we see here that the upper part of the subcorrelation can be explained with a single temperature.

Radiative transfer effects {#subsec:radtr}
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[If radiative transfer effects are considered,]{} FIR emission comes from dust at different optical depths, where the ISRF intensities are different because of dust extinction. As modeled in Section \[subsec:colour\], the FIR intensity is the sum of all contributions from different optical depths. @hibi06 expect that radiation transfer effects shift the FIR colours toward the subcorrelation on the colour–colour diagram.

In Fig. \[fig:clr\_varAv\], we show the FIR colours for $\chi =3$, 30, and 300 with various values of $A_V$ ($A_V=0$, 0.1, 0.2, 0.5, 1, and 2)[^5]. The FIR colours change little for $A_V\ga 2$, since the emission is negligible from such large optical depths in the wavelength range of interest because of low dust temperatures. As expected by @hibi06, there is a slight trend that the colours move toward the subcorrelation for $\chi\la 30$. However, we observe that the radiative transfer effects shift the FIR colours along the main correlation or the subcorrelation for $\chi\ga 30$. This indicates that the radiative transfer effects are hard to be distinguished from the change of $\chi$ on the colour–colour diagram.

![image](clr_radtr_sil.eps){width="8.5cm"} 