Mon. Not. R. Astron. Soc., 405, 800-820 (2010/June-3)
Stellar populations of Virgo cluster early-type dwarf galaxies with and without discs: a dichotomy in age ?
PAUDEL S., LISKER T., KUNTSCHNER H., GREBEL E.K. and GLATT K.
Abstract (from CDS):
Using Very Large Telescope/FORS2 spectroscopy, we have studied the properties of the central (inner 320pc) stellar populations of a sample of 26 nucleated early-type dwarf (dE) galaxies in the Virgo cluster with a magnitude range of -18.59 ≤ Mr ≤ -15.39 mag. With the addition of the data of the MAGPOP-IPP team (Michielsen et al.), extending the sample to 38 dEs, we find that these galaxies do not exhibit the same average stellar population characteristics for different morphological subclasses. The nucleated galaxies without discs, which are, on average, fainter than dEs with discs and distributed in regions of higher local density, are older and more metal-poor (mean ages 7.5±1.89 and 3.1±0.83Gyr, mean [Z/H] = -0.54±0.14 and -0.31±0.10dex, respectively). The α-element abundance ratio appears consistent with the solar value for both morphological types. Besides a well-defined relation between metallicity and luminosity, we also find a clear anticorrelation between age and luminosity. More specifically, there appears to be a bimodality: brighter galaxies (Mr ≤ -16.5 mag), including the disc galaxies, exhibit significantly younger ages than fainter dEs (Mr ≥ -17.0 mag). The magnitude overlap between these two subgroups appears to be resolved when considering, in addition, the weak correlation between age and local density, such that older galaxies at a given magnitude are located at higher densities. Thus, there seems to be no significant difference between the stellar populations of dEs with and without discs when compared at the same magnitude and density. Thereby, we revisit the question whether both could belong to the same intrinsic population, with discs surviving only in lower-density regions. However, it appears less likely that fainter and brighter dEs have experienced the same evolutionary history, as the well-established trend of decreasing average stellar age when going from the most luminous ellipticals towards low-luminosity ellipticals and bright dEs is broken here. The older and more metal-poor dEs could have had an early termination of star formation activity, possibly being `primordial' galaxies in the sense that they have formed along with the protocluster or experienced very early infall. By contrast, the younger and relatively metal-rich brighter dEs, most of which have discs, might have undergone structural transformation of infalling disc galaxies.
© 2010 The Authors. Journal compilation © 2010 RAS
galaxies: clusters: individual: Virgo - galaxies: dwarf - galaxies: elliptical and lenticular, cD - galaxies: evolution - galaxies: formation - galaxies: stellar content
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