2005ApJ...621..587C

We measure the near-infrared properties of 42 X-ray-detected sources from the Chandra Deep Fields North and South, the majority of which lie within the NICMOS Hubble Deep Field-North and Ultra Deep Field. We detect all 42 Chandra sources with NICMOS, with 95% brighter than H₁₆₀=24.5. We find that X-ray sources are most often in the brightest and most massive galaxies. Neither the X-ray fluxes nor the hardness ratios of the sample show any correlation with near-infrared flux, color, or morphology. This lack of correlation indicates that there is little connection between the two emission mechanisms and is consistent with the near-infrared emission being dominated by starlight rather than a Seyfert nonstellar continuum. Near-infrared X-ray sources make up roughly half of all extremely red (J₁₁₀-H₁₆₀>1.4) objects brighter than H₁₆₀<24.5. These red X-ray sources have a range of hardness ratios similar to the rest of the sample, decreasing the likelihood of dust-obscured active galactic nucleus (AGN) activity as the sole explanation for their red color. Using a combination of spectroscopic and photometric redshifts, we find that the red J₁₁₀-H₁₆₀objects are at high redshifts (z>1.5), which we propose as the primary explanation for their extreme J₁₁₀-H₁₆₀ color. Measurement of rest-wavelength absolute B magnitudes shows that X-ray sources are the brightest optical objects at all redshifts, which explains their dominance of the bright end of the red J₁₁₀-H₁₆₀population.