Infrared Properties of Close Pairs of Galaxies

We discuss spectroscopy and infrared photometry for a complete sample of ~800 galaxies in close pairs objectively selected from the CfA2 redshift survey.We use 2MASS to compare near infrared color-color diagrams for our sample withthe Nearby Field Galaxy Sample and with a set of IRAS flux-limited pairs fromSurace et al. We construct a basic statistical model to explore the physicalsources of the substantial differences among these samples. The model explainsthe spread of near infrared colors and is consistent with a picture wherecentral star formation is triggered by the galaxy-galaxy interaction before amerger occurs. For 160 galaxies we report new, deep JHK photometry within ourspectroscopic aperture and we use the combined spectroscopic and photometricdata to explore the physical conditions in the central bursts. We find a set ofobjects with H-K >= 0.45 and with a large F(FIR)/F(H). We interpret the veryred H-K colors as evidence for 600-1000 K dust within compact star-formingregions, perhaps similar to super-star clusters identified in individualwell-studied interacting galaxies. The galaxies in our sample are candidate``hidden'' bursts or, possibly, ``hidden'' AGN. Over the entire pair sample,both spectroscopic and photometric data show that the specific star formationrate decreases with the projected separation of the pair. The data suggest thatthe near infrared color-color diagram is also a function of the projectedseparation; all of the objects with central near infrared colors indicative ofbursts of star formation lie at small projected separation.Comment: 32 pages of text, 18 figures, accepted for publication (Astronomical Journal