TITLE: An Off-Center Density Peak in the Milky Way’s Dark Matter Halo?
AUTHORS: Michael Kuhlen, Javiera Guedes, Annalisa Pillepich, Piero Madau, Lucio Mayer
FIRST AUTHOR’S INSTITUTION: UC-Berkeley
However, today’s paper by Kuhlen et al. challenges this idea. They test the assumption that the dark matter peak is located at the dynamical center by examining four major galaxy formation simulations. Three of these simulations (ErisDark, Via Lactea II, and GHalo) are dark matter only, while one (Eris) models both dark matter and baryonic matter. They find that while the dark-matter-only galaxies keep their density peak at the dynamical center, the combined dark matter-baryonic matter simulation shows a well-defined, consistent offset, as much as 300 – 400 parsecs.The growth of this offset corresponds with the growth of a “core” in the Eris simulation, a possible clue to its origin. The typical dark matter profile of a galaxy has a “cusp” at the center – a region of sharply peaked density. However, observations of real galaxies suggest that the dark matter profile is closer to a “cored” profile – it has a region of largely uniform density at its center. Dark-matter-only galaxy simulations show a cusped profile, while dark matter plus baryonic matter simulations show a cored profile. This suggests that the same baryonic processes responsible for Eris’ core may be responsible for offsetting the density peak.The authors consider three explanations for this offset: statistical fluctuations; a partially dissolved subhalo core; and perturbation by some external force such as a galactic bar. Statistical tests rule out the fluctuation explanation for the peak. The subhalo explanation posits that the galaxy accreted a smaller dark matter halo that has not yet been completely disrupted. However, this would suggest that the offset density peak would be a bound structure of specific particles held together by gravity, but following the peak through the simulation shows that particles move in and out of the peak rather than staying bound to it. The last explanation, excitation by some perturber, seems the most likely. In this scenario the stellar bar in the Eris galaxy excites a resonance in the dark matter distribution, tugging the peak off-center. This conclusion is buoyed by the fact that the offset appears at roughly the same time as the bar, and tends to be offset in the direction of the bar.Kuhlen et al.’s results show that a dark matter peak might be displaced as much as 2.5 degrees from the galactic center, which puts the 130 GeV line detection well within the realm of possibility. However, they were not able to resolve the full structure of the peak, important for determining the expected contrast between the peak and its surroundings. Further results may have to wait for the completion of a higher-resolution run of the Eris simulation, not expected to finish for several more months. But it’s clear that, between this gamma ray signature and the detection of the Higgs, exciting things are happening around 130 GeV.