Galaxies in the Hubble Ultra Deep Field. I. Detection, Multiband Photometry, Photometric Redshifts, and Morphology

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Abstract
We present aperture-matched point-spread function (PSF)-corrected BVi'z'JH photometry and Bayesian photometric redshifts (BPZ) for objects detected in the Hubble Ultra Deep Field (UDF), 8042 of which are detected at the 10 σ level (e.g., i' < 29.01 or z' < 28.43). Most of our objects are defined identically to those in the public STScI catalogs, enabling a straightforward object-by-object comparison. We have combined detections from i', z', J + H, and B + V + i' + z' images into a single comprehensive segmentation map. Using a new program called SExSeg, we are able to force this segmentation map into SExtractor for photometric analysis. The resulting photometry is corrected for the wider NIC3 PSFs using our ColorPro software. We also correct for the ACS z'-band PSF halo. Offsets are applied to our NIC3 magnitudes, which are found to be too faint relative to the ACS fluxes. Based on BPZ spectral energy distribution (SED) fits to objects of known spectroscopic redshift, we derived corrections of -0.30 ± 0.03 mag in J and -0.18 ± 0.04 mag in H. Our offsets appear to be supported by a recent recalibration of the UDF NIC3 images combined with nonlinearity measured in NICMOS itself. The UDF reveals a large population of faint blue galaxies (presumably young starbursts), bluer than those observed in the original Hubble Deep Fields. To accommodate these galaxies, we have added two new starburst templates to the SED library used in previous BPZ papers. The resulting photometric redshifts are accurate to within 0.04(1 + zspec) out to z < 6. Our BPZ results include a full redshift probability distribution for each galaxy. By adding these distributions, we obtain the redshift probability histogram for galaxies in the UDF. Median redshifts are also provided for different magnitude-limited samples. Finally, we measure galaxy morphology, including Sérsic index and asymmetry. Simulations allow us to quantify the reliability of our morphological results. Our full catalog, along with our software packages SExSeg and ColorPro, is available from our ACS Web site.
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