2 The proposed new method and the database

In order to determine the total amount of extinction, we here propose a different method, using the PAH 7.7 $\mu$m to continuum 850 $\mu$m flux ratio. While the sub-mm radiation is optically thin and represents the emission from essentially all dust grains, the PAH strength (measured by the peak height of the Polycyclic Aromatic Hydocarbonates at 7.7 $\mu$m) is sensitive to dust extinction in the MIR.

Figure 1: Two-colour diagram PAH 7.7 $\mu$m to 850 $\mu$m flux versus PAH 7.7 $\mu$m to 100 $\mu$m. The symbols are: $\times$ = Seyfert-ULIRGs, $\blacklozenge$ = LINER- and HII/SB-ULIRGs, $\bullet$ = normal galaxies. The errors are less than 30%. The positions of Arp 220 and UGC 5101 after dereddening are marked with the encircled crosses. For Arp 220 two mixed case extinction curves are considered: interstellar dust (MRN: dotted line), and protostellar dust (KS: dashed line). The dash-dotted box indicates the min-to-max range of the typical ULIRGs after dereddenening with the KS model (Sect. 4.1) and AV values from Genzel et al. (1998), note that dereddening reduces the PAH 7.7 $\mu$m/850 $\mu$m dispersion

Figure 2: Two-"colour'' diagram PAH 7.7 $\mu$m to 850 $\mu$m flux versus $L_{\rm MIR}/L_{\rm FIR} (= L_{\rm 10-40~\mu m}/L_{\rm 40-1000~\mu m}$). Symbols as in Fig. 1. In addition, two PG quasars, where PAH spectra are available, are plotted. The dereddened positions of Arp 220 correspond to the KS and MRN cases listed in Table 1. The vertical dash-dotted line marks the division between warm and cool ULIRGs according to $F_{\rm 25~\mu m}/F_{\rm 60~\mu m} \approx 0.2$

We apply this tool to 850 and 1300 $\mu$m observations of a sample of nearby bright ULIRGs (Klaas et al. 2001) most of which we recently obtained at the JCMT with SCUBA and at the SEST. These data, supplemented by literature data (Rigopoulou et al. 1996; Lisenfeld et al. 2000) and with the 1300 $\mu$m fluxes interpolated to 850 $\mu$m, are compared with published PAH spectra of ULIRGs, obtained with ISO (Rigopoulou et al. 1999) and with the UKIRT (Smith et al. 1989; Dudley 1999). This results in a sample of 15 ULIRGs with $L_{\rm FIR} \approx 10^{\rm 12}~L_{\odot}$ and both the sub-mm and PAH fluxes available.

A comparison sample of 20 "normal galaxies'' was constructed as a subset of the SCUBA 850 $\mu$m local universe galaxy survey (Dunne et al. 2000) containing dusty galaxies with $L_{\rm FIR} < 10^{\rm 10}~L_{\odot}$. We selected those for which PAH spectra are available in the ISO data archive and for which the ISOPHOT-S aperture (24 $\hbox{$^{\prime\prime}$ }$) contains the entire 850 $\mu$m flux, as is the case for the ULIRGs.

The synchrotron contribution (clearly below 10%) has been subtracted from the 850 $\mu$m fluxes.