| Summary: | We carried out deep mapping observations of the atomic hydrogen (H i ) 21 cm line emission in a field centered on the famous galaxy group Stephan's Quintet (SQ), using the Five-hundred-meter Aperture Spherical Telescope (FAST) equipped with a 19-beam receiver. The final data cube reaches an H i column density sensitivity of 5 σ = 2.1 × 10 ^17 cm ^−2 per 20 km s ^−1 channel with an angular resolution of 4.′0. The discovery of a large diffuse feature of the H i emission in the outskirts of the intragroup medium of SQ was reported in a previous paper (Xu et al.). Here we present a new study of the total H i emission of SQ and the detection of several neighboring galaxies, exploiting the high sensitivity and the large sky coverage of the FAST observations. A total H i mass of M _H I = 3.48 ± 0.35 × 10 ^10 M _☉ is found for SQ, which is significantly higher than previous measurements in the literature. This indicates that, contrary to earlier claims, SQ is not H i deficient. The excessive H i gas is mainly found in the velocity ranges of 6200–6400 km s ^−1 and 6800–7000 km s ^−1 , which were undetected in previous observations that are less sensitive than ours. Our results suggest that the “missing H i ” in compact groups may be hidden in the low-density diffuse neutral gas instead of in the ionized gas.
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