2020ApJ...894...56D

We present a long temporal baseline variability survey in the Frontier Field MACSJ1149. In this study, we identify active galactic nuclei (AGNs) and other transient sources via their variability using over a decade of Hubble Space Telescope (HST) images for thousands of galaxies in the cluster region and detect significant variability in galaxies extending down to an apparent nuclear magnitude of m_i < 26.5. Our analysis utilizes HST images obtained in six different wavelengths from 435 nm to 1.6 microns and covers timescales 12 hr to 12 yr apart. We find that ∼2% of galaxies in these images are variable, with 49 AGN candidates and 4 new supernovae candidates detected. Half of the variables are in the cluster and these are primarily elliptical galaxies displaying variability only in the near-infrared bands. About 20% of the AGN candidates have morphologies and colors consistent with quasars, though most of the variables appear to be dominated by the host galaxy light. The structure function for these sources shows a greater amplitude of variability at shorter wavelengths, with slopes shallower than typical quasars. We also report a previously unknown Einstein cross identified in this field.