First Results on Dark Matter Substructure from Astrometric Weak Lensing
  Low-mass structures of dark matter (DM) are expected to be entirely devoid of
light-emitting regions and baryons. Precisely because of this lack of baryonic
feedback, small-scale substructures of the Milky Way are a relatively pristine
testing ground for discovering aspects of DM microphysics and primordial
fluctuations on subgalactic scales. In this work, we report results from the
first search for Galactic DM subhalos with time-domain astrometric weak
gravitational lensing. The analysis is based on a matched-filter template of
local lensing corrections to the proper motion of stars in the Magellanic
Clouds. We describe a data analysis pipeline detailing sample selection,
background subtraction, and handling outliers and other systematics. For
tentative candidate lenses, we identify a signature based on an anomalous
parallax template that can unequivocally confirm the presence of a DM lens,
opening up prospects for robust discovery potential with full time-series data.
We present our constraints on substructure fraction $f_l \lesssim 5$ at 90% CL
(and $f_l \lesssim 2$ at 50% CL) for compact lenses with radii $r_l <
1\,\mathrm{pc}$, with best sensitivity reached for lens masses $M_l$ around
$10^7$-$10^8\,M_\odot$. Parametric improvements are expected with future
astrometric data sets; by end of mission, $Gaia$ could reach $f_l \lesssim
10^{-3}$ for these massive point-like objects, and be sensitive to lighter
and/or more extended subhalos for $\mathcal{O}(1)$ substructure fractions.
