PoS(MULTIF2017)051 https://pos.sissa.it/ range, between the firmly established stellar-mass black holes ⊙ erent evolutionary scenarios for the creation of IMBHs. Through- ff M 5 10 − 2 10 * supermassive black holes. Predicted by a variety of theoretical models, IMBHs are ⊙ M 6 10 ≳ [email protected] ort to understand the formation of supermassive black holes and their co-evolution with their ff Speaker. the potential seeds of supermassiveest black extra-nuclear holes X-ray and sources. are This expecteddiscussion brief of to review IMBHs power is that the some took result of placefrequency of during the Behaviour a of the bright- High presentation 12th Energy and International Cosmic Frascati subsequent Sources”. workshopand The up-to-date manuscript on review aims “Multi- of to the provide di a concise and host galaxies. Intermediate mass black holes (IMBHs)pected are to an lie (as in yet) the elusive class of black holes that are ex- out the text I emphasize thee importance of the identification and classification of IMBHSs in our * Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). © XII Multifrequency Behaviour of High Energy12-17 Cosmic June, Sources 2017 Workshop Palermo, Italy Filippos Koliopanos CNRS, IRAP, 9 Av. colonel Roche, BPUniversité 44346, de F-31028 Toulouse; UPS-OMP; Toulouse cedex IRAP, Toulouse, 4, France E-mail: France Intermediate Mass Black Holes: A brief review PoS(MULTIF2017)051 ) ⊙ ⊙ ]), ]). M M 17 26 , ]), Pop 200 200 25 19 SMBH in ≲ , , ⊙ 24 18 ] to be the end , M ). Establishing 6 7 ]) and produced ⊙ ], Hilbert (1917) 23 1 , ], [ 10 M 33 Filippos Koliopanos 5 , 5 × 22 4 ciently and the Jeans , 32 10 15 (e.g. [ , ffi − 21 > ]. Numerous subsequent 4 , , while dynamical estima- 31 7 ⊙ 10 20 M 5 and . Furthermore, IMBHs are also 10 1 ⊙ M range, will provide tremendous insight ⊙ ]) have established the abundance of stel- M 9 , 8 300 ] used near infrared observations of the galactic O maser emission from a − 2 13 1 ], Chandrasekhar (1932, 1935) [ 60 4 ], H 15 for the “dark” companion of the Cyg X-1 binary star system, dark matter mini-haloes at redshift z ⊙ ⊙ ] M ]. 30 M 12 6 Myr) Pop III stars may collapse into BHs that can exceed , 3 erent mass regimes. 10 . Their high mass is the result of the lack of heavier elements in their 11 ≲ ⊙ ff , M – and are even further from establishing any characteristics of their popu- 10 ] as peculiar repercussions of Einstein’s theory of gravitation, black holes ) stands in stark contrast to the scarcity of observational evidence in fa- 3 ⊙ ⊙ M M line in MCG-6-30-15 [ 6 6 ] promoted the presence of SMBHs in the centers of massive galaxies. Approx- α 10 ]). Discovery of IMBHs, in the 10 ]), dynamical estimations of the SMBH in the center of the Milky Way (e.g. [ 14 ≳ ≲ 29 16 , BH 28 M , ≲ ⊙ 27 Due to their large distance and low surface number densities Pop III stars will most likely be outside the reach of The wealth of observational confirmations for the existence of smaller mass BHs ( In the 1970s Sanders and Lowinger (1972) [ In terms of stellar evolution, IMBHs are the expected relics of evolved Population III (Pop First theorized by Schwarzschild (1916) and reformulated by Droste (1917) [ M 1 ] and Weyl (1917) [ 2 III stars can exceed 200 progenitor cloud which maintains lowmass fragmentation, is as higher. the gas Thestellar cools resulting winds ine massive than Pop what III is expected stars in retain massive their metal mass rich as stars they (e.g. exhibit [ diminished the upcoming James Webb Telescope [ into the Pop IIIpredicted stars to that exist as in accretion of disks yet of active cannot galactic be nuclei (AGN, observed e.g. directly [ point of massive stars that collapse intoical singularities. estimation However, of it mass wasn’t until of 1972 several that and the an dynam- astrophysical source was firmly recognized as black hole (BH) [ imately a decade later anof abundant SMBHs accumulation in of solid active observational galacticbroadened evidence nuclei Fe for (beginning the K with presence theNGC remarkable 4258 detection [ of a relativistically along with strong indications for weaklythe accreting vast SMBHs majority in most of nearby the galaxies, astrophysics– has community galaxies. convinced that SMBHs lie in the centers of most – if not all After their short lifetime(e.g. ( [ lation (i.e. the existence of two sub-populations of 100-1000 III) stars. Produced inside vor of the existencestrong of candidates BHs (see between Section theseconfirm 4) two the and mass existence a regimes. intermediate modest mass100 With sample black the of holes exception (IMBHs) potential of – candidates, very which we few are have defined yet as to BHs with Review of IMBHs 1. Introduction [ observations of BH X-ray binaries (BH-XRBS:lar e.g. mass [ BHs in theof Universe, gravitational leading waves to [ the confirmation of BH mergers, in thecenter recent to discovery infer the presence oftions a massive suggested central the object presence exceeding ofRees a in supermassive 1984 black [ hole (SMBH) in the center of M87. Indeed and SMBHs ( these facts is a crucial andprobing necessary BH step feedback in in understanding di stellar and galactic evolution as well as (BHs) were later predicted by Landau (1932) [ PoS(MULTIF2017)051 ⊙ M erent erent 100 ff ff ∼ Filippos Koliopanos K. In addition to the 4 ]). In the massive seed – 10 must have formed within ], see relevant section on ers considerably, which in 46 ⊙ ff 68 , M 9 45 , 10 27 ≳ ]. While a considerable fraction of BH erent formation paths are presented in 67 M ff , erent “sizes” of IMBHs predicted by the ]. 66 ff by increasing the fraction of free electrons , 40 2 ]). Depending on the mass of the seed di 65 ] ). Therefore, discovering and measuring the 50 58 IMBHs seeds which in turn are the result of mul- 2 , ] (but also see Shirakata et al. 2016 implications , ⊙ 49 60 57 , erent formation paths will have a decisive impact on M , ff 3 48 ], Figure 1 in this article. See also Fig.1 in Greene 2012) ] or onto other massive stars in young massive clusters , 56 10 ]) also predict the existence of IMBHs in galactic halos , ∼ 59 34 47 64 55 , , 63 54 (e.g. [ reveals that SMBHs with , , ⊙ 6 62 53 M ∼ ]), or onto , 4 44 52 10 , , ∼ ]). cooling haloes with virial temperatures exceeding 43 51 , 39 α , 42 , 38 erent mass regimes. The “relics” of the di , ff 41 37 sec) quasars at z / 1 keV) are more penetrative and may heat the intergalactic medium, suppressing star , ≳ 36 erg , 47 35 In addition to central SMBHs, primordial BH formation scenarios based on phase transitions In the light seed scenario, the SMBH forms either via (super-Eddington) accretion onto In terms of galaxy evolution, the detection – and more importantly – the determination of the 10 ]). Gyr. This realization raises crucial questions with regard to the initial seeds of SMBHs and the ≳ 1 61 IMBHs (e.g. [ an illustrative plot by Mezcua (2017)([ in the early Universe (e.g. [ via photoionization, which in turnstar-formation facilitates (e.g. the cooling [ and collapse of gas, therefore promoting in her recent review of IMBHs. In addition to the di [ at a substantial distance from the galactic center [ formation paths of SMBHs see Volonteri et al. 2010 [ tiple mergers of Pop III IMBHor primordial direct X-ray collapse binaries – (e.g. scenario [ hydrogen the gas SMBH in seed Ly is thelack result of of metal line direct cooling, collapse the ofdestroy halo molecular primordial requires hydrogen pristine the and presence prevent oflapses early strong into Lyman-Werner collapse an UV and IMBH radiation of fragmentation, to untilevolutionary the paths structure lead col- to the creationited of accretion the or SMBH, episodic super- which Eddington may accretion involveMore (e.g. sustained Pacucci importantly, Eddington et the lim- al. energy 2015; range Inayoshi et ofturn al. the may 2016). resulting result X-ray in emission positive di (energies or negative feedback on theformation, formation while of soft early X-rays galaxies.ing (0.1 Hard at - X-rays lower 1 rates) keV: emitted can from promote more the massive formation accreting of IMBHs H accret- mass of a potential IMBHs populationformation is a at major di step in understanding stellar evolution and galaxy two scenarios, the direct collapse scenariogalaxies predicts than a the much Pop lower occupation III fraction remnant in scenario low-mass [ primordial IMBHs may merge toof produce them, central will remain SMBHs, in it the interemdiate ishalos. mass likely regime Members forming that a of a population such considerable of a IMBHs fraction if in population the they may galactic accrete manifest matter themselves as from hyper-luminous a X-ray companion sources, (e.g. ESO 243-49 HLX-1: [ (e.g.[ the star formation of thethe host presence galaxies. of They IMBHs) can involving be “light” divided or into massive two seeds. main For scenarios a (that detailed involve review of the di Review of IMBHs by repeated mergers ofwith compact a remnants dense of stellar massive population star (e.g. in [ circumnuclear giant H IIspatial regions and mass distribution and the sizeing of an of IMBH formation population of is a SMBHs pivotal and(L step in consequently our galaxy understand- formation itself.≲ The discovery of luminous process by which they are formed. The di PoS(MULTIF2017)051 ]). 77 , 7. SMBHs could 76 , ∼ Filippos Koliopanos 75 , by z ⊙ 74 ]), direct monitoring of O megamaser emission , M 2 9 71 73 , 10 , 70 72 , set of H 69 ff , 17 erent observational methods employed in the ff 3 6. Those seed BHs that did not grow into SMBHs can ∼ ]. 59 or adaptive optics assisted observations (e.g. [ / From Mezcua (2017), with permission from the author and IJMPD: Formation scenarios for ] or by 2D modeling of the nuclear gas kinematics using emission line imagery from high 16 Below, I will present a concise review of the di The most reliable method of detecting massive BHs (henceforth the term MBH will be used gas motion orbiting the central MBH (e.g. by measuring the o also directly form by mergers of protogalaxies at z to refer to massive central BHsof that galaxies may involves be direct either kinematic insurements the measurements. of SMBH individual or Such sources, the as methods IMBH is involve range) the the in case dynamical the for center mea- Sgrlines A* [ ([ spatial resolution and 2. Direct observational methods IMBHs. Seed BHs in theclusters early formed Universe out could either form from the fromcollapse second Population of generation III dense of stars, gas stars in from or from protogalaxies, mergers inflows and in in grow dense protogalaxies, via stellar or accretion from and direct mergingbe to found in the local Universe as leftover IMBHs. Figure 1: search for IMBHs and theirIMBHs, current see observational Mezcua constraints. 2017 [ For an extensive, recent review on Review of IMBHs ultraluminous X-ray sources below). PoS(MULTIF2017)051 ], by 87 , vir M 86 ]. An ex- ], however values esti- 93 81 [ is the time lag BH τ ⊙ M Filippos Koliopanos M estimated with the 5 BH 10 distribution ( [ is the BH mass). Inside ∼ M , where G sph / BH σ 2 the - M ]. τυ BH sph c ], 95 σ M [ = 79 ⊙ , vir M 78 values derived from the accretion disk M 5 the radius of influence would be 0.5 pc, s 10 BH / × M (e.g. [ ]). More precisely the value that is measured 6 . 2 3 relation and thus the virial factor is estimated , based on two-dimensional gas kinematic data 85 ∼ 30km ⊙ sph 4 sph = M er by a factor that is inversely proportional to the /σ σ 6 ff - sph − BH σ BH 5 M = ]). Therefore any potential issues with the validity of the GM ], placing it decisively in the IMBH range, but Peterson et and = 92 BH ⊙ 94 , [ M M in f erent modeling assumptions are evident in the mass estimation 91 ⊙ 5 R ff , log M 10 4 90 ∼ , 10 89 × , 6) . ), namely 88 2  sph 9 . σ (4 ]) = 84 , BH results (e.g. in the low-mass regime, see Section 3) will carry over to the virial mass 83 M , sph is the velocity of the material in the BLR. The actual mass of the BH is related to 82 σ - Another direct approach involves the reverberation mapping technique. Reverberation map- The radius of influence of the BH can be expressed in terms of the velocity dispersion of the In order to estimate the value of the virial factor, it is assumed that the υ ]. This places the MBH in NGC 4395 at the lower end of SMBHs and tentatively in the IMBH BH 80 M reverberation technique must be equaldicted (within using the the relation’s dynamically inherent estimated scatter) with the value pre- ample of the consequences ofof di the IMBH candidate invalue NGC of 4395 using reverberational. mapping. (2005) estimate Edri the et mass al. to (2012) be much measure higher, a at this estimation is dependent onthe underlying central model assumptions stellar and mass(e.g. is profile. [ strained by Only uncertainties in upper limits are provided for a handful more candidates ping involves accreting BHs andcessed relies on in the the fact high thatbroad velocity the line photo-ionized continuum region emission gas (BLR). of that Sincetions AGN the envelopes is observed continuum the repro- in emission central the is continuum sourceBLR variable, and provides – time an fluctuations delays known estimate of of between as the the fluctua- distance the The velocity between Doppler broadened the broadening BH of emission and the lines the lines BLR, ofBLR, can assuming yielding be the light an used travel to estimation time. infer for the thetailed velocity mass review of of of the the the orbiting method central material see BH in e.g.has assuming the Peterson dimensions Keplerian 2014 of rotation [ mass (for and a is de- known as the virial product see Section 3). The assumption is that for a given value of the for each source (e.g. [ estimations. Furthermore, recent analysis revealsproperties that versus the virial masswidth estimates of the di broad emission lines, leading to significant viral mass mis-estimations [ range. Dynamical gas modeling in NGC 404 yielded an estimation of galactic spheroid ( Review of IMBHs making it impossible tosuch, resolve so – far even there for areThe the no nearby dwarf definitive nearest galaxy direct galaxies NGC estimations 4395 –masses, of (see with using Section IMBHs, a 4) current even central hosts for MBH telescopes. one of the of the nearest As lowest candidates. directly measured BH and [ the sphere of influence, objects are expectedof to the follow Keplerian BH. orbits For under a the BH gravitational mass force of a factor of order unity,geometry known of as the BLR. the virial factor (f), which is introduced by the kinematics and mated by the reverberation mapping technique also follow the PoS(MULTIF2017)051 , 107 range , ⊙ emission 106 M , α 6 or accretion / 10 105 ≲ as suggested by ]). Furthermore, 5 BH and the size of the Filippos Koliopanos M 101 sph Å , ≲ σ ⊙ ∼ 100 M or optical continuum ]). While their approach / BH , 5 M relation (e.g [ 99 10 121 , , lines, Greene & Ho (2005) were sph 98 β σ , 120 ), demonstrated the high likelihood − , 97 sph luminosity and the optical continuum BH σ and H 119 and the properties of its host. The tight flux ratios), AGN are recognized among M − α , α α BH BH H / 118 M M , 5 117 , ] (which distinguish between LINERs, H II regions and 116 , and [S II] emission line (e.g., [ , α 102 β H / 115 , ]. The virial mass can then be estimated by inferring the velocity of 114 , [N II] , β 96 [ ] reproduce the scale of the relation, i.e. H 7 / . 113 0 , 105 5100 112 L , ∼ erent scaling relations between the 111 BLR ff ]). , R 104 110 , , ], the scaling relation between the mass of the central MBH and the velocity dispersion of the 103 109 Discovered simultaneously by two groups, Ferrarese & Merritt 2000 and Gebhardt et al. 2000 Direct observing methods are limited to only the most nearby galaxies and are vulnerable due Virial mass estimates of central MBHs can be inferred using single epoch observations by 87 , , 86 varies, all the above theoretical modelsgenerated rely during on the the accretion presence of episodescan massive be that outflows divided are or into winds two required broad that toand categories are the grow depending its on the host whether is central the “energy-driven” MBH. interaction orby “momentum-driven”. between The Silk The the models & energy-driven ouflow models Rees as (1998) suggested [ 108 [ stellar population of the spheroid bulge of galaxies ( AGN by measuring [O III] 3.2 Central IMBHs: Black hole mass - host bulge scaling relations more recent observational campaigns bythe McConnell normalization & of Ma the relation. 2013, but On the significantly other underestimate hand, momentum-driven solutions as suggested by 3.1 IMBHs in AGN: Empirical relations between emission lines and the BLR using the broadening of the H galactic surveys and their virialhas mass revealed can more be estimated than by 150 the candidates above with methods. a This central methodology MBH in the to modeling assumptions. Furthermore,observations the in order reverberation to mapping measure method thetantly requires spectral – time multi less lag. luminous epoch In and order less to massive candidates, probe a further variety and of – indirect most methods impor- can be employed. Review of IMBHs 3. Indirect observational methods making use of the empirical relationBLR, between namely optical AGN luminosity at 5100 by demonstrating a clearluminosity linear and a relation correlation between between the theable widths of to H the infer H the virialline. By mass way system of based optical spectroscopy solelyPhillips and & on the Terlevich” use (BPT) observations of diagrams of classification [ the techniques such broadened as H the “Baldwin, of a co-evolution betweenthe the study central of MBH di and its host galaxy and sparked a keen interest in history of the MBH. While the origindispersion of is the not coupling between yet the resolved, MBHto and there constrain the is host the a bulge physical velocity multitude mechanism of and analytic reproduce and the numerical models that attempt correlation between the mass of thesuggests that central there MBH is and mechanical feedback the between velocitybeyond the dispersion MBH the and of sphere it the of host galactic gravitational galaxy, which bulge influence extends of well the BH and transcends the merger and (e.g. [ PoS(MULTIF2017)051 , – ]). BH 142 ] and ] and , M 129 ) is the , 136 138 sph 141 , for a given , σ 128 er from lu- , ff − ]. Using this BH 137 140 M BH , 127 )[ 135 , L M Filippos Koliopanos 139 ] revisited the 126 ]). These issues result , ]. This correlation was 128 123 125 144 , , 122 124 relation itself is not well calibrated : e.g.[ sph L – σ - BH BH , but reproduce very well the M 4 relation can be used to infer the mass of the M sph L 6 – σ ∼ BH and the spiral pitch angle (PA) which measures the BH ]. Each of these scaling relations may reveal possible M ] and Graham & Scott (2013) [ relation in the low-mass regime (e.g see discussion in M BH 146 134 sph M , σ – 145 ]). Furthermore, there are large uncertainties regarding the shape BH , set towards lower masses and show little evidence of co-evolution M ] discovered 40 low luminosity AGN (LLAGN) that appear to have ff set towards lower masses (for similar values of 132 , 144 ff 128 ]). However, the sampling of these two relations is biased towards higher 127 ] predict a slope of holds. For a detailed discussion on this subject and also extensive reviews 131 , ]. L 111 – host galaxy relations involve a correlation between the mass of a galaxy’s central 130 133 / ] four independent mass scaling relations and the fundamental plane of black hole BH ]). M masses, as most galactic samples with directly measured SMBH masses are dominated 147 / relation in the low-mass regime (e.g. see discussion in [ 133 , sph σ 123 – Other MBH In recent studies, Graham (2012) [ Nevertheless, the observationally constrained Another well-known scaling relation (and one which actually preceded the ] contest this result arguing that the steeper slope is the result of inclusion of pseudobulge , . ]). Furthermore, a correlation between the tightness of the spiral arms and the mass of the including many more low-mass spheroids with directly measured SMBH masses. Their work BH sph 136 122 M L formulated as a relation between the tightness of the spiral armscandidates [ in the IMBHand regime, the however question there of their arepublication validity [ still in large the uncertainties low mass in regime their still estimations remains. Nevertheless, in a recent BH and its bulge concentration measured by the galactic Sérsic index (e.g. [ 143 central MBH in disk galaxies was discovered by Seigar et al. (2008) [ a mass of the central MBH that[ lies well within the IMBH range. However, Kormendy & Ho (2013) between the central MBHthat and a the (steeper host than galaxy. linear)which On relation indeed the does appear other exist to hand, for be pseudobulges Graham o (and (2012,2015) low-mass argues classical bulges), galaxies, which are randomly o or even the application[ of the improved scaling relation, [ the “broken” Graham (2016) [ luminosities dramatically revised the relation,relation, demonstrating becoming that steeper it at most low- likely and follows intermediate-mass a spheroids broken (see power-law also [ on SMBHs and their co-evolution with their host galaxies see Kormendy & Ho (2013) [ by luminous bulges (e.g. [ Review of IMBHs King et al. (2003) [ σ for low-mass galaxies, as theminosity galactic bias samples and employed large to uncertainties populate the that relation are su partially due to the precarious application of the in an incompleteness of MBHs and host galaxies in the low-mass regime. The tight scaling relationposing further questions underlines as the to co-relation howthat this between promotes is SMBH the regulated. and growth Namely, galaxy of isenables growth, its the the host feedback growth from galaxy, of the orcentral accreting is the MBH SMBHs the SMBHs? and environment created has The bymore the distant stellar advantage galaxies. activity that Studying that low-mass it galaxiescandidates does using (e.g. these not [ two require relations can spectroscopy reveal and new IMBH can be applied to one between the MBH mass and bulge luminosity ( PoS(MULTIF2017)051 , ]). 164 , 155 , 163 154 , , 5 GHz), the r ]), the study of L ] for reviews of 153 Filippos Koliopanos 162 157 , 161 , ]. While the study did not ers from very high intrinsic on average), it demonstrated ff 160 5 ] may also be registered along 128 . 6 ]. Based on these theoretical pre- ≈ 158 ⊙ ]). An additional, less pronounced ] probed a large sample of Galactic M 159 / 149 149 BH ] and Gilfanov 2010 [ M , e.g.[ 156 log Edd ˙ 7 M 01 . ]), the scaling relation su 0 ∼ 147 , multi-wavelength approach as a go-to method for estimating / ), accretion disks are often accompanied by relativistic jets ] and Falcke (2004) [ 171 ]). During episodes of low-luminosity advection-dominated ac- 148 ]) and therefore the method can be employed to put constraints on 149 , hard state 170 , 148 , 0.5-10 keV), and the BH mass, known as the FP-BH. 169 x , ]). During this state of accretion the accretion disk flow is interrupted at some L 168 152 , , 167 151 , , 150 166 Another approach in the search for IMBHs draws on the well established relation between The FP-BH can be used to infer the mass of accreting BHs during their hard-state. While , accreting compact objects). The entire broadbandparticularly emission if can accretion also be rates dominated arethermal by higher component the ( originating jet itself, in thewith truncated the accretion primary disk non-thermal, [ coronalstill emission. far from While resolved the it details isimportantly, of it understood the has that been disk-jet the shown jet mechanism that is are and the linked the luminosities directly disk-jet of to the mechanism the radio is accretion anddictions, independent process. X-ray of Merloni emission More the et are BH correlated al. mass (2003) [ [ the mass of central MBHs.galaxies as The they most are likely expectedlikely candidates to to to have host host undergone IMBHs lower-mass quiet are centralcan LLAGN merger reveal BHs. IMBH in histories candidates low-mass and While (e.g. are, the [ scatter, therefore therefore, X-ray an more and estimation radio using observations solely and the the FP-BH would FP-BH be very tentative. Nevertheless, as 165 cretion (also known as(e.g.[ a Therefore the combined emissionthe of radio the to accretion the disk, hard corona X-ray and band relativistic (see jets Done ranges 2007 from [ BHs and SMBHs andX-ray found luminosity a ( strong correlation between the radio luminosity ( unobscured AGN shows that they can also be found in a similar accretion state (e.g. [ the X-ray emission, theappear radio to emission be and the stronglyhole mass activity” correlated, (FP-BH of forming [ accreting what BHs. is These known three as quantities the “fundamental plane of black 3.3 IMBHs in AGN: The fundamental plane of black hole activity distance from the central object,result at of which this point new configuration, the aobject. material “corona” flows of When radially hot, towards photons, optically-thin theCompton-upscattered gas from BH. is to the As created energies (now a around of truncated) the tensanism central accretion to in hundreds disk, the of traverse relativistic keV. jets Furthermore the produces the “corona” emission synchrotron they primarily mech- are at the radio band (e.g. [ this accretion state is more clearly defined in X-ray binaries (e.g. [ Review of IMBHs activity (FP-BH: see next paragraph)seven were LLAGN combined that to had measure previously theconfirm been mass the reported of previous estimations to the (the central host MBHs MBH IMBHs had in [ a that all five (largely independent) methodsstudy produce introduced consistent this multiple-method results in the low-mass regime. The central MBH masses when their spheres ofcombined gravitational techniques influence protect cannot be against spatially outliers resolved. from any The prediction. one relation and yield a more robust average PoS(MULTIF2017)051 ) ], ⊙ M 202 , 50 ≳ 201 ]). While , ciently dense 178 relation to the 200 ffi , , ]). Integral field sph 38 Filippos Koliopanos , σ 199 188 − , 177 BH , 187 M , 176 ]). On the other hand, recent ] probed the dynamics of the , further considerations of the 186 ⊙ ]), more massive BHs ( 196 198 , , M ]. In the dense environment of GCs, 3 175 185 , 195 10 , , 181 , 174 , 184 194 , , 180 , 173 193 183 179 range – in the center of a globular cluster is ex- , 8 ⊙ ] demonstrated that although consecutive three-body 182 M ] it can provide a crucial, independent verification of 3 mass [ 172 ⊙ 10 147 M ]). Nevertheless, there is considerable controversy over the ], see section 4). 192 ] exploited 25 years of high precision observations of millisecond , 198 197 191 , . More recently Kızıltan et al. (2017) [ ⊙ IMBH is generated. Simulations have also shown that, in su M 190 ⊙ , M 3 189 7500 10 > ect its surrounding kinematics, in essence expanding the compact source ([ M ff ⊙ M range. Miller & Hamilton (2002) [ 3 ⊙ 10 Evidence for the existence of IMBHs in GCs and measurements of their mass can be obtained Under the right circumstances globular clusters (GCs) can become nurseries of IMBHs in the The presence of an IMBH – in the or radio emission may reveal the presence of an accreting BH and its mass can be measured M / × 3 3 . central region of GC 47 Tucanaemillisecond by X-ray precisely pulsars measuring found the inside location2 the and cluster. acceleration Their of results multiple inferred the presence of a single, if the IMBHs are accretingand matter from a companion,using the or FP-BH. from With the surrounding exceptiontected gas. of in GC those G1, Namely, GC no X-ray IMBH X-ray candidates emissionRadio or that observations of radio have numerous their emission GCs has presence have been inferred soIMBHs de- from far in only kinematic their center yielded indications. (assuming upper that limits they on are the there and mass they of are potential accreting, e.g. [ interpretation of these measurements, as theseconcentration results of can also stellar be BHs explained or by neutron an stars increased (e.g central [ pulsar PSR B1820-30A, located intime the derivative globular measurements cluster that NGC canIMBH 6624 be with to interepreted obtain as rotational frequency due to orbital motion around a central initial estimations of thisuncertainties process in predicted the stellar IMBHs evolution and inBHs mass the could loss be of as the little merged as systems a showed few that hundred the resulting lower mass regime. Estimationscandidates of in the the central GCs velocity M15 dispersion and in G1, GCs have (e.g. revealed [ IMBH temporal studies of millisecond pulsars inIMBH. globular clusters Perera have et indicated al. the presence (2017)[ of a central 10 interactions may expel lighter BHs from GCs (e.g. [ pected to e 3.4 Extra-nuclear IMBHs 3.4.1 IMBHs and Globular clusters Review of IMBHs was demonstrated in Koliopanos etestimations al. carried out [ using other methods. IMBHs have a higher likelihoodbinary of capturing is a greater companion. than Ifrepeated the the three mean body binding interactions energy stellar will of kinetic further the harden energy resulting the in binary the until accretion cluster begins. (a so called “hard binary”), spectroscopy combined with surface brightnessby profiling N-body and simulations, fitting to havecandidates modeled also slopes, (e.g. provided predicted [ best fit estimations or upper limits for several more may sink towards themassive gravitational BHs a center of theGCs, stellar cluster collisions could and result after in acreate successive stellar enormously “collision mergers runaway” massive during with which stars less repeated collisions which collapse into IMBHs (e.g. [ PoS(MULTIF2017)051 , ]. 221 VLA 187 , Centauri 186 ω ], for which ]. The only GC 208 , Filippos Koliopanos 205 ] yielded no results, 207 Jy. , 190 ]. Extra-nuclear IMBHs mu erent from standard BH- , 206 ff 210 ]. However such a scenario 189 ], an IMBH that has captured a 220 , 216 219 ] re-observed G1 M31 using the , reported by Gebhardt et al. [ ⊙ 209 218 M upper limit of 4.7 , 4 σ 10 217 × 2 9 erent state of (super-Eddington) accretion, dubbed ff ]). While there is no known binary star evolution ]. The astounding discoveries of a pulsating ULX ], who detect no radio emission in more than 300 ex- 215 229 204 , , 214 228 , , 213 227 , , -nuclear, extragalactic X-ray sources with isotropic luminosities that ff 212 226 [ ] or be gravitationally ejected from the cluster [ ] for a recent review). These sources were quickly considered as prime 37 211 ]. The theoretical predictions are strongly backed by observations that reveal 225 ] demonstrated that ULXs can also be powered by neutron stars. A number of , ] where no radio emission is detected in M15, M19, and M22, despite considerably 232 224 s. , 203 / , erg 231 223 There are numerous o The search for IMBHs is not confined in the central MBHs of galaxies. Extra-nuclear IMBHs, , 39 ultraluminous state , 10 230 would require most –not if the not case all for ULXsULX the population – majority can to be of powered reside ULXs. by stellar-mass inside BHs Furthermore, or accreting at it close super-Eddington rates has to (e.g. been [ globular demonstrated clusters, that which most is of the the compelling theoretical studies have suggested that it is very likely that many (or most) ULXs may companion could end up closeeither enough take with place its inside companion thefew-body to cluster dynamical initiate or recoils mass the as accretion. two the companions binary This may exits could be it driven [ closely, due to multiple that the temporal and spectralXRBs, characteristics suggesting of that ULXs they are indeed markedly are in di a di [ 222 candidates for hosting IMBHsstellar-mass accreting XRBs at (e.g. sub-Eddington [ rates in a similar fashion to standard for which kinematic estimations suggest the existence of an IMBH [ may also be the remnants ofIMBH past interactions finds of itself their in host a galaxiesbe with binary inferred other system dwarf by and galaxies. limits is If imposedaccretion-powered accreting an sources material on the from its Eddington its accretion limit companion, rateold is its in by imposed mass the physical by may luminosity laws, the (corresponding to fact i.e.pressure mass that will its accretion when exceed Eddington rate) the a of limit. ram certain the pressurethe thresh- source In of in-falling is the matter. reached, in-falling the In gas, radiation principleEddington thus hindering an limit, accretion accretion-powered which and source (as expelling cannot wasobject. be shown by more Assuming Eddington) luminous spherical scales than accretion linearly∼ its of with the hydrogen, mass the of Eddington the limit central for a stellar mass BH is scenario that would result in an XRB with an IMBH accretor [ produced inside dense globular clusters,tional can potential either well [ sink towards to center of the cluster’s gravita- exceed the Eddington limit for(see a stellar-mass Kaaret BH, 2017 known as [ ultraluminous X-ray sources (ULXs) tragalactic GCs in NGC 1032 using stacked observations). Deep X-ray observations of 3.4.2 IMBHs and ultraluminous X-ray sources Review of IMBHs but have also raised questions onal. their existence 2010 or [ whether they accretedeep any observations matter or (e.g. Wrobel Strader et et al. 2015 [ suggesting that either no IMBH is present or if it is, it is not accreting any mass [ telescope and detected no radio emission down to a 3 with detections of both X-raythe and FP-BH radio gives emission a is consistent G1 estimationHowever, with in we the galaxy must M31 note [ that Miller-Jones et al. (2012) [ PoS(MULTIF2017)051 , ]. α 245 240 shock- / Chandra Filippos Koliopanos ]. Building on these emission, while Soria α 235 , telescopes, revealed a strong 234 ]) and seem to follow the transi- , ATCA 238 233 , and , combined with complementary multi- ]. 237 VLT 249 ] demonstrated in a recently published study ], HLX-1 had an unabsorbed X-ray luminosity Chandra 236 , 242 10 erent origins, while also illustrating the results that ff Swift , HST , ] located the source’s optical counterpart, which shows an H ] suggested that the host star cluster or a photo-ionized ]). emission line is still unclear. If the line originates in an accretion 244 248 α 241 that are hard to explain even when invoking super-Eddington accretion 1 X-Shooter − ] in the 2XMM catalog [ (0.2-10.0 keV), for a distance of 95 Mpc. Even if one assumes that the erent techniques. ff 1 erg s 238 − 41 ] suggest a low density ionized nebula as the source. A more recent study of 10 erg s ∼ ], making HLX-1 a prime IMBH candidate. This claim relies on the assumption ], or M82 X-1: [ 245 42 238 10 240 ], Soria et al. (2010) [ ], the disk must be observed almost face on, as the emission line is considerably narrow [ , × ⊙ 243 247 HLX-1 – similarly to standard BH-XRBs – is also a source of occasional radio emission, HLX-1 is probably the best IMBH candidate we know so far. Discovered serendipitously by In this section, I present some notable IMBH candidates. The list is by far not comprehensive Nevertheless, there is a small group of hyper-luminous X-ray sources (HLXs) with luminosi- ]. Wiersema et al. (2010) [ 239 or beaming of the X-ray emission. HLXs are strong IMBH candidates, with a particular source, ]. The exact origin of the H / , 0.023. Indeed, in the years following the discovery of HLX-1 its association with ESO 243-49 = 246 68 which correlates with its transitions from hard (non thermal) to soft (thermal) spectral states [ ionized gas nebula veryet close al. to 2010 HLX-1 [ could be the source of the H that HLX-1 is indeed located inz galaxy ESO 243-49, whichhas lies been at confirmed the by aforementioned numerous distance studies. of Using the accurate X-ray position derived from 246 disk [ [ observations taken with the MUSE instrument on the Farrell et al. (2009) [ and only serves to presentcan candidates be with achieved di by di 4.1 ULXs: HLX-1 source is accreting at ten500 times M the Eddington limit, this luminosity implies a minimum mass of data [ emission line with a similar redshift to that of ESO 243-49, confirming it as its host galaxy [ wavelength data including Combining the X-ray and radio observations, the FP-BH allowed Webb et al. (2012) to place an 4. Notable candidates of 1.1 Review of IMBHs indeed be powered bytheoretical neutron predictions stars Koliopanos et rather al. than (2017) BHs [ (e.g. [ and known as ESO 243-49 HLX-1 (henceforthso HLX-1), far considered (see as details the best indo IMBH next not candidate section). feature found the Indeed spectral HLX-1 characteristicstions and of from also soft ULXs M82 to (e.g. hard X-1 [ state – similar[ the to two sub-Eddington stellar-mass brightest BH-XRBs HLXs (ESO – 243-49 HLX-1: variability of the emission lineconfirms that that was the associated emission with lineorigin the originates scenario) X-ray and close states validated to of the the distance HLX-1. to IMBH This HLX-1 (negating [ finding the nebula or star cluster ties exceeding that the spectral characteristics of a largemagnetized fraction neutron of ULXs stars are rather consistent than with BHs. accretion onto highly PoS(MULTIF2017)051 ] ]. 257 270 ]) and , 228 256 [ , ⊙ 254 M , ]. Other notable 4 ]. Compact radio Filippos Koliopanos 10 253 255 ]. Despite the detec- × ], Davis et al. (2011) 261 5 80 ∼ ]. This transition, along 250 keV, while the spectra of ], indicating a mass of the 250 10 ∼ 269 at s [ / ff relation. This value is in agreement range comes from reverberation map- ] and highly variable and non-thermal sph 30 km ⊙ σ ∼ (X-ray) and Very Large Array (radio) ob- - M line and from X-ray variability properties 263 ]. 5 BH β 10 M 259 Swift by Godet et al. (2012) [ − ⊙ 4 11 ]) solidified its AGN nature. More remarkably, the M 10 ]) BH. 4 268 10 238 , [ -nuclear AGN, belonging to a small low-mass galaxy that ∼ ff ⊙ based on the cult due to the source’s very high variability and the fact that 267 M ] M82 X-1 has similar characteristics to HLX-1. However, it , ⊙ ]), however the distinctness of NGC 4395 stems from the fact ffi ] using accretion disk modeling. The X-ray flux of HLX-1 varies M 241 5 500 266 260 for the mass of the BH in the system. This estimation was further , ≳ 252 ]) and from more recent kinematic estimations using integral field 10 ⊙ M 94 265 5 , , 10 95 264 ]. More recent optical integral-field observations of the surrounding kinematics ], the analysis of contemporaneous 258 ], suggestive of strong jet-like outflows [ 95 nuclear X-ray source in NGC 5252 which radio and X-ray observations also place in the 262 ff NGC 4395 is an Sd type, bulgeless galaxy with a Seyfert 1 nucleus. The presence of an ac- The hyperluminous source [ ]. Further confirmation of a mass in the ], and Straub et al. (2014) [ ], Merritt & Ferrarese 2001 [ 269 87 251 ping estimations (e.g. [ [ central MBH that is lesswith than estimations based on the broad profile of the H spectroscopy and fitting of the surface brightness of the nuclear star cluster [ is being accreted by the Seyfert galaxy NGC 5252 [ source’s central stellar velocity dispersion was less than servations has hinted at the possibility of NGC 4395 lying at a steeper track of the FP-BH [ tive MBH at the center of[ a bulgeless galaxy is exceptional in itself (see e.g. Ferrarese et al. 2000 [ nominal BH-XRB extend to energies1, of has often a hundreds luminosity ofand that keV temporal is during characteristics ten all of times spectral essentialyaccretion higher states. onto a than a scaled-up the HLX- very massive BH-XRBs, brightest ( ULX consistent and with displays sub-Eddington all the spectral sources in the same category involve theboth ULX in NGC the 2276-3C which X-ray exhibits and powerfuland radio jet the emission o wavelengths, with an FP-BHIMBH derived regime [ mass of of the source suggest that it may be an o 4.2 Dwarf galaxies: NGC 4395 that it is one ofoptical the and most UV likely emission candidates lines for a first central, revealed the active IMBH. AGN nature TheX-ray of presence emission NGC of (e.g. broadened 4395 [ [ tion of both X-ray and radio emissionusing from the the FP-BH core has of been NGC proven 4395,the di estimating X-ray the mass flux of variability the is MBH by only Peterson weakly [ coupled to the radio flux. Assuming theThe mass presence estimated of two distinct tracks in the FP-BH has been noted in stellar mass BH-XRBs, where supported by the estimation of a mass of has not been observed as extensively asalso HLX-1, be and modeled recent assuming works have super-Eddington shown accretion that onto its a spectrum stellar can mass BH [ Review of IMBHs upper limit of less than by a factor of 50 aswith the the source transitions periodical between appearance hard of anda jet soft crucial emission states feature (as [ that the sets most HLX-1which likely are out origin most as of likely an the super-Eddington outstanding radio accretors candidateand emission), – characteristic for do is “hard” an not and accreting “soft” transition spectral IMBH. between states ULXsmost the of – frequently strikingly standard BH-XRBs feature distinct ( a e.g. characteristic [ exponential spectral cuto emission [ PoS(MULTIF2017)051 , ] ]; ⊙ M 198 ], an opti- 275 6 , β 10 275 × 276 ] and Freire 285 Filippos Koliopanos ]); LEDA 87000 (or 277 ]. Kızıltan et al. (2017)[ ]). In addition to NGC 4395, 284 274 , ]. , 283 284 273 , , , 282 for the mass of its central MBH [ 282 272 ⊙ , M BH in its center (e.g. [ 5 271 ⊙ 12 10 × M and the mass of the globular cluster at 0.76 5 5 . ⊙ 1 10 M ∼ ]. Having had quiet merging and accretion histories, dwarf 1500 279 + 850 ], has the lowest BH mass measured in a dwarf galaxy and is 278 ]. 281 , 280 ] obtained 23 timing solutions for pulsars in 47 Tuc, including spin period deriva- 286 The extensive investigation by a large number of authors has yielded numerous tentative and The discovery of an IMBH candidate in the center of globular cluster 47 Tucanae is compelling This novel method opens up the possibility for estimating the mass of other potential IMBHs et al. (2017)[ several highly likely IMBH candidates, but aNevertheless, definitive detection the of increasing an IMBH number has of notat happened plausible multiple yet. candidates wavelengths suggests discovered that by a population multipleobservational of signatures methods IMBHs, lie and very at likely, the exists threshold but at of this our point current its observational limitations. 5. Conclusion: multi-wavelength future tives, but also orbital periodargued derivatives, that and Kızıltan proper et motion al. andestimation had jerk for underestimated measurements. the the distance distance, The to authors clusters Freire the can et be cluster account al. and for by show by using that a an simple the updated analytical all model, the without the accelerations need of for a the central pulsars IMBH. in the estimate that is further supported by X-ray and radio observations of its active nucleus [ galaxies are strong candidates forby hosting recent central extensive IMBHs. studies that A indicateto scenario the a that presence redshift is of of further an 2.4 IMBH strengthened [ population in dwarf galaxies up not just for its own sake,the but mass also of for proposing a aof gas-starved novel the IMBH method mass to that of infer is the asimulations not presence potential revealed and electromagnetically IMBH that measure visible. in velocity the dispersionpredictions Kinematic center that measurements did estimations of alone or 47 could did Tuc not nottook yielded include distinguish advantage inconclusive a between of central results IMBH the as [ resolution N-body presence timing of analysis 19 was millisecond available.additional pulsars The components authors located (to the measured in intrinsic the the source pulsarto spin-up), cluster, acceleration, the which for detecting presence they demonstrated of which can a high beIMBH central attributed candidate MBH to in a the valuein of cluster. agreement 2300 Using with this independent kinematic method estimations they [ inferred the mass of the 4.3 Globular clusters: 47 Tucanae Review of IMBHs state transitions occur at shortersome intervals other (e.g., strong [ central IMBHical candidates, estimations in provide dwarf an galaxies upper are: limit NGC of 404, fordwarf which AGN dynam- POX 52, wherecal multi-wavelength emission observations, including suggest X-ray, the radio, presence and of H a also a low luminosity X-ray source [ in globular clusters for whichestimate electromagnetic the detection mass is of not IMBH feasible,ertheless, candidates but in there also GCs are to measured caveats independently using to more the conventional findings methods. of Nev- Kızıltan et al.. Ridolfi et al. (2016)[ RGG 118) whose central MBH- lies host in the bulge intermediate scaling mass relations regime, [ according to most BH mass PoS(MULTIF2017)051 ] sph 40 σ - BH M ]) suggest the 288 Filippos Koliopanos , 287 ]). So far, galaxies with 40 erent shape of the ff mark (see also Volonteri 2010 [ ⊙ M Astrophysics Without Black Holes, and 5 range) will largely decide on the forma- 10 ⊙ M 5 APP.2014.01.0027 ] / 10 13 lines in AGN (starting with the galaxy MCG-6-30- ≳ α view or / ⊙ M 4 article / 10 − 3 APP erent scenarios predict a starkly di / ff Indeed, M82 X-1 is one of the two best candidate IMBH hyperlu- 10 As I am not a theoretical physicist or mathematician, I cannot com- How do you explain the problem of the naked singularity? [Author’s I would like to also point out the case of M82 X-1 as well known candidate index.php / ojs ] and references therein). While this preliminary, rough finding hints towards / 59 distribution is populated primarily by MBHs that have ended up above the IMBH sph σ - ojs.cvut.cz BH // Constraining this population and evaluating its characteristics is the next crucial step in the ef- M distribution in the low-mass regime (e.g. Fig. 10 from Volonteri 2010 [ for an IMBH [Author’s note: M82 X-1 had not been discussedFILIPPOS during KOLIOPANOS: the presentation ]. minous sources. The source showssub-Eddington similar BH-XRB in characteristics the as intermediate HLX-1 massin and range. appears the The reason to oral the behave presentation source like was andX-1 a not has is mentioned not not been monitored discussed as as extensivelywavelength, as and extensively HLX-1 also as which the has HLX-1 fact been that in systematicallydescribed more observed the as recent at review, a estimations all is super-Eddington point source. out that that M82 M82 X-1 can be plausibly WOLFGANG KUNDT: note: Prof. Kundt has arguedRoger for Penrose’s a cosmic long censorship time against hypothesis.Without the Extragalactic Gamma-Ray existence See Bursts of e.g. BHs, andhttps: has also criticized the dominance of the massiveremain seeds below scenarios, our it observational is capabilities as equallythe their plausible AGN that are the luminosity lower and mass distance IMBHs limited and DISCUSSION DANIELE FARGION: tion path of SMBHs, as the di regime, via accretion andreviewed above, mergers. that multi-wavelength observations It hold the becomeslation key of to apparent IMBHs. constraining from the As missing we the popu- enterobservational multitude the capabilities, era the of of detection studies, multi-messenger and astronomy classification briefly discoveries and of of push IMBHs the the promises next threshold to decade. of be our one of the big FILIPPOS KOLIOPANOS: ment – at an expert level – onas the an potential observer problems I of the am cosmic satisfiedence censorship by hypothesis. of the However, SMBHs currently – available and observational more evidence“evidence” importantly in I their favor am interpretation of mostly in the referring the pres- detection to context of three of highly general observational broadened, relativity. milestones 6.4 By keV in iron15 the K by SMBH Tanaka hypothesis. et al. The 1995), that are consistent with gravitational redshift by compact objects with a fort to understand how galaxies and globularcentral clusters IMBHs evolve. lie Determining the (i.e. mass in range in the which Review of IMBHs presence of a plume-like concentration of sources at the tentative measurements of their central MBH and velocity dispersion (e.g. [ Mezcua 2017 [ PoS(MULTIF2017)051 ⊙ 95 M 7 (Sept., 10 particle 44 ∼ a (May, MNRAS . of 77 AJ ARA&A Filippos Koliopanos motion the (1984) 471–506 and 22 ]. ]. ]. gravitation, ARA&A ]. of 1606.04855 1706.01812 1709.09660 theory ,[ ,[ ,[ 1311.5118 14 ,[ einstein’s X-Ray Properties of Black-Hole Binaries, in (1917) 197. ]. 19 . The Distribution of Mass in the Galactic Nucleus, centre . SA Cygnus X-1-a Spectroscopic Binary with a Heavy Companion ?, (June, 2017) 221101 (Oct., 2017) 141101 (June, 2016) 241103 Mass Measurements of Stellar and Intermediate-Mass Black Holes, 118 119 116 Wet., single a (Sept., 2014) 223–252 . 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