Detecting the Highest Redshift (z > 8) QSOs in a Wide, Near Infrared Slitless Spectroscopic Survey

We investigate the prospects of extending observations of high redshift QSOs to z>8 by means of a very wide-area near-infrared slitless spectroscopic survey, e.g. the planned survey with the European Space Agency's Euclid telescope. For any QSOs at z>8.06 the strong Lyman-alpha line will enter the wavelength range of the Euclid near-infrared spectrometer and imaging photometer (NISP). We perform a detailed simulation of Euclid NISP slitless spectroscopy (with the parameters of the wide survey) in an artificial field containing QSO spectra at all redshifts up to z=12. The simulation spectra are analysed with an automated redshift finder, and a detection rate estimated as a function of H magnitude and redshift. Spectroscopic identification of QSOs would reach deeper limits for the redshift ranges where either H-alpha (0.67<z<2.05) or Lyman-alpha (z>8.06) is visible. Furthermore, if photometrically-selected z>8 spectra can be re-examined and re-fitted to minimize the effects of spectral contamination, the QSO detection rate in the Lyman-alpha window will be increased by an estimated 60% and will then be better here than at any other redshift, with an effective limit H=21.5. With an extrapolated rate of QSO evolution, we predict the Euclid wide (15000 sq. deg.) spectroscopic survey will identify 20-35 (19-33 with a small correction for lineless objects) very luminous QSOs at z>8.06.