The Hydra I cluster core

II. Kinematic complexity in a rising velocity dispersion profile around the cD galaxy NGC 3311

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
e-mail: mhilker@eso.org
² Universidad de Concepción, Concepción, Chile
³ Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Rua do Matão 1226, São Paulo, SP, Brazil

Received: 8 August 2017
Accepted: 14 August 2018

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 h₃ and h₄.

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 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.