Strong Lensing, Stellar Dynamics and the IMF in Elliptical Galaxies

恒星初始质量函数(IMF)是在一组恒星中形成的恒星质量的分布，即大质量恒星和低质量恒星的相对数量。由于恒星的关键性质(光度、寿命、终点和重元素的释放)在很大程度上受质量的控制，星系的演化(质量与光比、化学丰度)在很大程度上取决于IMF的形式。IMF只能通过计算银河系及其少数卫星矮星系中不同质量的恒星来直接确定。在没有更好的信息的情况下，天文学家通常认为银河系IMF同样适用于整个宇宙中所有其他类型的星系，而不考虑星系质量、形成红移等；但这一假设有效吗？最近几年，国际货币基金组织是否真的“普遍存在”的问题引起了激烈的争论，尤其是大质量椭圆星系，它们的大部分恒星都是在早期宇宙纪元猛烈爆发时形成的。一些观测研究表明，这类星系拥有过多的极低质量恒星，即所谓的“底部重”的国际货币基金组织(van Dokkum&Conroy，2010；Treu等人)。2010年；Cappellari等人。对于银河系以外的星系，我们依靠间接方法来探测IMF，要么测量星系积分光谱中低质量恒星的微妙效应，要么通过恒星动力学或引力透镜探测它们的引力影响。利用在欧洲南方天文台甚大望远镜上实施的一种新的积分场光谱方法，我们已经开始了“Snells”计划，这是一项发现强透镜椭圆体的系统调查。附近透镜星系的质量估计相对对暗物质的假设不敏感，并允许使用补充方法对透镜进行更详细的研究。我们对前三种镜片的研究结果最近发表了(Smith，Lucey&Conroy 2015)。与其他研究中发现的底部重IMF的证据相反，我们发现这些星系的质光比与正常的银河系IMF一致。显然，需要三个以上的星系才能最终解决这一争议，所以我们现在正在扩大镜头搜索计划，以扩大样本。该项目将包括分析VLT仪器为新发现的系统提供的光谱数据，测量空间分辨运动学，并拟合动力学模型以恢复质量轮廓。该项目还将使用新的哈勃太空望远镜观测来改进星系的透镜模型。来自透镜和恒星动力学的质量之间的比较将检验每种方法中涉及的假设，并导致改进的质量光比估计。在开展这项工作的同时，该项目将包括正在进行的透镜发现和后续活动，其最终目标是从大量独特强大的透镜椭圆体样本中得出稳健和可靠的基金组织约束条件。

The stellar initial mass function (IMF) is the distribution of stellar masses formed in a population of stars, i.e. the relative number of high-mass and low-mass stars. Since the crucial properties of stars (their luminosities, lifetimes, end-points and release of heavyelements) are controlled to a great degree by their masses, the evolution of galaxies (their mass-to-light ratios, chemical enrichment) depends heavily on the form of the IMF.The IMF can only be determined directly, through counting individual stars of different masses in the Milky Way and in a few of its satellite dwarf galaxies. In the absence of better information, astronomers generally assume that the Milky Way IMF applies equally to all other types of galaxy throughout the universe, regardless of galaxy mass, formation redshift, etc; but is this assumption valid? The question of whether the IMF is truly "universal" has been hotly debated in recent years, with a particular focus on massive elliptical galaxies, which formed most of their stars in violent bursts at early cosmic epochs. Some observational studies suggest that such galaxies harbour an excess of very low-mass stars, i.e. a so-called "bottom-heavy" IMF (van Dokkum & Conroy 2010; Treu et al. 2010; Cappellari et al. 2012).For galaxies other than the Milky Way we rely on indirect methods to probe the IMF, either measuring the subtle effects of low-mass stars in the integrated-light spectra of galaxies, or detecting their gravitational influence through stellar dynamics or gravitational lensing. Using a novel integral-field spectroscopy approach, implemented on the ESO Very Large Telescope, we have begun the "SNELLS" project, a systematic survey to discover strong-lensing ellipticals. Mass estimates for nearby lens-galaxies are relatively insensitive to assumptions about dark matter and allow more detailed study of the lenses using complementary methods. Our results for the first three such lenses were recently published (Smith, Lucey & Conroy 2015). In contrast to the evidence for bottom-heavy IMFs found in other studies, we found mass-to-light ratios consistent with a normal Milky-Way-like IMF for these galaxies.Clearly more than three galaxies are required to resolve this controversy definitively, so we are now extending our lens search programme to enlarge the sample. The project will involve the analysis spectroscopic data from VLT instruments for the newly-discovered systems, measure spatially-resolved kinematics, and fit dynamical models to recover the mass profile. The project will also use new Hubble Space Telescope observations to improve the lens modelling of the galaxies. Comparison between the masses derived from lensing and from stellar dynamics will test the assumptions involved in each method, and lead to improved mass-to-light ratio estimates. In parallel with this work, the project will include the ongoing lens-discovery and follow-up campaigns, with the ultimate goal of deriving robust and reliable IMF constraints from a large and uniquely-powerful sample of lensing ellipticals.