The AGN/starburst content in high redshift ULIRGs
  We apply a simple model, tested on local ULIRGs, to disentangle the active
galactic nucleus (AGN) and starburst contributions in submillimiter and
24um-selected ULIRGs observed with the Spitzer-IRS spectrometer. We
quantitatively estimate the average AGN contribution to the stacked 6-8um
rest-frame spectra of these sources in different luminosity and redshift
ranges, and, under the assumption of similar infrared-to-bolometric ratios as
in local ULIRGs, the relative AGN/starburst contributions to the total infrared
luminosity. Though the starburst component is always dominant in
submillimeter-selected ULIRGs, we find a significant increase of the AGN
contribution at redshift z>2.3 with respect to lower z objects. Finally, we
quantitatively confirm that the mid-infrared emission of 24um-selected ULIRGs
is dominated by the AGN component, but the starburst component contributes
significantly to the bolometric luminosity.
