2023MNRAS.524.2911A

We present the results of our new H I observations for the nearby galaxy NGC 4449 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). More diffuse H I gas was detected by FAST compared with previous studies. The measured extent of H I gas is more than ∼19 arcmin (20 kpc) larger than that in the Very Large Array map, and the H I gas disc is ∼22 times the optical diameter of NGC 4449. The total H I mass detected for NGC 4449 is 3.07 $\times 10^9\, \rm M_{\odot }$, which is in agreement with that detected by the Effelsberg telescope. We found one H I cloud from the channel map that is in superposition with the optical dwarf galaxy NGC 4449B. This cloud is slightly offset from the NGC 4449B disc, suggesting that it could be gas stripped from NGC 4449B's optical body when it was disrupted and pulled into the nuclear region of NGC 4449. We also find a region in the outer part of the H I envelope with a peculiar velocity field and double-peaked spectral profiles, whose velocity distribution is consistent with the nearby companion dwarf galaxy DDO 125. This implies that NGC 4449 may have stripped gas out of DDO 125. We suggest that the irregularity of NGC 4449's huge H I gas halo might originate from interactions with NGC 4449B and DDO 125.