Peculiar galaxies I: A Catalog of Polar-Ring Galaxies from the TNG50 Simulation

TL;DR

The paper develops a robust catalog of polar ring galaxies (PRGs) in the high-resolution TNG50 cosmological simulation by dynamically decomposing galaxies into polar rings and hosts via a two-dimensional cos(\alpha)–j_r analysis and a CUPID clump finder. It finds 32 PRGs (≈1% in B-band, ≈0.5% in r-band) with bulge-dominated hosts (mean B/T ≈ 0.64) and rings that are typically less massive than their hosts (mean ring-to-host mass ratio ≈ 0.11) and more extended in gas than in stars. Stellar rings tend to be less perpendicular than gaseous rings, with mean inclinations around 83° and 80°, respectively, and rings often lie between ≈2–4 r_hm, though gaseous rings are distinctly larger (mean R_gas ≈ 3.4 r_hm, ≈13.9 kpc). Star formation in PRGs is generally modest but higher than in comparable control samples, with some rings driving notable recent activity. The study provides a valuable PRG catalog, reveals differences from observed systems that likely arise from selection and projection effects, and supports multiple formation channels (accretion, mergers, and cosmological gas inflow) as shaping PRG morphologies and evolution.

Abstract

The hydrodynamic cosmological simulation, TNG50, is employed to conduct an analysis of multi-spin galaxies that exhibit ringed structures composed of gas and stars that orbit nearly perpendicular around a host galaxy, known as polar ring galaxies (PRG). To ensure a robust sample, we select subhalos based on the angle subtended by the angular momentum profiles, as well as on a visual inspection. The analysis is focused on galaxies with stellar masses greater than 10$^{9}$ M$_\odot$. In addition, a dynamic decomposition is employed to separate the stellar and gaseous ring from the host galaxy. This results in a sample of 32 subhalos with PRGs. This sample exhibits properties similar to those observed. These include colours typical of early-type galaxies (ETGs) or those transitioning toward blue systems. Most host galaxies are classified as ETGs, with 37.5\% exhibiting a disk-dominated morphology. The mean bulge-to-total (B/T) ratio is 0.64. Rings have average radii that are 2.36 and 3.41 times larger than their effective radii for the stellar and gaseous components, respectively, with star formation occurring predominantly within the rings. In contrast with observations, rings in the simulation tend to be less massive and slightly less perpendicular. The obtained sample displays a variety of host galaxy morphologies, including wide and narrow rings, providing a robust framework for studying the varied structural characteristics of PRG variants.

Figures (17)

• Figure 1: Representation of the vectors of the specific total angular momentum of the disk (blue) and the ring (orange) of an idealised model of a PRG.
• Figure 2: Stellar and gaseous structure and kinematics of a representative PRG candidate (subhalo 167392). Top row: surface mass density of the stellar component in the $x-y$ (left), $x-z$ (middle), and $y-z$ planes (right) for the subhalo 167392. The $z$ axis was defined as the direction of the total angular momentum of the stellar particles within $0.1 r_{200}$ in the subhalo. Bottom row: radial profiles of the specific angular momentum (left), the relative angle of the specific angular momentum with the $z$ axis (middle), and the components of the specific angular momentum (right). In the three panels, both the angular momentum corresponding to the stellar ( solid lines) and gaseous ( dashed) galaxy components are shown.
• Figure 3: Mass weighted velocities of the stellar component in the line of sight along the main axes of the subhalo 167392. The rotating ring is clearly visible as polar structures in the left and middle panels, while the rotating host disk appears as an equatorial structure in the middle and right panels.
• Figure 4: Surface mass density of the stellar ( left) and gaseous ( right) components of the first sample of galaxies listed in the table \ref{['tab: kappa']}
• Figure 5: Continuation of Fig. \ref{['fig:collage_1']}