Letters of Intent received in 2024

LoI 2026-2221
Traversing the Galactic Center in Space and Time

Topics

- Compact and variable multiwavelength source Sgr A*: Even Horizon Telescope and GRAVITY interferometry results in synergy with other multiwavelength observations, stellar dynamics, detections and predictions of relativistic effects in the Galactic center
- Nuclear Stellar Cluster and its environment: formation history, interactions between gas and stars, kinematics and dynamics in the dense cluster environment, NIR and MIR observations (Very Large Telescope, Keck telescopes, JWST, 30- and 40-meter class telescopes) in synergy with other wavebands
- Star formation in the Galactic center: observations and models   
- Central Molecular Zone (CMZ): stellar and gas dynamics, star formation, astrochemistry in the CMZ, feeding and feedback mechanisms
- Comparison between Galactic center and nearby nuclei: Galactic center in the cosmological context

Rationale

Galactic center is the closest galactic nucleus to us, about 100- to 1000 times closer than other nuclei. It has been serving as a laboratory of accretion and dynamical processes in galactic nuclei since it is possible to resolve out its individual components (stars and gas structures) and to trace their motion and evolution in real time. Recently, the Event Horizon Telescope (EHT) and GRAVITY interferometers provided invaluable information about the innermost region of the accretion flow of Sgr A*, the compact radio source associated with the supemassive black hole (SMBH). In synergy with other instruments from radio, through infrared, to high-energy X-ray and gamma-ray domains it is possible to connect the observations on larger scales (Central Molecular Zone) with the innermost region (S cluster and Sgr A*).
The Galactic center, in particular Sgr A*, complements the observations of other galactic nuclei not only in terms of resolved spatial details but also because it is a representative of extremely low-luminosity galactic nuclei. To understand and to map the parameter space in accretion processes, it is essential to compare highly-accreting sources with low-accreting ones and understand how the accretion rate evolves on longer time scales. Despite its low luminosity, Sgr A* is quite active and exhibits infrared and X-ray flares a few times per day. Current 3D general relativistic magnetohydrodynamic simulations can capture some aspects of the observed variability, yet the detailed understanding of the processes across different wavelengths is still missing. In particular, the presence or absence of a large-scale jet emanating from Sgr A* is still an open problem at the border between observations and theory. Comparison with M87*, which is another EHT target, can provide insights into plasma dynamics close to low-accreting supermassive black holes.
Sgr A* is not isolated. On the contrary, it is surrounded by a dense Nuclear Star Cluster (NSC), which represents one of the densest stellar environments in the Galaxy, and a multiphase gas. The stars on the tightest bound orbits, so-called S stars, have been used to measure the mass and distance of Sgr A*. In addition, S2 star has been used to test predictions of General Theory of Relativity to a high precision. Several stars on even tighter orbits may be used for further tests of gravitational theories. The monitoring of S stars have also significantly constrained the contribution of the dark cusp and excluded the presence of an intermediate-mass black hole (IMBH) within the S cluster region. On the other hand, the presence of an IMBH on larger scales within the NSC cannot be ruled out. The studies of the formation and the continuous build-up of the NSC have favored an in-situ, episodic star-formation. The star-formation history depends on the photometric sensitivity of the observations and the applied extinction correction across the field. With the future improvements in the sensitivity towards lower-mass stars, the star-formation history as well as the initial mass function will further be constrained. The contribution of in-spiraling stellar clusters to the build-up of the NSC also cannot be entirely excluded.
Studies of star formation in the highly energetic environment around the SMBH are challenging both from an observational and a theoretical or a computational perspective. First, in the central parsec, there are just a few candidates of embedded young stellar objects. Second, in the theoretical and computational setups, the star-formation process depends significantly on the chosen boundary and initial conditions, which can differ greatly across the Galactic center history. Nevertheless, several star-formation channels (star formation within a dense gas disc or a cloud infall) have been found that appear promising to address the presence of stars that are only a few million years old. An additional challenge is including N-body dynamics into computational models as well as the effect of massive perturbers. These are crucial to constrain timescales for relaxation processes and understanding details of loss-cone dynamics. The latter is relevant for constraining the rate of extreme-mass ratio inspirals (EMRIs) for the future detectors of low-frequency gravitational waves.
To properly understand star-formation history in the Galactic center region and feeding - feedback mechanisms across different length-scales, detailed multiwavelength observations of the region are crucial. The NSC is embedded within a flatter, rotationally supported Nuclear Stellar Disc (NSD) on the scale of a few 100 pc. NSDs are typically found in barred spiral galaxies such as the Milky Way, where the bar potential provides the channel for the inflow of gas that fuels the star formation. The Central Molecular Zone (CMZ) containing the most massive molecular clouds in the Galaxy is the manifestation of this gas inflow. The star formation in this environment then gives rise to the NSD and eventually also the NSC in the very center. At the same time, with its large stellar densities, intense radiation, and strong magnetic field, the CMZ is one of the most extreme environments in the Galaxy. In this sense, the differences in star formation, the initial mass function, and stellar evolution between the CMZ and the Galactic plane are still not understood in detail. The studies of the gas and stellar dynamics in the CMZ are not only relevant for understanding the evolution of the Milky Way. With its high gas densities, large magnetic field, intense ionizing radiation, as well as the turbulent interstellar medium, the CMZ serves as a nearby analog of high-redshift star-forming regions.
At the proposed IAU symposium on the Galactic center, we will discuss the following pressing questions:
• What can the most recent EHT and GRAVITY data in synergy with multi-wavelength observations tell us about the accretion flow of Sgr A* (orientation, variability, presence of jet) ? How can fast-moving S stars be utilized for further tests of gravitational theories?
• What are the current constraints on the presence of compact remnants (stellar- and intermediate-mass black holes, neutron stars, and white dwarfs) in the NSC? Is there really a missing pulsar problem? Can millisecond or young pulsars/magnetars address the gamma-ray excess in the Galactic center?
• What is the present star-formation rate in the Galactic center and how precisely can we trace its history?
• How is the gas channeled towards the CMZ and from the CMZ towards the NSC and Sgr A*? What is the structure of the gas inflow and outflow? How do present and past outflows, including the putative jet, impact gas and stars in the surroundings? How is the gas in the CMZ enriched in metals? What are the most complex molecules that can exist within the CMZ?
We propose to organize the IAU symposium “Traversing the Galactic Center in Space and Time” in Brno, the Czech Republic, under the auspices of the Masaryk University and the Brno Observatory and Planetarium. We believe that Brno, with its rich history, including the past presence of distinct science personalities, such as Johann Gregor Mendel, Kurt Goedel, and Ernst Mach, as well as its current modern outlook towards high-tech space industry, can provide a comfortable venue for a vibrant exchange of information and ideas about the cutting-edge results concerning the Galactic center and beyond. Brno is conveniently located in Central Europe and it is well-connected by trains and other transport.