Infrared emission in paired galaxies. I - Detection fraction statistics

We present an analysis of the basic statistics for infrared emission from paired galaxies. Our pair sample is the catalog of galaxy pairs (CPG) assembled by Karachentsev in 1972. Our control sample is the catalog of isolated galaxies (CIG) assembled by Karachentseva in 1973. The IRAS Point Source Catalog (PSC) provides the infrared data for this study. Our principal motivation was to clarify the correlation of infrared emission with optical morphology in pairs. We concentrate in this paper on the correlation of the IRAS detection fraction (DF) with optical properties. We find that the DF is enhanced for pairs which contain late-type galaxies. Significantly more spiral-spiral (SS) and spiral-elliptical/lenticular (SE) pairs are detected than are expected based upon the control sample DF. Very few pairs involving only elliptical and lenticular galaxies (EE) are detected. The detection of excess FIR emission from the SE sample indicates that many such pairs are physical binaries. Most of our sample lies in the redshift range 0-11 000 km/s. We determine an average enhancement in DF for spiral components of pairs in this volume of 0.19 +/- 0.05. We do not know how the actual enhancement in SS pairs is distributed, but it is likely that only one component is active in SE pairs. We find that the DFs for Sa-c galaxies are more similar than would be expected if IR emission were principally correlated with gas/dust content. This suggests that the efficiency of the process responsible for the emission plays an important role. The use of interaction morphology in selecting IR active pairs is found to be important. The DFs for pairs with bridges LIN(br) or internal distortions (DIS) are higher than DFs for complete pair samples by up to 20%. Other interaction descriptors LIN(ta); ATM are not so useful. An excess DF is also found, however, among the fraction of pairs with no visible signs of interaction. Surprisingly little correlation is found between pair component separation (measured in terms of the larger component diameter) and DF. Similarly little correlation is found between DF and A/B, the ratio of component major-axis diameters. This result favors the idea that one component dominates the FIR emission even in SS pairs. Allowance for redshift and morphology distributions in pair and control samples can explain most previous published results for pairs and especially apparent discrepancies among these results.