Moving Groups as imprints of the non-axisymetric components of the Milky Way

Abstract

One of the most intriguing features of the stellar velocity distribution in the solar neighbourhood is the existence of moving groups. At present, the origin of these kinematic structures is far from completely understood although it is more than 140 years since they were discovered. Nowadays, several explanations for their origin are considered: cluster disruption, dynamical effects induced by the non-axisymmetric components (spiral arms and bar) of the Milky Way (MW), remnants of past accretion events and external dynamical effects on the disc resulting from interaction events. It has already been shown that the effects of the bar and the spiral arms of the MW can induce kinematic groups in the local stellar velocity distribution. <br/><br/>The aims of this thesis are: i) to characterise the observed moving groups, establishing observational insights into their origin, and ii) to explore to what extent we can use the kinematic imprints to constrain the large-scale structure of the MW and its recent evolution. <br/><br/>To undertake the observational study we have compiled an extensive compendium with the best available astrometric and photometric data for more than 24000 stars in the solar neighbourhood. We have applied the Wavelet Denoising multiscale technique to this sample to characterise and analyse the moving groups in the U-V-age-[Fe/H] space. We find that the dominant kinematic structures in the U-V plane are the branches of Sirius, Coma Berenices, Hyades-Pleiades and Hercules. From the large spread of ages and metallicities inside them, we refuse the models that relate their origin to cluster disruption. The Hercules branch is more conspicuous in the region of inner galactocentric radius, and for a region near the Sun in comparison to a region that is located further in the direction of rotation. For Hyades-Pleiades, Coma Berenices and Sirius the more negative the V component, the higher the mean metallicity. The Hercules branch does not follow this correlation and has a higher metallicity dispersion. <br/><br/>On the other hand, we have performed test particle simulations with the <br/>ame model to explore the phase space available to the local stellar distribution. This is a specific potential model for the MW which is very flexible and has been tuned to reproduce some recent observational constraints. We have also considered a great variety of initial conditions and integration procedures. This methodology has allowed to study the imprints of the spiral arms and the bar on the velocity distribution at different disc positions. We have also used a method to ascertain the regularity of the orbits in the U-V plane. Our results show that the bar and the spiral arms create strong kinematic imprints on the velocity distributions. When the spiral arms and the bar act together, individual imprints of each component can be still identified in the final velocity distributions. The spiral arms crowd the kinematic region of Hercules and not only the bar, as traditionally believed. The arms also induce slightly tilted kinematic branches that resemble some of the observed ones. The low angular momentum moving groups such as Arcturus can have an origin related to the bar acting on a relatively hot stellar disc, which introduces a new perspective on the interpretation of its extragalactic origin. <br/><br/>Our analysis of the observations and and simulations indicates that it is very feasible that some of the observed moving groups have an origin related to the spiral arms and the bar. We find that the induced stellar kinematics groups depend on the properties of the non-axisymmetric components. We discuss if it is currently possible to use the stellar phase space groups as constraints to the large-scale structure and evolution of the MW.<br/><br/>KEY WORDS: Galactic dynamics & kinematics, Milky Way, Solar Neighbourhood, Galactic Structure

<I>S'ha demostrat que els efectes de la barra i els braços espirals de la Via Làctia (MW) poden induir grups cinemàtics en la distribució de velocitats local. Els objectius d'aquesta tesi són: i) caracteritzar els grups mòbils observats, aportant més coneixement al seu origen, i ii) explorar fins a quin punt podem utilitzar les empremtes cinemàtiques per restringir la estructura a gran escala de la Via Làctia. D'una banda hem aplicat la tècnica del Wavelet Denoising a un compendi de dades cinemàtiques, edats i [Fe/H] per a més de 24000 estrelles de l'entorn solar. Hem trobat que les estructures cinemàtiques dominants en el pla cinemàtic són les branques de Sirius, Coma Berenices, Hyades-Pleiades i Hercules. Degut a la gran dispersió d'edats i metal·licitats dins de les branques, refusem els models que expliquen els grups mòbils com a restes de cúmuls estel·lars. La branca d'Hercules és més rellevant a la regió de radis interiors i en la regió propera al Sol en comparació a la zona cap a la rotació Galàctica. Per Hyades-Pleiades, Coma Berenices i Sirius, com més negativa és la component V de la branca, més gran és la seva metal·licitat mitja. Per altra banda, hem realitzat simulacions de partícules test en un potencial flexible de la MW, consistent amb diverses restriccions observacionals. El nostre treball ha mostrat que la barra Galàctica i els braços espirals creen fortes empremtes cinemàtiques. El braços poblen la regió cinemàtica d'Hercules i no únicament la barra. Els braços també indueixen branques cinemàtiques lleugerament inclinades semblants a les observades. Els grups a baix moment angular com Arcturus poden ser creats per la barra actuant sobre un disc calent i no necessàriament tenir origen extragalàctic. En les simulacions on els braços espirals i la barra actuen alhora, identifiquem empremtes individuals de cada component en les distribucions de velocitat finals. Trobem que els grups cinemàtics induïts depenen de l'estructura del model i de les condicions inicials dels experiments. Finalment discutim si es actualment possible utilitzar els grups estel·lars de l'espai fase com a restriccions de l'estructura a gran escala i evolució de la MW. </I>