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Published online by Cambridge University Press: 02 December 2009
We present long-slit spectra for 11 early-type galaxies observed with the Keck telescope. We measure rotation-velocity and velocity-dispersion profiles together with 20 Lick line-strength gradients. Gradients of indices are transformed into ages, metallicities and [α/Fe] using stellar-population models that take into account variations in chemical-abundance ratios. We find that the line-strength gradients are mainly due to radial variations of metallicity, although small gradients of [α/Fe] and age are also present. Contrary to what is expected in simple collapse models, galaxies in our sample have both positive and negative [α/Fe] profiles. This rules out a solely inside-out or outside-in formation mechanism for all early-type galaxies. Metallicity gradients correlate with the shape of the isophotes and the rotational velocity but do not correlate with the mass of the galaxies. Galaxies with younger populations in their centres have steeper metallicity gradients. Our results suggest a scenario whereby galaxies form through the merger of smaller structures and the degree of dissipation during those mergers increases when the masses of the progenitor galaxies decrease.
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