From deep Hα imaging data of Suprime-Cam/Subaru, we discovered nine extended ionized gas clouds (EIG) around galaxies in the Abell 851 cluster (A851) at z = 0.4. We surveyed a 30 x 25 arcmin region, and the EIGs were found only near the cluster center (< 2.3 arcmin ∼ 750 kpc). The parent galaxies of the EIGs are star-forming or post-starburst galaxies, all of which are spectroscopically confirmed members of the cluster. Four out of the nine parent galaxies show distortion of stellar distribution in the disk, which can be a sign of recent interaction, and the interaction may have made the EIGs. On the other hand, six parent galaxies (one overlaps those exhibiting distortion) show Hα emission without stars, which implies ram pressure stripping. The spectrum of the brightest parent galaxy shows a post-starburst signature and resembles the Hα stripped galaxies found in the Coma cluster. Meanwhile, two brightest parent galaxies in A851 are more massive than the EIG parent galaxies in the Coma cluster. This is consistent with a "downsizing" of star-forming galaxies, though it is still within the statistical variance. We also analyzed Suprime-Cam data of another z = 0.39 cluster, CL0024+17, but found no EIGs. The key difference between A851 and CL0024+17 would be the existence of a subcluster colliding with the main body of A851, in which six or seven out of the nine parent galaxies in A851 exist, and the fraction of EIGs in the subcluster is significantly higher than the main subcluster of A851 and CL0024+17.
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