X-UDS: The Chandra Legacy Survey of the UKIDSS Ultra Deep Survey Field

TL;DR

The X-UDS survey provides deep Chandra X-ray observations of the UDS field, detecting numerous sources, analyzing their properties, and exploring the evolution of obscured active galactic nuclei across cosmic time.

Contribution

This study presents the first comprehensive X-ray survey of the UDS field, including data reduction, source detection, spectral analysis, and insights into the evolution of Compton-thick AGN fractions.

Findings

Detected 868 X-ray sources with high confidence.

Identified 51 Compton-thick AGN candidates with a 30-35% obscured fraction.

Found evidence for a non-evolving or luminosity-dependent obscured AGN fraction over redshift.

Abstract

We present the X-UDS survey, a set of wide and deep Chandra observations of the Subaru-XMM Deep/UKIDSS Ultra Deep Survey (SXDS/UDS) field. The survey consists of 25 observations that cover a total area of 0.33 deg$^{2}$. The observations are combined to provide a nominal depth of ~600 ksec in the central 100 arcmin$^{2}$ region of the field that has been imaged with Hubble/WFC3 by the CANDELS survey and $\sim$200 ksec in the remainder of the field. In this paper, we outline the survey's scientific goals, describe our observing strategy, and detail our data reduction and point source detection algorithms. Our analysis has resulted in a total of 868 band-merged point sources detected with a false-positive Poisson probability of $<1\times10^{-4}$. In addition, we present the results of an X-ray spectral analysis and provide best-fitting neutral hydrogen column densities, $N_{\rm H}$, as well as a sample of 51 Compton-thick active galactic nucleus candidates. Using this sample, we find the intrinsic Compton-thick fraction to be 30-35% over a wide range in redshift ($z=0.1-3$), suggesting the obscured fraction does not evolve very strongly with epoch. However, if we assume that the Compton-thick fraction is dependent on luminosity, as is seen for Compton-thin sources, then our results are consistent with a rise in the obscured fraction out to $z\sim3$. Finally, an examination of the host morphologies of our Compton-thick candidates shows a high fraction of morphological disturbances, in agreement with our previous results. All data products described in this paper are made available via a public website.