Radial kinematics of brightest cluster galaxies

Abstract

This is the first of a series of papers devoted to the investigation of a large sample of brightest cluster galaxies (BCGs), their kinematic and stellar population properties, and the relationships between those and the properties of the cluster. We have obtained high signal-to-noise ratio, long-slit spectra of these galaxies with Gemini and WHT with the primary purpose of investigating their stellar population properties. This paper describes the selection methods and criteria used to compile a new sample of galaxies, concentrating on BCGs previously classified as containing a halo (cD galaxies), together with the observations and data reduction. Here, we present the full sample of galaxies, and the measurement and interpretation of the radial velocity and velocity dispersion profiles of 41 BCGs. We find clear rotation curves for a number of these giant galaxies. In particular, we find rapid rotation (> 100 km/s) for two BCGs, NGC6034 and NGC7768, indicating that it is unlikely that they formed through dissipationless mergers. Velocity substructure in the form of kinematically decoupled cores is detected in 12 galaxies, and we find five galaxies with velocity dispersion increasing with radius. The amount of rotation, the velocity substructure and the position of BCGs on the anisotropy-luminosity diagram are very similar to those of "ordinary" giant ellipticals in high density environments.Comment: Accepted for publication in MNRA

The dynamically hot stellar halo around NGC 3311: a small cluster-dominated central galaxy

“…The steep outward gradient is supported by two independent data sets and data reductions. The measurements of Loubser et al (2008) near the galaxy center had already hinted at a positive gradient from 190 km s −1 at R = 5 to ∼240 km s −1 at a radius of R = 10 , and data shown in Fig. 1 of Hau et al (2004) reach 300 km s −1 at ∼25 .…”

“…3 together with the previous measurements from Loubser et al (2008). Table 1, which is available in electronic form, contains the following information: source of data (Col. 1), distance from galaxy center (Col. 2), PA (Col. 3), v, σ with errors for each of the procedures 1.-4. described in the text, in Col. (4-5), (6-7), (8-9), and (10-11), respectively.…”

“…Gemini GMOS-South long slit spectra -We use Gemini archive long-slit spectra in the wavelength range from 3675 Å to 6266 Å observed with the B600 grating, a dispersion of 0.914 Å pixel −1 , a spectral resolution of σ = 135 km s −1 , and a spatial scale of 0. 146 pixel −1 ; a detailed description of the instrumental setup is presented in Loubser et al (2008). The seeing was typically in the range from 0.…”

“…Because the stellar populations in cD halos may have different metal abundances and ages from those of the inner regions Loubser et al (2008), based on Gemini-South GMOS data. The black asterisks are our independent measurements from these GMOS (archival) spectra, and the red asterisks show measurements from the new VLT-FORS2 spectra.…”

“…The procedure is described in Loubser et al (2008) for the wavelength calibration and background subtraction; also in this case the edges of the slit are used to interpolate the sky emission in the regions covered by the stellar continuum. Because our goal is to sample the kinematics well into the halo, the one-dimensional spectra for the absorption line measurements are summed along the slit direction so that a minimum S/N ∼ 20 per Å is obtained in each radial bin, out to a radial distance of about 40 from the center of NGC 3311.…”