The UV enigma of post-starburst galaxies

We have studied the panchromatic broad-band properties from the FUV to the MIR of a sample of 808 post-starburst galaxies. We find that in the optical and near-IR bands post-starburst galaxies (PSGs) form a remarkably uniform class of objects and that, on average, simple populations synthesis models (SSP) reproduce very well the SEDs of PSGs over a broad wavelength range, but not in the UV. We also find that, while the photometric variance in the optical and near-IR properties of the sample is small and comparable to the observational errors, both in the UV and the mid-IR the observed variance is much larger than the errors. We find a strong correlation between the UV fluxes and those in the mid-IR, indicating that the large variance in UV properties of PSGs could be related to a non-uniform distribution of dust covering the intermediate age populations. The disagreement between models and observations in the UV could be due to inadequate modelling; to the contribution of AGB and post-AGB stars; or to a non-uniform distribution of dust; possibly all three. Further progress in understanding this important class of galaxies, therefore, requires at the same time better modelling and better observations in the UV and mid-IR.