Final Report Summary - STREAMS (Measuring the Lumpiness of Dark Matter With Tidal Streams)
This Project set out to detect minuscule density perturbations along ghostly stellar streams in the halo of our Galaxy with the hope to pin down the abundances of the Dark Matter clumps devoid of stars. To achieve the proposed goals, our team had to build an inventory of Milky Way stellar sub-structures; find as many new streams as possible and carry out a search for barely detectable signs of stream-sub-halo interactions. Not only did the Project require a comprehensive observational campaign, we also needed to build a theoretical framework necessary to carry out stream-based inference of DM sub-halo properties. We have produced detailed predictions of the observable stream density perturbations, such as the widths of the stream gaps and their numbers. Five years later, we have discovered several new streams and detected at least three clear gaps in three separate structures. These stream gap discoveries are the most tangible result of this Project: before we started, the existence of such density perturbations was the object of speculation but had not been proven. Our work clearly shows the immense power of the stellar streams to probe the low-mass end of the spectrum of the Dark Matter sub-structure. We have finished the Project with a piece of research which combines all of the individual components described above into one groundbreaking discovery. Using the freshly released Gaia DR2 data, we have uncovered a long but previously unseen extension of the Orphan Stream (OS), and demonstrated that the stream has suffered a large-scale deflection from a massive perturber. By analysing the numerical simulations of the stream formation and evolution we have established that the culprit is the Large Magellanic Cloud (LMC). Taking advantage of the stream models developed as part of the Project, we have used the OS perturbation to measure the mass of the LMC - this is first determination of the total mass of an individual sub-halo (hosting a galaxy).