标题： M31富金属球状星团及其周围场区的HST-NICMOS观测：I. 技术方法 显示英文标题

标题： HST-NICMOS Observations of M31's Metal Rich Globular Clusters and Their Surrounding Fields: I. Techniques

摘要： 我们获得了哈勃空间望远镜（HST）近红外相机（NICMOS）对M31星系中五个最富金属含量的球状星团的观测数据。这些数据使我们可以对星团及其周围区域的单颗恒星进行测光研究。然而，为了实现我们的目标——获得与银河系对应天体精确对比的星等函数、通过巨星分支的斜率确定金属丰度、识别长周期变星，并从渐近巨星支顶端（AGB tip）的星等估计年龄，我们必须能够从非常拥挤的观测视场中分离出真实天体物理性质和观测效应。本文中，我们详细分析了拥挤视场中的测光数据，并展示了图像混合效应对数据结果和解释的影响。尽管这项分析是针对我们在M31中的NICMOS观测数据展开的，但我们所开发的技术可以应用于任何拥挤视场中的成像数据；我们还展示了图像混合效应对下一代空间望远镜（NGST）的影响。我们使用三种不同的技术来分析拥挤效应对数据的影响，包括插入人工恒星（传统的完备性测试）和创建完全人工合成的星团。这些方法用于为每个星团推导出阈值半径和关键混合半径，从而确定在多靠近星团中心的位置可以获得可靠的测光结果。模拟研究还允许我们量化并校正混合效应对不同表面亮度水平下红巨星支（RGB）斜率和宽度的影响。 显示英文摘要

摘要： We have obtained HST-NICMOS observations of five of M31's most metal rich globular clusters. These data allow photometry of individual stars in the clusters and their surrounding fields. However, to achieve our goals -- obtain accurate luminosity functions to compare with their Galactic counterparts, determine metallicities from the slope of the giant branch, identify long period variables, and estimate ages from the AGB tip luminosity, we must be able to disentangle the true properties of the population from the observational effects associated with measurements made in very crowded fields. In this paper we present a careful analysis of photometry in crowded regions, and show how image blending affects the results and interpretation of such data. Although this analysis is specifically for our NICMOS observations in M31, the techniques we develop can be applied to any imaging data taken in crowded fields; we show how the effects of image blending will even limit NGST. We use three different techniques to analyze the effects of crowding on our data, including the insertion of artificial stars (traditional completeness tests) and the creation of completely artificial clusters. They are used to derive threshold- and critical-blending radii for each cluster, which determine how close to the cluster center reliable photometry can be achieved. The simulations also allow us to quantify and correct for the effects of blending on the slope and width of the RGB at different surface brightness levels.