On the nature of Extremely Red Objects: the 2 Ms Chandra Deep   Field-North Survey results
  Understanding the nature of Extremely Red Objects [EROs; (I-K)>=4] is one of the most challenging issues in observational cosmology. Here we report on the X-ray constraints provided by the 2 Ms Chandra Deep Field-North Survey (CDF-N). X-ray emission has been detected from 11 out of 36 EROs (approx 30%). Five of these have hard X-ray emission and appear to be obscured AGNs, while non-AGN emission (star formation or normal elliptical galaxy emission) is likely to be the dominant source of X-rays from the soft X-ray sources detected at the faintest X-ray flux levels. 
C. Vignali, D.M. Alexander, F.E. Bauer, W.N. Brandt, A.E.   Hornschemeier, G.P. Garmire, and D.P. Schneider