标题： 波动光谱学：古老恒星群体的新探针 显示英文标题

标题： Fluctuation Spectroscopy: A New Probe of Old Stellar Populations

摘要： 我们引入了一种新方法来确定不同类型恒星对近邻早期星系综合光度的相对贡献。 众所周知，由于巨型星数量的泊松起伏，这些星系的面亮度表现出逐像素的变化。 根据波动强度（“波动光谱”）对像素进行差分光谱分析有效地测量了恒星随光度变化的光谱变化，这是很难单独获得的对于本地星系群外的单个恒星的信息。 我们应用这一技术到椭圆星系NGC 4472，使用HST/ACS在六个窄带斜坡滤波器中的成像，这些滤波器调谐到0.8-1.0微米范围内的光谱特征。 宽带变化在±5%以内的像素在窄带滤波器中显示出0.1%-1.0%的色差变化。 这些变化主要归因于上部巨星分支上TiO吸收强度随光度增加而系统性增加。 数据由Conroy & van Dokkum（2012）的恒星群体合成模型很好地再现，该模型是对综合光度的最佳拟合，残差在0.03%-0.09%范围内。 与综合光度值显著不同的年龄或金属丰度的模型不能给出良好的拟合。 我们还可以排除基础模型的一些修改，包括存在显著（>3%的光度）晚期M巨星群体。 目前的观测构成了对大型早期星系中富金属巨星预期光度和温度的强大测试。 研究具有更大（负）波动的像素将提供关于主序星和恒星初始质量函数的独特信息。 显示英文摘要

摘要： We introduce a new method to determine the relative contributions of different types of stars to the integrated light of nearby early-type galaxies. As is well known, the surface brightness of these galaxies shows pixel-to-pixel fluctuations due to Poisson variations in the number of giant stars. Differential spectroscopy of pixels as a function of fluctuation strength ("fluctuation spectroscopy") effectively measures the spectral variation of stars as a function of their luminosity, information that is otherwise difficult to obtain for individual stars outside of the Local Group. We apply this technique to the elliptical galaxy NGC 4472, using HST/ACS imaging in six narrow-band ramp filters tuned to spectral features in the range 0.8-1.0 micron. Pixels with +- 5% broad-band variations show differential color variations of 0.1% - 1.0% in the narrow-band filters. These variations are primarily due to the systematic increase in TiO absorption strength with increasing luminosity on the upper giant branch. The data are very well reproduced by the same Conroy & van Dokkum (2012) stellar population synthesis model that is the best fit to the integrated light, with residuals in the range 0.03% - 0.09%. Models with ages or metallicities that are significantly different from the integrated-light values do not yield good fits. We can also rule out several modifications to the underlying model, including the presence of a significant (>3% of the light) population of late M giants. The current observations constitute a powerful test of the expected luminosities and temperatures of metal-rich giants in massive early-type galaxies. Studies of pixels with much larger (negative) fluctuations will provide unique information on main sequence stars and the stellar initial mass function.