ORIGINS Black Hole Days

Oct 27, 2025, 9:00 AM → Oct 28, 2025, 7:00 PM Europe/Berlin

X5 1.1.18 (MPE, Garching)

The "Black Hole Days" aim to bring together researchers from the institutes in Munich and Garching who are interested in black holes. The event is being organized by the ORIGINS Connector 1 "Black Holes". The two-day workshop will have a broad range of topics, including:

• Supermassive Black Holes
• Active Galactic Nuclei
• Binaries, Gravitational Waves
• Primordial Black Holes

The event will take place at MPE in Garching. The event is planned to take place on Monday, the 27th of October and Tuesday, the 28th of October.

Overview/Invited talks:

• Dr. Aleksandra Olejak (MPA)
• Dr. Valeriya Korol (MPA)
• Dr. Andrea Merloni (MPE)
• Prof. Dr. Frank Eisenhauer (MPE/LMU)
• Prof. Dr. Volker Springel (MPA)
• Dr. Florian Kühnel (MPP/LMU)
• Prof. Dr. Gia Dvali (MPP/LMU)
• Prof. Dr. Dieter Lüst (MPP/LMU)

Main organizer: Valentin Thoss (LMU/MPE)

Monday, October 27

9:00 AM → 10:25 AM Primordial Black Holes & Fundamental Aspects

9:00 AM Welcome

9:05 AM Primordial Black Holes: Positivist Perspective and Galaxy Genesis

Speaker: Florian Kühnel (MPP & LMU Munich)

Kühnel.pdf

9:30 AM Black hole memory burden and its observational manifestations: spectroscopy of mergers and gravitational waves

The "memory burden" effect implies that the black hole's quantum information load, which is invisible in black hole's ground state and traditionally has not been paid the deserved attention, exerts macroscopic effects on perturbed black holes. First, it tends to stabilize a black hole against the Hawking evaporation, leading to dramatic consequences for PBH dark matter. Even more strikingly, it swiftly influences the classical dynamics of merging astrophysical black holes in Einstein gravity, predicting strong variations in classical gravitational radiation from black hole mergers with identical classical initial conditions.

Speaker: Gia Dvali

Dvali.pdf

9:55 AM Transitioning to Memory Burden: Detectable Small Primordial Black Holes as Dark Matter

Mounting theoretical evidence suggests that the information stored in black holes suppresses their decay rate. This quantum effect of memory burden opens up a new window for small primordial black holes (PBHs) below $10^{15}\,{\rm g}$ as dark matter candidates. In this talk, I show that the smooth transition from semi-classical evaporation to the memory-burdened phase strongly impacts observational bounds on the abundance of small PBHs. The most stringent constraints come from present-day fluxes of astrophysical particles and point towards an early onset of memory burden, after losing only a small fraction of the initial mass. Remarkably, currently-transitioning small PBHs are detectable through high-energetic neutrino events.

Based on:
G. Dvali, M. Zantedeschi, S. Z., Transitioning to Memory Burden: Detectable Small Primordial Black Holes as Dark Matter, arXiv:2503.21740.

Speaker: Sebastian Zell (Ludwig Maximilian University & Max Planck Institute for Physics, Munich)

BlackHoleDay_3.pdf

10:10 AM Light Micro Black Holes and their astrophysical consequences

The fundamental scale of gravity could be significantly lower than the Planck scale due to the existence of a large number of particle species. This would also mean that processes exceeding this energy scale could produce black holes. In this talk, I will discuss two consequences of such a situation: first, the production of micro black holes in the corona of active galactic nuclei, together with their accompanying phenomenology; and second, their prolonged lifetime due to the memory burden effect. In some regions of the parameter space, their lifetime exceeds the age of the universe, which opens the possibility that primordial versions of such black holes could be viable dark matter candidates.

Speaker: Manuel Ettengruber (Max-Planck-Institut für Physik (Werner-Heisenberg-Institut))

DPG_Slides-3.pdf

10:25 AM → 10:50 AM Coffee break

10:50 AM → 12:00 PM Primordial Black Holes & Fundamental Aspects

10:50 AM Primordial Black holes are 5D

We briefly revisit mechanisms for primordial black hole (PBH) production, namely inflation, phase transitions, and cosmic strings, in the context of the Dark Dimension Scenario, which is motivated by Swampland principles. Applying quantum gravity constraints, we demonstrate that any viable mechanism inevitably leads to the formation of five-dimensional PBHs. We comment on the observational implications of this result, including a potential connection to the recent detection of a high-energy neutrino by KM3NeT, whose energy is intriguingly close to the five-dimensional Planck scale in the Dark Dimension Scenario.

Speaker: Prof. Dieter Lüst

lüst.pdf

11:15 AM Minimal black holes and species thermodynamics

The thermodynamics of large black holes is reliably described by semiclassical quantum gravity, and contains the seeds of holography. Generically, it cannot be extrapolated beyond the validity of gravitational effective field theory. In some special regimes this is possible, and leads to novel constraints on weakly coupled completions of gravity. I will present the framework and discuss how the results are consistent with, and point toward, string theory.

Speaker: Ivano Basile (Max Planck Institute for Physics)

BHdays_Basile.pdf

11:30 AM Black Holes as Probes of UV Physics

Charged black hole solutions, which are ubiquitous in string theory, provide a natural setting for thought experiments probing the ultraviolet (UV) structure of quantum gravity. I will review recent progress in understanding how UV scales emerge in Effective Field Theories of quantum gravity, and explain how the attractor mechanism of charged black holes offers a controlled way to access UV physics at the horizon. I will conclude with comments on how these ideas might extend to more realistic black holes.

Speaker: Matilda Delgado (MPP & Harvard U.)

BHdays-Delgado.pdf

11:45 AM Black Holes as a laboratory for Quantum Gravity

Black holes remain one of the most fascinating windows into the quantum nature of gravity, yet their entropy is not fully captured by semiclassical considerations. Supersymmetric black holes, in particular, provide a controlled laboratory to explore subtle quantum effects and gain insights into more general black hole dynamics. In this work, we investigate genuinely non-perturbative contributions to the entropy of four-dimensional supersymmetric black holes and identify the precise conditions under which they emerge. Remarkably, while these corrections are generic, they vanish for specific background configurations, revealing a sharp criterion for their presence. The underlying mechanism is illuminated by the motion of charged particles in the near-horizon geometry, whose behaviour echoes aspects of black hole evaporation. By uncovering these structures in this setting, we gain new insights into the quantum physics of black holes and reveal patterns that may extend to more general gravitational systems.

Speaker: Carmine Montella (Max Planck Institute for Physics)

blackholedays_Montella.pdf

12:00 PM → 1:15 PM Catered lunch

1:15 PM → 1:55 PM Primordial Black Holes & Fundamental Aspects

1:15 PM Structure formation with primordial black holes

Dark matter could consist of primordial black holes (PBHs). I present the first simulation of structure formation with PBH dark matter that self-consistently incorporates collisional few-body effects, post-Newtonian orbit corrections, orbital decay due to gravitational radiation, and black-hole mergers. An interesting phenomenology emerges in this simulation. Many-body interactions eject PBHs at high speeds, making a subcomponent of dark matter that is hot enough to suppress structure growth on billion-PBH scales. Such interactions also suppress the abundance of small PBH clusters much more than was previously expected. Although collisional relaxation reshapes PBH halos, the abundance of binary pairs forestalls gravothermal core collapse. Mergers of PBH binaries give rise to an energy density in gravitational waves that remains steady at about 1/5000 the PBH mass density, but due to the frequency of interactions with other PBHs, binary merger times predicted by relativistic orbital decay do not correlate with when each pair actually merges. Accurate simulation results such as these will be crucial to sharpen constraints on PBH dark matter.

Speaker: M. Sten Delos (Max Planck Institute for Astrophysics)

delos.pptx

1:40 PM Detecting Primordial Black Holes in the Solar System

Speaker: Valentin Thoss (Ludwig-Maximilians-Universität München)

Thoss.pdf

1:55 PM → 2:35 PM The Galactic Center

1:55 PM The Black Hole in the Centre of the Milky Way

Black Holes are among the most mysterious objects in the Universe. They are so massive and compact that nothing - not even light - can escape their gravity. Our presentation will give an update on the overwhelming observational evidence for an extremely heavy and compact object in the Galactic Centre, for which a supermassive black hole is the best explanation. Using the world's largest telescopes and most advanced technology, we follow the stars orbiting the central object, precisely measure its mass, detect the stunning effects of general relativity, and measure the orbital motion of hot gas close to the event horizon. We will specifically highlight the spectacular observations from the GRAVITY interferometer, which has transformed the Galactic Center research in the last years.

Speaker: Frank Eisenhauer (MPE)

2:20 PM Finding faint stars in the Galactic Center with GRAVITY

Resolving the rich structure in the central 0.1 x 0.1’’ in the Galactic Center allows tracing stars orbiting the central massive black hole SgrA. Serving as test particles in a strong gravitational field, these S-stars are susceptible to effects of General Relativity. With its unmatched astrometric precision, the interferometric beam-combiner GRAVITY has already facilitated in the past a measurement of the Schwarzschild Precession and Gravitational Redshift of a bright S-star. This leaves the constraint of the spin of SgrA as a current quest. However, the effect of the Lense-Thirring precession on stars caused by the spin of SgrA drops off steeply with distance, demanding that stars orbit on short periods to be measurable for GRAVITY. In currently accessible magnitudes, no comparable star has been detected so far, requiring to look ever deeper into the Galactic Center. Existing imaging algorithms like CLEAN, which have been adopted from radio interferometry, are stretched to their limits when reconstructing GRAVITY data.
We present the development and results of the image reconstruction method GRAVITY-RESOLVE ($G^R$), which is designed for finding faint, yet undiscovered stars in the Galactic Center. The development includes an exact model of the Galactic Center, as well as all measured instrumental systematics of GRAVITY, allowing for the construction of an utmost precise forward model. The implemented model and inference are based on Information Field Theory.
The results comprise detections of two new stars in the Galactic Center, one of which could enable the measurement of the constraint on the spin of SgrA in the future.

Speaker: Felix Mang (Max Planck Institute for Extraterrestrial Physics)

Mang.pptx

2:35 PM → 3:00 PM Coffee break

3:00 PM → 4:00 PM The Galactic Center

3:00 PM Probing the gravitational potential around Sagittarius A* with stellar orbits

Since 2016, the GRAVITY interferometer at ESO’s Very Large Telescope has provided astrometric data with unprecedented accuracy for the S-stars orbiting Sagittarius A*, providing a powerful means to probe the gravitational potential surrounding the supermassive black hole at the center of our Galaxy. Notably, we have detected the in-plane, prograde Schwarzschild precession of the orbit of the star S2 and the gravitational redshift of its spectral lines, as predicted by General Relativity.

In this presentation, I will discuss the implications of an extended mass distribution around Sagittarius A, primarily consisting of a dynamically relaxed cusp of old stars and stellar remnants, along with a potential dark matter spike. By analyzing S-stars data, we establish stringent upper limits on the enclosed mass within S2’s orbit—approximately 1200 solar masses within the central 10 milliparsecs of our Galaxy—assuming a smooth, spherically symmetric mass distribution. Our observational constraint aligns closely with theoretical predictions for a dynamically relaxed stellar cusp, leaving little room for a significant enhancement of dark matter density near Sagittarius A.

I will then discuss the impact of granularity in the mass distribution on the orbit of the star S2, assuming it consists of a cluster of equal-mass objects surrounding Sagittarius A*. We find that this granularity can induce significant deviations from the orbit in case of a smooth potential, leading to precession of the orbital plane and variations in the in-plane precession. Specifically, I will show that if a cluster of stellar-mass black holes resides within S2’s orbit with a total mass consistent with our derived upper limit, the astrometric residuals during S2’s next apocenter passage in 2026 may exceed the accuracy threshold of GRAVITY. This presents a unique opportunity to detect scattering effects on S2’s orbit caused by stellar-mass black holes, leveraging the exceptional precision of GRAVITY and its future upgrade, GRAVITY+.

Speaker: Matteo Sadun Bordoni (Max Planck Institute for Extraterrestrial Physics)

SadunBordoni_bhday.key

3:15 PM Flares from SgrA*: Dynamical probes for the accretion flow around supermassive black holes

Photons emitted from near ultra-compact objects carry information not only about their emission environment but also about the spacetime geometry around such objects. In the case of Sagittarius A, the supermassive black hole at the centre of the Milky Way, the detected emission in the Near Infrared can flare to ten times its quiescent flux and be resolved temporally and spatially by the GRAVITY Instrument probing the near-event-horizon region of SgrA and the dynamics in its accretion flow. In this talk I will report on the recent polarimetric and astrometric measurements of NIR Flares obtained with the GRAVITY instrument, showcase the theoretical modelling performed by the GRAVITY Collaboration to interpret the observations. Additionally, I will highlight the current efforts to resolve the accretion flow and its internal dynamics.

Speaker: Diogo Ribeiro (Max Plank Institute for Extraterrestrial Physics)

ribeiro.pdf

3:30 PM Polarization of reflected X-ray emission from Sgr A molecular complex: multiple flares, multiple sources?

Extended X-ray emission observed in the direction of several molecular clouds in the Central Molecular Zone (CMZ) of our Galaxy exhibits spectral and temporal properties consistent with the `X-ray echo' scenario. It postulates that the observed signal is a light-travel-time delayed reflection of a short ($δt<$1.5 yr) and bright ($L_{\rm X}>10^{39}~{\rm erg~s^{-1}}$) flare, most probably produced a few hundred years ago by Sgr A. This scenario also predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction towards the primary source and polarization degree (PD) being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing Sgr A complex with higher significance, but also reveal an inconsistent polarization pattern for the brightest reflection region in its center. X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A illumination and shows a smaller PD compared to the large region. This could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located in the closer vicinity of the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

Speaker: Ildar Khabibullin (USM LMU, MPA)

Khabibulin.pdf

3:45 PM Feeding the Galactic Center: Understanding the G1-2-3 Gas Streamer

Sagittarius A*, the supermassive black hole at the center of our Galaxy, offers a unique laboratory to study black hole accretion at extremely low rates. Understanding what material is available to feed it, and how that material interacts with its accretion zone, is key to understanding black hole feeding mechanisms more broadly. Many gas clumps of the order of a few Earth Solar masses have been discovered in the vicinity; however, G1 and G2 have peaked in interest owing to their similar and highly eccentric orbit. In 2013, G2 made its pericenter passage. Along with detecting a tidal shear of G2, a post peri position was also detected. G2 also showed a tail following a similar orbit. In this talk, I will discuss our new ERIS observations of another gas cloud object that took shape from the outer tail of G2. This third object follows a similar orbit, and since the probability of finding three stars on the same orbit is negligible, we invoke a Gas Streamer picture. We also discuss the plausible origin of this G 1-2-3 Gas streamer to be the stellar wind from the massive binary star IRS16SW. The shifts in the orbital orientations and pericenter times synchronize with the motion of IRS16SW, pointing towards a shared dynamic origin.

Speaker: Simran Joharle (Max Planck Institute for Extraterrestrial Physics)

Black_Hole_Day.pptx

4:00 PM → 4:20 PM Coffee break

4:20 PM → 5:00 PM Stellar Black Holes & Gravitational Waves

4:20 PM Unveiling the milli-Herz Gravitational Wave Universe with LISA

Gravitational-wave observations have opened a new window onto compact objects across the Universe. The Laser Interferometer Space Antenna (LISA) is a flagship European-led mission that will explore the intermediate-frequency gravitational-wave spectrum between 0.1 and 100 mHz. With its unique discovery potential, LISA will detect an exceptionally rich variety of sources, from stellar binaries in our own Galaxy to black hole binaries of all masses and configurations throughout cosmic history. In this talk, I will give an update on the status of the LISA mission and highlight key science cases it will deliver concerning black holes.

Speaker: Valeriya Korol (Max Planck for Astrophysics)

27-10-2025_ORIGINS_BH_Days_Korol.pdf

4:45 PM Where are the isolated stellar-mass black holes?

Stellar evolution models predict that our galaxy should be filled with hundred of millions of isolated stellar-mass black holes (ISBH), yet only one has been detected conclusively as of today. Microlensing is the only method to detect ISBH, and only spatially resolving the event can break the degeneracy between distance and mass to the lens. I will present the latest developments in predicting the population of ISBH, as well as the promising new observation facilities which will enable ISBH detection, and measuring the mass function of this population distinct from the GW precursors.

Speaker: Antoine Mérand (European Southern Observatory)

BHday2025_AMerand.pptx

6:30 PM → 9:00 PM Conference dinner

Tuesday, October 28

9:00 AM → 10:10 AM Stellar Black Holes & Gravitational Waves

9:00 AM Gravitational Waves from Stars Stripped by Supermassive Black Holes

Recent discoveries of puzzling electromagnetic transients in galactic centers, such as quasi-periodic eruptions and oscillations, point to a new class of stellar-mass objects interacting with supermassive black holes (SMBH). One compelling scenario is that stars gradually spiral inward through gravitational-wave emission and eventually transfer mass to the SMBH, forming stellar extreme mass ratio inspirals: a possible new type of multimessenger source for the future LISA mission.
In my talk, I will present a case of a subgiant star transferring mass to an SMBH. We find that the system can undergo a long-lived, stable phase of mass transfer, stripping the star of its hydrogen envelope before it enters the LISA band. The resulting helium core then inspirals as a bright, persistent LISA source, potentially detectable for hundreds of thousands of years with signal-to-noise ratios approaching a million. Episodic hydrogen flashes may drive short bursts of rapid mass transfer at orbital periods of 20–30 hours, before the helium core is ultimately partially disrupted at millihertz GW frequencies. We estimate a ∼1% probability of such a system existing in the Galactic Center, with additional detectable events expected out to ∼1 Gpc.

Speaker: Aleksandra Olejak (Max Planck Institute for Astrophysics)

AOlejak_BHdays_slides_plus_backup.pdf

9:25 AM 3D Simulations of Stellar Black Hole Formation

Stellar-mass black holes (BHs) are formed in the collapse of massive stars. The detailed pathways to BH formation are, however, still under active debate. In my talk, I will present results from a set of three-dimensional (3D) neutrino-hydrodynamics simulations of BH formation in failed core-collapse supernova explosions (sometimes called "unnovae"). Because failed supernovae lack a bright electromagnetic signal, neutrinos and gravitational waves are the only messengers that carry valuable information on the physical processes in the deep interiors of the collapsing stellar cores. In my presentation, I will discuss how the (highly turbulent) hydrodynamics of the stellar plasma shapes the anisotropic emission (and absorption/scattering) of neutrinos, which has important implications for the natal kicks of the remnant BHs. Moreover, I will demonstrate that neutrinos from failed supernovae contribute significantly to (or may even dominate) the diffuse supernova neutrino background (DSNB), which is the cumulative neutrino flux from past stellar core-collapse events at cosmological distances. The almost guaranteed discovery of the DSNB with currently running and next-generation neutrino experiments (such as Super-Kamiokande, JUNO, and Hyper-Kamiokande) will mark the next milestone in neutrino astronomy after SN 1987A and will allow to directly probe the theory of stellar BH formation.

Speaker: Daniel Kresse (Max Planck Institute for Astrophysics)

Kresse_2025-10-28_ORIGINS_BH_Days.pdf

9:40 AM The Tip of the Iceberg: Unveiling the Origins of Compact Object Mergers through Gravitational-Wave Outlier Events

There is growing evidence that a substantial fraction of the compact object mergers detected through gravitational waves merge with non-zero eccentricity or large spin-orbit misalignment. In particular, recent evidence for an eccentric neutron star-black hole (NSBH) merger and NSBHs with anti-aligned BH spins challenge leading formation scenarios to date. Residual eccentricity rules out formation through isolated binary star evolution, while neutron star natal kicks and the unequal masses of NSBHs inhibit efficient pairing in dense stellar environments. I will show that all observed properties---NSBH merger rate, eccentricity, and spin-orbit misalignment---are explained by the high prevalence of massive stellar triples in the field. I will discuss that the formation of the neutron star (its natal kick and the mass-loss in supernova, often reduced via binary envelope stripping) decisively impacts the triple stability, preferentially leaving behind surviving NSBHs in compact triple architectures.
The rich three-body dynamics of compact, unequal-mass triples enables mergers across a wide range of orbital parameters without requiring fine-tuned highly inclined tertiary orbits and provides a natural explanation for an abundance of residual eccentricity and spin-orbit misalignment. Finally, I will discuss how spin measurements from future eccentric binary black hole mergers could provide key insights into their formation pathways.

Speaker: Jakob Stegmann (Max Planck Institute for Astrophysics)

BH-days.pptx

9:55 AM Very Massive, Rapidly Spinning Binary Black Hole Progenitors through Chemically Homogeneous Evolution -- The Case of GW231123

Among the over 200 gravitational wave detections reported so far, GW231123 is a remarkable event that not only holds the record for the most massive black hole merger, but also exhibits extreme spins. Its origin is actively debated. Proposed scenarios include dynamical formation involving a sequence of mergers, Population III stars, accretion in an AGN disk and also more exotic explanations including primordial black holes and cosmic strings, each facing different challenges.

Recent work showed that the incoming black holes of GW231123 can be formed out of massive rapidly rotating collapsing helium stars. Here, we address the question how such very massive rapidly rotating helium stars can be formed in very close binary systems. For this we explore chemically homogeneous evolution (CHE) involving progenitors with masses above the pair-instability mass gap.

We compute a grid of detailed massive binary models with the stellar evolution code MESA to follow the early evolution of binary progenitors and show that: (i) very massive (Mi>140M⊙) CHE binaries at low metallicity (Z=10−5) naturally produce rapidly rotating progenitors with high masses and high spins matching the properties of the black holes in GW231123 and (ii) the maximum spin of the progenitors is bound by their critical rotation rate leading to a tight correlation between the dimensionless spin and mass, a∝M−0.9, in models that have no hydrogen left.

We conclude that the CHE channel appears to be a viable and natural scenario to produce progenitors. We compare and discuss the differences with earlier studies and comment on the large uncertainties in the final fate and collapse.

Speaker: Silvia Popa (MPA)

GW_CHE_BH_days.pdf

10:10 AM → 10:35 AM Coffee break

10:35 AM → 12:00 PM Supermassive Black Holes & Active Galactic Nuclei

10:35 AM eROSITA AGN surveys: constraints on the local population of massive black holes

The statistical properties of Supermassive Black Holes in the local Universe provide both direct and indirect constraints on theoretical models of BH formation, growth and co-evolution with their host galaxies. In this talk, I will present studies of the local distribution of X-ray selected AGN carried out thanks to the largest survey of the kind, the eROSITA All-Sky Survey. I will focus in particular on robust statistical estimates of the incidence of active black hole as a function of the local galaxy properties across a wide range in stellar mass, including some of the most stringent constraints on the nuclear activity in low-mass galaxies.

Speaker: Andrea Merloni (MPE)

BHDays_Merloni_2025.pptx

11:00 AM Robust AGN and host galaxy decomposition in optical spectral fitting

Unraveling the mysteries of supermassive black holes and their relationship with their host galaxies is challenging and requires a proper disentanglement of the emission from the Active Galactic Nucleus (AGN) and the stellar populations. When observing the same AGN in different phases of its activity, some properties obtained from its optical spectra should be consistent, as the mass of the black hole, the stellar mass of the galaxy, and the velocity dispersion of the stars. Such quantities are often used for scaling relations to understand the co-evolution of supermassive black holes and their hosts. We describe a novel optical spectral fitting method that allows a robust decomposition of the spectra into the host galaxy’s stellar population and AGN emission. We apply our method to three Sloan Digital Sky Survey samples of multiple optical spectra of the same objects: 32 changing-look AGN in their bright and dim states, and 10 quasars and 10 galaxies with 5 single-epoch and one stacked spectra. We discuss the reliability of the AGN-galaxy spectral decomposition by assessing thresholds on the fraction of AGN contribution to the continuum (f_AGN) and setting uncertainties to the measurements. For properties obtained from stellar emission, as stellar velocity dispersion and stellar mass, f_AGN > 0.8 will provide unreliable measurements, while for the supermassive black hole mass, f_AGN < 0.2 spectra give unreliable results. We compare the results of the different samples for the above-mentioned properties, and analyze the results for the scaling relations. Our main goal is to assess and set thresholds for the reliability of the measurements from optical spectral fitting of diverse samples of AGN.

Speaker: Catarina Aydar (Max Planck Institute for Extraterrestrial Physics)

aydar.pdf

11:15 AM The Search for Ice Bands of Frosty Active Galactic Nuclei

Recent JWST mid-infrared observations of the central ~50pc region of the Seyfert 2 nucleus in the galaxy NGC 7582 have revealed prominent ice absorption features. Such icy bands have not been included in existing AGN or starburst dust radiative transfer models. In particular, template spectra from AGN model libraries severely underestimate the mid-infrared flux when compared with the JWST data. To address this discrepancy, we propose a new class of frosty AGN models. These models incorporate icy grain mantles into clumpy dust torus configurations. Using Monte Carlo radiative transfer calculations of the spectral energy distribution, we account for both the imprint of embedded ice features and the additional heating contribution from type-I quasar spectra, thereby extending and updating the AGN model library of Siebenmorgen et al. (2014).

This framework enables us to investigate several key scientific questions:

• At what radius in the accretion disk does the snow line emerge?
• Under what conditions can water freeze out onto dust particles in AGN environments?
• Is the ice reservoir preferentially located in the accretion disk, in clumps, or within the inner/outer torus regions?
• What is the contribution from starburst activity?
• Can the observed ice bands be attributed to a clumpy medium in the starburst or AGN torus?

Beyond these physical insights, our work also addresses the broader issue of why current AGN models fail to reproduce JWST mid-infrared spectra. Our findings underscore the need to revise torus models to include icy dust components, which may fundamentally alter the interpretation of AGN spectral energy distributions and their implications for galaxy evolution.

Speaker: Ms Khushboo - (LMU)

black_hole_days_ppt.key

11:30 AM Coronal gas emission - theory predicted, observationally confirmed - tracer of BH mass

I discuss the relevance of high ionisation coronal lines as genuine tracers of the ionising continuum in galactic nuclei, and in turn of the spectral shape and temperature of the accretion disk, hence of the BH mass. On the basis of bona-fide BH masses from reverberation mapping and the strong infrared coronal line [Si VI] 1.96 um, a tight correlation between the excitation of the gas and the BH mass is found. The correlation spans more than three order of magnitude in BH mass, the dispersion of the relation, 0.47 dex, is comparable with that of the M-sigma relation. The correlation is formally in line with the thin disk accretion theory prediction: Tdisc prop M_BH^(-1/4), and implies very high BH spins for the relation to hold. The relation predicts relatively cold accretion disks for low activity BHs - these being in the majority of Galactic Nuclei in the local Universe. I will show that despite their cold disc, the lowest active galactic nuclei also present coronal lines as revealed by JWST, and discussed their origin. This result, and the ubiquity of coronal gas in galactic nuclei regardless of activity level, opens a new window for BH demographics and accretion physics at high redshift.

Speaker: Almudena Prieto (IAC & Munich Observatory)

BHdays-GarchingOct2025-APrieto.pptx

11:45 AM Precision Black Hole Masses at Cosmic Noon

We present the first sample of cosmic noon quasars with spatially resolved broad-line regions (BLRs) and dynamical black hole mass measurements using GRAVITY+. Through deep integrations with GRAVITY+, we detect the differential phase signal across the H$\alpha$ broad emission line which allows for detailed kinematic modeling of the BLR. We find BLR sizes systematically smaller by a factor of 2-6 from the local radius-luminosity (R-L) relation which further leads to smaller black hole masses. All of our quasars are accreting at a super-Eddington rate. Using ALMA, we detect the integrated CO (3-2) emission line in the surrounding host galaxy which provides a measure of the galaxy mass. Comparing the black hole and galaxy mass, we find the majority of our quasars are over massive, similar to the recently discovered $z>4$ AGN from JWST.

Speaker: Taro Shimizu (MPE)

Shimizu_BlackHoleDays_2025.key

12:00 PM → 1:10 PM Catered lunch

1:10 PM → 2:10 PM Supermassive Black Holes & Active Galactic Nuclei

1:10 PM Elusive Plunges and Heavy Intermediate-mass-ratio Inspirals from Single and Binary Supermassive Black Holes

The most massive galaxies in the Universe also host the largest black holes, with masses of $10^9 \: \mathrm{M_{\odot}}$ and above (SMBHs). During their hierarchical assembly, these galaxies accreted many low-mass galaxies across cosmic time, possibly hosting IMBHs. They have experienced only a few major mergers at low redshift. If some of these IMBHs migrate to the galactic center, they may form compact subsystems around the central SMBH. We investigate the evolution of such hypothetical subsystems, consisting of ten $10^5 \: \mathrm{M_{\odot}}$ IMBHs at three different concentrations around a $10^9 \: \mathrm{M_{\odot}}$ SMBH. We evolve these systems both in isolation and in the presence of a secondary SMBH, using \texttt{MSTAR}, a regularized integration method including relativistic effects up to 3.5PN order. Our analysis focuses on gravitational-wave–driven inspirals (heavy IMRIs) and direct plunges. We show that perturbations from a secondary SMBH enhance the number of IMBH direct plunges by more than a factor of two, making them the dominant merger channel. These plunges and IMRIs with a central $10^9 \: \mathrm{M_{\odot}}$ SMBH will contribute to SMBH growth but will likely evade detection with future gravitational-wave interferometers and pulsar timing arrays. However, for galaxies with lower-mass SMBHs ($M_\bullet \le 10^8 \: \mathrm{M_{\odot}}$), heavy IMRIs will be detectable with LISA and can provide direct observational constraints on the existence of IMBHs, while the more dominant plunges will remain hidden.

Speaker: Lazaros Souvaitzis (Max Planck Institute for Astrophysics)

Souvaitzis_BlackHoleDays_talk.key

1:25 PM Ultramassive Black Holes at z < 0.1

Ultramassive black holes, with masses > 10^10 solar masses, sit at the very top of the black hole mass spectrum. These objects are rare and can’t be identified using standard scaling relations like the classic BH-sigma relation, which underestimate black hole masses at the high-mass end. Instead, the size of the core — a light-deficient central region compared to an inward extrapolation of the Sersic profile — is a much better predictor of black hole mass and it’s unaffected by projection effects.

We measure core sizes for around 15 BCGs at z < 0.1, for which we have both high-resolution and wide-field photometry. They suggest that black holes of at least 10^10 solar masses are indeed expected in these objects. We measure the Line-of-Sight Velocity Distributions (LOSVDs) of these objects from long-slit spectra acquired at the Large Binocular Telescope (LBT). We model the LOSVDs using our triaxial Schwarzschild code SMART and the deprojected light density determined with our semi-parameteric code.

The dynamically determined black hole masses confirm the hypermassive nature of these BCGs, almost doubling the number of objects in this mass range. This robustly confirms that core size is an excellent predictor of black hole mass. Using the superb imaging Euclid survey, this will allow us to study the evolution of the high mass black hole distribution as a function of redshift.

Speaker: Stefano de Nicola

deNicola_BHdays.pdf

1:40 PM The fine-tunning challenges of AGN feedback from galaxy groups to massive clusters

AGN feedback has become a standard ingredient in cosmological simulations to help regulate the development of cooling flows and star formation from the scales of galaxies to groups and clusters. However, it is difficult to fine-tune the feedback efficiency to reproduce the observed cool-core fractions at all scales. In the paper "How the cool-core population transitions from galaxy groups to massive clusters", we show that although a constant canonical feedback efficiency of ~0.10 in radio mode works at the scale of massive clusters, it results in an excess of AGN feedback at the scale of groups, suppressing the mid- and strong-cool-core populations in comparison with observations. To address this problem, we present a new model for the AGN efficiency in radio mode, based on the relative strength between the gravitational force and the Lorentz force driven by magnetic fields at the event horizon, which results in a feedback efficiency directly proportional to the accretion rate and scales well from the scales of galaxy groups up to clusters.

Speaker: Justo Gonzalez (European Southern Observatory)

The-fine-tunning-challenges-of-AGN-feedback-from-galaxy-groups-to-massive-clusters-Justo-Gonzalez.pdf

1:55 PM Supermassive black hole spin evolution in cosmological simulations

The mass and spin of massive black holes (BHs) at the centre of galaxies evolve due to gas accretion and mergers with other BHs. Besides affecting the evolution of relativistic jets, the BH spin determines the efficiency with which the BH radiates energy.
I implemented a sub-resolution prescription for cosmological, hydrodynamical simulations that models the evolution of the BH spin, accounting for both BH coalescence and misaligned accretion through a geometrically thin, optically thick accretion disc.
I will present a suite of simulations investigating the evolution of the BH spin across cosmic time and its role in controlling the joint growth of supermassive BHs and their host galaxies. Additionally, I will present results regarding the evolution and the distribution of BH spins as a function of BH properties.

Speaker: Luca Sala (C2PAP/LMU/USM)

Sala_BH_days.key

2:10 PM → 2:30 PM Coffee break

2:30 PM → 3:55 PM Supermassive Black Holes & Active Galactic Nuclei

2:30 PM Modelling the BH-galaxy connection in cosmological simulations

Speaker: Volker Springel (MPA)

2:55 PM Multiphase gas reservoirs around z~3 quasars and the impact of feedback from galaxy to halo scales

Extended Ly-alpha nebulae observed around quasars trace the cool gas
within the multiphase circumgalactic medium and can provide key insights
into the complex interplay between gas dynamics and active galactic
nuclei (AGN) feedback. However, the connection between this cool phase
and the cold molecular gas around the host galaxies remains largely
unexplored. In this talk, I leverage ALMA CO(4–3) observations of 37
quasars at z ~ 3 — all of which have been previously mapped in Ly-alpha
with VLT/MUSE — to investigate the relation between the CO emission, and
hence the molecular gas content, and halo properties. I show that
systems with higher black hole mass and lower Eddington ratio exhibit
higher CO detection rates while also demonstrating lower Ly-alpha
surface brightness. This result suggests that radiative feedback
regulates cold gas survival and life-cycles in dusty, gas-rich
environments, consequently affecting the reprocessed Ly-alpha emission
from halo gas. Additionally, as quasars trace overdensities, I report on
the incidence and properties of CO emitting companions and discuss any
evidence of gas depletion when approaching the quasar massive halo. This
investigation aims to provide a deeper understanding of the baryon cycle
in quasars around cosmic noon and constrain the processes that govern
the co-evolution of galaxies and black holes.

Speaker: Jelena Ritter (Max Planck Institute for Astrophysics)

Blackholedays_Ritter.pptx

3:10 PM Photometric redshifts for quasars in S-PLUS

Quasars are the most luminous type of AGN currently known, detectable across a vast range of redshifts extending beyond $z=7$. Thus, increasing the number of known quasars is essential for studying the accretion mechanisms of supermassive black holes, the evolution of galaxies, and the large-scale structure of the Universe, among others.
Over the last four decades, spectroscopic surveys have provided data for numerous quasars. However, such surveys are highly costly, in terms of telescope time, when applied to large areas. In this context, photometric surveys, such as the Southern Photometric Local Universe Survey (S-PLUS), offer the potential to cover wider regions of the sky with greater depth, thereby producing large data volumes within short time spans. The S-PLUS will cover a wide area of $\sim$9300 deg² of the southern celestial hemisphere with a unique optical filter system, which has 5 broad bands and 7 narrow bands centered on stellar parameters. This approach allows the determination of highly accurate estimates of photometric redshifts.
We have built the Quasar Catalog for S-PLUS DR4 (QuCatS; Nakazono & Valença et al., 2024), which contains over 640 thousand quasar candidates with photometric redshifts up to $z_\text{phot}=5$ across $\sim$3000 deg². To estimate the photometric redshifts, we trained 3 independent machine learning models with S-PLUS, infrared, and UV photometry when available, with training and validation of the method conducted on samples derived from a total sample of $\sim$33 thousand QSOs with spectroscopy from the Stripe 82. The three algorithms used are the Random Forest, the FlexCoDE, and a Bayesian Mixture Density Network (BMDN). Finally, by comparing the performance of the three algorithms trained with broad-band photometry alone and with the combined set of broad- and narrow-band filters, we find that the inclusion of narrow-band photometry as features improves the accuracy of the photometric redshift estimates. Moreover, the addition of narrow-band information provided better estimates of the probability density functions obtained with FlexCoDE and BMDN.

Speaker: Raquel Ruiz Valença (Universidade de São Paulo)

raquel.pdf

3:25 PM Spatially resolving the broad line region in a QSO at z=4

We present the first near-infrared interferometric data of the broad line region of a QSO at z=4. GRAVITY+ detects a differential phase signal tracing the spatially resolved kinematics for both the H$\beta$ and H$\gamma$ lines. Fitting these lines simultaneously shows that 80% of the HI line emission in the broad line region originates in an outflow with a velocity up to $10^4$ km/s, oriented so that our line of sight is along an edge of the conical structure. The model is also able to qualitatively match the outflowing CIV line profile and the systemic OI fluorescent emission. The derived black hole mass of 8e8 Msun is an order of magnitude lower than that inferred from various single epoch scaling relations, and implies that the accretion is super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region.

Speaker: Ric Davies (MPE)

Davies_BHdays_2025.10.pptx

3:40 PM Formation of massive seed BHs in the early universe

The formation of supermassive black holes (SMBHs) is one of the biggest challenges in astrophysics. The Direct Collapse (DC) model provides seed BHs with a mass of 10^5 Msun and explains the origin of high-redshift SMBHs. It has been assumed that these seed BHs can only form in primordial gas, a restrictive condition that yields too few seeds to explain the observed population. We investigate whether massive seed BHs can form in a metal-enriched universe, which is more prevalent. By conducting radiation hydrodynamical simulations across various metallicities (10^-6 < Z/Zsun < 0.01), we found that stars with masses greater than 10^4 - 10^5 Msun can form when Z/Z_sun <= 10^-3. Although dust cooling promotes low-mass star formation at finite metallicity, gas accretion still favors the growth of massive stars, leading to the formation of supermassive stars. This model offers a larger number of seed BHs compared to the previous DC model, providing a more universal explanation for the origin of SMBHs. We will further show how this new formation channel provides seed BHs in the early galaxies in our on-going high-resolution cosmological radiation hydro simulation.

Speaker: Sunmyon Chon (MPA)

20251028_MPA.pptx

3:55 PM → 4:15 PM Coffee break

4:15 PM → 5:15 PM Supermassive Black Holes & Active Galactic Nuclei

4:15 PM The Dichotomy in the Nuclear and Host Galaxy Properties of High-redshift Quasars

The early growth of high-redshift quasars and their host galaxies raises critical questions about their cosmic evolution. We exploit the angular resolution and sensitivity of NIRCam to investigate the host galaxies of 31 quasars at $4 < z <7$ drawn from multiple JWST surveys. Using a new multi-band forward-modeling code (\textsc{GalfitS}) that incorporates physically motivated priors, we securely detect and quantify the host emission in 30 objects, while simultaneously characterizing the nuclear spectral energy distribution. The host galaxies of high-redshift quasars are $\sim 0.3$~dex more compact than star-forming galaxies of comparable mass. A striking dichotomy emerges: luminous blue quasars ($L_{5100} > 10^{45}\,{\rm erg\,s^{-1}}$) reside in bulge-dominated galaxies ($n \approx 5$) and exhibit a narrow range of ultraviolet nuclear slopes (median $\beta_{\rm UV} \approx -1.4$), while fainter red quasars inhabit disk-like hosts ($n\approx 1$) and display a broad range of slopes ($\beta_{\rm UV}\approx-2$ to 4). These two populations differ markedly in their black hole-to-stellar mass ratios, with high-luminosity quasars showing $M_{\mathrm{BH}}/M_\ast = 1.2\%$ compared to $4.7\%$ for lower luminosity sources, placing them collectively $\sim$0.6~dex above the local $M_{\mathrm{BH}}-M_\ast$ relation. This offset likely reflects rapid black hole growth in early gas-rich environments, where feedback from the active galactic nucleus becomes effective only after substantial gas depletion. Our findings suggest that the observed dichotomy, whether due to intrinsic spectral differences or dust extinction, fundamentally shapes the coevolution of supermassive black holes and their host galaxies in the early Universe.

Speaker: Ruancun Li (Max-Planck-Institute for Extraterrestrial Physics)

MPE_blackhole.pptx

4:30 PM Accelerating Lensed Quasar Discovery in Euclid with Self-Supervised Learning

The formation channels of the first supermassive black holes (SMBHs) at z > 6 remain elusive, as current observations are dominated by the most luminous quasars, representing only the tip of the population. To uncover fainter quasars, gravitational lensing provides a uniquely effective natural telescope, extending the accessible luminosity range and offering essential probes of black hole seeds, host galaxy growth, and cosmic reionization.

The ESA Euclid mission will survey 14,000 deg² in optical and near-infrared bands and is expected to reveal ∼2000 strongly lensed quasars, including a rare yet valuable sample at z > 6. To meet this challenge, we present self-supervised learning techniques for discovering lensed quasars in Euclid data. Our framework leverages pretraining on simulated lenses and real sources (galaxies, stars, CCD artifacts) to learn robust image representations without extensive labels, improving completeness and reliability. Applied to the first 63 deg² of Euclid data, this method reduced an initial catalog of ∼1 million sources to ∼10,000 candidates. Preliminary inspection of the top 1,000 ranked systems revealed 40 potential dual point sources and one possible quadruple configuration.

With the forthcoming full data release, our approach will scale to the entire Euclid survey and enable the systematic discovery of lensed quasars at early epochs. This will open a unique window onto the faint end of the quasar luminosity function and the seeding and early growth of the first SMBHs.

Speaker: Dr Irham Taufik Andika (Ludwig-Maximilians-University Munich)

euclid_lensed_quasars_andika.pdf

4:45 PM Radiative Transfer Effects on Hydrogen Balmer Lines in Little Red Dots and Their Impact on the Black Hole Mass Estimates

Little Red Dots (LRDs), compact galaxies at z > 5 discovered by JWST, show hydrogen Balmer lines with broad wings (>1000 km/s), absorption features, and asymmetric profiles, indicating extremely dense hydrogen gas. When the broad components of Balmer lines are analyzed using standard AGN methods, they imply supermassive black holes with unusually high black hole–to–stellar mass ratios. However, the complex line shapes suggest that scattering processes in this dense medium may be reshaping the emission rather than tracing only black hole dynamics. Using 3D Monte Carlo radiative transfer simulations, we model resonance scattering in the n=2 state, Raman scattering of UV photons, and Thomson scattering by free electrons. We find that Thomson scattering alone can reproduce the observed broad wings, leading to black hole mass overestimates by factors of ≳10. Resonance scattering further modifies Balmer line ratios and profiles, while Raman scattering may contribute additional wing broadening. These results demonstrate that scattering is essential for interpreting Balmer lines in LRDs and has critical implications for understanding black hole growth in the early universe.

Speaker: Seok-Jun Chang (Max Planck Institut für Astrophysik)

RT_in_LRD_BH_day.ppsx

5:00 PM Multiple black holes in high-redshift galaxies with JWST/NIRSpec-IFS

JWST has discovered an unexpected ubiquity of massive, active black holes at high redshift. Understanding their formation and the properties of their host galaxies is an ongoing endeavour. I will present first results from BlackTHUNDER, a JWST/NIRSpec-IFS Large Program to study broad-line AGN and their immediate surroundings during the first few billion years of cosmic history. Specifically, I will discuss the discovery of a triple AGN system at z~5, in context with other confirmed and candidate dual AGN at high redshift. Our results indicate that multiple massive black holes may be common in galaxies in the early Universe, and highlight the important role of IFU observations for their discovery and characterisation.

Speaker: Hannah Übler (MPE)

Uebler_ORIGINS_BHdays.key