Hunting for a Dark Matter Wake

As the Large Magellanic Cloud plows through the Milky Ways dark matter halo, it may leave telltale signs of its passage. A recent study explores whether well be able to spot this evidence and what it can tell us about our galaxy and the nature of dark matter.The Milky Ways Large CompanionThe Large and Small Magellanic clouds, as observed from Earth. [ESO/S. Brunier]The Milky Way is far from lonely. Dozens of smaller satellite-galaxy companions orbit around our galaxy, charging through its larger dark matter halo. The most massive of these is the Large Magellanic Cloud (LMC), a galaxy of perhaps 10 or 100 billion solar masses thats about 14,000 light-years across.Studies suggest that the LMC is on its first pass around the Milky Way, traveling on a highly eccentric orbit; it likely only first got close to our galaxy (within about 200 kpc, or 650,000 light-years) about two billion years ago.There are still many uncertainties about this satellite and its travels, however. How massive, exactly, is the LMC? What does its past orbit look like? And how has it interacted with our galaxys dark matter halo, which its passing through?Density perturbations caused by the LMCs motion for one of the authors Milky Way models. The Milky Ways disk is in the xy plane; the black curve traces the LMCs past orbital path and the red star indicates its current position. Three primary overdense/underdense features are visible as signatures of the LMCs wake. [Adapted from Garavito-Camargo et al. 2019]A Telltale TrailA team of scientists led by Nicolas Garavito-Camargo (Steward Observatory, University of Arizona) thinks there may be evidence we can use to answer these questions.Like a boat, the LMC should generate a wake as it plows through the Milky Ways dark matter halo. This wake is caused by gravitational interactions between the satellite and dark matter particles that drag at the LMC, causing the galaxy to lose angular momentum as it orbits.The perturbations that make up this wake overdensities and underdensities in the dark matter and stellar distribution in the halo are signatures that we can predict and hunt for. In a new study, Garavito-Camargo and collaborators use high-resolution N-body simulations to explore the motion of the LMC through the Milky Ways halo and examine the perturbations caused by this charging satellite.Spotting the Evidence of PassageThe authors find that the LMCs motion produces a pronounced dark matter wake that can be decomposed into three parts:Transient response, a trailing wake of overdensity behind the satellite that traces its orbital historyGlobal underdensity, a large underdense region south of the transient responseCollective response, an extended overdensity leading the LMC in the galactic northThese features in the dark-matter distribution are echoed in how stars are distributed in the regions, and the stars should also show distinctive kinematic signatures.Observing strategies for identifying the LMCs wake using stellar densities. To avoid confusion with the Sagittarius stellar stream (the prominent yellow, orange, and red points indicated), the authors identify several regions for observation (colored rectangles) away from the stream where the wake should be detectable. Click to enlarge. [Garavito-Camargo et al. 2019]Garavito-Camargo and collaborators outline an observing strategy to spot the predicted overdensities and underdensities of the wake, and they show that the detection of just 2030 stars in specific regions could provide useful confirmation of their models. The measurements needed should be achievable with current and upcoming stellar surveys.What can we learn from these observations? The detection of the LMCs wake will track its past orbit, which will provide an indirect measure of our own galaxys mass. The specifics of the LMCs motion will also better constrain the satellites mass, as well as provide clues as to the nature of the dark-matter particles that drag on it.CitationHunting for the Dark Matter Wake Induced by the Large Magellanic Cloud, Nicolas Garavito-Camargo et al 2019 ApJ 884 51. doi:10.3847/1538-4357/ab32eb