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Abstract: Theoretical models of galaxy formation predict that galaxies acquire most oftheir baryons via cold mode accretion. Observations of high-redshift galaxies,while showing ubiquitous outflows, have so far not revealed convincing tracesof the predicted cold streams, which has been interpreted as a challenge forthe current models. Using high-resolution, zoom-in smooth particlehydrodynamics simulations of Lyman break galaxy LBG halos combined withionizing radiative transfer, we quantify the covering factor of the coldstreams at z=2-4. We focus specifically on Lyman limit systems LLSs anddamped Ly-alpha absorbers DLAs, which can be probed by absorptionspectroscopy using a background galaxy or quasar sightline, and which areclosely related to low-ionization metal absorbers. We show that the coveringfactor of these systems is relatively small and decreases with time. At z=2,the covering factor of DLAs within the virial radius of the simulated galaxiesis ~3% ~1% within twice this projected distance, and arises principally fromthe galaxy itself. The corresponding values for LLSs are ~10% and ~4%. Becauseof their small covering factor compared to the order unity covering fractionexpected for galactic winds, the cold streams are naturally dominated byoutflows in stacked spectra. We conclude that the existing observations areconsistent with the predictions of cold mode accretion, and outline promisingkinematic and chemical diagnostics to separate out the signatures of galacticaccretion and feedback.

Author: C.-A. Faucher-Giguere, D. Keres UC Berkeley


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