Signature of Galactic Outflows as Absorption-Free Gaps in the Ly-alpha Forest

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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 Mₘin 10¹0 Mₛun produce bubbles with a characteristic radius of Rb 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.