The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological?

Abstract

Context. Some galaxy clusters exhibit shallow or even cored dark matter density profiles in their central regions rather than the predicted steep or cuspy profiles, conflicting with the standard understanding of dark matter. NGC 3311 is the central cD galaxy of the Hydra I cluster (Abell 1060). Aims. 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. Methods. 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 the velocity dispersions of the stellar component from long-slit spectroscopy of NGC 3311 acquired with VLT/FORS1 out to 20 kpc. We present a new photometric model for NGC 3311, based on deep VLT/FORS1 images in the V-band. We search for a dark halo that, in the context of a spherical Jeans model, can reproduce the kinematical data. We also compare the radial velocity distributions of globular clusters and planetary nebulae. Results. The projected stellar velocity dispersion rises from a central low value of about 185 km s −1 to 350 km s −1 at a radius of 20 kpc. The globular cluster dispersion rises as well from 500 km s −1 at 10 kpc to about 800 km s −1 at 100 kpc, comparable to the velocity dispersion of the cluster galaxies. A dark matter halo with a core (Burkert halo) closely reproduces the velocity dispersions of stars and globular clusters simultaneously under isotropy. The central stellar velocity dispersions predicted by cosmological NFW halos do not agree well with those observed, while the globular clusters allow a wide range of halo parameters. A suspected radial anisotropy of the stellar population found in merger simulations aggravates the disagreement with observations. A slight tangential anisotropy would enable the data to be more accurately reproduced. However, we find discrepancies with previous kinematical data that we cannot resolve, which may indicate a more complicated velocity pattern. Conclusions. 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 most consistent with the present data. A more complete coverage of the velocity field and a more thorough analysis of the anisotropy is required to reach firm conclusions.

A tale of two tails and an off-centered envelope: diffuse light around the cD galaxy NGC 3311 in the Hydra I cluster

“…This study was based on Gemini long-slit spectra along PA = 63 • , and deep VLT/FORS2 long-slit spectra centered on the dwarf galaxy HCC 026 with a PA = 142 • . More recently, the VLT/FORS1 spectra along PA = 29 • published by Richtler et al (2011) confirmed the large outer velocity dispersion values and the asymmetry of the V LOS profiles, but also showed deviations between measurements at different PA. The mean LOS velocity V LOS (R) profiles along both PAs increase outwards from NGC 3311 by ∼100 km s −1 to the NE, while they remain close to the galaxy systemic velocity V NGC 3311 (R) = 3800 km s −1 to the SW.…”

“…However, it is not the extended light profile that distinguishes this central cluster galaxy from other luminous elliptical galaxies, but its very steeply rising velocity-dispersion profile (VDP) σ(R). From a central value of ∼170 km s −1 , the VDP rises to σ 230 km s −1 at R = 15 3.7 kpc, and then on to σ = 300−450 km s −1 at R = 50 12 kpc (Ventimiglia et al 2010;Richtler et al 2011). The steep rise in the velocity dispersion profile corresponds to a steep increase in the enclosed mass.…”

“…NGC 3309 does not have a dust lane at its core (Vasterberg et al 1991). Misgeld et al (2008) published VLT/FORS1 images of the Hydra cluster in the V-band, that were used by Richtler et al (2011) to derive the average, radial surface brightness profile for NGC 3311 down to μ V = 26.5 mag arcsec −2 . They classified NGC 3311 as a cD galaxy because of its low central surface brightness and extended radial profile.…”

Kinematic properties of the field elliptical NGC 7507

“…However, the same cannot be said conclusively for all elliptical galaxies. Central galaxies in galaxy clusters such as NGC 6166 (Kelson et al 2002), NGC 1399 (Richtler et al 2008;Schuberth et al 2010), and NGC 3311 (Richtler et al 2011) are embedded in massive dark halos as is the case of bright non-central cluster ellipticals such as NGC 4636 and NGC 4374 in Fornax (Schuberth et al 2006;Napolitano et al 2010). In constrast elliptical galaxies with lower luminosities in sparser environments (NGC 821,3379,4494,4697), whose mass profiles Based on observations taken at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and SECYT (Argentina).…”

Dark matter deprivation in the field elliptical galaxy NGC 7507

“…It seems that central cluster galaxies are also embedded in massive dark haloes (see e.g. Kelson et al 2002;Richtler et al 2008Richtler et al , 2011aSchuberth et al 2010), as are some of the most massive noncentral galaxies in the Fornax and Virgo clusters (e.g. Napolitano et al 2011;Schuberth et al 2012).…”