Using the publicly available vespa data base 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. By splitting the sample into bins of total stellar mass, we find a strong dependence, with stellar MWA increasing for more massive galaxies.
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. We again split the sample into bins of total stellar mass, and still find a strong dependence on stellar MWA for increasing mass galaxies, but additionally a second order trend of increasing stellar MWA with increasing dark matter mass of the host halo.
Furthermore we use public dark matter mass accretion history (MAH) code calibrated on simulations, to calculate the dark matter MWA 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 anticorrelated for large mass haloes, i.e, dark matter accretion does not seem to be the primary factor in determining when stellar mass was assembled. This effect can be described by downsizing.