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N. A. Grogin (JHU), A. T. Steffen, W. N. Brandt (PSU), M. Paolillo (U. of Naples), A. M. Koekemoer (STScI), E. J. Schreier (AUI)
In both GOODS fields, the survey's HST/ACS observations were taken in five epochs with uniform ~45-day spacing, for the primary purpose of high-z supernova detection and follow-up. However, this combination of sensitivity, high resolution, and time spacing is also well-suited to detecting optical variables (OVs) among ~50000 moderate-z galaxy nuclei on rest-frame timescales of weeks to months. The overwhelming majority of these variable galaxy nuclei will be AGN; the small fraction arising from SNe have already been meticulously culled by the high-z SNe search effort. Previous OV studies in the HDF-N have shown relatively poor overlap with X-ray-identified AGN from CXO 2-Msec observations. Thus OV galaxy nuclei potentially represent a significant addition to the census of distant AGN subject to multiwavelength scrutiny with GOODS.
We present the results of our z-band variability analysis, including the discovery of ~170 ``significantly variable'' galaxy nuclei at z850 < 24.5AB whose light-curve excess variance is statistically inconsistent with non-variability at the 10-4 level or better. Although the GOODS sensitivity is less than previous two-epoch OV searches in the HDF-N, the much larger GOODS area compared to HDF-N results in a several-fold increase to the number of known OV AGN in the Chandra Deep Fields. Moreover, the GOODS OVs are broadly distributed in redshift out to z~1.3, comparable to the redshift distribution of the Chandra Deep Field sources.
We compare the overlap of GOODS OVs with known Chandra sources; present the X-ray properties, including X-ray variability, of Chandra-detected OVs; and comment on X-ray stacking analyses of Chandra-undetected OVs to investigate the nature of these potentially X-ray underluminous AGN. We discuss future prospects, including the extension of the z-band light-curves to > 1yr baselines with existing and upcoming post-GOODS SNe-search epochs, and the potential detection of rest-frame NIR variability with SST/IRAC re-visits of the GOODS fields.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.