Extragalactic stellar streams as astrophysical tools to decipher dark matter

Project Information

EXTRADARK

Grant agreement ID: 101103129

DOI

10.3030/101103129

Project terminated on 31 May 2024

EC signature date 17 April 2023

Start date 15 December 2023

End date 14 December 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 230 774,40

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - EXTRADARK (Extragalactic stellar streams as astrophysical tools to decipher dark matter)

Reporting period: 2023-12-15 to 2025-12-14

Summary of the context and overall objectives of the project

Dark matter constitutes over 80% of the universe's matter, yet its true nature remains a mystery. Understanding dark matter is essential because it significantly influences the formation and structure of the universe. It affects how galaxies form and behave and offers insights into the fundamental laws of physics. It's called "dark" because it doesn’t emit, absorb, or reflect light, making it invisible and difficult to detect. However, we know it exists due to its gravitational effects on visible objects like stars and galaxies.

Dark matter forms large halos around galaxies, with many smaller clumps called subhalos. The characteristics of these subhalos depend on the properties of dark matter particles. If dark matter is made of light particles, it moves quickly in the early universe and forms fewer clumps. Conversely, if it's composed of massive particles, it moves slower and clumps together more easily. Since dark matter doesn’t interact with light, we can’t see it directly. Instead, we study its effects on visible matter, such as stars.

In this project, I will investigate stellar streams, which are long, thin groups of stars pulled out of star clusters or small galaxies by the gravitational forces of a larger galaxy. Imagine a string of pearls stretched into a line. These stellar streams help us study the invisible dark matter surrounding galaxies, as the way the stars are pulled and stretched reveals important information about the dark matter's presence and distribution. New telescopes like the Vera C. Rubin Observatory, Euclid, and NASA's Nancy Grace Roman Telescope will soon discover many more streams in galaxies beyond our own. My project aims to fill the gaps in our current knowledge so we can make sense of this new data and extract the most information about dark matter from stellar streams.

We don't know how many streams exist in the outer regions of galaxies or their exact locations. This information is crucial for comparing theoretical models with actual data. In this project, I'll predict the number and location of streams in various galaxies, helping future observations focus on the right areas. By predicting where thin streams from globular clusters are in external galaxies using theoretical models and simulations, I will provide a foundation for understanding the number of streams per galaxy.

Additionally, we need to understand how to use streams in other galaxies to learn about dark matter. I'll study how multiple streams can provide better constraints on dark matter properties than individual streams. This involves creating methods to analyze stream data and predict dark matter distributions. Using simulations and theories, I'll develop tools and methods that can be used by the wider scientific community to understand how streams in different types of galaxies can reveal dark matter properties.

Finally, to make meaningful comparisons, we need data from many galaxies, especially dwarf galaxies. I'll develop models to predict how streams form and evolve in dwarf galaxies, considering different dark matter scenarios. This will prepare us to interpret new data from upcoming observations and understand how they fit with existing dark matter theories. By creating these models, I'll help us anticipate what to expect from future observations and how they align with current dark matter paradigms.

By addressing these gaps, my research will advance our understanding of dark matter and enhance our ability to interpret new astronomical data.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Since I ended my Marie Curie Fellowship after 5.5 months, I only filled the first gap and completed the first scientific objective, which was to predict where thin streams from globular clusters reside in external galaxies using theoretical models and simulations.

Thin star streams from gravitationally bound groups of ~50,000 stars (globular clusters) are excellent tools for detecting small dark matter clumps. Analyzing all star streams together will help us understand the distribution of dark matter and its properties. To investigate how many and where thin streams from globular clusters exist, we used a model to predict the number, size, and locations of dissolved globular clusters in galaxies like the Milky Way. The model builds upon previous studies which simulate the formation and evolution of individual globular clusters. This model helps us create detailed mock catalogs of GCs with properties like mass, age, metallicity, positions, and velocities that match real GCs. We then use these catalogs to model star streams in our study. The catalogs are based on an analytical model of globular cluster formation and evolution in selected galaxies from large scale simulations of our Universe. This model includes a detailed process of how tidal forces break up clusters, which shapes their initial masses to match current observations. The model works in four steps:

1) Formation of globular clusters: globular clusters form when their parent galaxy undergoes a major merger or rapid growth.
2) Cluster sampling: We use observed relationships to calculate the gas mass and metallicity of the parent galaxy.
3) Particle assignment: The total mass of the new clusters is assumed to be proportional to the cold gas mass, and their metallicity is inherited from the parent galaxy.
4) Tidal disruption: The model estimates the time it takes for clusters to break apart, which is crucial for accurately modeling the star streams that come from these clusters.
In essence, this model helps us understand how globular clusters form, evolve, and eventually break apart, providing a reliable basis for studying star streams.

We also created simulated star streams from these clusters based on their history, formation time, mass, and metal content.

We found that a Milky Way type galaxy, should have around 10,000 fully dissolved clusters, of which about 9,000 dissolved in the central bulge and are now fully mixed, while the remaining 1,000 still exist as distinct stellar streams in the Galaxy. This means our current count of about 80 globular cluster streams in the Milky Way is far too low. Beyond 15,000 light-years from the center of the galaxy, we're missing hundreds of streams, mostly from clusters that merged with our galaxy. Deep photometry from the Rubin Observatory could detect these streams, even the distant ones beyond 75,000 light-years. We also expect the Andromeda galaxy to have many streams between 30,000 and 100,000 light-years from its center. Future surveys will likely find many more star streams from globular clusters, which can help in searching for dark matter clumps, as these clumps leave signatures of gaps in streams if the clumps pass near a stellar stream.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During this fellowship, I mapped the missing stellar streams in Milky Way type galaxies, and predicted how many streams are there in the outskirts of galaxies. This filled gap1 in the propsal. Here are the main findings:

-There are over 10,000 completely dissolved globular clusters in galaxies like the Milky Way, but around 1,000 still exist as clear star streams today.
-We have observed less than 10% of these dissolved GC streams in the Milky Way. There are hundreds of missing streams within 30,000 light-years from the center, and dozens more beyond that distance.
-In the outer regions of galaxies like the Milky Way (beyond 30,000 light-years), the existing GC streams come from clusters that merged with the galaxy.
-The Large Synoptic Survey Telescope will be able to detect even the most distant missing GC streams.
- We expect to find many GC streams in the outer regions of the Andromeda galaxy (M31) and nearby dwarf galaxies. Future observations with the Nancy Grace Roman Space Telescope should be able to detect these streams.

With this work, we now know that many stellar streams exist in the outskirts of the Milky Way galaxy, and in the outskirts of other galaxies. These streams have yet to be detected. With this work, we have shown that future surveys can detect these streams. Since these streams are sensitive to the clumping of dark matter, which is sensitive to the properties of the dark matter particle, this work is an important step towards using stellar streams for dark matter science.

Dark matter's affect on stellar streams