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Register a new userThe Hydra I cluster core - II. Kinematic complexity in a rising velocity dispersion profile around the cD galaxy NGC 3311
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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. 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. We use pPXF to extract the kinematic information. We find a homogeneous velocity and velocity dispersion field within r<10 kpc. Beyond this radius, both the velocities and dispersions 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. If the radial dependence alone is considered, the velocity dispersion does not simply rise but fills an increasingly large range of values with two well defined envelopes. The lower envelope is about constant at 200 km/s. The upper envelope rises smoothly, joining the velocity dispersion of the outer 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 with different anisotropies can account for all observed velocity dispersions, including radial MOND models. 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.
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(Abridged for arXiv) The history of the mass assembly of brightest cluster galaxies may be studied by the mapping the stellar populations at large radial distances from the galaxy centre. We provide extended and robust measurements of the stellar population parameters in NGC 3311, the cD galaxy at the centre of the Hydra I cluster and out to three effective radii. Using seven absorption-features defined in the Lick/IDS system and single stellar populations models, we obtained luminosity-weighted ages, metallicities and alpha element abundances. The trends in the Lick indices and the distribution of the stellar population parameters indicate that the stars of NGC 3311 may be divided into two radial regimes, one within and the another beyond one effective radius, $R_e = 8.4$ kpc, similar to the distinction between inner galaxy and external halo derived from the NGC 3311 velocity dispersion profile. The inner galaxy ($Rleq R_e$) is old (age $sim 14$ Gyr), have negative metallicity gradients and positive alpha element gradients. The external halo is also very old, but the metal and element abundances of the external halo have both a large scatter, indicating that stars from a variety of satellites with different masses have been accreted. The region in the extended halo associated with the off-centred envelope at 0$^o$ < P.A.< 90$^o$ (Arnaboldi et al. 2012) has higher metallicity with respect to the symmetric external halo. The different stellar populations in the inner galaxy and extended halo reflect the dominance of in situ stars in the former and the accreted origin for the large majority of the stars in the latter. These results provide supporting evidence to the recent theoretical models of formation of massive ellipticals as a two-phase process.
We used FORS2 in MXU mode to mimic a coarse IFU in order to measure the 3D large-scale kinematics around the central Hydra I cluster galaxy NGC 3311. Our data show that the velocity dispersion field varies as a function of radius and azimuthal angle and violates point symmetry. Also, the velocity field shows similar dependence, hence the stellar halo of NGC 3311 is a dynamically young structure. The kinematic irregularities coincide in position with a displaced diffuse halo North-East of NGC 3311 and with tidal features of a group of disrupting dwarf galaxies. This suggests that the superposition of different velocity components is responsible for the kinematic substructure in the Hydra I cluster core.
The formation of intracluster light and of the extended halos around brightest cluster galaxies is closely related to morphological transformation, tidal stripping, and disruption of galaxies in clusters. We analyze Ks- and V-band surface photometry as well as deep long-slit spectra, and establish a link between the structures in the light distribution, the absorption line kinematics, and the LOS velocity distributions of nearby galaxies and planetary nebulae (PNs). The central galaxy NGC 3311 is surrounded by an extended symmetric outer halo with n=10 and an additional, off-centered envelope ~ 50 to the North-East. Its luminosity L_V= 1.2x10^{10} +/- 6.0 x 10^8 L_sun corresponds to ~50 % of the luminosity of the symmetric halo in the same region. Based on measured PN velocities, at least part of the off-centered envelope consists of high-velocity accreted stars. We have also discovered two tidal streams in the cluster center, emerging from the dwarf galaxy HCC 026 and from the S0 galaxy HCC 007. The HCC 026 stream is redshifted by ~1200 km/s with respect to NGC 3311, similarly as HCC 026 itself, a fraction of PNs in the off-centered envelope, and several other dwarf galaxies nearby. The stars in one of the HCC 026 tails are known to be consistent with the low-metallicity population of HCC 026, and our photometry shows that this galaxy is already mostly dissolved in the tidal field. The tidal stream around HCC 007 extends over at least 110 kpc. It is fairly thick and is brighter on the side of the asymmetric outer halo of NGC 3311, which it may join. Its luminosity is several 10^9 L_sun, similar to the luminosity of the stripped-down galaxy HCC 007. The redshift of the stream is determined from a few PN velocities and is similar to that for HCC 007 and HCC 026.
NGC 3311 is the central cD galaxy of the Hydra I cluster. We use globular clusters around NGC 3311, combined with kinematical data of the galaxy itself, to investigate the dark matter distribution in the central region of Hydra I. Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask spectroscopy, are used to calculate velocity dispersions which are well defined out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of study has been performed. We also determine velocity dispersions of the stellar component from long slit spectroscopy out to 20 kpc. Moreover, we present a new photometric model for NGC 3311 in the V-band. We search for a dark halo which in the context of a spherical Jeans model. We also compare the radial velocity distributions of globular clusters and planetary nebulae. The projected stellar velocity dispersion rises from 185 km/s to 350 km/s at a radius of 20 kpc. The globular cluster dispersion rises as well from 500 km/s at 10 kpc to about 800 km/s at 100 kpc, comparable to the velocity dispersion of the cluster galaxies. A dark matter halo with a core reproduces well the velocity dispersions of stars and globular clusters simultaneously under isotropy. The central stellar velocity dispersions predicted by cosmological NFW halos are less good representations, while the globular clusters allow a wide range of halo parameters. A suspected radial anisotropy of the stellar population aggravates the deviations. However, we find discrepancies with previous kinematical data, which we cannot resolve and may indicate a more complicated velocity pattern. Although one cannot conclusively demonstrate that the dark matter halo of NGC 3311 has a core rather than a cusp, a core seems to be preferred by the present data. A more complete velocity field and an analysis of the anisotropy is required to reach firm conclusions.
Several observations of the central region of the Hydra I galaxy cluster point to a multi-epoch assembly history. Using our novel FORS2/VLT spectroscopic data set, we were able to map the luminosity-weighted age, [Fe/H] and [$alpha$/Fe] distributions for the stellar populations around the cD galaxy NGC 3311. Our results indicate that the stellar populations follow the trends of the photometric substructures, with distinct properties that may aid to constrain the evolutionary scenarios for the formation of the cluster core.
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