Constraining the Initial Mass Function in Massive Early-Type Galaxies

Abstract

The initial mass function (IMF) is a key parameter in understanding the formation and evolution of galaxies that describes the distribution of stellar masses at birth in a star formation event. Constraining the value of the IMF is important to ensure simulations and observational analysis are accurate. Two common methods of constraining the IMF, stellar population modeling and dynamical modeling, are applied to a sample of three massive, early-type galaxies (NGC0665, NGC0997, and NGC1684). Massive early-type galaxies are important due to evidence that the form of the IMF for these galaxies diverges from the standard Milky Way-like IMF. Using five spectra from LDSS-3 for each of these three galaxies, we fit the data using alf, an absorption line fitter and a complex population synthesis modeling tool that creates a best-fit model of the spectra to place constraints on the M/L ratio and key IMF parameters. Additionally, CO(2-1) observations from ALMA are used to dynamically model the M/L ratio within NGC0997. From the stellar population modeling, we find a IMF gradient that decreases with increasing radii from the center of the galaxy for at least one side (3 spectra) of each galaxy. However, some fits had problematic emission lines and dust such that general conclusions about IMF gradients are complicated due to these outer spectra. There does not seem to be consistency between the dynamical and stellar population M/L at the center of NGC0997, but there is consistency at the outer radii. With only NGC0997 modeled with dynamics, further analysis of the CO(2-1) observations of NGC1684 and NGC0997 would benefit this work by providing a larger sample size to compare these two methods for constraining the IMF.