CCA Colloquium: Monica Valluri

Title: Two new ways to probe the nature of dark matter with tidal streams

Abstract: Lambda CDM predicts that dark matter halos are moderately triaxial and not static, but tumbling slowly and coherently out to 60% of the virial radius. The tumbling rate (pattern speed) inferred from cosmological simulations is so small (15-40 degrees/Gyr) that it has been assumed to have negligible effects on baryons. We recently showed that the torque from even a slowly tumbling halo can cause detectable perturbations to tidal streams in the Milky Way. We also showed, from the first analysis of figure rotation in cosmological simulations with baryons, that the prevalence and magnitude of figure rotation is one of the only predictions of Lambda CDM on scales of galaxies that is relatively unaffected by the presence of baryons. The standard method of measuring figure rotation in simulations uses a quantity called the shape tensor which is extremely sensitive to the presence of sub-halos and satellites. Although the Milky Way is one of the best places to attempt to measure figure rotation, the fact that it is currently being perturbed by a massive companion like the Large Magellanic Cloud (LMC) has led us to develop a new method to measure figure rotation based on Basis Function Expansions (BFEs). This method can not only measure the pattern speeds and rotation axes of halos but can help to determine why and when halos rotate. I will also describe a new probe of the cusp/core issue in dwarf galaxies that uses the structure of tidal streams from accreted globular clusters. Finally, I will give a briefly overview of the Milky Way survey being carried out by the Dark Energy Spectroscopic Instrument (DESI) and describe some recent results on the GD-1 tidal stream obtained with DESI.