Infrared Counterparts to Chandra X-Ray Sources in the Antennae

Abstract

We use deep J and Ks images of the Antennae (NGC 4038/9) obtained with WIRC on the Palomar 200-inch telescope, together with the Chandra X-ray source list of Zezas et al. (2002a), to search for IR counterparts to X-ray point sources. We establish an X-ray/IR astrometric frame tie with 0.5&#34; rms residuals over a \~4.3&#39; field. We find 13 ``strong&#39;&#39; IR counterparts brighter than Ks = 17.8 mag and < 1.0&#34; from X-ray sources, and an additional 6 ``possible&#39;&#39; IR counterparts between 1.0&#34; and 1.5&#34; from X-ray sources. The surface density of IR sources near the X-ray sources suggests only ~2 of the ``strong&#39;&#39; counterparts and ~3 of the ``possible&#39;&#39; counterparts are chance superpositions of unrelated objects. Comparing both strong and possible IR counterparts to our photometric study of ~220 Antennae, IR clusters, we find the IR counterparts to X-ray sources are \~1.2 mag more luminous in Ks than average non-X-ray clusters. We also note that the X-ray/IR matches are concentrated in the spiral arms and ``overlap&#39;&#39; regions of the Antennae. This implies that these X-ray sources lie in the most ``super&#39;&#39; of the Antennae&#39;s Super Star Clusters, and thus trace the recent massive star formation history here. Based on the N_H inferred from the X-ray sources without IR counterparts, we determine that the absence of most of the ``missing&#39;&#39; IR counterparts is because they are intrinsically less luminous in the IR, implying that they trace a different (possibly older) stellar population.