The spatially resolved stellar populations of isolated early-type galaxies

We present radial stellar population parameters for a subsample of 12 galaxies from the 36 isolated early-type galaxies of Reda et al. Using new long-slit spectra, central values and radial gradients for the stellar age, metallicity [Z/H] and ?-element abundance [E/Fe] are measured. Similarly, the central stellar population parameters are derived for a further five isolated early-type galaxies using their Lick indices from the literature. On average, the 17 isolated galaxies have mean central [Z/H]o and [E/Fe]o of 0.29 ± 0.03 and 0.17 ± 0.03, respectively, and span a wide range of ages from 1.7 to 15 Gyr. We find that isolated galaxies follow similar scaling relations between central stellar population parameters and galaxy velocity dispersion to their counterparts in high-density environments. However, we note a tendency for isolated galaxies to have slightly younger ages, higher metallicities and lower abundance ratios. Such properties are qualitatively consistent with the expectation of an extended star formation history for galaxies in lower density environments. Generally, we measure constant age and [E/Fe] radial gradients. However, three galaxies show remarkable positive age gradients and two galaxies have negative age gradients. We find that the age gradients anticorrelate with the central galaxy age. Thus as a young starburst evolves, the age gradient flattens from positive to almost zero. Metallicity gradients range from near zero to strongly negative. For our high-mass galaxies metallicity gradients are shallower with increasing mass. Such behaviour is not predicted in dissipational collapse models but might be expected in multiple mergers. The metallicity gradients are also found to be correlated with the central age and metallicity, as well as to the age gradients. In conclusion, our stellar population data for a sample of isolated early-type galaxies are more compatible with an extended merger/accretion history than early dissipative collapse.