Absorption by Spinning Dust: a Contaminant for High-Redshift 21 cm
  Observations
  Spinning dust grains in front of the bright Galactic synchrotron background
can produce a weak absorption signal that could affect measurements of high
redshift 21 cm absorption. At frequencies near 80 MHz where the EDGES
experiment has reported 21\,cm absorption at $z \approx 17$, absorption could
be produced by interstellar nanoparticles with radii $a \approx 50\AA$ in the
cold interstellar medium at temperature $T \approx 50$ K. Atmospheric aerosols
could contribute additional absorption. The strength of the absorption depends
on the abundance of such grains and on their dipole moments, which are
uncertain. The breadth of the absorption spectrum of spinning dust limits its
possible impact on measurement of a relatively narrow 21 cm absorption feature.
