The GECKOS survey: Jeans anisotropic models of edge-on discs uncover the impact of dust and kinematic structures

TL;DR

The GECKOS survey applies Jeans Anisotropic Multi-Gaussian Expansion (JAM) modelling to seven edge-on disc galaxies using 3.6 μm imaging for the light distribution and MUSE spectroscopy for stellar kinematics. By masking dust using $E(B-V)$ maps, the authors assess how dust affects the fidelity of axisymmetric JAM fits and explore whether residual velocity structures reveal non-axisymmetric components such as bars and boxy-peanut bulges. They find that global dynamical quantities like enclosed mass are robust to dust masking (within ~10%), even as disc regions are reasonably well fit with $ ext{χ}^2_{ ext{reduced}} \,\leq 5$, and residual velocity maps uncover coherent bar- and BP-related signatures in at least three galaxies. The work demonstrates both the capabilities and limitations of JAM in dusty, edge-on systems and provides practical guidelines for using residuals to diagnose internal structure, with implications for future orbit-superposition analyses and radiative-transfer enhancements.

Abstract

The central regions of disc galaxies host a rich variety of stellar structures: nuclear discs, bars, bulges, and boxy-peanut (BP) bulges. These components are often difficult to disentangle, both photometrically and kinematically, particularly in star-forming galaxies where dust obscuration and complex stellar motions complicate interpretation. In this work, we use data from the GECKOS-MUSE survey to investigate the impact of dust on axisymmetric Jeans Anisotropic Multi-Gaussian Expansion (JAM) models, and assess their ability to recover kinematic structure in edge-on disc galaxies. We construct JAM models for a sample of seven edge-on ($i \gtrapprox 85^\circ$) galaxies that span a range of star formation rates, dust content, and kinematic complexity. We find that when dust is appropriately masked, the disc regions of each galaxy are fit to $χ^2_{\text{reduced}}\leq 5$. We analyse two-dimensional residual velocity fields to identify signatures of non-axisymmetric structure. We find that derived dynamical masses are constant within 10% for each galaxy across all dust masking levels. In NGC 3957, a barred boxy galaxy in our sample, we identify velocity residuals that persist even under aggressive dust masking, aligned with bar orbits and supported by photometric bar signatures. We extend this analysis to reveal a bar in IC 1711 and a possible side-on bar in NGC 0522. Our results highlight both the capabilities and limitations of JAM in dusty, edge-on systems and attempt to link residual velocities to known non-axisymmetric kinematic structure.

Figures (13)

• Figure 1: The Spitzer IRAC 3.6$\mu$m imaging of our sample of seven GECKOS galaxies. The images are scaled with an inverse hyperbolic sine (arcsinh) scaling, and masked regions (bright stars, imaging artefacts) are shown in white. There are clear star-forming spiral arms in NGC 5775 and NGC 0360.
• Figure 2: MGE fit to the galaxy IC 1711. Spitzer 3.6$\mu$m imaging contours are in black, the MGE model contours are in red, and masked regions (other sources) are in yellow. The contours are spaced by 0.5 mag arcsec$^{-2}$. The right side panel is a zoomed-in version of the left side panel, with the scale shown by a box around the central region in the left side panel.
• Figure 3: Illustration of masking for two E(B–V) thresholds across our sample, overlaid on the $V$ maps. Each row corresponds to a galaxy, ordered from top to bottom by decreasing star formation rate. The left column shows masks where Voronoi bins with E(B–V) > 0.7 are excluded (greyed out), while the right column shows masks for E(B–V) > 0.2. Galaxies are also labelled with their star formation rates in $\rm{M}_\odot~\text{yr}^{-1}$. We show the range in the velocity colourbar in the bottom right of the right-side panels. Aggressive masking removes most mid-plane bins in highly star-forming galaxies.
• Figure 4: JAM model for galaxy NGC 3957. The upper row shows the $V_{\text{rms}}$ map, the central row shows the $V$ map, and the lower row shows the $\sigma$ map. The left column shows nGIST mean light-weighted velocity $V$ and $\sigma$ binned to S/N=100, as well as the derived quantity $V_{\text{rms}}=\sqrt{V^2+\sigma^2}$. The central column shows the dynamical model, and the right column shows the residuals (data-model). The model is fit to the $V_{\text{rms}}$ map, and a $\kappa$ value is fit to find the $V$ map from $V_{\text{rms}}$. Additionally, we circle structure in the $V$ residual map that we believe corresponds to kinematic components that JAM has failed to successfully model.
• Figure 5: As for Figure \ref{['fig:NGC3957']}, but for IC 1711, and without circling structure.