Hunting for a Dark Matter Wake

As the Large Magellanic Cloud plows through the Milky Way’s dark matter halo, it may leave telltale signs of its passage. A recent study explores whether we’ll be able to spot this evidence — and what it can tell us about our galaxy and the nature of dark matter.

The Milky Way’s Large Companion

LMC and SMC

The 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 that’s 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 galaxy’s dark matter halo, which it’s passing through? 

LMC wake

Density perturbations caused by the LMC’s motion for one of the authors’ Milky Way models. The Milky Way’s disk is in the x–y plane; the black curve traces the LMC’s past orbital path and the red star indicates its current position. Three primary overdense/underdense features are visible as signatures of the LMC’s wake. [Adapted from Garavito-Camargo et al. 2019]

A Telltale Trail

A 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 Way’s 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 Way’s halo and examine the perturbations caused by this charging satellite.

Spotting the Evidence of Passage

The authors find that the LMC’s motion produces a pronounced dark matter wake that can be decomposed into three parts:

  1. Transient response, a trailing wake of overdensity behind the satellite that traces its orbital history
  2. Global underdensity, a large underdense region south of the transient response
  3. Collective response, an extended overdensity leading the LMC in the galactic north

These 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 strategy

Observing strategies for identifying the LMC’s 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 20–30 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 LMC’s wake will track its past orbit, which will provide an indirect measure of our own galaxy’s mass. The specifics of the LMC’s motion will also better constrain the satellite’s mass, as well as provide clues as to the nature of the dark-matter particles that drag on it.

Citation

“Hunting 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