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 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.

Figures (6)

• Figure 1: The H i spectra of PRGs. In each panel, the galaxy name is shown in the upper left corner, and the optical central velocity is shown as the red vertical line. The colors distinguish the origin of H i spectra: black (FAST or this work), magenta Masters2014MNRAS.443.1044M, blue Haynes2018ApJ...861...49H, lime Koribalski2004AJ....128...16K, green Stark2021MNRAS.503.1345S, and violet Springob2005ApJS..160..149S.
• Figure 2: The optical image and H i contours/profiles for three mapping galaxies (Top: PRC A-03, middle: PRC B-01, bottom: SPRC 260 ). In each row, the left panel shows the $g$-band image from the DESI survey Dey2019AJ....157..168D following the method in Akhlaghi2015ApJS..220....1A to highlight the galaxy structure, the middle panel represents the zoom-out optical composite image overlaid with the H i contours (the outer one is the 2$\sigma$ level), and the right panel displays the integrated H i spectrum from FAST (black). In the top and bottom right panel, the blue spectrum is from Peterson1974AJ.....79..767P and the ALFALFA survey Haynes2018ApJ...861...49H, respectively.
• Figure 3: Left: the optical composite image of SPRC 39 and its two companions with the red circle show the FAST beam size. Right: The FAST H i spectrum of this pointing, and we mark the central velocities of these three galaxies as vertical lines.
• Figure 4: The NUV$-$r color (left) and atomic gas fraction (log $M_{\rm H\, \textsc{i}}$/$M_{*}$) as a function of stellar mass. In each panel, the black contours show the distribution of ALFALFA-NSA galaxies, the blue symbols are xGASS galaxies, and the red circles are kinematically confirmed PRGs. In the right panel, down-triangles (navy: xGASS, magenta: PRGs) are H i mass upper limits of non-detections, and the cyan curve shows the weighted average distribution of the xGASS galaxies from Catinella2018MNRAS.476..875C. The green solid and dashed curves illustrate the relationship described by Scholte2024MNRAS.535.2341S, as well as the 0.6 dex upper shift.
• Figure 5: The H i profile asymmetry ($A_F$, left) and profile shape ($K$, right) as a function of stellar mass for the ALFALFA-NSA sample and PRGs. In each panel, the black contours show the 20%, 40%, 60%, and 80% distribution of ALFALFA-NSA galaxies from inside out, and the red circles are kinematically confirmed PRGs.