Number of additional galaxies in a 1.5° × 1.5° area around the target galaxies, within a velocity range of −500 to +500 km s⁻¹ with respect to the central velocity of the target galaxy. This number is plotted against the H I mass of the target galaxy in the left panel and against the stellar mass in the right panel. Note the sudden increase at M_H I ∼ 10⁹ M_⊙ (left) and M_⋆ ∼ 10⁸ M_⊙ (right).