The FAST H; I; 21 cm Absorption Blind Survey. III. OH Absorption Search in the 21 cm Absorber Sample and Continued H; I; Absorption Search
Journal Article
Overview
abstract
Abstract; ; We present the first blind search for OH 18 cm absorption with the Five-hundred-meter Aperture Spherical Telescope (FAST), conducted alongside the H; i; 21 cm absorption search. Our previous FAST blind H; i; absorption search identified 34 systems. In this work, we extend the search using 2024 and part of the 2025 CRAFTS and FAST All Sky H; i; Survey data (394.4 hr and 1622.1 deg; 2; ) together with FATHOMER observations, yielding four known and three new H; i; absorbers, for a total of 41 H; i; absorption systems. We search for OH absorption in 19 H; i; absorption systems whose OH redshifted frequencies fall within the FAST band. The known OH absorber toward PKS 1413+135 was redetected, making our survey the first blind survey to detect OH absorption. No new OH absorbers were identified. We examine the relationship between; N; OH; and; N; HI; , applying survival analysis to account for upper limits. The analysis does not provide statistically significant evidence for either an; N; OH; –; N; HI; correlation or redshift evolution of; N; OH; /; N; HI; . Finally, spectral stacking sets 3; σ; OH column density upper limits of 4.93, 1.64, and 1.72; T; ex; /; c; f,OH; × 10; 12; cm; −2; K; −1; for associated, intervening, and combined samples, corresponding to [OH]/[H; i; ] ratios of <1.66 × 10; −8; , <1.42 × 10; −8; , and <0.90 × 10; −8; , assuming; T; ex; = 10 K for OH and; T; s; = 100 K for H; i; . These results place the strongest constraints to date on the OH content in radio-selected H; i; absorbers and establish a blind-survey benchmark for future studies of molecular gas in H; i; -selected systems. They also demonstrate that known H; i; 21 cm absorbers provide an effective parent sample for systematic OH absorption searches, paving the way for future larger surveys.;
