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 Hα 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.