3 Comparison with ISO photometry and spectra

If the non-stellar 3.8 $\mu$m flux is truly associated with hot dust, we would expect to find correlations between ISO MIR observations and $L^\prime$, if both arise from a similar small-grain population. Also, observations of the two most important photo-dissociation region (PDR) cooling lines, [C  II] and [O  I], suggest that gas and dust temperatures increase together (Malhotra et al. 2001); we might therefore expect trends with these line fluxes and ratios and K-L.

   
3.1 Broadband fluxes

We first checked that our ground-based (small-aperture) $L^\prime$ fluxes were broadly consistent with the ISO fluxes at F_6.75 and F₁₅ (Dale et al. 2000). All three are mutually correlated (not shown), except for two cases (NGC 278 and NGC 5713) where the ISO flux is substantially larger than expected from the general trend. This discrepancy is almost certainly due to aperture effects, since the ground-based data are acquired in a 14 $^{\prime\prime}$ aperture, while the ISO values published in Dale et al. (2000) are total fluxes as extrapolated from curves of growth. NGC 1569 is generally an outlier; however, its properties are similar to those of other low-metallicity dwarf irregulars (see Hunter et al. 2001). The other dwarf irregular in our observed sample is NGC 1156, again a clear outlier in most of the subsequent plots and correlations. As noted in Sect. 2, these galaxies are also the two closest galaxies, which considerably complicates the comparison of the ISO and our ground-based 14 $^{\prime\prime}$ aperture data.

Figure 2: Hybrid ground-based/ISO color-color plot: the top panel shows the ratio of the 3.8 $\mu$m flux (not transformed to L) and F15plotted against K-L, and the bottom panel the ratio of F6.75 and F15 plotted against K-L. The dotted line shows the best-fit regression; the correlation coefficients are shown in each panel (parentheses denote the coefficient without the two nearest galaxies NGC 1569 and NGC 1156). Solid dots are used when the corrected H-K>0.35.

We next compare ISO colors with K-L in Fig. 2 where F_6.75/F₁₅ and the "hybrid'' color F_3.8/F₁₅ are plotted against K-L. Ground-based K-L turns out to be correlated (2.5 $\sigma$) with F_6.75/F₁₅, but is uncorrelated with the hybrid color. The sense of the F_6.75/F₁₅ vs. K-L correlation is such that redder K-L implies lower F_6.75/F₁₅. NGC 1569 stands out since it has a very low F_6.75/F₁₅ratio for its K-L color. As an extreme case, in Fig. 2, we have also plotted SBS 0335-052 (not considered in the correlation), using the ISO data from Thuan et al. (1999) and K-L from Hunt et al. (2001). SBS 0335-052 is an unusual BCD with 1/40 solar metallicity, and the ISO spectrum shows no AFEs; not surprisingly Fig. 2 shows that this galaxy has a lower F_6.75/F₁₅ ratio than any of the galaxies in the ISO sample.

Figure 3: IRAS 12 $\rm\mu m$/25 $\rm\mu m$ flux ratio vs. K-L. No IRAS upper limits are shown. Solid points refer to galaxies with a corrected H-K>0.35. The best-fit regression is shown together with the correlation coefficient (in parentheses is that without NGC 1156 and NGC 1569, the two nearest galaxies).

Since dust temperatures in large "classical'' grains are connected with the IRAS flux ratio F₆₀/F₁₀₀, and since this ratio and F_6.75/F₁₅ are anticorrelated (Dale et al. 2000), we might expect K-L to also be anticorrelated with F₆₀/F₁₀₀. We found no such correlation but note, instead, that IRAS F₁₂/F₂₅ is anticorrelated (2.6 $\sigma$) with K-L, as shown in Fig. 3. Together with the trend with K-L and F_6.75/F₁₅, this means that the presence of hot dust is: a) usually linked to the suppression of AFEs which dominate the $12~\mu$m band (Helou et al. 1991), and b) largely independent of the temperature and characteristics of the large grains.

   
3.2 Line measurements

In PDRs and H  II regions, gas and dust are intimately related. In ionized regions, gas and dust compete for far-ultraviolet (FUV) photons, but the neutral gas in PDRs is heated predominantly by photoelectrons from small dust grains (Watson 1972; Hollenbach & Tielens 1997). Neutral gas is cooled primarily by atomic and ionic fine-structure lines, [C  II] (158 $\mu$m) and [O  I] (63 $\mu$m), and these lines can be used as diagnostics for the physical conditions in the PDR gas (Tielens & Hollenbach 1985): the [O  I] line is expected to become more important relative to [C  II] in warmer and denser gas. [C  II] and [O  I] line fluxes for the galaxies in our sample have been measured by ISO (Malhotra et al. 2001), and in this section we analyze those measurements in the context of our new photometry.

Figure 4: ISO emission-line ratios vs. K-L. The top panel shows the log of the ratio of [C  II] (158 $\mu$m) and [O  I] (63 $\mu$m) line fluxes vs. K-L, and the bottom panel the log of the ratio of [C  II] and FIR vs. K-L. The correlation coefficients are shown in each panel (the correlation coefficient in parentheses is that without NGC 1156 and NGC 1569, the two nearest objects). Solid dots refer to galaxies with corrected H-K > 0.35.

An important diagnostic is the ratio of [C  II] and the far-infrared flux (FIR), since it measures essentially the efficiency of the photoelectric heating of the gas by dust grain ejection. This ratio [C  II]/FIR tends to decrease with warmer FIR colors F₆₀/F₁₀₀ and increasing star-formation activity as indicated by FIR/B (Malhotra et al. 2001). Moreover, warmer gas, as signified by smaller [C  II]/[O  I], correlates with warm dust or larger F₆₀/F₁₀₀; this last is the most significant correlation in the study by Malhotra et al. (2001). Since red K-L should be related to hot dust and its temperature, we might expect to find similar correlations with normalized FIR line fluxes and ratios. However, this supposition is not borne out by the data (see Fig. 4): we find no correlation between K-L and [C  II]/FIR, and only a weak anticorrelation between K-L and [C  II]/[O  I]. Again, K-L appears to be measuring a hot-dust phase, not closely connected to the properties of the cooler dust.