6 Conclusions

With our JHK$L^\prime$ photometry, we have analyzed the 4 $\mu$m continuum and its relation with the MIR spectrum and FIR emission lines. We find the following:

1.

The majority of the 26 KP galaxies have a falling 4 $\mu$m continuum, K-L < 1.0, consistent with stellar photospheres and moderate dust extinction. 10 of them have a flat or rising 4 $\mu$m continuum, $K-L\ga 1.0$, consistent with a measurable fraction of 600-1000 K hot dust.

2.

K-L is anticorrelated with ISO ratios F_6.75/F₁₅ and IRAS ratio F₁₂/F₂₅, but only weakly with [C  II]/[O  I], and not at all with [C  II]/FIR/ or IRAS ratio F₆₀/F₁₀₀.

3.

PDR models for these galaxies show that the hot dust measured by red K-Lis associated with high pressures and intense far-ultraviolet radiation fields in compact ($\la$100 pc) regions.

4.

These results taken together suggest that star formation in these galaxies occurs in two "extreme forms'':

(a)

a relatively rare "active'' mode characterized by hot dust, suppressed AFEs, high pressure, intense ultraviolet radiation field, and compact size;

(b)

a more common "passive'' mode characterized by photospheric K-L colors, with moderate extinction, and less extreme physical conditions.

5.

The physical conditions we infer for the star-forming regions containing hot dust are similar to those created by interactions and mergers. We speculate that such intense episodes may have been more common in the past, so that the "active'' regime could dominate star formation at high redshift.

Acknowledgements

The TIRGO staff was instrumental in the success of these observations. We are grateful to Giovanni Moriondo who valiantly attempted to observe for this project, and to the TIRGO Time Allocation Committee for generous time allocations. We gladly acknowledge enlightening discussions with Marc Sauvage, and would like to thank an anonymous referee for several insightful comments.