Analysis of tidal interactions in IllustrisTNG galaxy clusters

Abstract

Each galaxy is unique in its physical and morphological properties, for which the surrounding environment plays a significant role in the observed diversity. Galaxies in dense environments, such as groups and clusters, frequently interact gravitationally with each other. In contrast, in sparse, isolated environments, there are fewer interactions. These forces felt by gravitational encounters with perturbers are called tidal forces. From an evolutionary aspect, studying tidal forces is a fascinating topic because they have the power to start or stop star formation in galaxies by tearing or compressing the interstellar matter of the galaxy. Strong tidal forces can change the whole morphology of a galaxy by increasing its internal velocity dispersion, transforming disk-like galaxies into spheroidal-like galaxies, and in some cases, even destroying them through shredding.

Although observational methods and instruments are evolving and more data is available, studying the effects of tidal forces purely through observation is challenging as the data can only capture short snapshots in time. Due to the long time scales of the galactic phenomena, one needs to study them using numerical N-body simulations. When it comes to simulations, the IllustrisTNG project is one of the state-of-the-art numerical cosmological simulations, including both the gravitational dynamics of baryonic and non-baryonic components as well as detailed recipes for the physical processes of baryonic matter. In this thesis, I use the IllustrisTNG galaxy cluster data (TNG100 at z=0) together with my self-written test-particle simulation code to study the effects of tidal interactions.

In my simulations, I integrate orbits of cluster galaxies using analytical models for the cluster potential and perturbing galaxies by fitting NFW and Hernquist profiles for the TNG clusters and cluster members, respectively. For the galaxy of interest, I use a disc made of test-particles. In order to have realistic cluster properties such as galaxy densities and velocity dispersion, I use the initial values for positions and velocities of galaxies selected from TNG100 clusters. My simplified simulations allow analyses of the effects and strengths of the tidal interactions that the galaxy of interest experiences in different types of orbits.

Interesting dynamical factors studied in this thesis include the impact of disc’s rotational direction on tidal radii, the frequency of close encounters between galaxies, a potential drift of more massive galaxies towards the group’s centres, and analysis of axial ratios of TNG100 galaxies and whether they are elongated towards the cluster centres. Analyses of these factors led to the following findings and demonstrations throughout the thesis. 1) The significance of the rotational direction of the disc was demonstrated using test-particle code, showcasing that retrograde orbits are more durable than prograde orbits against perturbing tidal forces. 2) Using the test-particle code, the impact of dynamical friction on galaxies causing them to drift towards the centre of the cluster was studied, along with a demonstration of how dwarf galaxies could be shredded into the intra-cluster medium at central regions of the cluster. 3) Simulating TNG100 galaxies further in time led to the finding that physically close encounters between galaxies are rather rare events. However, in the case of dynamically significant encounters, a fast and significant deformation of a galaxy or galaxies could happen. 4) Galaxies from the eight selected TNG100 clusters have a surprisingly strong preference for elongation direction towards the clusters’ centres, with this elongation preference being stronger for dark matter particles than stellar particles.

At the end of this thesis, in Section 6, future research and development of the test-particle simulation code are discussed.

Tämä pro gradu -tutkielma tarkastelee vuorovesivuorovaikutusten roolia galaksien morfologiseen kehitykseen erilaisissa ympäristöissä. Vuorovesivoimat ovat siitä mielenkiintoinen tutkimuskohde, että ne pystyvät aloittamaan tai lopettamaan tähtien muodostumisen galakseissa repimällä tai puristamalla galaksin tähtienvälistä ainetta. Vahvat vuorovesivoimat voivat muuttaa koko galaksin morfologiaa kasvattamalla sen sisäistä nopeushajontaa, muuttaen kiekkomaisia galakseja pallomaisiksi ja joissakin tapauksissa jopa tuhoamaan ne täysin.

Koska galaktiset ilmiöt tapahtuvat pitkällä aikavälillä, niiden tutkiminen pelkästään havaintoihin pohjautuen on haastavaa. Tästä johtuen, tutkielmassa hyödynnetään N-kappaleen numeerisia simulaatioita. IllustrisTNG-projekti on edistyksellinen numeerinen kosmologinen simulaatio, johon kuuluvat barionisen ja ei-barionisen aineen gravitaatiodynamiikka. Tutkielmassa on käytetty IllustrisTNG:n galaksiklusteridataa (TNG100 z=0) yhdessä itse kirjoitetun testihiukkassimulaatiokoodin kanssa vuorovesivuorovaikutusten tutkimiseen.

Tutkielmassa tehtiin seuraavat havainnot. 1) Kiekkojen pyörimissuunnan merkittävyys havainnollistettiin, näyttäen että retrogradiset radat vastustavat ulkoisia häiriöitä progradisia ratoja paremmin. 2) Dynaaminen kitka saa galaksit ajautumaan kohti klusterin keskustaa. Galaksijoukon keskustassa kääpiögalaksit voivat hajota liittäen nämä osaksi galaksijoukon väliainetta. 3) Galaksien väliset lähikohtaamiset ovat harvinaisia, mutta dynaamisesti merkittävissä kohtaamisissa yksi tai useampi galaksi voi kokea morfologisesti suuria muutoksia. 4) Galakseilla havaittiin olevan erityisen vahva taipumus venyä kohti galaksijoukon keskustaa, ilmiön ollessa vahvempi pimeän aineen jakaumalle, kuin tähtien jakaumalle.