Age and metallicity gradients in early-type galaxies: a dwarf-to-giant sequence

Abstract

We studied the stellar populations of 40 early-type galaxies using medium\nresolution long-slit spectroscopy along their major axes (and along the minor\naxis for two of them), from 10^7 Msol to 10^12 Msol (-9.2 > M_B > -22.4 mag).\nAll the studied galaxies lie on the mass-metallicity and age-mass relations.\nThe transition type dwarfs deviate from the latter relation having younger mean\nage, and the low-mass dwarf spheroidals have older ages, marking a\ndiscontinuity in the relation, possibly due to selection effects.\n In all mass regimes, the mean metallicity gradients are approximately -0.2\nand the mean age gradients +0.1 dex per decade of radius. The individual\ngradients are widely spread: $ -0.1 < \\nabla_{\\rm Age} < 0.4 $ and $-0.54 <\n\\nabla_{[{\\rm Fe/H}]} < +0.2 $. We do not find evidence for a correlation\nbetween the metallicity gradient and luminosity, velocity dispersion, central\nage or age gradient. Likewise, we do not find a correlation between the age\ngradient and any other parameter in bright early-type galaxies. In faint\nearly-types with $M_B \\gtrsim -17$ mag, on the other hand, we find a\ncorrelation between the age gradient and luminosity: the age gradient becomes\nmore positive for fainter galaxies.\n We conclude that various physical mechanisms can lead to similar gradients\nand that these gradients are robust against the environmental effects. In\nparticular, the gradients observed in dwarfs galaxies certainly survived the\ntransformation of the progenitors through tidal harassment or/and ram-pressure\nstripping. The diversity of metallicity gradients amongst dwarf elliptical\ngalaxies may reflect a plurality of progenitors' morphologies. The dwarfs with\nsteep metallicity gradients could have originated from blue compact dwarfs and\nthose with flat profiles from dwarf irregulars and late type spirals.\n(Abridged)\n