Is the Initial Mass Function universal?

星系建模的一个关键参数是对恒星质量分布的假设，这是恒星从气体云形成时的结果，即初始质量函数或IMF。质量最大的恒星的数量决定了超新星的数量，从而决定了反馈到星际介质的能量和发生的化学富集的数量。低质量恒星的数量决定了“隐藏”在低质量矮星中的星系质量的比例，而低质量矮星对光输出几乎没有贡献。因此，高质量和低质量恒星的相对数量会对星系的光输出产生根本性的影响。最简单的假设是，IMF在任何时候都是所有星系的通用函数。然而最近有人提出最大规模早型星系可能增强矮星相比更少的巨大星系(如看到康罗伊& 2012 - http://iopscience.iop.org/0004 - 637 x/760/1/71/pdf/apj_760_1_71.pdf和冯·多库姆Cappellari et al - http://www.nature.com/nature/journal/v484/n7395/full/nature10972.html)这些索赔是基于高精度的测量特定的光谱特性敏感小矮人恒星的存在。这种现象是否仅限于大质量星系中心的特殊环境，对星系的质量估计和恒星形成历史具有重要意义，但仍然未知。牛津大学在帕洛玛5m望远镜上建造的SWIFT积分场光谱仪（IFS）是远红光谱中最敏感的仪器之一，在远红光谱中发现了钠、钙和氢化铁的IMF临界吸收特征。该项目将使用SWIFT IFS来测量一系列星系中的IMF敏感特征，特别是确定其他人报告的IMF变化是否仅发生在星系核中还是遍布整个星系。来自WHT-ISIS、SAMI和MaNGA的光谱将用于利用光学光谱特征将这一分析扩展到更多的星系。

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.