Using the publicly available VESPA database of SDSS Data Release 7 spectra,
we calculate the stellar Mass Weighted Age (hereafter MWA) as a function of
local galaxy density and dark matter halo mass. We compare our results with
semi-analytic models from the public Millennium Simulation. We find that the
stellar MWA has a large scatter which is inherent in the data and consistent
with that seen in semi-analytic models. The stellar MWA is consistent with
being independent (to first order) with local galaxy density, which is also
seen in semi-analytic models.
As a function of increasing dark matter halo mass (using the SDSS New York
Value Added Group catalogues), we find that the average stellar MWA for member
galaxies increases, which is again found in semi-analytic models. Furthermore
we use public dark matter Mass Accretion History (MAH) code calibrated on
simulations, to calculate the dark matter Mass Weighted Age as a function of
dark matter halo mass. In agreement with earlier analyses, we find that the
stellar MWA and the dark matter MWA are anti correlated for large mass halos,
i.e, dark matter accretion does not seem to be the primary factor in
determining when stellar mass was compiled. This effect can be described by
down-sizing.Comment: 11 pages, 3 figures, submitted to MNRA
