Metallicity and age gradients in round elliptical galaxies^*

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
² Sternberg Astronomical Institute, University av. 13, Moscow 119992, Russia e-mail: olga@sai.msu.su
³ Isaac Newton Institute of Chile, Moscow Branch
⁴ Special Astrophysical Observatory, Nizhnij Arkhyz 369167, Russia

Received: 16 November 2006
Accepted: 26 January 2007

Abstract

Aims.We probe the stellar population age and metallicity distributions in nearby elliptical galaxies over the largest extension to date.

Methods. Long-slit spectroscopy is made by using the spectrograph SCORPIO of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciencies. The Lick indices Hβ, Mg b, Fe 5270, and Fe 5335 are calculated along the slit up to radii of 1.3 to 3r_e in 4 galaxies and up to 0.5r_e in the fifth one. The comparison with evolutionary synthesis models of simple stellar populations allows us to disentangle age and metallicity and to measure both.

Results. We have found that the mean stellar age is constant along the radius only in one galaxy out of 5. The other 4 galaxies demonstrate quite different behaviour of the mean stellar age: the outer parts are older than the centres in 3 cases and younger – in one case. The metallicity gradients cannot be approximated by a single power law over the full radial extension in 4 galaxies of 5. The inner metallicity gradients within 0.5r_e are all rather steep, steeper than –0.4 metallicity dex per radius dex, and are inconsistent with the origin of the elliptical galaxies by a major merger.