Optical And Near-infrared Variability Among Distant Galactic Nuclei Of The CANDELS UDS Field

The CANDELS HST Multi-cycle Treasury Program completed its observations of the UKIDSS UDS field in January 2011. The coverage comprises WFC3/IR exposures in J and H across a contiguous ? square arcminutes, and coordinated parallel ACS/WFC exposures in V and I across a contiguous ? square arcminutes that largely overlaps the WFC3/IR coverage. These observations were split between two epochs with ≈ 52-day spacing for the primary purpose of high-z supernovae (SNe) detection and follow-up. However, this combination of sensitivity, high resolution, and time spacing is also well-suited to detect optical and near-infrared variability ("ONIV") among moderate-z galaxy nuclei (in the near-infrared to AB∼24 mag; in the overlapping optical to AB∼25 mag) on rest-frame timescales of up to several weeks.

The overwhelming majority of these variable galaxy nuclei will be AGN; the small fraction arising from SNe have already been meticulously culled by the CANDELS high-z SNe search effort. These ONIV galaxy nuclei potentially represent a significant addition to the census of distant AGN subject to multi-wavelength scrutiny with CANDELS. We present the preliminary results of our variability analysis, including a comparison of the HST ONIVs with the known AGN candidates in the field from deep Spitzer and XMM-Newton imaging across the field, and from extensive optical spectroscopy. We also assess the redshift distribution of the ONIVs from both spectroscopy and from robust SED-fitting incorporating ancillary deep ground-based imaging along with the CANDELS VIJH photometry.