标题： 哈勃空间望远镜的球状星团测光 第五部分：M15中心密度尖峰的WFPC2研究 显示英文标题

标题： Globular Cluster Photometry with the Hubble Space Telescope. V. WFPC2 Study of M15's Central Density Cusp

摘要： 我们描述了使用哈勃空间望远镜广角和行星相机2（WFPC2）获得的密集球状星团M15（NGC 7078）中心区域的图像。 在F336W、F439W和F555W滤光片（大约为U、B和V）下获取的数据用于研究在距离星团中心$r<2'$（6.7$\,$pc）范围内的$^2$区域内检测到的$\sim3\times10^4$星星的表面密度分布。 星团M15的恒星表面密度（经过不完全性和光度偏差/散射效应的修正后）在半径范围内由一个幂律很好地表示：$N(r)\sim{r}^{-0.82\pm0.12}$，从$0.3''$（0.017$\,$pc）到$6''$。 观测到的幂律与如果星团中心存在一个大质量黑洞时所预期的结果非常相似。 非参数化的密度分布估计显示，随着半径减小，密度持续增加，一直到$r=0.3''$，这是可以可靠测量密度的最小半径；没有迹象表明在更小半径处分布趋于平坦。 任何半径大于$2''$(0.11$\,$pc) 的平坦核心在$>95$% 显著性水平上被排除。星计数轮廓与核心坍缩模型预期的相符，或者与大质量（几倍$10^3\,M_\odot$）黑洞周围的预测分布相符。明亮恒星的紧密三元组 AC~214 位于集群中心位置的$0.5''$($1.5\sigma$) 范围内。 显示英文摘要

摘要： We describe images of the center of the dense globular cluster M15 (NGC 7078) obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 (WFPC2). Data taken in the F336W, F439W, and F555W filters (approximately U, B, and V) are used to study the surface density distribution of the $\sim3\times10^4$~stars detected in a 5~arcmin$^2$ region within $r<2'$ (6.7$\,$pc) of the cluster center. The surface density of stars in M15 (after correction for the effects of incompleteness and photometric bias/scatter) is well represented by a power law in radius: $N(r)\sim{r}^{-0.82\pm0.12}$, over the radial range $0.3''$ (0.017$\,$pc) to $6''$. The observed power law is remarkably similar to what is expected if the center of the cluster harbors a massive black hole. Non-parametric estimates of the density profile show a monotonic rise with decreasing radius all the way in to $r=0.3''$, the smallest radius at which the density can be reliably measured; there is no indication that the profile flattens at smaller radii. Any flat core of radius larger than $2''$ (0.11$\,$pc) in the stellar distribution is ruled out at the $>95$\% significance level. The star count profile is consistent with that expected from core-collapse models or with the predicted distribution around a massive (few times $10^3\,M_\odot$) black hole. The close triplet of bright stars, AC~214, is within $0.5''$ ($1.5\sigma$) of the cluster centroid position.