H i Gas and Star Formation in Major Galaxy Pairs from the FAST All Sky H i Survey (FASHI)

Atomic hydrogen (H i ) plays a fundamental role in fueling star formation in galaxies. However, the behavior of H i gas in interacting systems, particularly galaxy pairs, remains elusive. In this work, we investigate the H i content of major mergers by crossmatching the extragalactic H i catalog from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) All-Sky H i Survey with a previously established sample of isolated galaxy pairs. With the superior sensitivity of FAST, we have constructed the largest sample of major mergers with H i detections, consisting of 440 galaxy pairs: 364 spiral-spiral (S+S) and 76 spiral-elliptical (S+E) systems. We examine the H i gas fraction ( f H I ), star formation rate (SFR), and H i star formation efficiency (SFE H I = SFR/ M H I ) for individual galaxies in pairs. The control sample is matched in both stellar mass and redshift. We find that paired galaxies, particularly those in pairs with small projected separations ( d p < 50 h −1 kpc), exhibit systematically lower (by 8.8%) H i gas fractions compared to the control galaxies. The SFR is enhanced for galaxies in the S+S pairs. The SFE H I is ∼15% higher for galaxies in the S+S pairs than in the control galaxies, while spiral galaxies in the S+E pairs show no significant difference in SFE H I compared to the control sample. These findings suggest that the merger process triggers efficient H i gas depletion and enhances star formation, especially in close S+S pairs. Notably, our sample includes 26 red spirals in paired systems. These galaxies exhibit H i deficiency and suppressed star formation activity compared to the isolated galaxies, indicating that interactions may affect quiescent spirals differently, potentially due to mechanisms similar to those of ellipticals.