the spectra plotted are: in black the 5.6-11.4 $\mu$ m ISO-PHT-S spectrum and in light grey the 8-13 $\mu$ m and 17-23 $\mu$ m UCL spectra of Roche et al. (1986). IRTF photometry (Wynn-Williams & Becklin 1993) is marked by filled circles, ISO-CAM (LW3) photometry by a triangle and the IRAS 25 $\mu$ m flux by a cross. The positions of absorption and emission bands are indicated, as are several choices for the local continuum for each object.
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In the text

$\begin{figure} \par\includegraphics[width=17.9cm,clip]{H4645F6.ps}\end{figure}$ Figure 6: PAH and continuum templates used in the decomposition of the observed mid-IR spectrum of Arp 220. Left panel: four mid-IR absorbed continuum spectra. The spectra are shown at their instrumental resolution: $R \sim 90$ for IRAS 03344-02103 and NGC 4418; $R \sim 40$ for IRAS 00183-7111; $R \sim 1000$ for Sgr A^*. The spectra have been normalized to their 7.5-8.5 $\mu$ m peak fluxes and are sorted according to the central wavelength of their 7.5-8.5 $\mu$ m peak flux. Right panel: three mid-IR PAH-dominated spectra, shown at a spectral resolution $R \sim 90$ . The spectra have been normalized to their 7.7 $\mu$ m peak fluxes and are sorted in order of increasing 9-12 $\mu$ m continuum.
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{H4645F7.ps}\end{figure}$ Figure 7: Comparison of two mid-IR extinction laws, overlayed on the PAH emission spectrum of reflection nebula NGC 7023 ( grey area). The Galactic center extinction law of Lutz (1999) is shown as a black line, the extinction law of Draine (1989) for the local solar neigbourhood as a dotted line. The extinction laws are refered to as Lutz_gc and draine_local, respectively.
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In the text

$\begin{figure} \par\includegraphics[width=17.9cm,clip]{H4645F8.ps}\end{figure}$ Figure 8: Four 2-component fits to the Arp 220 mid-IR ISO spectra (ISO-PHT-S: dark grey histogram; ISO-CAM-CVF: dark grey line). In each panel the dark grey area represents the contribution of the continuum component and the light grey area the contribution of the PAH component. The PAH component is the same in all panels: the spectrum of the starburst galaxy M 83. The absorbed continuum component differs between left and right panels. In the left panels it is NGC 4418 as observed; in the right panels it is NGC 4418 as seen through an additional screen of A(9.6 $\mu$ m) = 2.5, equivalent to a factor ten more attenuation at 9.6 $\mu$ m. The fractional contribution of each fit component to the total 6-12 $\mu$ m fit is stated in brackets behind the name of the template. Also indicated is the amount of 9.6 $\mu$ m screen extinction on the PAH component.
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In the text

$\begin{figure} \par\includegraphics[width=17.8cm,clip]{H4645F9.ps}\end{figure}$ Figure 9: The 2-3000 $\mu$ m spectrum of Arp 220 ( grey) overplotted with a three component fit. The black dashed curve represents the model SED for the two identical nuclei. The black dotted curve represents grey body emission from 30 K cold dust. The black dash-dotted curve is the sum of these two components. The black continuous curve is the sum of the model SED and the weakly absorbed diffuse PAH component (M 83) from our 2-component decomposition (Sect. 3).
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In the text

$\begin{figure} \par\includegraphics[width=8.7cm,clip]{H4645F2.ps} \end{figure}$	Figure 2: A comparison of the ISO-PHT-S spectra of M 83, Arp 220 and NGC 4418. While the spectrum of M 83 is dominated by PAH emission bands, the spectrum of NGC 4418 is dominated by absorption bands of ices and silicates. The spectrum of Arp 220 shows characteristics of both. The spectra of M 83 and NGC 4418 have been scaled and offset.
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$\begin{figure} \par\includegraphics[width=8.6cm,clip]{H4645F7.ps}\end{figure}$	Figure 7: Comparison of two mid-IR extinction laws, overlayed on the PAH emission spectrum of reflection nebula NGC 7023 ( grey area). The Galactic center extinction law of Lutz (1999) is shown as a black line, the extinction law of Draine (1989) for the local solar neigbourhood as a dotted line. The extinction laws are refered to as `Lutz_gc` and `draine_local`, respectively.
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