Near-infrared properties of faint X-ray sources from NICMOS imaging in the Chandra Deep Fields

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 H-160 = 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 ( J(110) - H-160 > 1.4) objects brighter than H-160 < 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 J(110) - H-160 objects are at high redshifts ( z > 1.5), which we propose as the primary explanation for their extreme J(110) - H-160 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 J(110) - H-160 population.