Letters of Intent received in 2022

LoI 2024-2190
Measures of luminous and dark matter in galaxies across time

Date: 8 August 2024 to 9 August 2024
Category: Focus meetings (GA)
Location: Cape Town, South Africa
Contact: Sabine Thater (sabine.thater@univie.ac.at)
Coordinating division: Division J Galaxies and Cosmology
Other divisions:
Co-Chairs of SOC: Gauri Sharma (University of the Western Cape)
Sabine Thater (University of Vienna)
Jonathan Freundlich (Observatoire Astronomique de Strasbourg)
Co-Chairs of LOC: Gauri Sharma (University of the Western Cape)
Ed Elson (University of the Western Cape)
Lerothodi Leeuw (University of the Western Cape)



1) Robust mass measurement techniques for local and high-z galaxies
2) Dynamical models with different kinematical tracers
3) Constraining dark matter across the electromagnetic spectrum
4) Cosmological simulations: validating and connecting galaxy mass measurements across time
5) Interplay between baryonic processes and dark matter
6) Cold dark matter paradigm: challenges at galactic scales
7) Impact of robust mass determination on dark matter searches
8) Utilising archival data to unravel the nature of dark matter
9) Strategies for upcoming observing facilities in the context of dark matter
10) Alternatives to cold dark matter cosmology



In 1933, Fritz Zwicky used the Doppler velocities and luminosities of galaxies in the Coma cluster to estimate the total mass. He reported an enormous discrepancy between, respectively, the gravitating and luminous mass of the system. This study raised the question of a 'missing mass' and coined the term "dark matter". Although the discrepancy was overestimated, still a factor of six remains today. In the late 1970s, Vera Rubin showed that the rotation velocity of disc galaxies does not decrease beyond their visible domain. That is, the gravitating mass increases as a function of radius, suggesting that "galaxies are surrounded by a dark matter halo which extends much further than their visible matter." In the early 1980s, theoretical models of structure formation supported these observational findings. Since then, dark matter is rooted in theory and has become one of the main pillars of the current cosmological model.

In the last half-century, the field of astronomy has provided a large number of detailed observations, across the electromagnetic spectrum, of galaxies, clusters, and the cosmic web. These observations allow us to measure velocities of gas and stars to trace the gravitating mass and luminosities to infer the visible mass, along with maps of the distribution and chemistry of gas and stars in galaxies. All these observations and the associated scaling relations between the properties of galaxies indicate the presence of dark matter, which acts via gravity and fills 27% of the Universe, whereas baryonic matter only accounts for about 4%. Moreover, dark matter typically dominates the total masses of galaxies, but its nature remains unknown.

In parallel to the boom in observations, the advancement in technology has enabled astrophysicists and cosmologists (1) to simulate the Universe from large to small scales, (2) to construct detailed dynamical models of galaxies, and (3) to compare both these simulations and models to observations in a statistically robust way. From these outcomes, the current consensus is that without dark matter, it is impossible to explain various phenomena that occur in the Universe, such as the amplitude of baryonic acoustic oscillations, the formation and evolution of cosmological structures, and the motion of stars and gas on galactic scales. To explain the nature of dark matter, many particle candidates have been proposed, from ultra-light axions to massive compact objects like primordial black holes. However, they have been unsuccessful in representing the nature of dark matter, opening the ground for alternative theories of gravity.

Now, it is obvious to ask: What is lacking in overall efforts? How do we improve upon them as a community, and what should be the main focus for the next five years? Especially when the era of precision astrophysics and cosmology provides an enormous amount of quality data that was impossible to obtain in the past. For example, new generation radio telescopes like MeerKAT and SKA will now allow the study of neutral hydrogen up to higher redshifts; spectroscopic surveys with JWST and ALMA can resolve the hot and cold gas kinematics of galaxies up to the peak of cosmic dawn; upcoming gamma-ray facilities such as CTA, in combination with the Fermi-LAT in orbit, will put at test the most preferred dark matter particle models. But, are we sufficiently prepared to interpret these data to enlighten the nature of dark matter? In particular, do we have (1) the models to robustly measure and disentangle the distribution of luminous and dark matter, and (2) the simulations to accurately mimic galaxy-scale baryonic processes and their impact on the total mass distribution? If so, then what keeps hindering the searches for the dark matter particle?

In line with the aforementioned considerations, in IAU GA2024, we plan to draw the attention of the astrophysical community toward the measures of luminous and dark matter in galaxies across cosmic time. Robust estimates of galaxy masses are paramount for investigating the nature of dark matter: mass is the primary driver of the formation and evolution of structures in the Universe, and, to the best of our current knowledge, dark matter interacts with baryonic matter only through gravity. Furthermore, by combining the knowledge of gravitating masses with galaxy stellar and gas mass estimates from complementary observations, we can provide new insights into the interplay between baryonic and dark matter to help unravel the nature of dark matter.

In the proposed focus meeting, we plan to combine different galaxy mass tracers covering a broad range of the electromagnetic spectrum (gamma-radio) and across cosmic time. The wide wavelength range will allow us to cover the different galactic scales: from inner galaxy regions (including stellar kinematics, ionised and molecular gas kinematics, and strong lensing) to their outskirts (including atomic gas kinematics, weak lensing, systematic velocities of planetary nebulae, globular clusters, and satellite galaxies). Combining different tracers will allow us to measure the galaxy masses as well as constrain their amount, radial profile and even shape of dark matter haloes. Moreover, these galaxy mass measurements at different cosmic scales will give a snapshot of galaxy evolution in real time. In addition, we will discuss the abilities of cosmological simulations in validating and connecting galaxy mass measurements across scales and cosmic time. As such, this focus meeting aims to answer the following key open questions:

1) How consistent are galaxy mass measurements based on different tracers?
2) How robust are galaxy mass measurements against modelling assumptions and degeneracies?
3) How accurate are state-of-the-art cosmological simulations reproducing current galaxy mass measurements across scales and cosmic time?
4) Are dark matter halo properties the result of baryonic feedback, or are they due to the nature of dark matter itself?
5) What are the missing observations to understand the nature of dark matter?
6) What is the impact of having accurate mass measurements on current dark matter particle constraints, and how will this help us to strategically plan (and succeed with) future dark matter searches?
7) Do we need alternative theories of dark matter? If yes, how capable are they in representing galaxy mass measurements across scales and cosmic time?

SOCs: Gauri Sharma (SA), Sabine Thater (AT), Jonathan Freundlich (FR), Ed Elson (SA), Benoit Famaey (FR), Marie Korsaga (FR), Julien Lavalle (FR), Miguel A. Sánchez-Conde (ESP), Glenn van de Ven (AT), Alice Zocchi(AT),...

The proposed meeting dates are a suggestion but are flexible between 05.08.2024 - 16.08.2024.