NUCLEI 2026

November 16-20, 2026

ESO Santiago, Chile

Workshop Rationale and Topics

The nuclear regions of galaxies display a rich structural and dynamical diversity, including nuclear star clusters (NSCs), intermediate-mass black holes (IMBHs), and supermassive black holes (SMBHs) spanning a wide range of accretion states. Many SMBHs reside in galaxies without strong ongoing accretion, particularly at lower masses. 
 
Observations have revealed that some ultra-compact dwarfs (UCDs) are likely the stripped remnants of more massive galaxies, retaining massive central black holes while having lost their outer stellar envelopes. These findings suggest that a significant population of black holes may be hidden in systems that no longer resemble their original hosts. 
 
Furthermore, dynamical measurements in compact, low-luminosity galaxies have extended black hole mass estimates into previously inaccessible regimes, helping to populate the lower end of the mass spectrum. 
 
Advances in the study of NSCs have also revealed diverse stellar populations, internal substructures, and evidence for complex formation pathways. The demographics of NSCs has been found to vary with environment, while their formation appears to vary with their mass. Theoretical models now also highlight NSCs as active players in the evolution of galactic nuclei, possibly feeding or regulating central black holes, particularly in dwarf galaxies. 
 
These developments raise several pressing questions: Under what conditions do NSCs and black holes co-exist or compete? How does AGN activity influence the creation or survival of an NSC? What determines whether a galaxy retains or forms a central black hole? How common are IMBHs in dwarf galaxies and compact stellar systems? How do current constraints on IMBHs constrain black hole seed formation mechanisms?  How many SMBHs remain hidden in stripped galactic cores or displaced from their original centers? 
 
To address these questions, we would like to focus this workshop mostly on quiescent and non-accreting galactic nuclei. These systems are essential for understanding the full range of nuclear evolutionary pathways, complementary to studies of accreting AGNs. At the same time, new observing facilities provide fresh opportunities: GRAVITY+, operational from 2026, will deliver early high-precision interferometric results on both quiescent galactic nuclei and AGN, extending stellar dynamical studies into regimes not previously accessible. The Vera C. Rubin Observatory (LSST) will begin uncovering large samples of tidal disruption events (TDEs) and other nuclear transients, providing powerful probes of otherwise dormant black holes. Other current and future facilities will add complementary perspectives, broadening the discovery space. 
 
The workshop will therefore provide a timely forum to assess recent progress, present first results from new facilities, and discuss how the scientific focus of this field will develop in the coming decades. 

Proposed Topics

Intermediate-Mass Black Holes (IMBHs)

  • Formation pathways and host environment
  • Observational techniques and detection challenges in quiescent systems
  • Occupation fraction in dwarf galaxies and the transition to the supermassive regime

Nuclear Star Clusters (NSCs)

  • Formation and growth: in-situ star formation vs. cluster infall
  • Co-existence and interaction with central black holes
  • Nuclear star cluster  demographics

Stripped Galactic Nuclei and Ultra-Compact Dwarfs (UCDs)

  • Dynamical and photometric signatures of stripped former galactic cores
  • Evidence for massive black holes in compact stellar systems
  • Consequences for the local black hole census and galaxy evolution

Observational Pathways to Black Holes in Quiescent and Active Nuclei

  • Stellar dynamical mass measurements and interferometric astrometry, including early science from GRAVITY+ on galactic nuclei and AGN
  • Transient probes: tidal disruption events (TDEs) and nuclear variability (e.g. LSST, ZTF)
  • Modeling challenges in crowded and low-luminosity environments

Looking Ahead: The Next Decades of Galaxy Nuclei Science

  • Anticipated breakthroughs with current and future facilities (e.g. ELT, JWST, ALMA, Roman, LISA, Rubin Observatory)
  • Theoretical challenges: black hole seed formation and nuclear star cluster evolution
  • Strategies for building a comprehensive black hole census across all mass scales

We invite you to join us for this exciting workshop at the ESO in Santiago Chile, with remote participation available via MS Teams. 

E-mail: NUCLEI2026@eso.org

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