Letters of Intent received in 2024

LoI 2026-2208
Towards a More Precise Determination of the Dark Matter Content of Galaxies

Topics

Dynamics of galaxies; Astrometry and 3D velocities; Large surveys of resolved stars; The Galaxy and its halo; Rotation curves of galaxies; Dwarf spheroidal and ultra-faint dwarfs; Dwarf irregular galaxies; Magellanic System; Andromeda System; Near field cosmology; Dark matter; Galaxy formation

Rationale

(a) Type: Symposium;
(b) Title: Towards a More Precise Determination of the Dark Matter Content of Galaxies
(c) Coordinating Division: H, The Local Universe
Division J
Division G
(c) SOC members (gender in lower case):
Francois Hammer (co-chair, FRANCE, m)
Roger Ianjamasimanana (co-chair, SPAIN, m)
Magda Arnaboldi (ESO, f)
Zuhui Fan (CHINA, f)
Stacy McGaugh (USA, m)
Marcel Pawlowski (GERMANY, m)
Zara-Nomena Randriamanakoto (MADAGASCAR, f)

We are thinking to invite (maximum 3), other scientists for a better coverage of the distant, and of the theoretical aspects.

(d) Scientific Rationale and Science Topics
The first evidence for dark matter (DM) in galaxies came from early studies of galaxy rotation curves (RCs). During the 70s, studies of RCs from ionized (Rubin et al.) and neutral gas (Bosma) have revealed that a significant part of mass is missing, which lie mostly in galactic halos. These studies were based on nearby (almost edge-on) spiral galaxies, including our neighbor, M31.
Since that time, considerable advances have been made in understanding galaxy formation and in studying the most distant galaxies thanks to large ground-based telescopes, including ALMA, to the HST and the first results from the James Webb Space Telescope. In particular, it has been shown that most spiral galaxies, including the Galaxy and M31, result from gas-rich mergers, which underlines the success of the hierarchical scenario including by solving the problem posed by the acquisition of their large angular momenta. Furthermore, studies of very distant galaxies suggested that several of them may lack DM when compared to their nearby analogs. This emphasizes that to determine a galaxy’s DM content, one needs to understand its history.
HST and Gaia have provided considerable advances in establishing 3D-velocities in the Milky Way (MW) and its immediate neighborhood. Combined with a very large number of spatial and ground-based surveys (APOGEE, 2MASS, WISE, SDSS, DESI), our knowledge of the Galaxy and its neighborhood has reached a level of maturity not attainable at larger distances. As a result, the Milky Way now has the most precise rotation curve amongst other giant spiral galaxies. Significant works from PAndAs, PHAT, PHAST, have revealed important properties of M31 and its halo, pointing to a recent merger that has occurred in that galaxy.
All the above results have generated debates about the dynamical or total mass in most galaxies, i.e. about their DM content. This has considerable scientific impact, because mass uncertainties limit all studies aiming at understanding the past and future of galaxies. For example, the total mass inferred for the Milky Way depends on the a priori choice of dynamical tracers in the halo, and may still vary by factors larger than two. There is also some recent evidence that the Large Magellanic Cloud could be far much more massive than initially thought, which could modify the dynamical status of many galaxies in the Local Group, including our own. Yet, here the current uncertainty reaches a factor close to 10! The wealth of observational data has also revealed the importance of non-equilibrium dynamics for interpreting galaxies, which again can be best studied in the Local Group where full 6D phase-space information can be accessed. Furthermore, the status of several dwarf galaxies remains uncertain, and some of the smallest ones could be confused with globular clusters, resulting in very uncertain total masses. The question of dynamical equilibrium has a considerable impact, since one needs to verify if stars and gas have completed a sufficiently large number of orbits to allow a precise mass determination.
The range of galaxy masses studied for DM content is huge, from the several hundred of billion solar masses for the Milky Way or M31, down to ultra-faint galaxies like Segue 1 that have only a few hundreds of stars. Uncertainties in mass determination reach factors from two to thousands, from the more to the least massive galaxies respectively. There are also several attempts for indirect detections of DM, which are conducted in the MW and in nearby dwarf galaxies. These have substantial implications, including in the area of cosmology, and in studies of the nature of dark matter and its alternatives.
The goal of this symposium is to gather specialists of the many different types of galaxies from the Local Group to distant galaxies. It aims at including observers and theoreticians, in order to discuss the status of their dynamical mass estimates, to present novel approaches to observations, modeling and theory, as well as to establish common strategies for future advances. The meeting will give a large room to methodological considerations, including on the different dynamical methods used to estimate DM content. It will start with a session on our Galaxy and its neighborhood, including indirect detection of DM, on M31 and its halo, and then on nearby galaxies. Comparison with studies made at large distances will be discussed in a session at the end of the Symposium. Much time will be allocated for discussions, which is essential for collecting the opinions and contributions of all participants.
We endeavor to address the following questions, among other proposed by the attendance:
1. What is the amount and distribution of luminous and dark substructure around galaxies in the Local Group? Cosmological simulations predict a hierarchical accretion of smaller structures onto larger ones, whose Dark Matter (DM) halos have almost universal density profiles and with a well-defined substructure mass function. Hence, the detection and characterization of substructures around the massive galaxies in the Local group enables crucial tests of predictions from the Standard Cosmological Model. The quantitative study of substructures can either decree the success of CDM cosmology or lead to a rethinking of the whole paradigm, perhaps introducing more exotic forms of Dark Matter.
2. How is mass accreted onto galaxies and deposited in different radial regions? The hierarchical assembly of massive galaxies, as well as gas-dynamics, can lead to complex and extended star formation, as testified by color gradients and multiple populations of Globular Clusters (GCs) and Planetary Nebulae (PNe) out to the outskirts of their hosts. The chemistry and extended kinematics of starlight and tracers around galaxies in the local group can then be used to probe their DM halos and accretion histories in detail.
3. What is the role of baryons in shaping the central parts of galaxies? The buildup of stellar mass in the central regions of galaxies can severely alter the simple predictions from DM-only simulations. Depending on the timescales involved and amount of feedback from star-formation and Active galactic Nuclei/black holes, the initial DM profiles can be made steeper or shallower. Also, the accretion of circum-galactic gas can happen in radically different modes depending on the mass of the central object. These processes all concur to shape the final mass-profiles and stellar densities of galaxies in the Local group.

(e) Venue and preferred dates:
Antananarivo
Dates: TBC (Spring to Fall 2026)

LOC members (preliminary):
Solohery Randriamampandry (Chair)
Zara-Nomena Randriamanakoto
Sambatra Andrianomena
Roger Ianjamasimanana

(f) Editor: main Editor to be defined
Cambridge University Press

Expected fees: 400 €/participants, with support from IAU grants
Total persons in venue: about 75
Possibility to include on-line participants