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Title:
PEARS AGN: HST/ACS Grism Spectroscopy of Chandra Deepest Field Optical Counterparts to i = 26AB.
Authors:
Grogin, Norman A.; Malhotra, S.; Rhoads, J.; Cohen, S.; Hathi, N.; Windhorst, R.; Pirzkal, N.
Affiliation:
AA(Arizona State University), AB(Arizona State University), AC(Arizona State University), AD(Arizona State University), AE(Arizona State University), AF(Arizona State University), AG(Space Telescope Science Institute)
Publication:
American Astronomical Society, AAS Meeting #211, #46.05; Bulletin of the American Astronomical Society, Vol. 39, p.801
Publication Date:
12/2007
Origin:
AAS
Bibliographic Code:
2007AAS...211.4605G

Abstract

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 1<z<5. Our grism sensitivity to prominent Type 1 AGN emission lines (CIV, CIII, and MgII) throughout this redshift range allows us to comment upon the poorly-constrained ratio of obscured to unobscured AGN at z>1.

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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