ESO 358-60: Kinematics and dynamics

In this work, now accepted for publication in The Astrophysical Journal, we analyzed the velocity maps of neutral hydrogen (HI) in the galaxy ESO 358-60, derived from measurements of the line-of-sight velocity. In these maps, red and blue colors indicate regions where the gas is moving away from us or toward us, respectively. To first order, this pattern can be interpreted as the projection of a rotating disk onto the plane of the sky. Because the disk is inclined with respect to the line of sight, its rotation produces the observed velocity gradient. By measuring the line-of-sight velocity across the entire disk, we can reconstruct the galaxy’s kinematic structure.

Starting from this velocity map and using the Velocity Ring Model, a technique we introduced a few years ago, we reconstructed both the tangential velocity field and the radial velocity field of the galaxy.

The main result emerging from these velocity maps is that the central region of the galaxy appears to be close to dynamical equilibrium, with matter rotating coherently around the galactic center. In contrast, the outer regions show significant velocity gradients and asymmetries, suggesting the presence of non-equilibrium phenomena, likely induced by tidal interactions or environmental effects.

For this reason, we focus our analysis on the central region and fit the observed rotation curve using two alternative mass-distribution models: the standard dark matter halo model and the dark matter disk model.

In the halo model, the stellar and gaseous disk in rotational equilibrium is embedded in a spherical dark matter halo whose support is provided by an isotropic velocity dispersion and which, by construction, does not participate in the disk rotation. In the dark matter disk model, by contrast, an additional mass component is assumed to be distributed within the disk itself, following a spatial distribution similar to that of the neutral hydrogen. This component is not directly observable through electromagnetic radiation; one possible interpretation is that it consists of very cold neutral hydrogen clouds with negligible emission.

From a statistical point of view, the main difference between the two models concerns the number of free parameters. The dark matter disk (DMD) model has only one free parameter, whereas the standard halo model requires two. Despite its greater simplicity, the DMD model provides a statistically better fit to the observed rotation curve, reproducing the data more accurately.

Furthermore, the total mass inferred in the DMD model is approximately one order of magnitude smaller than that required by the spherical halo model. This difference is not only statistically significant but also physically important, as it implies substantially different dynamical interpretations of the galaxy’s mass distribution.

In summary, the dark matter disk model requires less dark matter and proposes a simpler configuration in which all the matter participates in the rotation of the disk.