Near‐Infrared Properties of Faint X‐Ray Sources from NICMOS Imaging in the Chandra Deep Fields

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Abstract
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 - H160 objects 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 - H160 population.
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