The fast hi 21 cm absorption blind survey. Iii. Oh absorption search in the 21 cm absorber sample and continued hi absorption search

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 deg2) 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 NOH and NHI, applying survival analysis to account for upper limits. The analysis does not provide statistically significant evidence for either an NOH–NHI correlation or redshift evolution of NOH/NHI. Finally, spectral stacking sets 3σ OH column density upper limits of 4.93, 1.64, and 1.72 Tex/cf,OH × 1012 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 Tex = 10 K for OH and Ts = 100 K for H I. These results place the strongest constraints to date on the OH content in radioselected 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.