Tobias Goerdt (1), A. Dekel (1), A. Sternberg (2), D. Ceverino (1),
R. Teyssier (3), J.R. Primack (4) (1: HU Jerusalem, 2: Tel Aviv U, 3:
U Zurich, 4: UCSC)

We use high-res cosmological hydro AMR simulations to predict the hydrogen 
Lalpha emission from the cold gas streams that fed galaxies in massive 
haloes at high z. The local emissivities due to collisional excitation are 
calculated from the simulated gas properties, while photoionization is 
less important. The Lalpha surface density is mapped assuming that 85% of 
the Lalpha photons are observed. Typical haloes of mass Mv~10^{12-13}Msun 
at z ~ 3 emit as Lalpha blobs (LABs) with luminosities 10^{43-44} erg/s 
and 50-100kpc extent. Most of the Lalpha comes from the extended, narrow, 
partly clumpy, inflowing, cold streams of (1-5)*10^4K that feed the 
galaxy. Dust absorption is negligible in the streams. The predicted LAB 
morphology is irregular, with dense clumps and elongated extensions. The 
area contained within isophotes with surface brightnesses of 2.2*10^{-18} 
erg s^{-1} cm^{-2} arcsec^{-2} is ~20-200 arcsec^2. The linewidth is 
expected to range from a few hundreds to above 1000 km/s with a large 
variance. The typical Lalpha surface brightness profile is proportional to 
r^{-1.2}. The Lalpha emission in our simulations is powered by the 
gravitational energy gained by the streaming into the halo potential well, 
while the UV background contributes <20%. A toy model of gravitational 
heating explains the simulated results. The simulated LABs are similar in 
luminosity, morphology and extent to the observed LABs, and they have 
distinct kinematic features. The predicted luminosity function is 
consistent with observations, and the predicted areas and linewidths 
reproduce the observed scaling relations. The LABs can be regarded as 
direct detections of the cold streams that drive galaxy evolution at high 
z. This mechanism for producing LABs appears inevitable in most high-z 
galaxies.