Dark Matter Velocity Spectroscopy

Dark matter decays or annihilations that produce linelike spectra may be smoking-gun signals. However, even such distinctive signatures can be mimicked by astrophysical or instrumental causes. We show that velocity spectroscopy—the measurement of energy shifts induced by relative motion of source and observer—can separate these three causes with minimal theoretical uncertainties. The principal obstacle has been energy resolution, but upcoming experiments will have the precision needed. As an example, we show that the imminent Astro-H mission can use Milky Way observations to separate possible causes of the 3.5-keV line. We discuss other applications.

Figure 1

Top: How DM, astrophysical, and detector lines shift with Galactic longitude is starkly different. Bottom: For DM signals at positive longitude, our motion through the nonrotating DM halo yields a negative LOS velocity and thus a blueshift. In contrast, for astrophysical lines (e.g., from gas), corotation in the disk leads to a positive LOS velocity and thus a redshift. These signs reverse at negative longitude. Detector lines have zero shift.

Figure 2

Comparison of received spectra for DM and gas (G2). The emitted spectra are taken to have equal flux and to be centered at 3.5 keV before velocity effects. The line profiles include velocity dispersion and shift effects, as well as the energy resolution of Astro-H. Vertical bands indicate the $1\sigma$ centroid uncertainties after 2-Ms observations. For contrast, the brown line in the figure and the inset shows the same signal if Astro-H had the energy resolution of XMM.

Figure 3

LOS velocity for DM and various gas models (the realistic version of Fig. 1). Uncertainties are computed assuming 2-Ms Astro-H exposures on each point.