  Powerful outflows from star-forming galaxies are expected to push away the
neutral intergalactic medium (IGM) around those galaxies, and produce
absorption-free gaps in the Ly-alpha forest. We analyze the abundance of gaps
of various sizes in three high resolution spectra of quasars at z ~ 3 - 3.5.
The gap statistics agrees well with a model in which galactic halos above a
minimum mass scale of M_min ~ 10^10 M_sun produce bubbles with a characteristic
radius of R_b ~ 0.48 Mpc/h. Both numbers are consistent with naive theoretical
expectations, where the minimum galaxy mass reflects the threshold for infall
of gas out of a photo-ionized IGM. The observed gaps are typically bounded by
deep absorption features as expected from the accumulation of swept-up gas on
the bubble walls.
