Abstract

This workshop constitues the third of a series, that we have renamed as « October in Paris Series » and that we have been organizing starting in 2023 (with a workshop entitled « Milky Way research : connecting the near and far field ») and then in 2025 (with a workshop entitles « ALMA, JWST, Gaia – the Milky Way connection » ). As every year, we will ensure a balanced participation of institutions, permanent and non-permanent staff, women and men.

While for these first two workshop, the focus was on the formation and evolution of discs in galaxies like our own, this year we wish to gather the community working on the the most ancient, and metal-poor components of disc galaxies, ie. their stellar halos.

Scientific justification

Stellar halos of galaxies, and in particular that of the Milky Way, are a heterogeneous assembly of different systems : remnants of accretion and mergers of satellite galaxies, globular clusters, tidal tails, dwarf galaxies, so-called “ultrafaint” galaxies, and perhaps a component resulting from the initial collapse just after the Big Bang. It extends to at least 300 kpc but represents less than 1% of the stellar mass of the Galaxy. Its low density and size make it a difficult subject to study, but it is nevertheless essential for understanding how galaxies like ours came into being, and provides a valuable test for the Lambda CDM scenario of structure growth in our universe.
Theory predicts that halos of galaxies such as the Milky Way are formed by the assembly of smaller sub-halos, with the smaller sub-halos being the most numerous. Typically, in a galaxy like the Milky Way, the halo is expected to be the result of two or three major mergers (in a ratio of 1/3 or more between the halo of our Galaxy and the subhalo).

The halo of our Galaxy therefore provides a test of the Lambda CDM scenario, and thanks to the arrival of new data (Gaia, APOGEE, DESI, soon WEAVE, 4MOST, etc.), its exploration has begun, revealing a multitude of structures. In particular, it has revealed that the Milky Way has indeed undergone at least one major merger, called “Gaia Sausage Enceladus”. It has also made it possible to find dozens of tidal tails left by globular clusters, and to measure the dark matter profiles in the Milky Way’s dwarf satellite galaxies.

Numerical simulations are not to be outdone, and are attempting to produce increasingly realistic simulations of galaxies and their environment, such as the Local Group, including an increasingly detailed description of baryonic physics in a cosmological context.

The aim of comparing observations and these simulations is to answer the following questions, in a list that is not exhaustive : what was the history of the formation of our Galaxy’s halo ? How many major mergers contributed to its formation ? What is the history of the formation of the Milky Way’s globular cluster system ? Is there an “in situ” component of the halo, formed in a phase of primordial collapse ? How was the formation of our Galaxy’s halo determined by the history of the Local Group ? Is there any difference between the stellar halo of our own Galaxy and those of external ones ?

Preliminary program

– Chervin Laporte, CPJ (LIRA, Paris Obs)
– Misha Haywood, permanent (LIRA, Paris Obs)
– Pierre Boldrini, postdoc (LIRA, Paris Obs)
– Paola Di Matteo, permanent (LIRA, Paris Obs)
– Ewould Wempe, postdoc (LIRA, Obs de Paris)
– Jonathan Bland Hawthorn, permanent (University of Sydney)
– Giulia Pagnini, postdoc (Obs de Strasbourg)
– Alice Mori, PhD student (University of Florence)
– Nicolas Martin, permanent (Observatoire de Strasbourg)
– Rodrigo Ibata, permanent (Observatoire de Strasbourg)
– Salvatore Ferrone, postdoc (Obs de Strasbourg)
– Vanessa Hill, permanent (Obs de la Côte d’Azur)
– Georges Kordopatis, permanent (Obs de la Côte d’Azur)
– Eline Tolstoy, permanent (Groningen)
– Vasily Belokurov, permanent (IoA, Cambridge)
– Sergey Khoperskov, postdoc (AIP, Potsdam)
– Alis Deason, permanent (Durham University)
– Sarah Pearson, permanent (University of Copenhagen)
– Matthew Orkney, postdoc (University of Barcelona)
– Eduardo Vitral, postdoc (University of Edimburgh)
– Jorge Penarrubia, permanent (University of Edimburgh)
– Stefania Salvadori, permanent (University of Florence)
– Giuseppina Battaglia, permanent (IAC, Tenerife)
– Daniel Horta, postdoc (University of Edimburgh)
– Carme Gallart, permanent (IAC, Tenerife)
– Rohan Naidu, NASA Hubble fellow (MIT, USA)
– Ricardo Schiavon, permanent (Liverpool John Moores University)
– Antonino Milone, permanent (University of Padua)
– Oscar Agertz, permanent (Lund UNiversity)
– Florent Renaud, permanent (Obs de Strasbourg)
– Claire Guillaume, PhD student (Obs de Strasbourg)