Infrared Counterparts to Chandra X-Ray Sources in the Antennae

We use deep J (1.25 μm) and Ks (2.15 μm) images of the Antennae (NGC 4038/9) obtained with the Wide-field InfraRed Camera on the Palomar 200-inch telescope, together with the Chandra X-ray source list of Zezas et al. (2002a), to search for infrared counterparts to X-ray point sources. We establish an Xray/IR astrometric frame tie with ∼ 0. 5 rms residuals over a ∼ 4.3 field. We find 13 “strong” IR counterparts brighter than Ks = 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 ∆MKs ∼ 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 NH inferred from the X-ray sources without IR counterparts, we determine Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville, FL 32611; [email protected]. Leiden University, P.O. Box 9513, 2300 RA Leiden, Netherlands. Department of Astronomy, P.O. Box 400325, University of Virginia, Charlottesville, VA 22904. Jet Propulsion Laboratory, Earth and Space Science, Pasadena, CA 91109. Department of Astronomy, Cornell University, 610 Space Sciences Building, Ithaca, NY 14853-6801. Gemini Observatory, AURA, Inc./Casilla 603, La Serena, Chile. Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.