Faculty

Multimessenger Astrophysics

Dark Universe

Nature of particle dark matter is still under intense debate, and depending on its particle property, it yields different distribution pattern at small scales (i.e., scales smaller than galaxies). If dark matter is made of "cold" particles as is conventionally assumed, then dark matter structure forms hierarchically from small to large scales. This means that a large dark matter halo contains many smaller subhalos. On the other hand, if dark matter is made of "warm" particles, then their higher velocity dispersion washes away smaller structures, suppressing the number of subhalos. By comparing with various observations, one can constrain various properties of dark matter particles.

I offer several projects on the studies of small-scale dark matter distribution as follows.
1. Inner cups at the center of halos and subhalos. It has been pointed out that a steep cusp at halo center might survive against merger and accretion processes. The presence of such cusps will increase the rate of annihilation of dark matter particles if they annihilate at all. Since halos are made of smaller subhalos, this means a strong enhancement of the rate of dark matter annihilation in each halo. The purpose of this project is to accurately compute the rate of annihilation by adopting the semi-analytical subhalo models, SASHIMI.


Dark matter distribution at small scales

Future gamma-ray telescopes such as CTA will allow comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external galaxies such as Andromeda. Measurements with the Fermi-LAT telescope revealed that the flux from Andromeda is confined to the inner regions of the galaxy and does not fill its galactic disk or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star-formation activity suggesting that the emission is not interstellar in origin. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of Andromeda by disrupted globular clusters or from the decay or annihilation of dark matter particles. This project will estimate the sensitivity of the upcoming CTA gamma-ray telescope to DM annihilation and MSPs at the Andromeda Galaxy. We will introduce a statistical framework for including systematic errors and estimate the consequent degradation in sensitivity. The morphology of the signal at very high energies might allow to distinguish the DM from MSPs hypothesis in the Andromeda galaxy.

A realistic assessment of CTA sensitivities to dark matter and millisecond pulsars in the Andromeda Galaxy

Understanding the nature of dark matter (DM) has become one of the most pressing questions in modern physics and cosmology. Evidence for its existence is exclusively based on its gravitational interactions, as we still know very little about its particle properties. Recent studies are investigating the possibility that DM particles experience collisions against each other. The rate of such collisions can be constrained from observed galaxy density profiles, spanning from dwarf galaxy scales (e.g. Correa 2020) to cluster scales (e.g. Sagunski et al. 2020). 

However, a theoretical model for the DM particle interactions, that connects the recent observational estimations at various galaxy scales, is still missing. The goals of this research project are to derive such model arguing that DM exists in a ‘hidden sector’, where forces between DM particles are mediated by analogues to electroweak or strong forces. The following steps will be to implement the model in a numerical simulation of galaxy formation, produce simulation runs using the supercomputer cartesius, and investigate observational signatures of self-interacting DM.

Numerical simulations have become an almost indispensable tool in astrophysical research. By carrying out this project, the student will learn about cosmological simulations of galaxy formation, parallelisation and high-performance computing. The student will also connect particle physics modelling with cosmology while building a solid understanding of structure formation.

__References__

[Correa (2020)](https://arxiv.org/abs/2007.02958)

[Sagunksi et al. (2020)](https://arxiv.org/abs/2006.12515)


Galactic clues to the nature of dark matter

In recent years, discussions on statistical properties of the all-sky gamma-ray data have been found very efficient to constrain properties of astrophysical sources and dark matter that can contribute to the gamma-ray background. Thus far, however, people argued only up to a second-order moment, i.e., variance or the angular power spectrum, of the distribution of the photon counts. There is however no need to stop there, and therefore, it is natural to question what extra information the next third-order moment, skewness, would bring on top of what has been achieved. Maybe using the skewness or the angular bi-spectrum will be essential in revealing the nature of particle dark matter still hidden in the existing data. Master student will explore this.

Higher order statistics of the gamma-ray data

With the increased sensitivity of gamma-ray detectors such as Fermi-LAT the number of presently known gamma-ray globular clusters has grown by a factor of ~2 in the last year. The new detections are beginning to provide clues about the origin of the high-energy radiation in the form of emerging patterns and correlations among observed quantities such as gamma-ray luminosity, stellar mass and interstellar radiation energy density. But there are still many questions about the mechanisms of emission and intracluster environmental properties. This project will re-examine these emerging patterns and correlations by carefully studying the population of currently undetected globular clusters.

Understanding the high energy emission mechanisms in globular clusters of the Milky Way

Dwarf galaxies are dark matter (DM) dominated systems, and therefore  provide ideal laboratories to study not only the DM halo and galaxy structure, but also the nature of DM. It has been proposed that DM particles experience collisions against each other, and to further constrain the rate of these collisions, recent studies (e.g. Valli &  Yu 2018) have derived a semi-analytic approach for describing          self-interacting DM halo profiles, called the Jeans model. This          approach is only valid in the regime where the mean free path is          larger than the typical size of the collisional region (corresponding          to a small collision cross section). A recent study (Correa 2020),          however, is predicting that the dwarf galaxies that orbit around the          Milky Way are in gravothermal core-collapse, a state where DM-DM          collisions increase the density in the central regions. This scenario          invalidates the Jeans model analysis of these systems, as well as the          current upper limits of the DM collision cross sections.

The goals of this research project are to derive an alternative model          for describing self-interacting DM halo profiles under gravothermal          collapse, use the latest observational data from the local dwarfs to constrain their density profile and provide new upper limits to the DM collision cross sections. During the first part of the project the student will study the density profile of self-interacting DM haloes using cosmological simulations of galaxy formation.

By carrying out this project, the student will learn about the          observations of galaxies from the local group, cosmological          simulations of galaxy formation, as well as the theory of structure   formation.

__References__

Valli & Yu (2018)

Correa (2020)


Confronting self-interacting dark matter with observational data

Light Dark Matter (DM) [with a mass between 1 MeV to 1 GeV], provides an intriguing theoretical and phenomenological possibility within current DM research. Candidates for which the DM interaction cross-section is velocity-suppressed are well motivated but need better study. Furthermore, the scarcity of experiments in the MeV-GeV region makes indirect detection via gamma rays difficult. This project aims to address these issues by studying lower-energy X-ray data from various telescopes. Crucially, we plan to include state-of-the-art DM distribution models of dark matter in the center of the Galaxy and the contribution from inverse Compton scattering on galactic radiation fields and the CMB. We expect to obtain much stronger constraints than in previous studies for DM masses above 20 MeV.


Shin’ichiro Ando

Research Areas

Masters Thesis Projects