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Register a new userA 3D view of the Hydra I galaxy cluster core - I. Kinematic substructures
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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.
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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.
(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.
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.
We analyse the properties of the early-type dwarf galaxy population in the Hydra I cluster. We investigate the galaxy luminosity function (LF), the colour-magnitude relation (CMR), and the magnitude-surface brightness relation down to M_V~-10 mag. Another goal of this study is to find candidates for ultra-compact dwarf galaxies (UCDs). Two spectroscopic surveys performed with Magellan I/LDSS2 and VLT/VIMOS, as well as deep VLT/FORS1 images in V and I bands were examined. We identify cluster members by radial velocity measurements and select other cluster galaxy candidates by their morphology. One possible UCD candidate with M_V=-13.26 mag is found. Our sample of 100 morphologically selected dwarf galaxies defines a CMR that extends the CMR of the giant cluster galaxies to the magnitude limit of our survey (M_V~-10 mag). It matches the relations found for the Local Group and the Fornax cluster dwarfs almost perfectly. The Hydra I dwarfs also follow a magnitude-surface brightness relation similar to that of the LG dwarfs. Moreover, we observe a continuous relation for dwarf galaxies and giant early-type galaxies when plotting the central surface brightness mu_0 of a Sersic model vs. the galaxy magnitude. The effective radius is found to be largely independent of the luminosity for M_V>-18 mag, being R_e~0.8 kpc. We derive a very flat faint-end slope of the LF (alpha = -1.13 pm 0.04) from fitting a Schechter function, whereas fitting a power law for M_V>-14 mag gives alpha = -1.40 pm 0.18. Our findings suggest that early-type dwarf and giant galaxies are the same class of objects. The similarity of the dwarf galaxy scaling relations to other environments implies that internal processes could be more important for their global photometric properties than external influences. (abridged)
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.
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