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 \sim 3 – 3.5 .
The gap statistics agrees well with a model in which galactic halos above a minimum mass scale of M _ { min } \sim 10 ^ { 10 } ~ { } M _ { \odot } produce bubbles with a characteristic radius of R _ { b } \sim 0.48 h ^ { -1 } Mpc .
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 .