2007ApJ...658..319C

We use deep J (1.25 µm) and K_s(2.15 µm) images of the Antennae (NGC 4038/4039) obtained with the Wide-field InfraRed Camera on the Palomar 200 inch (5 m) telescope, together with the Chandra X-ray source list of Zezas and coworkers to search for infrared counterparts to X-ray point sources. We establish an X-ray/IR astrometric frame tie with ∼0.5" rms residuals over a ∼4.3' field. We find 13 ``strong'' IR counterparts brighter than K<sub>s</sub>=17.8 mag and <1.0" from X-ray sources, and an additional 6 ``possible'' IR counterparts between 1.0'' and 1.5'' from X-ray sources. Based on a detailed study of the surface density of IR sources near the X-ray sources, we expect only ∼2 of the ``strong'' counterparts and ∼3 of the ``possible'' counterparts to be chance superpositions of unrelated objects. Comparing both strong and possible IR counterparts to our photometric study of ∼220 IR clusters in the Antennae, we find with a >99.9% confidence level that IR counterparts to X-ray sources are ΔM_Ks∼1.2 mag more luminous than average non-X-ray clusters. We also note that the X-ray/IR matches are concentrated in the spiral arms and ``overlap'' regions of the Antennae. This implies that these X-ray sources lie in the most ``super'' of the Antennae's super star clusters, and thus trace the recent massive star formation history here. Based on the N_Hinferred from the X-ray sources without IR counterparts, we determine that the absence of most of the ``missing'' IR counterparts is not due to extinction, but that these sources are intrinsically less luminous in the IR, implying that they trace a different (possibly older) stellar population. We find no clear correlation between X-ray luminosity classes and IR properties of the sources, although small-number statistics hamper this analysis.