4 PDR models

Figure 5: Log n (upper panel) and Log 0 (lower) plotted against K-L. The PDR parameters are taken from Malhotra et al. (2001). The dotted line in the lower panel shows the best-fit regression; the correlation coefficients are shown in each panel. Solid points refer to galaxies with corrected H-K > 0.35. Estimate of correlation coefficients and regression include all objects (the parentheses contain the coefficient without NGC 1156 and NGC 1569, the two nearest galaxies).

Photodissociation models for all the galaxies in our sample have been reported in Malhotra et al. (2001), calculated from the model grids of Kaufman et al. (1999). These models calculate the line emission in [C  II] and in [O  I] (including the line at 145 $\mu$m) and the dust continuum emission for a plane slab of gas illuminated on one side by FUV radiation. Gas heating in the models is dominated by photoelectrons ejected from classical (large) grains and from VSGs according to the recipe of Bakes & Tielens (1994). From these models, it is possible to infer values for the neutral gas density n and the FUV radiation field 0, as well as for the pressure P and temperature T of the gas.

Figure 6: Log T (upper panel) and Log P (lower) plotted against K-L. The PDR parameters are taken from Malhotra et al. (2001). The dotted lines show the best-fit regression; the correlation coefficients are reported in each panel. Solid points refer to galaxies with corrected H-K>0.35. Estimates of regression line and correlation coefficients include all galaxies (the parentheses contain the coefficient without the two nearest objects, NGC 1156 and NGC 1569).

Figure 5 shows gas density n and FUV flux 0, as derived from PDR models, plotted against K-L. Without NGC 1156 (one of the two nearest galaxies), K-L is well correlated with 0, at the 3.5 $\sigma$ level. This is a clear indication that red K-L is connected to strong FUV radiation fields. Also shown in Fig. 5 is SBS 0335-052, with a $G_0 \sim10~000$ times stronger than the local value (Dale et al. 2001); this galaxy follows the same correlation. We then examine whether the primary dependency of red K-L color is on neutral gas temperature T or on its pressure P, given that the K-L color is independent, or roughly so, of neutral gas density n. The relevant PDR parameters are plotted in Fig. 6 where T and P as inferred from the PDR models are plotted versus K-L. The thermal pressure P of the PDR is expected to be approximately equal to the thermal pressure of the adjacent H  II region (Malhotra et al. 2001). The correlation between P and K-L is weak but becomes significant (2.8$\sigma$) once we exclude NGC 1156. There is no apparent correlation between T and K-L. Taken together, these results suggest that the PDR pressure (roughly equal to the H  II pressure) and the FUV radiation field 0 are the main factors which govern the existence of hot dust in star-forming galaxies; neutral gas density and temperature by themselves are less important influences.