Clues to Active Galactic Nucleus Growth from Optically Variable Objects in the Hubble Ultra Deep Field

We present a photometric search for objects with point-source components that are optically variable on timescales of weeks to months in the Hubble Ultra Deep Field (HUDF) to i = 28.0 mag. The data are split into four substacks of approximately equal exposure times. Objects exhibiting the signature of optical variability are selected by studying the photometric error distribution between the four different epochs and selecting 622 candidates as 3.0 σ outliers from the original catalog of 4644 objects. Of these, 45 are visually confirmed as free of contamination from close neighbors or various types of image defects. Four lie within the positional error boxes of Chandra X-ray sources, and two of these are spectroscopically confirmed AGNs. The photometric redshift distribution of the selected variable sample is compared to that of field galaxies, and we find that a constant fraction of ~1% of all field objects show variability over the range of 0.1 ≲ z ≲ 4.5. Combined with other recent HUDF results, as well as those of recent state-of-the-art numerical simulations, we discuss a potential link between the hierarchical merging of galaxies and the growth of AGNs.