Radially extended kinematics and stellar populations of the massive ellipticals NGC 1600, NGC 4125, and NGC 7619^*

Constraints on the outer dark halo density profile

¹ National Astronomical Observatory/Yunnan Observatory, the Chinese Academy of Sciences, Kunming 650011, PR China e-mail: pushibi@hotmail.com
² Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
³ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁴ Universitätssternwarte, München, Scheinerstrae 1, 81679 München, Germany
⁵ Key Laboratory of the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650011, PR China

Received: 10 March 2010
Accepted: 1 April 2010

Abstract

We present high quality long slit spectra along the major and minor axes out to 1.5–2 R_e (14–22 kpc) of three bright elliptical galaxies (NGC 1600, NGC 4125, NGC 7619) obtained at the Hobby-Eberly Telescope (HET). We derive stellar kinematic profiles and Lick/IDS indices (Hβ, Mgb, Fe₅₀₁₅, Fe₅₂₇₀, Fe₅₃₃₅, Fe₅₄₀₆). Moreover, for NGC 4125 we derive gas kinematics and emission line strengths. We model the absorption line strengths using Simple Stellar Populations models that take into account the variation of [ α/Fe] and derive ages, total metallicity and element abundances. Overall, we find that the three galaxies have old and [ α/Fe] overabundant stellar populations with no significant gradients. The metallicity is supersolar at the center with a strong negative radial gradient. For NGC 4125, several pieces of evidence point to a recent dissipational merger event. We calculate the broad band color profiles with the help of SSP models. All of the colors show sharp peaks at the center of the galaxies, mainly caused by the metallicity gradients, and agree well with the measured colors. Using the Schwarzschild's axisymmetric orbit superposition technique, we model the stellar kinematics to constrain the dark halos of the galaxies. We use the tight correlation between the Mgb strength and local escape velocity to set limits on the extent of the halos by testing different halo sizes. Logarithmic halos – cut at 60 kpc – minimize the overall scatter of the Mgb-V_esc relation. Larger cutoff radii are found if the dark matter density profile is decreasing more steeply at large radii.