The global mass-to-light ratio of SLACS lenses

¹ Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Complesso Universitario Monte S. Angelo - Edificio 6, via Cinthia, 80126 Napoli, Italy e-mail: winnyenodrac@gmail.com
² Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
³ Dipartimento di Fisica Generale “A. Avogadro”, Università di Torino and Istituto Nazionale di Fisica Nucleare, Sezione di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁴ Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁵ Dipartimento di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
⁶ Istituto Nazionale di Fisica Nucleare, sezione di Napoli, Complesso Universitario di Monte S. Angelo, Edificio 6, via Cinthia, 80126 Napoli, Italy

Received: 5 October 2008
Accepted: 27 May 2009

Abstract

Aims. The dark matter content of early-type galaxies (ETGs) is a hotly debated topic with contrasting results arguing in favour of or against the presence of significant dark mass within the effective radius and the change with luminosity and mass. To address this question, we investigate here the global mass-to-light ratio of a sample of 21 lenses observed within the Sloan Lens ACS (SLACS) survey.

Methods. We follow the usual approach of the galaxy as a two component systems, but we use a phenomenological ansatz for , proposed by some of us, that is able to smoothly interpolate between constant  models and a wide class of dark matter haloes. The resulting galaxy model is then fitted to the data on the Einstein radius and aperture velocity dispersion.

Results. Our phenomenological model turns out to agree with the data suggesting the presence of massive dark matter haloes to explain the lensing and dynamics properties of the SLACS lenses. According to the values of the dark matter mass fraction, we argue that the halo may play a significant role in the inner regions probed by the data, but such a conclusion strongly depends on the adopted initial mass function of the stellar population. Finally, we find that the dark matter mass fraction within R_eff scales with both the total luminosity and stellar mass in such a way that more luminous (and hence more massive) galaxies have a greater dark matter content.