Hot dust in normal star-forming galaxies: JHKL' photometry of the ISO Key Project sample

November 2002 • 2002A&A...394..873H

Authors • Hunt, L. K. • Giovanardi, C. • Helou, G.

Abstract • We present JHK and 3.8 mu m (L') photometry of 26 galaxies in the Infrared Space Observatory (ISO) Normal Galaxy Key Project (KP) sample and of seven normal ellipticals with the aim of investigating the origin of the 4 mu m emission. The majority of the KP galaxies, and all the ellipticals, have K-L<~ 1.0, consistent with stellar photospheres plus moderate dust extinction. Ten of the 26 KP galaxies have K-L>~ 1.0, corresponding to a flat or rising 4 mu m continuum, consistent with significant emission from hot dust at 600-1000 K. K-L is anticorrelated with ISO flux ratio F_6.75/F₁₅, weakly correlated with line ratio [O I]/[C II], but not with [C II]/FIR or IRAS ratio F₆₀/F₁₀₀. Photodissociation-region models for these galaxies show that the hot dust responsible for red K-L resides in regions of high pressure and intense far-ultraviolet radiation field. Taken together, these results suggest that star formation in normal star-forming galaxies can assume two basic forms: an ``active'', relatively rare, mode characterized by hot dust, suppressed Aromatic Features in Emission (AFEs), high pressure, and intense radiation field; and the more common ``passive'' mode that occurs under more quiescent physical conditions, with AFEs, and without hot dust. The occurrence of these modes appears to only weakly depend on the star-formation rate per unit area. Passive star formation over large scales makes up the bulk of star-forming activity locally, while the ``active'' regime may dominate at high redshifts. Based on data obtained at TIRGO, Gornergrat, Switzerland.

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George Helou

IPAC Executive Director