We describe ongoing searches for intermediate-mass black holes with M_BH ~ 100-10^5 M_sun. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find that dynamical and accretion signatures alike point to a high fraction of 10^9-10^10 M_sun galaxies hosting black holes with M_BH<10^5 M_sun. In contrast, there are no solid detections of black holes in globular clusters. There are few observational constraints on black holes in any environment with M_BH ~ 100-10^4 M_sun. Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the M_BH-sigma_* relation continues unbroken to M_BH~10^5 M_sun, albeit with large scatter. We believe the scatter is at least partially driven by a broad range in black hole mass, since the occupation fraction appears to be relatively high in these galaxies. We fold the observed scaling relations with our empirical limits on occupation fraction and the galaxy mass function to put observational bounds on the black hole mass function in galaxy nuclei. We are pessimistic that local demographic observations of galaxy nuclei alone could constrain seeding mechanisms, although either high-redshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models.

中文翻译：

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中等质量黑洞

我们描述了对 M_BH ~ 100-10^5 M_sun 的中等质量黑洞的持续搜索。我们回顾了一系列搜索机制，包括动态搜索机制和依赖吸积签名的搜索机制。我们发现动力学和吸积特征都指向 10^9-10^10 M_sun 星系中的大部分，其中包含 M_BH<10^5 M_sun 的黑洞。相比之下，在球状星团中没有可靠的黑洞探测。在 M_BH ~ 100-10^4 M_sun 的任何环境中，对黑洞的观测限制很少。考虑到具有动态黑洞质量和约束限制的低质量星系，我们发现 M_BH-sigma_* 关系持续到 M_BH~10^5 M_sun，尽管散布很大。我们相信散射至少部分是由大范围的黑洞质量驱动的，因为在这些星系中占据的比例似乎相对较高。我们将观测到的比例关系与我们对占据率和星系质量函数的经验限制折叠起来，以对星系核中的黑洞质量函数施加观测界限。我们感到悲观的是，单独对星系核的局部人口统计观察可能会限制播种机制，尽管高红移光度函数或对离核黑洞的稳健测量可能会开始区分模型。