Amplified J-factors in the Galactic Center for velocity-dependent darkmatter annihilation in FIRE simulations

Kavli Affiliate: Lina Necib

| First 5 Authors: Daniel McKeown, James S. Bullock, Francisco J. Mercado, Zachary Hafen, Michael Boylan-Kolchin

| Summary:

We use FIRE-2 zoom cosmological simulations of Milky Way size galaxy halos to
calculate astrophysical J-factors for dark matter annihilation and indirect
detection studies. In addition to velocity-independent (s-wave) annihilation
cross sections $sigma_v$, we also calculate effective J-factors for
velocity-dependent models, where the annihilation cross section is either
either p-wave ($propto v^2/c^2$) or d-wave ($propto v^4/c^4$). We use 12
pairs of simulations, each run with dark-matter-only (DMO) physics and FIRE-2
physics. We observe FIRE runs produce central dark matter velocity dispersions
that are systematically larger than in DMO runs by factors of $sim 2.5-4$.
They also have a larger range of central ($sim 400$ pc) dark matter densities
than the DMO runs ($rho_{rm FIRE}/rho_{rm DMO} simeq 0.5 – 3$) owing to
the competing effects of baryonic contraction and feedback. At 3 degrees from
the Galactic Center, FIRE J-factors are $5-50$ (p-wave) and $15-500$ (d-wave)
times higher than in the DMO runs. The change in s-wave signal at 3 degrees is
more modest and can be higher or lower ($sim 0.3-6$), though the shape of the
emission profile is flatter (less peaked towards the Galactic Center) and more
circular on the sky in FIRE runs. Our results for s-wave are broadly consistent
with the range of assumptions in most indirect detection studies. We observe
p-wave J-factors that are significantly enhanced compared to most past
estimates. We find that thermal models with p-wave annihilation may be within
range of detection in the near future.

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