AGB populations in post-starburst galaxies

Abstract

In a previous paper we compared the spectral energy distributions of a sample of 808 K+A galaxies from the far ultraviolet to the mid infrared to the predictions of the spectrum synthesis models explicitly using asymptotic giant branch components. Here we use the new AGB-light models from C. Maraston (including less fuel for the later stages of stellar evolution and improved calibrations) to address the discrepancies between our observations and the AGB-heavy models used in our previous paper, which over-predict the infrared fluxes of post-starburst galaxies by an order of magnitude. The new models yield a much better fit to the data, especially in the near-IR, compared to previous realisations where AGB stars caused a large excess in the H and K bands. We also compare the predictions of the M2013 models to those with BC03 and find that both reproduce the observations equally well.We still find a significant discrepancy with both sets of models in the Y and J bands, which however is probably due to the spectral features of AGB stars. We also find that both the M2013 and the BC03 models still over-predict the observed fluxes in the UV bands, even invoking extinction laws that are stronger in these bands. While there may be some simple explanations for this discrepancy, we find that further progress requires new observations and better modelling. Excess mid-infrared emission longwards of 5 μm is well modelled by a Tdust = 300 K black body, which may arise from dust emission from the circumstellar envelopes of oxygen rich M stars (expected for a metal-rich population of AGB stars).