How well can we measure and understand foregrounds with 21 cm
  experiments?
  Before it becomes a sensitive probe of the Epoch of Reionization, the Dark
Ages, and fundamental physics, 21 cm tomography must successfully contend with
the issue of foreground contamination. Broadband foreground sources are
expected to be roughly four orders of magnitude larger than any cosmological
signals, so precise foreground models will be necessary. Such foreground models
often contain a large number of parameters, reflecting the complicated physics
that governs foreground sources. In this paper, we concentrate on spectral
modeling (neglecting, for instance, bright point source removal from spatial
maps) and show that 21 cm tomography experiments will likely not be able to
measure these parameters without large degeneracies, simply because the
foreground spectra are so featureless and generic. However, we show that this
is also an advantage, because it means that the foregrounds can be
characterized to a high degree of accuracy once a small number of parameters
(likely three or four, depending on one's instrumental specifications) are
measured. This provides a simple understanding for why 21 cm foreground
subtraction schemes are able to remove most of the contaminants by suppressing
just a small handful of simple spectral forms. In addition, this suggests that
the foreground modeling process should be relatively simple and will likely not
be an impediment to the foreground subtraction schemes that are necessary for a
successful 21 cm tomography experiment.
