2025ApJ...986L..17F

Sulfur is essential for life, but its abundance and distribution in the interstellar medium remain uncertain, with over 90% of sulfur undetected in cold molecular clouds. Sulfur allotropes (S_n) have been proposed as possible reservoirs, but the only detected interstellar molecule with a disulfide bond is S₂H in the Horsehead Nebula, making the estimation of sulfur chains abundances difficult. Here we present total-power Atacama Large Millimeter/submillimeter Array images of H₂S, S₂H, SO₂, CO⁺, and SH⁺ toward the Horsehead Nebula. These observations, with unprecedented sensitivity (rms ∼1.5 mK), provide the first detections of SH⁺ and CO⁺ in this region, together with the identification of a new S₂H line. The comparison of the spectroscopic images of H₂S, S₂H, SO₂, CO⁺, and SH⁺ shows that the S₂H emission originates from a warm gas layer adjacent to the photodissociation front. The emission peak of S₂H is offset from those of reactive ions such as SH⁺, CO⁺, and SO⁺, suggesting that gas-phase reactions involving SH⁺ and H₂S are not the dominant formation pathway of S₂H. Instead, we propose that S₂H is desorbed from irradiated grain surfaces by nonthermal processes. The SH⁺ detection indicates that sulfur is not significantly depleted at the UV-irradiated edge of the molecular cloud, arguing against a major refractory sulfur reservoir in the interior of molecular clouds.