Abstract
We present the most distant detection of cosmic voids ($z 2.3$) and the
first detection of three-dimensional voids in the Lyman-$\alpha$ forest using
the 3D tomographic map from the CLAMATO survey. Using the LRIS spectrograph on
the 10.3m Keck-I telescope, CLAMATO obtained moderate-resolution spectra from
240 background Lyman-break galaxies and quasars in a 26.6' by 21.3' section of
the COSMOS field to reconstruct the Ly$\alpha$ absorption field on 2.5 $h^-1$
Mpc scales over a comoving volume of $3.15 10^5$ $h^-3$ Mpc$^3$. We
detect voids using a spherical overdensity finder with thresholds calibrated
from hydrodynamical simulations of the intergalactic medium. We find that the
same thresholds produce a consistent volume fraction of voids in both data
(19.1%) and simulations (18.1%). We fit the void radius function using
excursion set models at $z 2.3$ and we compare the two-dimensional and
radially averaged stacked profiles of large voids ($r > 5$ $h^-1$ Mpc) to
stacked voids in mock observations and the simulated density field. Finally,
using 432 coeval galaxies in the same volume as the IGM map, we find that the
tomography-identified voids are underdense in galaxies by 5.91$\sigma$ compared
to random cells.
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