A Search for Optical AGN Variability in 35,000 Low-mass Galaxies with the Palomar Transient Factory

We present an analysis of the long-term optical variability for ∼50,000 nearby (z < 0.055) galaxies from the NASA-Sloan Atlas, 35,000 of which are low-mass (M* < 1010 M ). We use difference imaging of Palomar Transient Factory (PTF) R-band observations to construct light curves with typical baselines of several years. We then search for subtle variations in the nuclear light output. We determine whether detected variability is AGN-like by assessing the fit quality to a damped random walk model. We identify 417 variability-selected AGNs, including 237 with stellar masses between 107 and 1010 M . 75% of low-mass galaxies with AGN-like variability have narrow emission lines dominated by star formation. After controlling for nucleus magnitude, the fraction of variable AGNs is constant down to M* = 109 M , suggesting no drastic decline in the BH occupation fraction down to this stellar mass regime. Combining our NASA-Sloan Atlas sample with samples of nearby galaxies with broad H emission, we find no dependence of variability properties with black hole mass. Our PTF work demonstrates the promise of long-term optical variability searches in low-mass galaxies for finding AGNs missed by other selection techniques.