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

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 insight 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 FAST, we detect H I emission in 22 sources, identify one potential H I absorption feature, and find four non-detections 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 that 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 disentangle the gas distribution and kinematics in PRGs, which are key to understanding their formation mechanisms.

💡 Research Summary

This paper presents a comprehensive H I 21‑cm study of polar‑ring galaxies (PRGs), aiming to elucidate their gas content, dynamics, and evolutionary pathways. The authors assembled a complete list of 40 kinematically confirmed PRGs drawn from the literature and supplemented it with new observations obtained with the Five‑hundred‑meter Aperture Spherical Telescope (FAST). In total, 22 galaxies show clear H I emission, one exhibits a possible absorption feature, and four are non‑detections; the remaining objects have archival H I measurements. The FAST campaign targeted 20 PRGs using both tracking and multi‑beam on‑the‑fly (OTF) modes, achieving reliable detections in 12 of them.

The H I spectra were analyzed for line width, asymmetry, and profile shape (single‑peaked versus double‑peaked). PRGs display a markedly higher incidence of asymmetric and double‑peaked profiles compared with the large, blind ALFALFA survey, a difference that is statistically significant (p < 0.05). This suggests that the neutral gas in PRGs is dynamically disturbed, likely reflecting its recent acquisition or the influence of a triaxial dark‑matter halo that stabilizes the polar structure.

When placed on a colour–mass diagram, PRGs predominantly occupy the green valley or quenched region, yet they possess H I gas fractions that exceed those of typical early‑type (S0) galaxies of comparable stellar mass by ~0.2 dex. This elevated gas content supports scenarios in which PRGs have accreted external material, either through tidal interactions with companions or direct inflow from the intergalactic medium.

The authors then examine the classic Tully‑Fisher relation (TFR) and the baryonic TFR (bTFR). Two contrasting assumptions are tested: (1) the H I resides in the host galaxy’s main disk, and (2) the H I is primarily located in the polar ring. Under assumption (1) PRGs scatter widely from the established TFR, indicating that using the host‑disk rotation velocity misrepresents their dynamical mass. Under assumption (2), the line widths better trace the rotation of the polar ring, and the scatter around both the TFR and bTFR is dramatically reduced. This result underscores that accurate placement of PRGs on scaling relations hinges on knowing the true spatial distribution of the neutral gas.

The paper discusses formation mechanisms in light of these findings. The high gas fractions, asymmetric H I profiles, and improved TFR alignment when the gas is assigned to the ring all point toward external gas acquisition as a dominant channel—either via minor mergers, tidal stripping, or cold‑flow accretion. The necessity of a triaxial halo to maintain the polar configuration is also reinforced.

In summary, the study provides three key insights: (i) PRGs are gas‑rich systems that sit in the green‑valley/quiescent regime, (ii) their H I properties differ statistically from those of typical field galaxies, and (iii) the placement of PRGs on TFR and bTFR depends critically on whether the neutral gas is in the host or the polar ring. The authors conclude that spatially resolved H I imaging with interferometers such as VLA, MeerKAT, or ASKAP is essential to disentangle the gas distribution, constrain the dark‑matter halo shape, and ultimately discriminate among competing formation scenarios for polar‑ring galaxies.