Statistical detection of tidal streams generated by globular clusters and their application to measure the axis ratio of the Milky Way dark matter halo

Abstract

In this thesis we have presented a new method to search for tidal streams generated by globular clusters in the Gaia catalogue. This statistical method is based on the maximum likelihood technique to distinguish stream stars from foreground field stars. It uses the phase- space location of each star and a realistic phase-space model of the stellar stream and of the Milky Way galaxy to evaluate a statistical test to determine whether the stream exists or not. If the statistical test is positive, the method is designed to select the stars with the highest probability of being members of the stream using their phase-space coordinates. Finally, in our final selection, we include only those that are colour and magnitude compatible with the progenitor cluster. We apply the statistical method to Gaia Data Release 2. We detect a clear tidal stream generated by the globular cluster M68 (NGC 4590) that spans the entire North Galactic hemisphere. A section of this stream coincides with a previously discovered stream named Fj¨orm. We select 115 stream stars that are colour and magnitude compatible with the progenitor cluster. We also detect the tidal stream generated by the globular cluster NGC 3201 extending about 140 deg in the sky. A section of the trailing arm of this stream coincides with the previously catalogued stream Gj¨oll. We select 170 highly likely member stars of this stream. We have also applied the statistical method to identify 126 stars belonging to the already known Palomar 5 stellar stream. We use these three streams to constrain the shape of the dark halo of the Milky Way within 20 kpc from the Galactic centre. We assume an axisymmetric mass model of the Galaxy constructed from the sum of two exponential profiles for the thin and thick disc, a flattened bulge, and a two power-law density profile for the dark halo. We compute the best- fitting values of the parameters characterising the model using a Bayesian statistical method, which includes constraints on the mass distribution, the circular velocity curve of the Milky Way, and several kinematical and dynamical properties of the Galaxy in the vicinity of the Sun. We focus our analysis on the shape of the dark matter halo out of the plane of the disc. In particular, we study the halo density axis ratio q. The stream of NGC 3201 does not provide a strong constraint on this parameter giving q = 2.06 ± 0.93. The stream of M68 favours a moderately prolate dark halo but compatible with a spherical shape of q = 1.14+0.21. Palomar 5 requires a prolate or oblate halo of q = 1.01 ± 0.09 which fits well with the spherical shape. All three streams together favour a prolate dark halo of q = 1.06 ± 0.06 compatible with the spherical shape. These results are compatible with previous studies using stellar streams and recent studies using different fitting methodologies and based on different observational data, such as globular clusters or halo stars in equilibrium with the dark halo. These results are in tension with cosmological simulations in which the influence of baryons on the distribution of dark matter have been included. In general, the simulations predict oblate halos for Milky Way-like galaxies, with axis ratios typically in the range q ∈ [0.6 - 0.8].

En aquesta tesi s’ha presentat un nou mètode estadístic per cercar rieres de marea generades per cúmuls globulars en el catàleg Gaia. Aquest mètode es basa en la tècnica de màxima probabilitat per distingir les estrelles de la riera de les estrelles que pertanyen a la Via Làctia. L’aplicació d’aquest mètode a la segona versió del catàleg de Gaia (Gaia Data Release 2) ha permès detectar una riera generada pel cúmul globular M68 (NGC 4590). S’han seleccionat 115 estrelles de la riera compatibles en color i magnitud amb el seu cúmul progenitor. També ha permès detectar la riera de marea generada pel cúmul globular NGC 3201. S’ha seleccionat 170 estrelles membres d’aquesta riera. També s’ha aplicat el mètode per identificar 126 estrelles que pertanyen a la prèviament coneguda riera de Palomar 5. Aquestes tres rieres s’han utilitzat per constrènyer la forma de l’halo de matèria fosca de la Via Làctia. L’anàlisi s’ha concentrat en la forma de l’halo fora del pla del disc, en particular, en la relació d’eixos que caracteritzen la densitat de l’halo q. La riera de NGC 3201 no proporciona un constrenyiment significatiu obtenint q = 2.06 ± 0.93. La riera de M68 afavoreix un halo moderadament prolat però compatible amb una forma esfèrica de q = 1.14+0.21. La riera de Palomar 5 requereix un halo prolat o oblat de q = 1.01 ± 0.09 que encaixa bé amb la forma esfèrica. Les tres rieres juntes afavoreixen un halo de matèria fosca prolat de q = 1.06 ± 0.06 compatible amb la forma esfèrica. Aquests resultats són compatibles amb estudis anteriors que utilitzen dades observacionals aplicant diferents metodologies, però estan en tensió amb els resultats obtinguts amb simulacions cosmològiques en les quals s’ha inclòs la influència dels barions. En general, les simulacions prediuen halos oblats per galàxies semblants a la Via Làctia, amb valors típicament en el rang q ∈ [0.6 - 0.8].