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Kinematic and dynamics of the galaxy ESO 358-60

Francesco Sylos Labini, Matteo Straccamore

Abstract

We investigate the velocity field derived from HI measurements of the irregular galaxy ESO 358-60 using the Velocity Ring Model (VRM) method. This technique, which assumes a coplanar disk, allows us to reconstruct coarse-grained maps of both radial and tangential velocity components from the observed line-of-sight velocity field. Such maps reveal that tangential motions dominate the inner regions, while radial motions become increasingly significant toward the outskirts. This kinematic behavior contrasts with that inferred from the Tilted Ring Model (TRM), which suggests that radial motions are more prominent in the intermediate disk and negligible in the outskirts and detects a pronounced warp of approximately $20^\circ$, with the inner disk nearly edge-on and the outer regions inclined by approximately $60^\circ$. In contrast, the VRM analysis finds that the disk exhibits a bar-like structure in its central regions. This interpretation is further supported by the intensity and velocity dispersion maps. To test the origin of the TRM-derived warp, we construct a toy model based on the TRM results and analyze it with the VRM technique, finding evidence that the warp is likely an artifact arising from the TRM's assumptions. Finally, we estimate the galaxy's mass using both the standard dark matter halo model and a dark matter disk (DMD) model, where all mass lies in the disk plane. The DMD yields a total mass approximately three times lower and provides a slightly better fit to the rotation curve.

Kinematic and dynamics of the galaxy ESO 358-60

Abstract

We investigate the velocity field derived from HI measurements of the irregular galaxy ESO 358-60 using the Velocity Ring Model (VRM) method. This technique, which assumes a coplanar disk, allows us to reconstruct coarse-grained maps of both radial and tangential velocity components from the observed line-of-sight velocity field. Such maps reveal that tangential motions dominate the inner regions, while radial motions become increasingly significant toward the outskirts. This kinematic behavior contrasts with that inferred from the Tilted Ring Model (TRM), which suggests that radial motions are more prominent in the intermediate disk and negligible in the outskirts and detects a pronounced warp of approximately , with the inner disk nearly edge-on and the outer regions inclined by approximately . In contrast, the VRM analysis finds that the disk exhibits a bar-like structure in its central regions. This interpretation is further supported by the intensity and velocity dispersion maps. To test the origin of the TRM-derived warp, we construct a toy model based on the TRM results and analyze it with the VRM technique, finding evidence that the warp is likely an artifact arising from the TRM's assumptions. Finally, we estimate the galaxy's mass using both the standard dark matter halo model and a dark matter disk (DMD) model, where all mass lies in the disk plane. The DMD yields a total mass approximately three times lower and provides a slightly better fit to the rotation curve.
Paper Structure (14 sections, 17 equations, 11 figures)

This paper contains 14 sections, 17 equations, 11 figures.

Table of Contents

  1. Introduction
  2. Data
  3. Kinematics and morphology
  4. Tilted Ring Model
  5. Surface brightness and velocity dispersion profiles
  6. Velocity ring model
  7. Cross correlation
  8. Test on warp
  9. Discussion
  10. Dynamical mass model
  11. On the physical origin of dark matter disks
  12. Formation of disks in CDM models
  13. Quasi-stationary disks from monolithic collapse
  14. Conclusion

Figures (11)

  • Figure 1: Left panel: neutral hydrogen surface brightness map (moment 0). The color scale indicates the column density in units of $10^{2}\,\mathrm{cm}^{-2}$. Middle panel: velocity dispersion map (moment 2). The color scale is given in $\mathrm{km\,s^{-1}}$. In both the left and middle panels, the $X$ and $Y$ axes (the coordinates are in the plane of the sky) are in arc-seconds. Right panel: Digitized Sky Survey (DSS) image from the HyperLeda galaxy catalog Makarov_etal_2014 displayed as a color composite. The field of view is about 600 arc-seconds as the neutral hydrogen intensity map.
  • Figure 2: Left panel: radial profile of the surface brightness. For comparison, a reference line with an exponential decay of characteristic length $40"$ is also shown. Right panel: radial profile of the velocity dispersion. The error bars are determined from the angular average within each ring.
  • Figure 3: Left panel: the observed 2D line-of-sight velocity field. Right panel: the residuals from the fit to the VRM model with $N_r=20$ and $N_a=32$. The $X$ and $Y$ axes (the coordinates are in the plane of the sky) are in arc-seconds.
  • Figure 4: Results of the VRM model for $N_r=20$ concentric rings and $N_a=4, 8, 16, 32$ azimuthal sectors: respectively, upper panels show the transversal velocities while bottom panels the radial ones. The $X$ and $Y$ axes (the coordinates are in the plane of the galaxy) are in arc-seconds; the color code in km s$^{-1}$.
  • Figure 5: Transversal (Left panel) and radial (Right panel) velocity profiles for ESO 358-60: the number of rings is the same, $N_r=20$, whereas the number of arcs varies $N_a=4, 8, 16, 32$. Black dots (Kam) represents the results based on the TMR procedure (see text) obtained by Kamphuis_etal_2025.
  • ...and 6 more figures