The Hydra I cluster core - II. Kinematic complexity in a rising velocity dispersion profile around the cD galaxy NGC 3311

Abstract

Context. NGC 3311, the central galaxy of the Hydra I cluster, shows signatures of recent infall of satellite galaxies from the cluster environment. Previous work has shown that the line-of-sight velocity dispersion of the stars and globular clusters in the extended halo of NGC 3311 rises up to the value of the cluster velocity dispersion. In the context of Jeans models, a massive dark halo with a large core is needed to explain this finding. However, position dependent long-slit measurements show that the kinematics are still not understood. Aims. We aim to find kinematic signatures of sub-structures in the extended halo of NGC 3311. Methods. We performed multi-object spectroscopic observations of the diffuse stellar halo of NGC 3311 using VLT/FORS2 in MXU mode to mimic a coarse 'IFU'. The slits of the outermost masks reach out to about 35 kpc of galactocentric distance. We use pPXF to extract the kinematic information of velocities, velocity dispersions and the high-order moments h 3 and h 4 . Results. We find a homogeneous velocity field and velocity dispersion field within a radius of about 10 kpc. Beyond this radius, both the velocities and the velocity dispersion start to depend on azimuth angle and show a significant intrinsic scatter. The inner spheroid of NGC 3311 can be described as a slow rotator. Outside 10 kpc the cumulative angular momentum is rising, however, without showing an ordered rotation signal. If the radial dependence alone is considered, the velocity dispersion does not simply rise but fills an increasingly large range of dispersion values with two well defined envelopes. The lower envelope is about constant at 200 km s −1 . The upper envelope rises smoothly, joining the velocity dispersion of the outer globular clusters and the cluster galaxies. We interpret this behaviour as the superposition of tracer populations with increasingly shallower radial distributions between the extremes of the inner stellar populations and the cluster galaxies. Simple Jeans models illustrate that a range of of mass profiles can account for all observed velocity dispersions, including radial MOND models. Conclusions. The rising velocity dispersion of NGC 3311 apparently is a result of averaging over a range of velocity dispersions related to different tracer populations in the sense of different density profiles and anisotropies. Jeans models using one tracer population with a unique density profile are not able to explain the large range of the observed kinematics. Previous claims about the cored dark halo of NGC 3311 are therefore probably not valid. This may in general apply to central cluster galaxies with rising velocity dispersion profiles, where infall processes are important.

Study of gravitational fields and globular cluster systems of early-type galaxies

“…Galaxy encounters however also increase velocity dispersion in another way apart from tidal heating (Hilker et al 2018). This mechanism is based on the fact that the baryonic halos of central cluster galaxies are grown by material released from the galaxies that fall on the central galaxy from the outskirts of the galaxy cluster or from the field.…”

“…Because of the large difference of gravitational potential, the new GCs have a large velocity and increase the velocity dispersion of their new host substantially before the system relaxes dynamically. Employing this mechanism, Hilker et al (2018) were able to explain the apparent disagreement of MOND with the stellar kinematics in the central galaxy of the Hydra I cluster, NGC 3311. This mechanism suggests itself particularly for the galaxies NGC 1399 and NGC 4365 showing multiple signs of capturing GCs of other galaxies.…”

“…Other mechanisms influencing the velocity dispersion include the following: the contamination of the GC population by the GCs moving in the total gravitational field of the cluster; the contamination by the GCs orbiting the satellite galaxies of the investigated galaxies; the presence of binary GCs, just as binary stars increase the observed velocity dispersion in dwarf galaxies (Dabringhausen et al 2016); the anisotropic spatial distribution of GCs, such as that observed around the Milky Way (e.g., Pawlowski et al 2012;Arakelyan et al 2018); the substructures in the GC system resulting from stripping of other galaxies (e.g., like in NGC 3311, Hilker et al 2018); or the systematic error introduced by our approximative treatment of the systemic rotation of the GC systems by Eq. 8.…”