Desika Narayanan, Romeel Davé
We examine the cosmic evolution of a stellar initial mass function (IMF) in galaxies that varies with the Jeans mass in the interstellar medium, paying particular attention to the K-band stellar mass to light ratio (M/L_K) of present-epoch massive galaxies. We calculate the typical Jeans mass using high-resolution hydrodynamic simulations coupled with a fully radiative model for the ISM, which yields a parameterisation of the IMF characteristic mass as a function of galaxy star formation rate (SFR). We then calculate the star formation histories of galaxies utilising an equilibrium galaxy growth model coupled with constraints on the star formation histories set by abundance matching models. Our main result is that at early times, energetic coupling between dust and gas drive warm conditions in the ISM, and hence bottom-light/top-heavy IMFs associated with large ISM Jeans masses for massive star-forming galaxies. At late times, lower cosmic ray fluxes allow for cooler ISM temperatures in massive galaxies, and hence bottom-heavy IMFs. Because the massive stars (M >~ 1 Msun) formed during the top-heavy phases at early times have all disappeared by today, the resultant M/L_K ratios in massive galaxies at the present epoch is increased relative to the non-varying IMF case. A key result is that a given galaxy may go through both top-heavy and bottom-heavy phases during its lifetime. Quantitatively, the variations in M/L_K with galaxy mass are slightly smaller than what is observed. Nonetheless, these results are encouraging because they can, at least qualitatively, reconcile a bottom-light IMF that would help explain a number of observations of massive high-z galaxies with the bottom-heavy IMF inferred for the descendants of those galaxies today.
View original:
http://arxiv.org/abs/1210.6037
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