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Kinematics and excitation of ionized gas in the stellar counterrotating galaxies PGC 35706 and LEDA 2220522

This study compares two stellar counterrotating galaxies, revealing that while both exhibit opposing stellar and gas velocity gradients, only LEDA 2220522 displays regular gas rotation and star formation, whereas PGC 35706 shows highly irregular kinematics and weak emission, demonstrating that confirming gas counterrotation requires separate assessments of velocity gradients, field regularity, and gas excitation.

Original authors: V. S. Goradzhanov, K. O. Parfenov, D. Gasymov

Published 2026-10-29

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CC BY 4.0

Original authors: V. S. Goradzhanov, K. O. Parfenov, D. Gasymov

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Galaxies are not static islands of stars; they are dynamic systems where gas and stars move in complex, often surprising ways. In many galaxies, the gas that fuels new star formation orbits in the same direction as the stars, a natural alignment expected from their shared birth. However, some galaxies host a peculiar phenomenon where a significant portion of their stars spins in the opposite direction to the gas, or vice versa. This "counterrotation" is a cosmic fingerprint that hints at a violent or messy past, such as a galaxy swallowing a smaller neighbor or a massive cloud of gas with a different spin. Astronomers study these misalignments to understand how galaxies grow and evolve, but simply seeing that two things spin in opposite directions is not enough. To truly understand the story, researchers must determine if the gas is moving in a smooth, orderly disc like a well-behaved record, or if it is chaotic and disturbed, and they must also check what kind of energy is lighting up that gas.

In a recent study, researchers examined two specific galaxies, PGC 35706 and LEDA 2220522, both of which display this counterrotating behavior. The team wanted to know if the gas in these galaxies was simply a calm, rotating disc moving in the opposite direction to the stars, or if the picture was more complicated. They used powerful telescopes to map the speed and direction of the gas across the entire face of each galaxy. By comparing the gas motion to the motion of the stars and to theoretical models of how stars orbit, they could test whether the gas was behaving like a regular disc or if it was turbulent and irregular. They also analyzed the light emitted by the gas to see if it was being energized by the birth of new stars or by older, dying stars.

The results revealed a striking difference between the two galaxies, despite their similar appearance of counterrotation. In LEDA 2220522, the gas behaves exactly as a regular, spinning disc should. The researchers found that the gas moves in a smooth, predictable pattern, with only tiny deviations from a perfect rotation. When they compared their detailed measurements with a different method of analyzing the same telescope data, the results matched almost perfectly, confirming that the gas is indeed rotating in a stable, orderly fashion opposite to the stars. Furthermore, the light from this gas showed clear signs of active star formation, indicating that fresh stars are being born within this counter-rotating disc. This suggests a scenario where the galaxy likely captured a cloud of gas from the outside that was spinning the wrong way, and that gas has since settled into a calm disc to form new stars.

In contrast, the story for PGC 35706 is far more chaotic. While the gas in this galaxy also moves in the opposite direction to the stars on a large scale, it does not form a smooth disc. When the researchers tried to fit the gas motion to a regular rotation model, the data did not match well; the differences between the model and the actual observations were large and structured, driven by a few specific areas where the gas was moving at extreme speeds. Unlike in LEDA, the two different methods of analyzing the data disagreed significantly on the local speeds of the gas in PGC 35706, suggesting the motion is not stable or regular. Additionally, the gas in this galaxy showed no signs of new star formation. Instead, the light indicated it was being energized by an older population of stars, with weak, low-energy emissions that do not support the idea of a calm, star-forming disc.

The study concludes that seeing gas and stars spin in opposite directions is only the beginning of the story. It is not enough to simply note the misalignment; astronomers must also check if the gas is moving in a regular, smooth pattern and what is causing it to glow. In LEDA 2220522, the evidence points to a clean, regular counterrotating disc formed by accreted gas. In PGC 35706, the gas is likely disturbed or irregular, and its motion does not represent a stable disc. This work highlights that the history of a galaxy is written in the details of its gas, requiring a careful look at both its motion and its energy to distinguish between a peaceful merger and a more turbulent past.

Technical Summary: Kinematics and Excitation of Ionized Gas in the Stellar Counterrotating Galaxies PGC 35706 and LEDA 2220522

Problem Statement
Galaxies with counterrotating stellar components offer a unique laboratory for studying the formation of kinematically distinct structures. While mechanisms such as gas accretion with opposite angular momentum, mergers, and internal orbital redistribution have been proposed, the specific nature of the gas in these systems remains complex. Integral-field spectroscopic surveys have revealed that stellar-gas kinematic misalignment occurs across diverse conditions, but distinguishing a regularly rotating gas disc from irregular or disturbed gas is challenging. This study addresses the specific problem of determining whether gas motion opposite to the stellar component in two counterrotating galaxies (PGC 35706 and LEDA 2220522) corresponds to regular disc rotation or represents a more complex, irregular state. The authors aim to quantify departures from axisymmetric models, compare independent spectral analyses, and investigate gas excitation to differentiate between regular counterrotation and other dynamical states.

Methodology
The study utilizes MaNGA DR17 integral-field spectroscopic observations of PGC 35706 and LEDA 2220522. The analysis employs a multi-faceted approach:

Spectral Analysis: Two independent spectral fitting pipelines are compared: NBursts (used for the primary analysis and previous dynamical studies) and the MaNGA Data Analysis Pipeline (DAP). Both analyses fit stellar continua and emission lines (H $\beta$ , [O III], H $\alpha$ , [N II], [S II], [O I]) simultaneously on identical Voronoi tessellations.
Kinematic Modeling: Gas velocity fields are fitted with an axisymmetric arctangent rotation model to assess regularity. Residuals are calculated to identify deviations from smooth rotation. Additionally, a weighted plane fit is used to determine the large-scale velocity gradient direction independently of the rotation model.
Dynamical Context: The gas kinematics are compared against projected velocity fields derived from axisymmetric Schwarzschild dynamical models (constructed using Forstand and AGAMA) of the stellar components. These models include broad prograde and retrograde orbital families.
Excitation Diagnostics: Gas excitation is classified using Baldwin–Phillips–Terlevich (BPT) diagrams ([N II], [S II], [O I]) and the WHAN (Weak Active Galactic Nucleus) equivalent-width diagram. Star formation rates (SFR) and oxygen abundances are estimated for regions meeting specific signal-to-noise and classification criteria.

Key Contributions and Results
The paper provides a comparative analysis of two galaxies that share similar large-scale stellar-gas kinematic misalignment but exhibit fundamentally different gas properties:

LEDA 2220522 (Regular Counterrotation):
- Kinematics: The ionized gas exhibits a highly regular velocity field. The root-mean-square (RMS) residual of the axisymmetric model is small (5.90 km s $^{-1}$ ).
- Analysis Consistency: There is strong agreement between NBursts and DAP analyses regarding both the direction of the velocity gradient and local velocities (Pearson correlation $r=0.994$ ).
- Excitation: A significant fraction of the gas (66.7% of bins) is classified as star-forming. The study derives a star formation rate of $\approx 1.12$ M $_{\odot}$ yr $^{-1}$ and an oxygen abundance of $12 + \log(\text{O/H}) \approx 8.6$ .
- Dynamics: The gas velocity field correlates strongly ( $r=0.947$ ) with the projected field of the broad retrograde orbital families in the stellar model.
PGC 35706 (Irregular/Disordered Gas):
- Kinematics: The gas velocity field is irregular. The axisymmetric model leaves structured residuals with a large RMS value of 45.5 km s $^{-1}$ , dominated by a few peripheral bins with extreme velocities.
- Analysis Consistency: Agreement between NBursts and DAP is substantially poorer at the level of local velocities (RMS difference 55.1 km s $^{-1}$ ), though the large-scale gradient direction remains consistent.
- Excitation: The gas shows weak emission relative to the continuum with low-ionization characteristics. No bins are classified as star-forming; the gas is consistent with ionization by an old stellar population (retired sector in WHAN). Consequently, no SFR can be estimated from H $\alpha$ .
- Dynamics: Despite the irregularity, the gas velocity field still correlates with the retrograde orbital families ( $r=0.856$ ), though with a larger gradient difference (3.9 $^{\circ}$ ) compared to LEDA.
Sensitivity to Spectral Processing: The study highlights that while large-scale gradient directions are robust across different spectral fitting methods, local velocity measurements and derived quantities (such as SFR in LEDA) can be sensitive to the specific treatment of emission lines (particularly H $\beta$ fluxes) and continuum subtraction.

Significance and Claims
The authors conclude that the mere presence of nearly opposite stellar and gas velocity gradients is insufficient to confirm regular counterrotating gas discs. The interpretation of gas counterrotation requires a tripartite assessment:

The direction of the velocity gradient.
The spatial regularity of the velocity field (quantified by model residuals).
The excitation state of the gas.

The paper demonstrates that two galaxies with similar kinematic misalignment can host gas in vastly different physical states: one supporting a regular, star-forming disc (LEDA 2220522) and the other exhibiting disordered, low-ionization gas (PGC 35706). This distinction is crucial for correctly identifying formation scenarios; for instance, the regular counterrotation in LEDA supports external gas accretion followed by star formation, whereas the state of PGC 35706 suggests a different evolutionary path or dynamical history. The study emphasizes that detailed, spatially resolved diagnostics are necessary to distinguish between regular counterrotating discs and irregular gas systems in counterrotating galaxies.

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