We explore the properties of high-redshift Lyman-alpha emitters (LAE), and their link with the Lyman-Break galaxy population (LBG), using a semi-analytic model of galaxy formation that takes into account resonant scattering of Lya photons in gas outf
lows. We can reasonably reproduce the abundances of LAEs and LBGs from redshift 3 to 7, as well as most UV LFs of LAEs. The stronger dust attenuation for (resonant) Lya photons compared to UV continuum photons in bright LBGs provides a natural interpretation to the increase of the LAE fraction in LBG samples, X_LAE, towards fainter magnitudes. The redshift evolution of X_LAE seems however very sensitive to UV magnitudes limits and EW cuts. In spite of the apparent good match between the statistical properties predicted by the model and the observations, we find that the tail of the Lya equivalent width distribution (EW > 100 A) cannot be explained by our model, and we need to invoke additional mechanisms. We find that LAEs and LBGs span a very similar dynamical range, but bright LAEs are about 4 times rarer than LBGs in massive halos. Moreover, massive halos mainly contain weak LAEs in our model, which might introduce a bias towards low-mass halos in surveys which select sources with high EW cuts. Overall, our results are consistent with the idea that LAEs and LBGs make a very similar galaxy population. Their apparent differences seem mainly due to EW selections, UV detection limits, and a decreasing Lya-to-UV escape fraction ratio in high SFR galaxies.
{Abridged} We investigate the observability of cold accretion streams at redshift 3 via Lyman-alpha (Lya) emission and the feasibility of cold accretion as the main driver of Lya blobs (LABs). We run cosmological zoom simulations focusing on 3 halos
spanning two orders of magnitude in mass, roughly from 10^11 to 10^13 solar masses. We use a version of the Ramses code that includes radiative transfer of UV photons, and we employ a refinement strategy that allows us to resolve accretion streams in their natural environment to an unprecedented level. For the first time, we self-consistently model self-shielding in the cold streams from the cosmological UV background, which enables us to predict their temperatures, ionization states and Lya luminosities with improved accuracy. We find the efficiency of gravitational heating in cold streams in a ~10^11 solar mass halo is around 10-20% throughout most of the halo but reaching much higher values close to the center. As a result most of the Lya luminosity comes from gas which is concentrated at the central 20% of the halo radius, leading to Lya emission which is not extended. In more massive halos, of >10^12 solar masses, cold accretion is complex and disrupted, and gravitational heating does not happen as a steady process. Ignoring the factors of Lya scattering, local UV enhancement, and SNe feedback, cold accretion alone in these massive halos can produce LABs that largely agree with observations in terms of morphology, extent, and luminosity. Our simulations slightly and systematically over-predict LAB abundances, perhaps hinting that the interplay of these ignored factors may have a negative net effect on extent and luminosity. We predict that a factor of a few increase in sensitivity from current observational limits should unambiguously reveal continuum-free accretion streams around massive galaxies at z=3.
