Testing Claims of Different Stellar Initial Mass Functions

University Of Arizona, Tucson AZ

Investigators

Abstract

They will study whether stars form in the same manner throughout the Universe. Their specific question is whether stars form in nearby galaxies follow the same distribution of star birth weights as is found in our galaxy. Some stars in our galaxy are born as massive giants and others are small dwarfs, but astronomers have found a pattern in the sizes of stars, called the Initial Mass Function (IMF). They have devised an experiment to determine whether this pattern is universal. The answer to their question is important for understanding how galaxies form and whether these galaxies contain dark matter. Conventional wisdom, based on extensive studies of Galactic star formation regions, is that the IMF is invariant. However, indirect evidence based on observation of starburst galaxies, elliptical galaxies, and the high redshift universe suggests that some variations exist. Neither argument is convincing. Galactic studies do not constrain the IMF in all environments because they probe a limited parameter range (interstellar radiation field, density). On the other hand, the extragalactic arguments are based on highly simplified stellar population modeling, with few constraints on star formation histories and the luminosities of evolved stars. Zaritsky has previously uncovered straightforward evidence for IMF variations within the Local Group of galaxies, where it is possible to study the stellar populations in detail and over a wider range of environments than found near the Sun. In mostly older stellar clusters, the inferred IMF is consistent with distribution of stars in the local neighborhood near our Sun. They will attempt to prove the significance of the variations in birth weights using a remarkable new camera system for observing distant star clusters. Their project will train a graduate student and prove the capabilities of the new high sensitivity camera system. In addition, they plan to use their research results as the basis for a set of public lectures to illustrate how scientific thought develops.