FEASTS Combined with Interferometry. I. Overall Properties of Diffuse H i and Implications for Gas Accretion in Nearby Galaxies

Abstract

We present a statistical study of the properties of diffuse H i in 10 nearby galaxies, comparing the H i detected by the single-dish telescope FAST (FEASTS program) and the interferometer Very Large Array (THINGS program), respectively. The THINGS observation missed H i with a median of 23% due to the short-spacing problem of interferometry and limited sensitivity. We extract the diffuse H i by subtracting the dense H i, which is obtained from the THINGS data with a uniform flux-density threshold, from the total H i detected by FAST. Among the sample, the median diffuse-H i fraction is 34%, and more diffuse H i is found in galaxies exhibiting more prominent tidal-interaction signatures. The diffuse H i we detected seems to be distributed in disk-like layers within a typical thickness of 1 kpc, different from the more halo-like diffuse H i detected around NGC 4631 in a previous study. Most of the diffuse H i is cospatial with the dense H i and has a typical column density of 1017.7–1020.1 cm−2. The diffuse and dense H i exhibit a similar rotational motion, but the former lags by a median of 25% in at least the inner disks, and its velocity dispersions are typically twice as high. Based on a simplified estimation of circumgalactic medium properties and assuming pressure equilibrium, the volume density of diffuse H i appears to be constant within each individual galaxy, implying its role as a cooling interface. Comparing with existing models, these results are consistent with a possible link between tidal interactions, the formation of diffuse H i, and gas accretion.

HiFAST: An H i Data Calibration and Imaging Pipeline for FAST. II. Flux Density Calibration

“…Ongoing initiatives such as the Commensal Radio Astronomy FAST Survey (CRAFTS, Li et al 2018) are underway, with Zhang et al (2019) providing insights into the survey's extragalactic H I sensitivity and projecting FAST's future survey capabilities. By combining observational data from FAST with that from WSRT and VLA, Wang et al (2023Wang et al ( , 2024 have achieved a more comprehensive understanding of the diffuse H I distribution surrounding interacting galaxies. Additionally, the FASHI source catalog published by Zhang et al (2023) highlights FAST's exceptional performance in blind survey projects.…”