Analysis of shape and angular orientation parameters of velocity-dependent dark matter annihilation signals in Galactic Centres for FIRE simulations

ABSTRACT We use Feedback In Realistic Environments 2 (FIRE-2) zoom cosmological simulations of Milky Way-sized galaxy haloes to characterize the shape of the J-factor emission signal on the sky. We find that, at a fixed $\mathrm{d}J/\mathrm{d}\Omega$ contour, the shape is well fit by an ellipse, with semimajor axis $R_{\rm major}$ and semiminor axis $R_{\rm minor}$ measured in degrees on the sky. We use least-squares fitting to fit ellipses to the J-factor emission, viewed from the solar location. The ratio of minor to major axes ($R_{\rm major}/R_{\rm minor}$) allows us to characterize the shape of each contour, with a ratio of 1.0 corresponding to circular/spherical emission. These results provide new expectations for the shape of dark matter annihilation emission signals we might expect to see if dark matter is annihilating with its own antiparticle. We find that both the shape and angular orientation of the emission signal are different from results predicted from dark matter-only simulations. In terms of shape, we find that the ratios of semiminor to semimajor axis is always consistently at 0.8, revealing a consistent circular emission shape, whereas with dark matter-only emission signals the range of ratios from halo to halo is broader, and often closer to 0.5, showing a much more elliptical shape in general. In terms of angular orientation, we find that the major axis of the J-factor emission signal maps for FIRE haloes is consistently aligned with the Galactic plane within a few degrees, meaning that excess emission out of the plane would be hard to explain with a dark matter annihilation signal. However, we also find that the expected emission signal would be consistent with Fermi-Large Area Telescope measurements showing a galactic centre excess in full hydrodynamical simulations with stars and gas included. In contrast, dark matter-only simulations do not produce the expected signal.