(Active)GalacticNucleiAndreaMerloni1,MarcusBrüggen2,StefanGillessen1,NadineNeumayer3

1Max-PlanckInstitutfürExtraterrestrischePhysik,Garching2UniversityofHamburg3Max-PlanckInstitutfürAstronomie,Heidelberg

DraftVersion,20.6.2016

ExecutivesummaryThestudyofgalacticnuclei,andofthesupermassiveblackholesthatresideinthere, isacomplex,and far-reaching area of intense research within the astrophysical community worldwide, and inGermany in particular. Active Galactic Nuclei (AGN) and their dormant counterparts are uniquephysical laboratories (providing tests of strong gravity, extreme plasma conditions, high-energyparticle acceleration and gravitational waves generation), but may also hold the keys of ourunderstanding of galaxy evolution and Cosmology. Operatively, the study of galactic nuclei is ashowcase for multi-wavelength astronomy, highlighting epistemological trends that will becomemoreandmorecommonplaceinastrophysicsinthecomingdecades.Overthelastdecade,researchcarriedout inGermanyhashadsignificant impacton the fieldofAGNstudies.Wesummarize thisherebypresentingten"successstories",followedbyanoutlineofwhichwillbethekeyquestionsinthe field for the comingdecades. Inour Summary,wepresenta sketchofa roadmap intended toprovideguidance towardsaddressingsuchquestion (including instrumentaldevelopments)andwehighlightstrengthsandweaknessesoftheGermancommunityinthisareaofresearch.

1. Introduction:WhatisaGalacticNucleus?Sincethediscovery inthe late ‘90sthatmost, ifnotall,galaxiesharborasupermassiveblackhole,the study of galactic nuclei has received a vigorous impulse. These objects, whether in an active,growingphase,or inquiescence, representamong thebest laboratories to studyextremephysicalprocesses pertaining to gravity theories and plasma physics. Because of the complex multi-scalenatureof theAGNphenomenon, the interpretationof thedata requiresaprofoundknowledgeofthemanyphysicalprocessesinvolved.Thus,astrophysicalstudiesofblackholesingalacticnucleiare,by their very nature, pan-chromatic andmulti-messenger: high energy neutrinos, cosmic rays andgravitationalwavesareallusefulcomplementstoelectromagneticobservations.Assuch,thestudyof galactic nuclei represents a showcaseofmulti-tracer astronomy, andmay serve as amodel forfutureholisticviewsoftheCosmos.

In the last decade, a combination of high sensitivity, high spatial resolution observations and ofcoordinatedmulti-wavelength surveyshas revolutionizedour viewofblackhole (BH) astrophysics.Wenowknowthatsupermassiveblackholesinthenucleiofgalaxiesgrowovercosmologicaltimesby accreting matter, interact and merge with each other, and in the process liberate enormousamountsofenergythatinfluencedramaticallytheevolutionofthesurroundinggasandstars,andfilltheir immediate surroundings, and the whole Universe, with cosmic rays and magnetic fields,providingpowerfulself-regulatorymechanismsforgalaxyandstructureformation.

Indeed, the local supermassive black hole (SMBH) distribution is the outcomeof the cosmologicalgrowthofstructuresandoftheevolutionofmassinflowtowards(andwithin)thenuclearregionsofgalaxies,likelymodulatedbythemergersthesenuclearblackholeswillexperienceasaresultofthehierarchical galaxy-galaxy coalescences. Both in-depth studies of individual objects and populationstudies of evolving SMBHs via largemulti-wavelength surveys have been fundamental avenues ofresearchinthelastdecade,andwillcontinuetodosointhenextone.AbetterunderstandingoftheSMBHpopulation,andofitsevolution,willinformandprovidecriticalconstraintstogalaxyevolution

studiesandCosmology,andwillserveasaframeofreferenceinthepioneeringyearsofgravitationalwave astronomy (see Chapter 10, Kramer et al.) and astro-particle physics (see Chapter 16,Mannheimetal.).

In this document, we highlight the fundamental contributions that the German Astrophysicscommunityhasprovidedtothefieldinthelasttenyears,andoutlineasetoffundamentalquestionsthatwebelievewilldominatethelandscapeof(active)galacticnucleiresearchinthenextdecade(s).

2. TenGermansuccessstoriesResearchersworkingatGermaninstitutionsongalacticnucleihaveachievedaremarkablenumberoffundamentalresults.Here,wepresentaselectionoftensuccessstories.

2.1 TheCenteroftheMilkyWayAtadistanceof25,000lightyearsonly,thegalacticcenterisatrulyuniqueastrophysicallaboratoryfor studying in unparalleled detail stars and gas around amassive black hole (Genzel et al. 2010).Usinghigh-angularresolutiontechniquesinthenear-infraredwehavemovedcloserandclosertoanobservationalproofofexistenceofanastrophysicalmassiveblackhole. The compact radio sourceSgrA* in thegalactic center is associatedwithablackholeof roughly4million solarmasses. Theproofitselfisasbeautifulassimple:bymonitoringindividual,short-periodstarsonKeplerianorbitsonecanweighthecentralmasstopercent-levelprecision(seeFigure 1;seealsoSchödel,Merritt,Eckart2009).

Figure 1: The orbit of the star S2 in the galactic center. Left: the data points are measurements of thepositionsfrom1992to2015.ThestarisorbitingSgrA*every16yearsonaKeplerianellipse(bestfit:blackline).Right: The radial velocitydataare fit simultaneouslyby the sameorbitmodel (Gillessenetal. 2009,2013).

As a surprise came in 2011 the discovery of a little 3-Earth-mass gas cloud, called G2, that wasapproachingquicklythemassiveblackhole(Gillessenetal.2012).ItpassedthepericenterofitsorbitinSpring2014.ThedatarecordedshowsinbeautifuldetailhowG2hasbeentidallydisruptedbythegravitational force of the massive black hole - the first time that one can follow and study thisprocessobservationally.Since2013wehavebeenable toseehowthegashasswirledaroundtheblackhole.Inthisway,SgrA*representsalsoalaboratoryforthestudyofplasmaphysicsprocessesin the strong gravitational field of a SMBH. The observed spectral energy distribution, both in

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quiescenceandintheregularlyobservedflares,canbemodeledtorevealdetailsoftheinteractionbetweenaccretingmatterandtheblackholeenergyrelease(eeee.g.Dodds-Edenetal.2011;Eckartetal.2012;Pontietal.2015).

Thenearfuturewillbeexcitingingalacticcenterresearch:Thenear-infraredobserverswillbeableto use the VLT interferometer GRAVITY soon, increasing resolution and accuracy bymore than afactor10ofwhat ispossible today (Eisenhaueretal.2011).Similarly,observers in themm-regimewill soon use global VLBI to synthesize a radio telescope that has almost Earth diameter. Such aninstrumentmightbeableto formanactual imageof themassiveblackhole in thegalacticcenter.TheEuropeaneffort isbundled inthe"BlackHoleCam"project,whiletheUScolleaguesbrandthesameideaas"EventHorizonTelescope".

Figure 2: Simulatedimageofthemassiveblackholeinthegalacticcenter.Globalmm-VLBIaimsatobtainingsuchan image, inwhichtheshadowfromtheeventhorizonappearsasadistinct feature(Falcke,Melia&Agol2000).

2.2 ResolvedstudiesoffeedingandfeedbackinnearbygalacticnucleiInfrared high-resolution techniques (namely adaptive optics and interferometric observations onverylargetelescopes)alsoexcelatrevealingthephysicalprocessesatplayinother,nearbygalacticnuclei.TheGermancommunityoccupiesafavorablepositioninthefieldthankstothedevelopmentof PI instruments such as NACO, SINFONI, MIDI, GRAVITY, PACS. This has led to a number offundamental discoveries, among which we mention here: (i) spatially and kinematically resolvedstudies of feeding and feedback in many galaxies (see e.g. Davies et al. 2007, 2014, and variouspapersbasedonIFUdata)and(ii)interferometricobservationsresolvingthe(sub-)parsec-scaleduststructuresinnearbyAGNs,leadingtotherealisationthatthereareatleasttwodistinctcomponents,oneofwhichappearstotracetheoutflow(Meisenheimeretal.2007,Tristrametal.2014,Burtscheret al. 2013). The former provided the first clues of spatial and temporal correlation between gasconsumptionduetostarformationandaccretionontotheblackingalacticnuclei;thelatterhelpedustracingthewarmgasanddustthatsurroundstheinnermostregionsofAGN,providesthematerialfor accretion onto the super-massive black hole, and is held responsible for the orientation-dependentobscurationofthecentralengine.

2.3 TidaldisruptioneventsAt leastsomepartof the fuelsupply forAGNmustcomefromstars,whichaboundwithingalacticnuclei,andwillbe tidallydisruptedwhendislodged intoorbitspassingcloseenoughto thecentralblack hole. The frequency of such events depends on the stellar dynamical properties of galacticnuclei, and is a non-trivial outcome of a series of complex processes. Indeed, there is substantialuncertainty in the true rateof tidal stellardisruption ingalacticnuclei.Observational studieshavemostly reported constraints on the tidal disruption event (TDE) rate that are up to one order ofmagnitude lower than most theoretical predictions. This apparent contradiction should not besurprising,as,observationally,thefield isstill in its infancy;onlyaroundtwodozenTDEcandidates

havebeenidentifiedsofarbymeansofX-rayUVandopticalobservations(seeGezari2013;Komossa2015,forrecentreviews).Germanscientistshaveplayedanimportantroleinthisearlyphase,withthefirstTDEdiscoveredintheROSATdatamorethan10yearsago(Komossaetal.2004).Today,therateofdiscovery is increasingdramaticallywith theadventof largewide-areaoptical time-domainsurveys(mostlydrivenbysupernovaesearches),suchastheCatalinaReal-TimeTransientSurvey,thePalomar Transient Factory, PanSTARRS and the All-Sky Automated Survey for Supernovae. Thefuture promises even more rapid advances with next generation wide area X-ray (SRG/eROSITA,Merlonietal.2012,Khabibullinetal.,2014),andoptical(Skymapper,ZTFandLSST)surveys.

2.4 Blackholemassmeasurementsbasedonresolvedgasandstellarkinematics;BH-galaxyscalingrelations

AstronomersinGermanyhavecontributedsignificantlytothecensusofblackholesatthecentersofgalaxiesbyobservingandmodelingthemotionofgasandstarsatveryhighspatialresolutionusingspace observatories and ground based adaptive optics assisted facilities (Bender et al. 2005,Neumayeretal.2007,Sagliaetal.2016).IthasobservationallybeenshownthateverygalaxymoremassivethantheMilkyWayhostsamassiveblackholeatitscenter,withmassesbetweenamilliontoseveralbillionsolarmasses.Surprisingly,themassofthecentralblackholecorrelatestightlywiththemassofthesurroundinggalaxyorgalaxybulge(seeFigure 3)-oftenexpressedintermsofthegalaxy’svelocitydispersionσorbythegalaxyluminosity(seee.g.HäringandRix2004,Beifiorietal.2012,Sagliaetal.2016,Figure 4).Thismeans themoremassive thecentralblackhole, themoremassiveisitshostgalaxy,andviceversa.

The tightness of this correlation seems surprising,becausetheblackholeandthegalaxy itsits inactonvery different spatial scales. The galaxy does not feelthegravitationalinfluenceoftheblackholeandhence,the origin of the correlation was sought for in thecommon evolution of the black hole and its hostgalaxy. The cosmic evolution of the observed scalingrelations isan importantdiagnostic inuncovering thepossiblecoevolution(Merlonietal.2010,Bongiornoetal.2012),however, selectioneffectsplaya significantrole in the analysis (Schulze & Wisotzki 2014). Theeffect of feedback of an active black hole on itssurroundings (e.g. Di Matteo et al. 2005) has beenchallenged, as theobserved correlation could alsobeexplained solely by the hierarchical assembly of theblack hole and its ‘parent’ galaxy (Jahnke & Macciò2011).

Given all the widespread relevance attached to thescaling relations discussed above, one importantquestion still remains open: How massive are the

original“seed”blackholes?(Neumayer&Walcher2012,Latifetal.2013).Thefuturelooksbrightforresearchinthisdirection.Highspatialresolutioninstrumentation,bothfromspace(JWST)andfromtheground(E-ELT)willbekeyinansweringthisimportantopenquestion.

Figure 3 Therelationbetweenblackholemassandbulgemassforasampleof30galaxies.Asthesegalaxieshavebeenselectedtobemainlyellipticalgalaxies,thebulgemassisequaltothetotalgalaxymassformostofthegalaxies(fromHäring&Rix2004).

Figure 4: Therelationbetweenblackholemassandstellarvelocitydispersionσ(left)andbulgemass(right)aspresentedinSagliaetal.(2016).

2.5 VLBIandrelativisticjetkinematicsHigh-resolutionradioobservationswithVeryLongBaselineInterferometry(VLBI)areapowerfultooltorevealtheinnermostregionsofactiveblackholes,inparticularthesitesofparticleaccelerationatthebaseofrelativisticjets.Inthelastfewyears,majorprogresshasbeenmadeinunderstandingthecharacteristics of relativistic outflows at parsec scales and their kinematics from the MOJAVEprogram (Lister et al. 2009; 2013).Within Germany, theMPIfR and the Effelsberg telescope haveplayedamajorroleintheseendeavors.Abdoetal.(2010)andFuhrmannetal.(2014)showedhowthe combination of radio and Fermi gamma-ray data reveals key features of the Spectral EnergyDistribution (SED) of Blazars, those jetted AGN seen along the axis of the relativistic flow, andconstrain themodels for the emission processes in these systems. VLBI observations also provedcrucial to map the acceleration of relativistic jets in nearby radio galaxies, providing the highestresolutionviewyetofsuchcomplexprocesses.Alsohere,mm-VLBIinthecomingyears(seesection2.1), by resolving sizes of the order of a few gravitational radii inM87 (see e.g. Krichbaum et al.2006),isboundtorevolutionizeourunderstandingofjetformationandacceleration.

2.6 AGNinVHEgamma-raysAlmostattheoppositeendoftheelectromagneticspectrum,Blazarsrepresenttheprototypicalclassof Very High Energy gamma-ray sources. Most discoveries of astronomical sources, theirclassification,andthefirstpopulationstudiesweredonewithinstrumentswithastrong(andinmostcases leading) German participation, such as the MAGIC and HESS telescopes. Among the keydiscoveriesof the last tenyears,we listhere (i) thediscoveryofaPeVatron in theGalactic center(HESS), and (ii) the discovery of the ultra-short (onminute timescale) variability of blazars in VHEenergies (HESS and MAGIC). Such a success story deserves more than the little space we haveavailablehere,andwereferthereadertoChapter11(Pohletal.)foramorein-depthdiscussionofVHEgamma-rayastrophysics,includingAGNstudies.

2.7 AGNevolutionGiven the observed population of supermassive black holes in galactic nuclei, a key question thathavekeptbusyobserversandtheoreticiansalike isthefollowing:canweconstrainmodelsoftheircosmological evolution to trace back the local population to their formationmechanisms and themainobservablephasesofgrowth,asidentifiedbytheentireAGNpopulation?

As opposed to the case of galaxies, where the direct relationship between the evolving massfunctionsofthevariousgalaxytypesandthestarformationdistributionisnotstraightforward,duetotheirnever-endingmorphologicalandphotometrictransformation,blackholesarecharacterizedonly by two physical properties (mass and spin), the evolution ofwhich is regulated by analyticalformulae, to the firstorder functionsof the rateofmassaccretiononto them.Thus, foranygiven“seed” black hole population, their full cosmological evolution can be reconstructed, and its end-point directly compared to any local observation, provided that their growth phases are fullysampledobservationally.Ontheotherhand,blackholesgrowtherasesallinformationontheinitialconditions, while in galaxies dynamics and stellar populations can help us disentangling theirformationhistory.

Figure 5 Top panel: Evolution of the X-ray luminosity density of AGN with LX > 10

43.2 erg/s, for variousinterveningcolumndensities.TheluminosityoutputofAGNexperiencesariseandfallindensityinthez=1-3.5range(totalastopgrayshadedregion).Thestrongestcontributiontotheluminositydensityisduetoobscured, Compton-thin (blue shaded region) and Compton-thick AGN (green shaded region), whichcontributeinequalpartstotheluminosity.Theemissionfromun-obscuredAGN(redshadedregion,bottom)is significantly smaller. Bottom panel: Redshift evolution of space density of AGN split by the level ofobscuration.Differentpanelscorrespondtodifferenthydrogencolumndensity intervalas indicatedatthetop.FromBuchneretal.(2015).

This motivates ever more complete AGN searches (surveys). The level to which the desiredcompletenesscanbeachievedinpracticedependsonthelevelofourunderstandingofthephysicalandelectromagneticprocessesthat takeplacearoundaccretingblackholes.So,wecannotdiscusstheevolutionofsupermassiveblackholeswithoutanin-depthunderstandingofAGNsurveys,andoftheir results; but at the same time, we cannot understand properly these surveys if we do notunderstandthephysicsbehindtheobservedAGNphenomenology.

For thestudyofAGN,X-rayshavemeritsoverother selection techniques,primarilyauniformandquantifiable selection function at all redshifts, and a reduced incidence of absorption due tointerveningmatteralongthelineofsight(mostlylocatedintheimmediatevicinityoftheblackholeitself)andhostgalaxylightdilution.Thisiscritical,becauseinordertodiscriminateamongthemanyanddifferentmodelsfortheco-evolutionofbackholesandgalaxies,weneedaninventoryofAGNas

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Fig. 17 Top panel: Evolution of the X-ray luminosity density of AGN with LX > 1043.2 erg/s, forvarious column densities. The luminosity output of AGN experiences a rise and fall in density in thez = 1−−3.5 range (total as top gray shaded region). The strongest contribution to the luminositydensity is due to obscured, Compton-thin (blue shaded region) and Compton-thick AGN (greenshaded region), which contribute in equal parts to the luminosity. The emission from un-obscuredAGN (red shaded region, bottom) is significantly smaller. Bottom panel: Redshift evolution ofspace density of AGN split by the level of obscuration. Different panels correspond to differenthydrogen column density interval as indicated at the top. From Buchner et al. (2015).

Adopting a general form of luminosity-dependent bolometric correction, andwith a relatively simple parametrization of the effect of the obscuration bias onthe observed LF, Hopkins et al. (2007) were able to project the different observedluminosity functions in various bands into a single bolometric one, φ(Lbol). As acorollary from such an exercise, we can then provide a simple figure of merit forAGN selection in various bands by measuring the bolometric energy density asso-ciated with AGN selected in that particular band as a function of redshift. I show

completeandhomogeneousaspossible(seeMerloni2016,forarecentreview).ThedeepestsurveyssofarcarriedoutinthesoftX-rayenergyrange(0.5–2keV),supplementedbythepainstakingworkofopticalidentificationandredshiftdeterminationofthedetectedsourceshaveprovidedthemostaccuratedescriptionoftheoverallevolutionoftheAGNluminosityfunction.NeitherpureluminositynorpuredensityevolutionprovideasatisfactorydescriptionoftheX-rayLFevolution,withagoodfittothedataachievedwitha“LuminosityDependentDensityEvolution” (LDDE)model,orvariationsthereof. In their influential work, Hasinger et al. (2005) unambiguously demonstrated that in theobservedsoftX-rayenergybandmoreluminousAGNpeakedathigherredshiftthanlowerluminosityones.Thus,aqualitativelyconsistentpictureofthemainfeaturesofAGNevolutionisemergingfromthe largest surveys of the sky in various energy bands. Strong (positive) redshift evolution of theoverallnumberdensity,aswellasmarkeddifferentialevolution(withmoreluminoussourcesbeingmoredominantathigher redshift) characterizes thepopulation (seee.g. Schulze&Wisotzki 2010;Buchneretal.2015,Figure 5).

2.8 PhysicsofLow-LuminosityAGN,fundamentalplaneofBHactivityThepossibility toapply scaling relations to thepropertiesofaccretingblackholesacross themassscale has been a long-standing goal of relativistic astrophysics.One step in that directionwas thestudyofMerloni,Heinz&DiMatteo(2003),whoexaminedthedisc--jetconnection instellarmassandsupermassiveblackholesbyinvestigatingthepropertiesoftheircompactemissionintheX-rayand radio bands, and demonstrate that the sources define a ``fundamental plane'' in the three-dimensional (log LR, log LX, log MBH) space. This important empirical correlation has an eleganttheoreticalinterpretationintermsofscaleinvariantjetmodelsandcanbedirectlyusedtoconstrainthedynamicsoftheaccretionflow.TheobservedrelationimpliesthattheX-rayemissionfromblackholesaccretingatlessthanafewpercentoftheEddingtonrateisproducedbyradiativelyinefficientaccretionflows.

Inrecentyears,thisresulthasbeenexploitedtogaininsightonthephysicalstateofLow-LuminosityAGN (LLAGN), and on the impact of their energy release on their environments. The first directevidenceforfeedbackonthegassurroundingablackholecamewiththearrivalofhigh-resolutionX-ray imaging: usingROSATdata, Böhringer et al. (1993) discovered that the radio galaxy PerseusA(powered by the supermassive black hole in the central cluster galaxy NGC 1275) excavates largecavities in the hot, X-ray emitting thermal gas that fills the Perseus cluster. The physics of thisfeedbackwasfirstsimulatedbyChurazovetal.(2001)andBrüggenetal.(2002)forM87intheVirgocluster.ThegasispushedasideintodenseshellsandtheexcavatedX-raycavitiesarefilledwithradioemissionby the lobesof the radiogalaxy. In theChandraandXMMera,directevidenceof LLAGNfeedbackinactionhasbeenfoundintheX-rayobservationsofmanynearbygalaxyclustersshowinghowblackholesdepositlargeamountsofenergyonkpcscalesinresponsetoradiativelossesoftheclustergas.Bystudyingthecavities,bubblesandweakshocksgeneratedbytheradioemittingjetsinthe intra-clustermedium (ICM) it appears that (low-luminosity, radio loud) AGN are energeticallyabletobalanceradiativelossesfromtheICMinthemajorityofcases.

Ataglobal level,AGNfeedbackmodelshingeontheunknownefficiencywithwhichgrowingblackholesconvertaccretedrestmassintokineticand/orradiativepower.Merloni&Heinz(2007)showedthattheoutputoflow-luminosityAGNistrulydominatedbykineticenergyratherthanbyradiation,andhaveallowedestimatesofthekinetic luminosityfunctionofAGNbasedontheobservedradioemissionof their jets. In thoseworks, tantalizingevidencewas found for thekineticpoweroutputfrom low-luminosity AGN dominating the feedback at low redshift, which has been confirmed bystudyingofthez<1radiogalaxiespopulationintheCOSMOSfield(Kördingetal.2006;Smolcicetal.2009).

2.9 TheroleofAGNfeedbackinmodelsofstructureformationThetightscalingrelationsobservedbetweenthecentralblackholesmassandvariouspropertiesoftheir host spheroids that characterise the structure of nearby inactive galaxies havemodified thewayweconceivethephysicallinkbetweengalaxyandAGNevolution.AsSMBHgrowthisduemainlytoaccretionduringactiveAGNepisodes, it ispossiblethatmost, ifnotall,galaxieswentthroughaphase of nuclear activity in the past, during which a strong physical coupling (generally termed“feedback”)mighthaveestablisheda long-lasting linkbetweenhostsandblackholeproperties. Inrecent years, attempts have been made to connect AGN and galaxy growth via ‘evolutionary’scenarios, in which key observational properties of AGN, such as the degree of dust and gasobscurationofthenuclearactivityarerelatedtooverallpropertiesofgalaxyevolution,suchasgasconsumptionandstarformation.

Distinguishing nuclear AGN emission from the surrounding galactic one requires high spatialresolution observations, but these are challenging, particularly because the resolving power oftelescopes varies widely across the electromagnetic spectrum. Indeed, for more distant AGN,combining observations at the highest possible resolution at different wavelength on large,statistically significant samples, is nearly impossible, andoneoften resorts to less directmeansofseparatingnuclearfromgalacticlight.

Thus, most relevant studies require at the same large sample, and deep multi-wavelengthcharacterizationof thehostpropertiesofAGN.However,only for relatively fewsmallpencil-beamfields(suchasCOSMOS,CANDLES,GOODSNandS)thisisavailable.Thefirstresultsonthestudyofthe AGN hosts, however, give contradictory answers, with some indication that AGN populateuniformlybothstar-formingandpassivegalaxiesandnoclearevidenceofa“smokinggun”forAGNfeedback (Bongiorno et al. 2012; Santini et al. 2012; Rosario et al. 2013). More recent in-depthanalysis, takingadvantageofVLTNIR IntegralFieldspectrographs(SINFONI,KMOS),havebeguntounveil the true incidence of AGN-driven outflows in the z~2 star-forming galaxies, i.e. thoseexperiencing the most rapid and sustained growth phase (see e.g. Förster-Schreiber et al. 2014,Genzeletal.2014).

2.10 Systematicsearchesforhigh-zQSOsTheAGNpopulationathighredshift isfundamentaltoconstrainstructureformationmodels,butisstill very poorly known. Until about ten years ago, only a handful of very luminous quasarswereknownatz>6.Verydeepnear-IRimagesarecrucialtoimprovethissituationandtostudytheroleofAGNattheepochofreionisation.

Today,morethan120quasarshavebeenfoundatredshiftz>6,whentheuniversewas lessthan1Gyr old. More than half of these have been found by astronomers in Germany. A physicalcharacterizationof thesequasarshas revealedthepresenceofbillionsolarmassblackholes, largereservoirsofmoleculargas,andactivestarformationinthehostgalaxiesoftheseveryearlysystems.Assuch,theyprovideuniqueconstraintsonearlyquasaractivityandgalaxyevolution.

Alongthese lines,onemainfocuswillbepushingtheredshift frontiersforquasarstoz>8.Thiswillenablestudiesoftheearlygrowthofblackholesandtheirhostgalaxieswaybackintotheepochofreionisation. As such, they provide unique constraints on black hole- galaxy growth and earlystructureformation.Germanastrophysicsiswellpositionedtocontinuetobeamajorplayerinthefield.Thesearchforthez>8quasarswillrequirelargescale(~allsky)imagingintheIRbands,whichwill beprovidedbyEuclid. Thisdatawill have tobe complementedbyoptical surveysatmatcheddepth(e.g.LSSTorsomeotherPan-STARRSssuccessor).

3. Futurekeyquestions

Inordertofocusonthelong-termperspectiveofthisfieldofresearch,wehavesurveyed70seniorscientists working on AGN and galactic nuclei in Germany. The list was compiled bythe authors,withoutpretenseofcompleteness,butwiththeaimofhaving itrepresentative,relevant,balancedand, incaseofdoubt,broad.Theterm“senior”wasnotdefinedintermsoftypesofpositionheld,but rather by judgment of the authors, as broadly defined “impact” in the field, and the level ofseniority in the list typically ranges from full professor to senior postdocs. The list comprised 11women and 59men. Geographically, three clusters are apparent: 26 of the scientist work in thelargerMunicharea,14inHeidelbergand9intheBonn/Colognearea(seeFigure 6formoredetails).The list contained all relevantwavebands, from radio, overmm, submm, IR, optical, X-ray to veryhighenergies;itcontained10theorists,and2scientistsfromthegravitationalwavecommunity.

Figure 6: Map of the geographical distribution of the scientists active in the field of (active) galactic nuclei in Germany, as surveyed by the authors of this chapter before the RDS Denkschrift meeting in December 2015.

Below,wedistiltheoutcomeofoursurveyintowhatwebelieveconstitutesthelistofmajorkeyquestionsopeninthefield,whichwilldriveitsdevelopment,withinandoutsideGermany,inthecomingdecades.

1. Do central compact objects in galactic nuclei obey the general relativistic definition of a“BlackHole”?

Inotherwords,are compactobjects ingalacticnucleidescribedbyEinstein’sgeneral relativity(GR),andcanweprobethegeometryofspacetimearoundthem?Despitethe longandrobustchain of evidences that point towards GR’s black holes as the explanations of all thephenomenology we have described above, a formal, iron-clad proof of their existence is stillmissing, and undoubtedly scientists will continue to look formeans to achieve this goal. Thisposesthequestionofwhetheritwillbepossibletoimagetheeventhorizonofablackholeand,more generically, of what will constitute an observational proof of the existence of

(supermassive) black hole. The impact of these questions goes way beyond the realm ofastrophysicalresearch,andwebelievethesignsarepositivethattheGermancommunitywillbeattheforefrontoftheenterprise.2. Canwecharacterize thephysicalprocesses that takeplace in the immediatevicinityofa

blackholeeventhorizon(accretion,jetacceleration)?Morespecifically,howdoaccretionflowsreleaseenergyintheir innerregions,andwhat isthenature of disc turbulence and viscosity? Accretion disc theory is a mature subject, but,somewhatuncomfortably,fundamentalopenquestionsremainonhowexactlydodiscsandjetstransport angular momentum, and what is the actual geometry of the inner accretion flowaroundaccretingblackholes.Ontheotherhand,thetheoryofjetproductionandaccelerationincompactaccretingobjects isstill lackingpredictivepowerandmanydetailedquestionsarestillopen.Howareparticlesaccelerated inrelativistic jetsandneartheeventhorizon?What isthestructureof the inner jet?AreAGN jetVHEemissionprocesses leptonicorhadronic?AreAGNtheprimary sourceofUHECR?ArehighlyenergeticneutrinosproducedbyAGN?AreAGN thesource of fast radio transients? Here our expectation is that progresses in multi-messengerastronomy,andacloserconnectionbetweentheorists,observationalastronomers,astro-particlephysicists and gravitational wave scientists will bring forward many of the answers thecommunityhasbeensearchingfor.3. Howdoblackholesgrowingalaxies,andhowdotheyinfluenceeachother?Oneofthemostimportantopenquestionsishowblackholesgettothecentersofgalaxiesinthefirst place. How do they evolve over cosmic time and from which initial seed mass dosupermassiveblackholesstart togrow?Starting fromthere, itwillbe importantto findouttowhatextentdoblackholeandgalaxiesgrowinlockstep,andwhatrolesgalaxiesandblackholemergersplaythere.Dogalaxymergerstriggeraccretionepisodes?DogalaxymergersnecessarilyleadtoSMBH-SMBHmergers?Howarethecentralblackholesfueled,andwhatistheAGNdutycycleandthepowerspectrumofvariabilityoverdifferenttimescales?TheissueofAGNfeedbackand its role in the overall process of galaxy evolution will most likely remain subject of veryintense investigation. The reason is the inherent complexity of galaxies as the nodes of thebaryon cycle,where pristine gas gets reprocessed into stars. Tomake progress,we’ll need tounderstandhowstarformationproceedsintheimmediateenvironmentofaSMBH,and,insomecases,resolveindividualstars innearbygalaxies,andfollowthedynamical interactionbetweenstars and SMBHs. At the nuclear level, the interaction between SMBH and their host galaxiesrequiresnewtestsoffundamentalstellardynamicaltheory(inarelativisticregime),aswellasadeeperunderstandingofhowSMBHsandtheenergytheyreleaseaffectthe ISM,andhowthevariousphasesof theobservedoutflows (ionizedgas,molecular gas,dust) are related toeachother. Proceeding towards larger and larger scales, wewill be busy in the coming years withtrying tounderstandwhat the relationshipbetweenoutflowsandstar formationquenching is,andhowblackholesaffectthethermodynamicsofthecircum-galacticandintra-clustermedium.4. Canweuseblackholestotracelarge-scalestructure,andconstrainCosmology? Finally, luminous, accreting black holes (quasars,QSOs) have a long tradition of being used asbeacons to probe themost distant reaches of the Universe. Still, because only the tip of theiceberg has been unveiled, we need to quantify QSO/AGN contribution to the reionizationprocess.ThepossibilitytoobserveQSOsuptohighdistancesalsomakesthemuniquetracersoflarge-scalestructure.Widearea(imagingandspectroscopicsurveys)havebeeninstrumental inmapping the three-dimensional structure of the Universe, and future sensitive instruments(WEAVE,DESI,PFS,4MOST)arebeingdesignedtoallowtomeasurebaryonicacousticoscillations(a powerful “standard ruler” for cosmology) with AGN, which would give access to a widerredshiftrangethanprobedwithgalaxiesalone.Ontheotherhand,abetterunderstandingofthephysicalprocessesgivingrisetoAGNemission(seequestion2above)mayunlockthepotential

of quasars as “standardizable candles”, able to probe the geometry of theUniverse up to thehighestredshifts.

4. SummaryandConclusionsThere is littledoubtthattheareaofresearchongalacticnuclei representsamajorstrengthof theGerman astrophysics community. Almost every key topic is covered, and observations across theentireelectromagneticspectrumfromworld-classfacilitiesprovidehighlyvaluabledatasets.Whenprojecting such developments into the coming decades,we need tomake sure that all themanyareas of excellence are preserved, trying, at the same time, to steer away from the “wavelengthchauvinism”,i.e.theperceptionthatone’sowninstrumentofchoiceistheuniquecarrierofthefield.TheverynatureofAGNandgalacticnuclei,aswehavearguedthroughoutthisdocument,demandscoordinated approaches. Nonetheless, in the section below, we briefly try to outline where webelieve future developments will be most impetuous. At the end, we close by summarizing thestrengthsofourcommunityandtheareaswhereroomforimprovementhasbeenidentified.

3.1 InstrumentalroadmapWethinkthatinthefuturetwoprominentbranchesofAGNresearchwillbepursued.Thefirstaimsatobserving individualobjectsatthehighestpossibleprecisionandinasmuch(spatial,temporaland spectral) detail as possible. The aim is to understand in depth the astrophysical processesreigninggalacticnuclei.Nearbygalaxiesandourowngalacticcenterwillbetheprimetargets,andthe main parameter will be the achievable resolution. Near-Future instruments to this end areglobal-mm VLBI, the second-generation VLTI instruments (GRAVITY & MATISSE), next-generationadaptive optics instruments and the James-Webb Space Telescope (JWST). The high-resolutioninstrument suite of the European ELT and the ATHENA X-ray observatory are suited for the sameapproach,butfurtherinthefuture.

The second approach will be of more statistical nature, i.e. one tries to gather large data setscoveringmanysources.Themainparameterherewillbethenumberofsourcesused.Fromalargesampleonecanderivenotonlyaveragequantities,butalsotrends.Asofteninastronomy,thisisoneoftheveryfewwaystolearnaboutevolutionarysequences.InstrumentstothatendwillbeeROSITA(an X-ray all-sky survey); the Euclid ESAmission, andWFIRST on the NASA side; themulti-objectwide-areaopticalspectroscopicinstrument4MOSTatESO'sVISTAtelescope,andtheLargeSynopticSurveyTelescope (LSST),aswellas the radio telescopeSquareKilometerArray (SKA).AprominentroleofAGNinplanninglargesurveyswillguaranteedueattentionisgiventomulti-wavelengthandmulti-messengersynergies.

Furthertrendswillbetheshifttohigherredshift(enabledbytheuseoflargertelescopes,e.g.JWSTandE-ELT),andtheopeningofacompletelynewchannelbymeansofthespace-basedgravitationalwave observatory eLISA. At the highest energies, the Cherenkov Telescope Array (CTA) will bringforwardTeVground-basedastronomy,whilethenextgenerationofneutrinotelescopeswillenrichthemulti-messengerapproach.

3.2 StrengthsGermangroupsareveryvisible,andhavemademanyseminalcontributions inthepast.There isahealthy level of diversity in the approaches and scientific outputs, brought by the wide spectralcoverage,verygoodmulti-wavelengthcase,stronglinkbetweensimulationsandobservations.Inthelast decade, research on galactic nuclei in Germany has produced outstanding results, both onindividual objects, and on population studies. We firmly believe the main key to this string ofsuccesses is the privileged access of German groups to cutting-edge, top instrumentation, over abroad rangeofwavebands.Astrophysics is still,byand large,anobservationallydriven field.Mosttopresearchgroupsareembeddedininternationalcollaborationsthatgobeyondanationalcontext,butahighlevelofvisibilityseemstobewelldefendedinmanycases.

3.3 RoomforimprovementA fewareashave comeupduring the surveywhere theoverall positivepicturewouldprofit fromimprovements:

● Aneven largerdegreeofcoordinationbetweendifferentwavelengthswouldbedesirable-since a singlewaveband is insufficient to fully understand the inherently complex galacticnuclei

● A major worry is that German researchers might miss prime access to two major, newfacilities:TheLSST(fortime-domainastronomy)andtheSKA(forradioastronomy).

● On the theory side,more support for theoretical/numerical studieswouldbedesirable.Alink to computer scientists might be an example how to achieve more efficient codedevelopment.