Intermediate-Mass Black Holes Jenny E. Greene,1 Jay Strader,2 and Luis C. Ho3 1Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA; email: [email protected] 2Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA 3Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China; Department of Astronomy, School of Physics, Peking University, Beijing 100871, China xxxxxx 0000. 00:1{69 Keywords Copyright c 0000 by Annual Reviews. All rights reserved black holes, active galactic nuclei, globular clusters, gravitational waves, tidal disruption, ultra-luminous X-ray sources Abstract We describe ongoing searches for intermediate-mass black holes with 5 MBH≈ 100 − 10 M . We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find: • Dynamical and accretion signatures alike point to a high fraction of 9 10 5 10 − 10 M galaxies hosting black holes with MBH ∼ 10 M . In contrast, there are no solid detections of black holes in globular clusters. • There are few observational constraints on black holes in any envi- 4 ronment with MBH ≈ 100 − 10 M . • Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the MBH-σ∗ relation continues unbro- 5 ken to MBH∼ 10 M , 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 arXiv:1911.09678v2 [astro-ph.GA] 20 Mar 2020 bounds on the black hole mass function in galaxy nuclei. • We are pessimistic that local demographic observations of galaxy nu- clei alone could constrain seeding mechanisms, although either high- redshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models. 1 Contents 1. Introduction .................................................................................................. 3 1.1. Motivation............................................................................................... 3 1.2. Definition ................................................................................................ 3 1.3. Historical Context ....................................................................................... 4 2. Formation Paths: Where to Find IMBHs .................................................................... 5 2.1. Seeding Models ......................................................................................... 6 2.2. From Seeds to Local Predictions: Population III and Direct Collapse Channels ...................... 7 2.3. From Seeds to Local Predictions: Gravitational Runaway ............................................. 9 2.4. From Seeds to Local Predictions: Wandering Black Holes ............................................ 11 2.5. Tests of Seeding Mechanisms ........................................................................... 12 3. Stellar and Gas Dynamical Searches for IMBHs ............................................................. 13 3.1. Dynamics From Integrated Light Measurements ....................................................... 13 3.2. Proper Motions.......................................................................................... 15 3.3. IMBH Demographics in Galaxy Nuclei From Dynamics ................................................ 16 3.4. Dynamical Searches for IMBHs in the Milky Way ..................................................... 17 3.5. IMBH Demographics in Globular Clusters From Stellar Dynamics .................................... 18 3.6. Hypervelocity stars ...................................................................................... 19 4. Searches for Accreting IMBHs in Galaxy Nuclei ............................................................. 20 4.1. Optical Spectroscopic Selection ........................................................................ 21 4.2. Multi-wavelength Searches and Confusion With Star Formation ...................................... 23 4.3. Pushing Down the Luminosity Function with X-ray or Radio Observations ........................... 24 4.4. Going Further with the Fundamental Plane of Radio Activity ......................................... 25 4.5. The Promise of Variability Selection.................................................................... 27 4.6. Moving to Higher Redshift .............................................................................. 27 5. Searches for Accreting Black Holes Outside of Galaxy Nuclei .............................................. 28 5.1. Ultra-Luminous X-ray Sources .......................................................................... 28 5.2. Fundamental Plane Searches in Globular Clusters...................................................... 30 6. IMBH Searches with Transient Phenomena ................................................................. 32 6.1. Tidal Disruption Events ................................................................................. 32 6.2. Gravitational Waves ..................................................................................... 34 7. IMBH Candidate Tables...................................................................................... 35 8. Scaling Relations ............................................................................................. 35 8.1. MBH − σ∗ ............................................................................................... 36 8.2. MBH − M∗ .............................................................................................. 37 8.3. Using AGN to Probe Scaling Relations ................................................................. 38 8.4. Scaling With Nuclear Star Clusters ..................................................................... 38 9. Demographics ................................................................................................ 41 9.1. Existing limits on the occupation fraction .............................................................. 41 9.2. Inferred Black Hole Mass Functions at Low Mass ..................................................... 43 10.The Future ................................................................................................... 44 11.Summary ..................................................................................................... 46 12.Acknowledgements ........................................................................................... 47 13.Supplementary Material 1: The Future ...................................................................... 62 13.1.Next-generation Opportunities .......................................................................... 62 14.Supplement II: Additional Tables ............................................................................ 63 2 Greene et al. 1. Introduction 1.1. Motivation \Intermediate-mass" black holes (IMBHs) are often introduced as what they are not. They are not stellar-mass black holes, which are formed in the deaths of massive stars and are historically thought to be ∼ 10 M (Remillard & McClintock 2006). They are not super- 6 10 massive black holes, which are historically considered to have masses of 10 − 10 M . The question is often framed: are there black holes with masses between these two classes? We frame the central question differently. At some point in cosmic time, black holes 6 9 between 10 and 10 M had to exist, in order to make the 10 M black holes that are seen only hundreds of millions of years after the Big Bang (e.g., Ba~nadoset al. 2018). The central question of this review is whether we can find evidence for these \intermediate-mass" black holes. The corollary is whether, based on the mass distributions and environments of the black holes, we can determine how these IMBHs were formed. Currently we have no concrete 5 evidence for black holes with masses ∼> 100 − 10 M , although there are some important candidates in this mass range that we will discuss. Finding objects, and characterizing the black hole mass function in this range, is of interest for many reasons. Extending scaling relations to this regime may provide unique insight into the evolution of black holes (e.g., Kormendy & Ho 2013, Shankar et al. 2016, Pacucci et al. 2018), along with the possible importance of feedback for dwarf galaxies (Silk 2017, Bradford et al. 2018, Penny et al. 2018, Dickey et al. 2019). Demographics of black holes at lower masses will help elucidate the dynamical evolution of dense stellar systems (e.g., Miller & Hamilton 2002, Portegies Zwart & McMillan 2002, G¨urkan et al. 2004, Antonini & Rasio 2016). Intermediate-mass black holes will also be prime sources of gravitational radiation for upcoming gravitational wave detectors in space (Laser Interferometer Space Antenna [LISA], e.g., Amaro-Seoane et al. 2015). To determine the rates and interpret the gravita- tional wave signals, we need independent measurements of the black hole number densities (e.g., Stone & Metzger 2016, MacLeod et al. 2016a, Eracleous et al. 2019). Furthermore, on- going and future time-domain surveys are detecting more and more tidal disruption events. In principle, lower-mass black holes should be significant contributors to the detected