FAR-INFRARED PROPERTIES OF MARKARIAN GALAXIES WITH MULTIPLE NUCLEI - WARM DUST EMISSION IN MERGERS

Coadded IRAS data are investigated for 187 Markarian galaxies with multiple optical nuclei or extreme morphological peculiarities suggestive of galaxy interactions and mergers. The sample also contains spiral and irregular galaxies with giant H II regions which were described as multiple-nucleus galaxies in early studies. Their far-infrared (FIR) properties are compared to the IRAS Bright Galaxy Sample, a complete sample of University of Michigan emission-line galaxies, and normal galaxies selected from the Uppsala General Catalogue. Statistically significant differences among the samples are mainly due to selection biases inevitably associated with the dominant dust heating components. The multiple-nucleus Markarian galaxies have a significantly higher median dust temperature than each of these samples, indicating a greater contribution of active star formation to the total FIR luminosity. Correlations between integral FIR properties and nuclear optical measurements suggest that warm dust concentrated in the nuclei dominates the FIR emission detected by IRAS. Inspection of currently available Markarian galaxy detections indicates that use of the FIR two-color plane misses 50% of the known Seyfert 2 galaxies and 20% of the Seyfert 1s in the Markarian sample. This confirms previous indications that both optical/UV and FIR selection techniques must be employed to obtain complete samples of AGNs. A subsample of merger candidates is compared statistically to other galaxy samples in the framework of a two-component dust heating model which incorporates a realistic grain size distribution, including PAH molecules. A systematic increase in the relative contribution of warm dust emission due to active star formation and AGNs is found, ranging from almost-equal-to 35% ([f60/f100] = 0.41) in isolated optically selected galaxies, to almost-equal-to 40% ([f60/f100] = 0.47) in interacting galaxies chosen for their optical tidal features, almost-equal-to 55% ([f60/f100] = 0.60) in the double-nucleus Markarian galaxies, and almost-equal-to 80% ([f60/f100] = 1.05) in dusty FIR ultraluminous galaxies which are apparently advanced mergers. However, the inferred enhancement of FIR luminosity induced by galaxy collisions is shown to depend strongly on the assumed nature of the precursor galaxies. A significant correlation is found which demonstrates that f60/f100 increases with decreasing projected nuclear separation. These aspects can be qualitatively understood in a model which considers enhanced OB star formation and alteration of the grain size distribution by shocks generated during interpenetrating collisions between galaxies separated by less-than-or-similar-to 10 kpc. The observation of substantially higher dust temperatures than found in isolated disk galaxies of comparable optical luminosities, and mounting evidence that the warm dust emission is concentrated in the nuclear regions, are inconsistent with a "subdued" interpretation of ultraluminous merging galaxies proposed by H. A. Thronson and co-workers which appeals to dispersed dust heated by a dilute interstellar radiation field that is dominated by old stars.