Testing the Extremes of Initial Mass Function Variability using Compact Stellar Systems

Abstract

The initial mass function (IMF) is a cornerstone in star formation and galaxy evolution studies. It has traditionally been assumed to be universal, but this has been challenged by increasingly detailed observations of diverse, extragalactic stellar populations. However, this observed variability in the IMF is debated as a sufficient theoretical framework to explain it has not yet been substantiated. A major limitation is that these observations have only probed narrow regions of mass-metallicity-density parameter space (i.e. metal-rich, early-type galaxies). We present an unprecedented sample of integrated light spectroscopy of diverse objects, including "compact" stellar systems (CSSs, i.e. globular clusters and ultra-compact dwarf galaxies) and brightest cluster galaxies. Our sample covers a wide range of metallicities (-1.7 < [Fe/H] < 0.01) and velocity dispersions (7.4 km/s < σ < 275 km/s). We carefully reduce high S/N Keck LRIS spectra and measure the IMF by employing a new suite of full-spectrum stellar population synthesis models. These simultaneously fit for ages, metallicities, and detailed chemical abundances, allowing us to robustly measure the effects of the IMF. We show that CSSs do not follow trends with physical parameters that have been found for early-type galaxies (ETGs). In particular, previously established metallicity-dependent trends with the IMF may change in complex ways. We examine potential factors that could be causing the population of CSSs to contrast with the ETGs in this parameter space.