Insights into the Physical Nature of Polar-ring Galaxies from H I Observations

Abstract Polar-ring galaxies (PRGs) host an outer ring of gas and stars oriented nearly perpendicular to the main stellar body. They represent extreme examples of misaligned systems and provide valuable insights into galaxy interactions, gas accretion, and peculiar gas dynamics. We compile a complete sample of kinematically confirmed PRGs and collect their H i measurements. Combining literature data with new observations from the Five-hundred-meter Aperture Spherical Telescope, we detect H i emission in 22 sources, identify 1 potential H i absorption feature, and find 4 nondetections among 40 confirmed PRGs. Compared to galaxies in the ALFALFA and xGASS surveys, PRGs predominantly occupy the green valley or quenched regimes but exhibit higher gas fractions than typical early-type galaxies, suggesting gas accretion. The H i profile asymmetry and shape for PRGs are not consistent with those of the ALFALFA sample with p < 0.05. We examine their Tully–Fisher relation (TFR) and baryonic TFR (bTFR), linking the systems’ rotation velocities to their masses. The extreme outliers in TFRs for the control sample tend to display single-peaked H i profiles. PRGs do not follow a tight TFR or bTFR if the H I resides primarily in the host galaxy. But the scatter decreases significantly if we assume the gas is mainly distributed in the polar ring. Spatially resolved H i observations are essential to disentangling the gas distributions and kinematics in PRGs, which are key to understanding their formation mechanisms.