PEARS AGN: HST/ACS Grism Spectroscopy of Chandra Deepest Field Optical Counterparts to i = 26AB.

Astronomy and Astrophysics – Astronomy

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We investigate sensitive, high spatial resolution, grism spectroscopy for 194 of the 231 X-ray sources within the Chandra Deep Fields North (CDF-N) and South (CDF-S) covered by the PEARS (Probing Evolution And Reionization Spectroscopically) program. These HST/ACS G800L spectra (6000-9500Å) of the faintest known X-ray sources, most of which host moderate-luminosity active galactic nuclei (AGN) at moderate to high redshifts, reach some two magnitudes fainter (to iAB = 26 mag) than currently possible from ground-based observatories.
Among our 102 CDF-N spectra are 22 without prior ground-based redshifts, and similarly 22 among the 92 CDF-S spectra. Most of these sources have iAB > 25 mag, making redshift determination challenging even for the sensitive grism observations presented here. We measure redshifts for 52(48) of the CDF-N(S) counterparts, including 5(3) without prior redshift. We compare grism redshift estimates to ground-based redshifts where available, and to PEARS spectro-photometric redshift estimates derived from template-matching to the combination of (U)BViz(JHK) photometry and grism spectra.
For the 44 faintest optical counterparts beyond the reach of ground-based spectroscopy, most or all of which are moderate-luminosity AGN at z>1, we also assess whether a lack of spectral features in our grism observations can rule out a Type 1 (unobscured) AGN classification. We simulate an unobscured AGN grism extraction for each of these sources, using the Chandra X-ray flux to appropriately normalize the faintest currently available Type 1 AGN template rest-frame UV spectrum (from the VIMOS VLT Deep Survey) at putative redshifts 11.
This work was supported by HST grant GO-10530 from the Space Telescope Science Institute, which is operated by AURA under NASA contract NAS5-26555.

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