Astronomical News Report on the ESO Workshop Satellites and Streams in Santiago held at ESO Vitacura, Santiago, Chile, 13–17 April 2015 Andreas H. W. Küpper1 the Milky Way (MW) and M31 pose chal- Steffen Mieske2 lenges to our understanding of structure and star formation in the early Universe, and the feedback between baryons and 1 Department of Astronomy, Columbia dark matter. University, New York, USA 2 ESO But how well do we understand what it means to be a satellite of the MW or M31? Even in the era of high­precision Galactic satellites and tidal streams are cosmology, we are still uncertain about arguably the two most direct imprints the total masses of the two dominant of hierarchical structure formation in the galaxies in the Local Group, their assem- haloes of galaxies. At this ESO work- bly histories and the shape and extent of shop we sought to create the big picture their dark matter haloes — key aspects of the galactic accretion process, and for gaining a consistent picture of these shed light on the interplay between galaxies and their satellite systems in a satellites and streams in the Milky Way, ΛCDM context. On the contrary, the dis- Andromeda and beyond. The Scientific covery of transition objects at the star Organising Committee prepared a cluster–dwarf galaxy interface has made well-balanced programme with 60 talks things more complicated. It has blurred and 30 poster contributions, resulting the historical distinction between satellite in a meeting which was greatly enjoyed classes, putting in question our under- by the more than 110 participants standing of tidal transformation and the at the venue, and worldwide via Twitter census of small stellar systems. Figure 1. Conference poster showing the Milky Way (#SSS15). with its most prominent satellite, the Large Magellanic Cloud, over Santiago. The upper part of the poster Although these aspects of near­field shows a collection of tidal streams from the Vía Lactea cosmology have become more and more Cauda simulation. Introduction prominent in the age of surveys, there has not been a conference on both satel- by presentations on observations and Near­field cosmology has become lites and streams in over a decade. This modelling of tidal streams, and the final increasingly important over the past few five­day ESO workshop (see the work- part of the workshop was dedicated decades. While the current concordance shop poster, Figure 1) therefore met with to the star cluster–dwarf galaxy divide. cosmological model (Lambda Cold Dark a great demand for presentation slots, The grouping into these sessions was Matter [ΛCDM]) has been very successful and the registration had to close early of course not strict, and, as intended, in reproducing and predicting the proper- due to the overwhelming interest. many presenters pointed out important ties of the Universe on large scales, connections between satellites and several possible tensions have been iden- Based on the scope of the meeting, the streams. The workshop programme, with tified on small scales (≤ 1 Mpc). Issues week was divided into three parts. First, links to many of the presentations, can like the “missing satellite problem”, the satellites and the satellite systems of the “core/cusp problem”, “too big to fail”, Milky Way, M31 and other nearby galax- Figure 2. Conference photo taken in the garden of and detections of satellite discs around ies were discussed. This was followed the ESO Vitacura premises. Stephane Courteau Stephane 38 The Messenger 161 – September 2015 be accessed online1. The participants are topic at the end of the meeting. Collins hosts and making them interesting test- shown in the gardens of ESO Vitacura in furthermore pointed out that kinematic beds for ΛCDM. Else Starkenburg showed Figure 2. data may often be ambiguous as some results from her semi-analytical models of the satellites seem to have had recent of dwarf galaxies, which are particularly The meeting was opened by the ESO gravitational interactions and encounters, useful to test the physics inside the satel- Director General Tim de Zeeuw, giving an while others like Hercules and Willman 1 lites and gain intuition about their evolution. overview of the ESO facilities and an appear to be entirely out of equilibrium, exciting outlook for the field into the Euro- making mass determinations hard, if not pean Extremely Large Telescope era, impossible. Accretion of satellites when it will be possible to resolve stellar populations out to the Virgo Cluster. Finding new satellites and following up Accretion and disruption of satellites the discoveries with deeper photo metry can give us important insights into the and/or spectroscopy has become a build-up of bulges and haloes of galaxies. Satellite systems sport. Yet only through this important Benjamin Hendricks demonstrated this exercise will we eventually get a complete approach for globular clusters (GCs) in Galactic satellites give us an account of census of substructure in the local Uni- Fornax, and Ryan Leaman shed light on low-mass substructures at the present verse. After seeing the great successes the accretion history of the Milky Way day. However, in order to put these satel- of PAndAS, we heard further results and using its GCs as tracers of dwarf galaxy lites into the context of structure forma- outlooks from ongoing and upcoming infalls. Similarly, tidal streams provide tion within the ΛCDM framework, their surveys with the Large Sky Area Multi­ insights on the star formation histories of masses, and especially their dark matter Object Fibre Spectroscopic Telescope individual dwarf galaxies, as Thomas content, have to be understood. In the (LAMOST) by Jeff Carlin, on the Large de Boer demonstrated in the example of first talk of the satellite session, Jorge Binocular Telescope (by Giacomo Beccari) Sagittarius. Peñarrubia discussed the advances, prob- and the Magellan Telescope (Denija lems and challenges of mass modelling Crnojevic), all looking for more substruc- But larger infalling galaxies, or major of dwarf galaxies. Although modelling ture in the local Universe. mergers, could also trigger the formation might help to put constraints on the nature of new satellites — known as tidal dwarf of dark matter (density cores/cusps), he Eduardo Balbinot then presented the rich galaxies. Pavel Kroupa argued that these clearly called for more (kinematic) data to harvest from the Dark Energy Survey satellites could be long-lived, and pollute inform the models. In the spirit of the (DES) data release 1, giving us a detailed (or entirely make up) the satellite popula- meeting’s title, and like many other speak- look at the structure and formation his- tions of host galaxies. Kinematically cor- ers, he also presented an interesting new tory of the Large Magellanic Cloud (LMC), related satellite populations in the Milky idea, using the streams of satellites to as well as of the nine newly discovered Way and M31 halo could point towards break the core/cusp degeneracy. Calling satellites around the Magellanic Clouds. such a formation scenario. However, it a “diversity problem” rather than a Later in the meeting, Vasily Belokurov Pierre­Alain Duc argued that the forma- cusp/core problem, Chervin Laporte dem- talked more about the independent dis- tion of tidal dwarf galaxies through onstrated the large phenomenology of covery of these satellites in the DES data. mergers is less likely than expected, and dark matter in galactic satellites due to a Both groups have follow-ups on one that the objects thus formed do not possible re-growth of cusps in cored satellite, Reticulum II, confirming that it is resemble the satellites found in the local galaxies via accretion of dark matter sub- in fact a dwarf galaxy. Similarly, Erik Universe. From the numerical side, Sylvia haloes or other dwarf galaxies — further Tollerud presented his newest findings in Ploeckinger is developing the tools to complicated by baryonic effects on the the Galactic Arecibo L­band Feed Array perform full hydrodynamical simulations central density profiles of satellites. (GALFA) survey. The two newly discov- of tidal dwarf galaxies to study their long- ered, gas-rich, low-mass galaxies repre- term survivability. Michelle Collins then gave a recount of sent the progenitors of the dwarf galaxies the dozens of newly discovered satellites that we find around larger host galaxies. Marcel Pawlowski, Rodrigo Ibata and in the Milky Way and M31 haloes from Understanding this transformation from Noam Libeskind then gave detailed surveys like the Pan­Andromeda Archae- gas-rich to quenched galaxy is not descriptions of the co-rotating structures ological Survey (PAndAS; see Figure 3). straightforward, as Thorsten Lisker ex­­ around the Milky Way, Andromeda and In her presentation, a mass–size diagram plained and Rachael Beaton later on con- Centaurus A. The speakers pointed out (Figure 4) made its first appearance and firmed. that the chances of finding such planes in ended up being the plot shown most ΛCDM appears to be rather low, but may often during the meeting. In particular the Numerical simulations are getting up have to do with either larger mergers divide between dwarf galaxies and glob- to speed with the flood of observational bringing in lots of satellites or infall of satel- ular clusters in this diagram is currently data. Coral Wheeler presented high-­ lites along the cosmic web. Gurtina Besla being populated with newly discovered resolution hydrodynamical simulations added the Large and Small Magellanic objects, most of which cannot be clas­ of isolated dwarf galaxies to understand Clouds to this picture, discussing different sified by their photometry alone — which their satellites, finding a few ultra­faint infall scenarios for the two satellite galax- is why we had an entire session on this satellites around each of these low-mass ies. She argued that if the LMC originally The Messenger 161 – September 2015 39 Astronomical News Küpper A.