2025ApJ...987..109Y

We present the results of a deep neutral hydrogen (H I) observation of the early-type galaxy NGC 2768 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Leveraging the high sensitivity of FAST, we discover an extended gas envelope around NGC 2768. The total H I mass is measured to be 8.1 × 10⁸M_⊙, representing a magnitude increase compared to previous Westerbork Synthesis Radio Telescope (WSRT) studies. A position-velocity (PV) diagram indicates the envelope mainly involves two components: an H I disk of NGC 2768 and a newly discovered satellite galaxy without detectable counterparts in currently deep optical surveys. The center of the gas disk is misaligned with the optical disk of NGC 2768, with more gas redshifted, indicating it has been disturbed. Our study indicates NGC 2768 is currently undergoing a transition from a spiral galaxy to an S0. Previous deep WSRT observations reveal two dense clumps (named “Clumps A” and “Clump B” throughout this paper) in the center of the envelope. We find Clump A corresponds to the densest part of the disk, while Clump B might be a newly discovered satellite galaxy that probably collided with NGC 2768 about 0.38 Gyr ago. We also find tidal interactions between Clump B and PGC 2599651, NGC 2768, and UGC 4808. Based on these new findings, we finally analyze the hierarchical accretion history of NGC 2768.