Identifying AGNs in Low-mass Galaxies via Long-term Optical Variability

We present an analysis of the nuclear variability of similar to 28,000 nearby (z < 0.15) galaxies with Sloan Digital Sky Survey (SDSS) spectroscopy in Stripe 82. We construct light curves using difference imaging of SDSS g-band images, which allows us to detect subtle variations in the central light output. We select variable active galactic nuclei (AGNs) by assessing whether detected variability is well-described by a damped random walk model. We find 135 galaxies with AGN-like nuclear variability. While most of the variability-selected AGNs have narrow emission lines consistent with the presence of an AGN, a small fraction have narrow emission lines dominated by star formation. The star-forming systems with nuclear AGN-like variability tend to be low mass (M-* < 10(10) M-circle dot), and may be AGNs missed by other selection techniques due to star formation dilution or low metallicities. We explore the AGN fraction as a function of stellar mass, and find that the fraction of variable AGN increases with stellar mass, even after taking into account the fact that lower-mass systems are fainter. There are several possible explanations for an observed decline in the fraction of variable AGN with decreasing stellar mass, including a drop in the supermassive black hole occupation fraction, a decrease in the ratio of black hole mass to galaxy stellar mass, or a change in the variability properties of lower-mass AGNs. We demonstrate that optical photometric variability is a promising avenue for detecting AGNs in low-mass, star formation-dominated galaxies, which has implications for the upcoming Large Synoptic Survey Telescope.