Milky Way-like galaxy simulations in the Gaia era: disk large scale structures and baryonic content

Abstract

Simulations have shown to be one of the best tools to study properties of galactic large scale structures and their effects on to the local kinematics of stars. The aims of this thesis are: i) To obtain realistic N-body models that allow us to analyse kinematics, dynamics and internal structure of non-axisymmetric components in galaxies, ii) to learn how to control numerical effects and also how to distinguish them from the proper physical ones, iii) to find observable parameters from stellar kinematics that can give us information about formation, evolution and nature of the large scale structures in galaxies and, iv) to test which of such methods can be used to distinguish among spiral arm natures in real galaxies.

In this thesis, using high resolution simulations obtained with different codes and initial condition techniques, we find that exist two different behaviours for the rotation frequency of transient spiral arms like structures. Whereas unbarred disks present spiral arms nearly corotating with disk particles, strong barred models (bulged or bulge-less) quickly develop a bar-spiral structure dominant in density, with a pattern speed almost constant in radius (Roca-Fàbrega et al. 2013). Preliminary results also indicate that particles in barred models move inside the spiral structures.

A second result we present in the thesis has been obtained mapping the kinematics of stars in the simulated galaxy disks with spiral arms using the velocity ellipsoid vertex deviation (lv). For this study we have used both test particle and high resolution N-body simulations. What we have found is that for all barred models, spiral arms rotate closely to a rigid body manner and there the vertex deviation values correlate with the density peaks position bounded by overdense and underdense regions. However, the most interesting result is that In such cases, vertex deviation sign changes from negative to positive when crossing the spiral arms toward disk rotation, in regions where the spiral arms are in between corotation (CR) and the Outer Lindblad Resonance (OLR). By contrast, when the arm sections are inside the CR and outside the OLR, lv changes from negative to positive. We propose that measurements of the vertex deviations pattern can be used to trace the position of the main resonances of the spiral arms (Roca-Fàbrega et al. 2014).

Finally we present a new cosmological Milky Way like galaxy simulation that includes both the collisionless N-body and also the gas components. This simulation has been obtained using the adaptive mesh refinement (AMR) N-body code ART (Kravtsov et al 1999) plus the hydrodynamics and physical processes presented by Kravtsov et al 2003. The MW like system has been evolved inside a 28 Mpc cosmological box with a spatial resolution of 109 pc. At z=0 the system has an Mvir = 7.33·10^11Msun. We have observed how a well defined disk is formed inside the dark matter halo and the overall amount of gas and stars is comparable with MW observations. Several non-axisymmetric structures arise out of the disk. We have also observed that a huge reservoir of hot gas is present at large distances from the disk. Gas column density, emission and dispersion measures have been computed from inside the simulated disk at a position of 8 kpc from the center and in several different directions. Our preliminary results reveal that the distribution of hot gas is non-isotropic according with observations. After a careful analysis we confirm that due to the anisotropy in the gas distribution more than 50 random observations of different sky regions are needed to recover the real distribution of hot gas in the galactic halo.

En aquesta tesi, fent servir simulacions d'alta resolució i mitjançant diferents codis i tècniques de generació de condicions inicials hem pogut detectar que existeixen dos tipus diferents d'estructura espiral segons el seu perfil de rotació. Primer, en el cas de galàxies no barrades, les estructures roten a la mateixa velocitat que el disc, i segon, en el cas de galàxies barrades, aquestes estructures roten com un sòlid rígid.

El segon resultat que presentem en aquesta tesi s'ha obtingut analitzant la cinemàtica estel·lar en simulacions de galàxies espirals. Per dur a terme aquest segon estudi hem utilitzat aproximacions analítiques i simulacions d'N-cossos pures i de partícules test. A partir d'aquests anàlisis hem pogut constatar que la desviació del vèrtex és un bon traçador de la posició de les estructures de densitat i també que el canvi de signe que pateix en creuar els pics de densitat de les espirals i les regions inter-braç ens permeten conèixer la posició de les principals ressonàncies, és a dir, la corrotació i la ressonància externa de Lindblad.

Finalment, desprès de l'estudi exhaustiu dels codis i els processos físics relacionats amb la física de la component gasosa de les galàxies hem aconseguit obtenir una simulació cosmològica d'una galàxia molt semblant a la Via Làctia. Tot just hem començar a analitzar-la però ja hem obtingut resultats molt interessants tals com la presència de gran quantitat de gas calent a la regió de l'halo de matèria fosca o l'aparició de dues barres desalineades 90 graus a la zona del disc galàctic, una de jove, procedent del disc i una de vella, que és el fòssil d'una fusió de dues galàxies el·líptiques a un desplaçament cap el vermell de 3.