Stellar Wakes from Dark Matter Subhalos

We propose a novel method utilizing stellar kinematic data to detect low-mass substructure in the Milky Way’s dark matter halo. By probing characteristic wakes that a passing dark matter subhalo leaves in the phase-space distribution of ambient halo stars, we estimate sensitivities down to subhalo masses of $\sim {10}^7{M}_{\odot }$ or below. The detection of such subhalos would have implications for dark matter and cosmological models that predict modifications to the halo-mass function at low halo masses. We develop an analytic formalism for describing the perturbed stellar phase-space distributions, and we demonstrate through idealized simulations the ability to detect subhalos using the phase-space model and a likelihood framework. Our method complements existing methods for low-mass subhalo searches, such as searches for gaps in stellar streams, in that we can localize the positions and velocities of the subhalos today.

Figure 1

(Left) Moments of the stellar phase-space distribution (at $z=0\mathrm{kpc}$), perturbed by a passing DM subhalo at the origin. The subhalo is described by a Plummer sphere with $M_{\mathrm{sh}}=2\times {10}^7{M}_{\odot }$ and $r_s\approx 0.72\mathrm{kpc}$ and is traveling in the $\widehat{\mathbf{x}}$ direction with $v_{\mathrm{sh}}=200\mathrm{km}/\mathrm{s}$. The background phase-space distribution is described by a Maxwell-Boltzmann distribution [see (5)] with $v_0=100\mathrm{km}/\mathrm{s}$. (Middle) The same as the left, but selecting only stars that are comoving with the subhalo with $v_x>150\mathrm{km}/\mathrm{s}$. (Right) The stellar-wakes likelihood profile, defined in (8), as a function of the assumed subhalo mass. We show results for a simulation with the same background and subhalo parameters as in the left panel (black solid curve) and for a background-only simulation without a subhalo (blue dashed curve). The corresponding uncertainties, described in the text, are shown in gray and light blue, respectively, for a distance of 5 kpc from the center of the ROI to Earth. The unperturbed stellar number density is $n_0=5\times {10}^3{\mathrm{kpc}}^{-3}$, and we use an ROI with radius $R=3\mathrm{kpc}$ (dashed green circle in the left panel). For the simulation including a subhalo, the likelihood profile peaks at the correct subhalo mass, marked by a vertical dotted blue line. For the background-only simulation, we find good agreement with analytic results based on the Asimov data set (red curve); see (15).