We use high-resolution cosmological hydrodynamical AMR simulations to predict the characteristics of { L } \alpha emission from the cold gas streams that fed galaxies in massive haloes at high redshift .
The { L } \alpha luminosity in our simulations is powered by the release of gravitational energy as gas flows from the intergalactic medium into the halo potential wells .
The UV background contributes only < 20 \% to the gas heating .
The { L } \alpha emissivity is due primarily to electron-impact excitation cooling radiation in gas \sim 2 \times 10 ^ { 4 } K. We calculate the { L } \alpha emissivities assuming collisional ionisation equilibrium ( CIE ) at all gas temperatures .
The simulated streams are self-shielded against the UV background , so photoionisation and recombination contribute negligibly to the { L } \alpha line formation .
We produce theoretical maps of the { L } \alpha surface brightnesses , assuming that \sim 85 \% of the { L } \alpha photons are directly observable .
We do not consider transfer of the { L } \alpha radiation , nor do we include the possible effects of internal sources of photoionisation such as star-forming regions .
Dust absorption is expected to obscure a small fraction of the luminosity in the streams .
We find that typical haloes of mass M _ { v } \sim 10 ^ { 12 - 13 } M _ { \odot } at z \sim 3 emit as { L } \alpha blobs ( LABs ) with luminosities 10 ^ { 43 - 44 } { erg s } ^ { -1 } .
Most of the { L } \alpha comes from the extended ( 50 - 100 { kpc } ) narrow , partly clumpy , inflowing , cold streams of ( 1 - 5 ) \times 10 ^ { 4 } K that feed the growing galaxies .
The predicted LAB morphology is therefore irregular , with dense clumps and elongated extensions .
The integrated area contained within surface-brightness isophotes of 2 \times 10 ^ { -18 } { erg s } ^ { -1 } { cm } ^ { -2 } { arcsec } ^ { -2 } is \sim 2 - 100 { arcsec } ^ { 2 } , consistent with observations .
The linewidth is expected to range from 10 ^ { 2 } to more than 10 ^ { 3 } { km } { s } ^ { -1 } with a large variance .
The typical { L } \alpha surface brightness profile is \propto r ^ { -1.2 } where r is the distance from the halo centre .
Our simulated LABs are similar in luminosity , morphology and extent to the observed LABs , with distinct kinematic features .
The predicted { L } \alpha luminosity function is consistent with observations , and the predicted areas and linewidths roughly recover the observed scaling relations .
This mechanism for producing LABs appears inevitable in many high- z galaxies , though it may work in parallel with other mechanisms .
Some of the LABs may thus be regarded as direct detections of the cold streams that drove galaxy evolution at high z .