Kinematics and line strength indices in the halos of the Coma brightest cluster galaxies NGC 4874 and NGC 4889^*

¹ Max-Plank-Institut für Extraterrestrische Physik, Giessenbachstrae 1, 85741 Garching bei München, Germany e-mail: lcoccato@mpe.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Strae 2, 85748 Garching bei München, Germany
³ INAF, Osservatorio Astronomico di Pino Torinese, 10025 Pino Torinese, Italy
⁴ Research School of Astronomy & Astrophysics, ANU, Canberra, Australia
⁵ Institute for the Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba 277-8568, Japan

Received: 22 March 2010
Accepted: 6 May 2010

Abstract

Aims. We investigate the stellar kinematics and line strength indices in the outer halos of brightest cluster galaxies (BCGs) in the Coma cluster to obtain the outer halo V_rot and σ profiles and to derive constraints on the formation history of these objects.

Methods. We analyzed absorption lines in deep, medium-resolution, long-slit spectra in the wavelength range ~ 4500–5900 Å, out to ~ 50 kpc for NGC 4874 and ~ 65 kpc for NGC 4889, probing regions with a surface brightness down to μ_R ~ 24 mag arcsec^-2.

Results. These data provide stellar velocity and velocity dispersion profiles along the major axes of both BCGs, and also along the minor axis of NGC 4889. The kinematic properties of NGC 4874 and NGC 4889 halos extend the previous relations of early-type galaxy halos to bright luminosities and indicate that the stars in the outer regions are still bound to these galaxies. For NGC 4889 we also determine Hβ Mg and Fe line strength indices, finding strong radial gradients for Mg and Fe. The current dataset for NGC 4889 is one of the most extended in radius, including both stellar kinematics and line strength index measurements.