Extracting the Global Signal from 21-cm Fluctuations: the Multi-Tracer
  Approach
  The multi-tracer technique employs a ratio of densities of two differently
biased galaxy samples that trace the same underlying matter density field, and
was proposed to alleviate the cosmic variance problem. Here we propose a novel
application of this approach, applying it to two different tracers one of which
is the 21-cm signal of neutral hydrogen from the epochs of reionization and
comic dawn. The second tracer is assumed to be a sample of high-redshift
galaxies, but the approach can be generalized and applied to other
high-redshift tracers. We show that the anisotropy of the ratio of the two
density fields can be used to measure the sky-averaged 21-cm signal, probe the
spectral energy distribution of radiative sources that drive this signal, and
extract large-scale properties of the second tracer, e.g., the galaxy bias.
Using simulated 21-cm maps and mock galaxy samples, we find that the method
works well for an idealized galaxy survey. However, in the case of a more
realistic galaxy survey which only probes highly biased luminous galaxies, the
inevitable Poisson noise makes the reconstruction far more challenging. This
difficulty can be mitigated with the greater sensitivity of future telescopes
along with larger survey volumes.
