2015MNRAS.446.3199G

We present a study of galaxies showing mid-infrared variability in data taken in the deepest Spitzer/MIPS 24 µm surveys in the Great Observatory Origins Deep Survey South field. We divide the data set in epochs and subepochs to study the long-term (months-years) and the short-term (days) variability. We use a χ2-statistics method to select active galactic nucleus (AGN) candidates with a probability ≤ 1 percent that the observed variability is due to statistical errors alone. We find 39 (1.7 percent of the parent sample) sources that show long-term variability and 55 (2.2 percent of the parent sample) showing short-term variability. That is, 0.03 sourcesx/arcmin2 for both, long-term and short-term variable sources. After removing the expected number of false positives inherent to the method, the estimated percentages are 1.0 and 1.4 percent of the parent sample for the long term and short term, respectively. We compare our candidates with AGN selected in the X-ray and radio bands, and AGN candidates selected by their IR emission. Approximately, 50 percent of the MIPS (Multiband Imaging Photometer for Spitzer) 24 µm variable sources would be identified as AGN with these other methods. Therefore, MIPS 24 µm variability is a new method to identify AGN candidates, possibly dust obscured and low-luminosity AGN, that might be missed by other methods. However, the contribution of the MIPS 24 µm variable identified AGN to the general AGN population is small ( ≤ 13 percent) in GOODS-South.