Chandra Constraints on the AGN Fraction and Star Formation Rate of Red z>~2 Galaxies in the FIRES MS1054-03 Field

Abstract

Very deep near-infrared observations have recently uncovered a significant population of red galaxies at redshifts z>2. These distant red galaxies (DRGs) are efficiently selected by the criterion J-K>2.3. We use Chandra data to examine the X-ray emission from DRGs in the 5'x5' FIRES MS1054-03 field. Two of 40 DRGs with K<22 are detected by Chandra, and we infer that ~5 % of DRGs host active nuclei with L>1.2x10^43 ergs/s. This fraction is smaller than that inferred from optical and near-IR spectroscopy, probably largely due to strong spectroscopic selection biases. By stacking all undetected DRGs we find that their average X-ray flux in the 0.5-8 keV band is ~4.6x10^-17 ergs/s/cm^2. The detection is only significant in the soft and full energy bands. The mean detection may result from star formation, the presence of low luminosity AGN, or a combination of both. Assuming the detection is due exclusively to star formation, we find an average instantaneous star formation rate of 214 +- 68 (random) +- 73 (systematic) solar masses per year, in excellent agreement with previous results from spectral energy distribution fitting when constant star formation histories are assumed. These results may imply that DRGs contribute significantly to the cosmic star formation rate at z~2.5. However, the mean X-ray flux strictly provides only an upper limit to the star formation rate due to the uncertain contribution of low luminosity, possibly obscured AGNs. Observations at other wavelengths are needed to provide independent estimates of the star formation rate of DRGs.