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We present an analysis of the nuclear variability of $\sim28,000$ nearby($z<0.15$) galaxies with Sloan Digital Sky Survey (SDSS) spectroscopy in Stripe82. We construct light curves using difference imaging of SDSS g-band images,which allows us to detect subtle variations in the central light output. Weselect variable AGN by assessing whether detected variability is well-describedby a damped random walk model. We find 135 galaxies with AGN-like nuclearvariability. While most of the variability-selected AGNs have narrow emissionlines consistent with the presence of an AGN, a small fraction have narrowemission lines dominated by star formation. The star-forming systems withnuclear AGN-like variability tend to be low-mass($M_{\ast}<10^{10}~M_{\odot}$), and may be AGNs missed by other selectiontechniques due to star formation dilution or low-metallicities. We explore theAGN fraction as a function of stellar mass, and find that the fraction ofvariable AGN increases with stellar mass, even after taking into account thefact that lower mass systems are fainter. There are several possibleexplanations for an observed decline in the fraction of variable AGN withdecreasing stellar mass, including a drop in the supermassive black holeoccupation fraction, a decrease in the ratio of black hole mass to galaxystellar mass, or a change in the variability properties of lower-mass AGNs. Wedemonstrate that optical photometric variability is a promising avenue fordetecting AGNs in low-mass, star formation-dominated galaxies, which hasimplications for the upcoming Large Synoptic Survey Telescope.

Identifying AGNs in low-mass galaxies via long-term optical variability