Charles Steinhardt (Niels Bohr Institute): "Is the Stellar Initial Mass Function Truly Universal?"

Current techniques for analyzing large photometric catalogs are generally forced to assume a single, universal stellar initial mass function (IMF). However, the IMF is predicted to depend upon the temperature of gas in star-forming molecular clouds, and thus should be expected to vary depending upon conditions within a star-forming galaxy. The introduction of an additional parameter into photometric template fitting allows galaxies to be fit with a range of different IMFs. Three surprising new features appear: (1) most star-forming galaxies are best fit with a bottom-lighter IMF than the Milky Way; (2) most star-forming galaxies at fixed redshift are fit with a very similar, but non-Milky Way IMF; and (3) the most massive star-forming galaxies at fixed redshift instead exhibit a less bottom-light IMF, which suggests quiescence may be the result of gas depletion rather than stochastic or environmental processes. Finally, the lowest-mass star-forming galaxies appear to exhibit a distinct relationship between IMF and star formation rate, possibly hinting at distinct feedback mechanisms in the earliest stages of star formation.