Is the Initial Mass Function universal?

点击翻译按钮获取中文摘要

A key parameter in modelling galaxies is the assumption made about the distribution of stellar masses that result when stars form from a cloud of gas, the initial mass function or IMF. The number of the highest mass stars determines the number of supernovae and thus the amount of energy fed back into the interstellar medium and the chemical enrichment that occurs. The number of low mass stars determines the fraction of the galaxy mass that is "hidden" in low mass dwarf stars that contribute almost nothing to the light output. So the relative numbers of high and low mass stars can have a radical effect on the light output of a galaxy. The simplest assumption is that the IMF is a universal function for all galaxies at all times. However recently it has been proposed that the most massive early-type galaxies may be enhanced in dwarf stars compared to less massive galaxies ( e.g. see Conroy & van Dokkum 2012 - http://iopscience.iop.org/0004-637X/760/1/71/pdf/apj_760_1_71.pdf and Cappellari et al - http://www.nature.com/nature/journal/v484/n7395/full/nature10972.html)These claims are based on high precision measurements of specific spectral features that are sensitive to the presence of dwarfs stars. Whether this phenomenon is limited only to the rather special environment at the centres of massive galaxies has significant implications for the mass estimates and star formation history of galaxies and remains unknown. The Oxford built SWIFT integral field spectrograph (IFS) on the Palomar 5m telescope is one of the most sensitive instruments in the far red region of the spectrum where the IMF critical absorption features of sodium, calcium and iron hydride are found. This project will use the SWIFT IFS to measure the IMF sensitive features in a range of galaxies and in particular to determine whether the IMF changes reported by others occur only in the galaxy nuclei or pervade the entire galaxy. Spectra from WHT-ISIS, SAMI & MaNGA will be used to extend this analysis to larger numbers of galaxies using spectral features in the optical.