RSAA Colloquia / Seminars / Feast-of-Facts: Friday, 17 March 2017, 14:30-14:50; Duffield Lecture Theatre

Giuliano Iorio

"A First Gaia look at the inner halo"

The stellar halo hosts only a small fraction of the baryonic component of the Galaxy (< 1%), however the study of its structural properties is fundamental both to understand the formation and evolution of the Milky Way and to study the Galactic dynamics. The easiest way to infer the spatial distribution of objects in the halo is counting the stars that are tracers of the halo population as for example RRLyrae (RRL). Results of past works are only in roughly agreement and are limited in the volume sampling of the halo by the footprint of the used survey. In this context, the advent of GAIA offers a dramatic improvement. Although the final catalogue of variable stars will be released only by the end of the mission (about 2020), I present a method to select a sample of RRLs by using only time integrated quantities from the first Gaia data release. I analysed the sample to retrieve the properties of the the inner halo (Rg<28 kpc): its shape is well reproduced by a oblate ellipsoid flattened along the z-axis (axes ratio about 0.55). while the density follows a single power-law with index about -2.7. There is no evidence of a change of slope of the density, but for the first time we are able to show directly on the data a change of halo flattening at large radii (Rg>20-25 kpc). Finally, I discuss the possible extensions and improvement of this work that can be obtained with the future GAIA data releases.