Nature of Lyman Alpha Blobs: Powered by Extreme Starbursts

Abstract: We present a new model for the observed Lyman alpha blobs (LABs) within the context of the standard cold dark matter model. In this model, LABs are the most massive halos with the strongest clustering (proto-clusters) undergoing extreme starbursts in the high-z universe. Aided by calculations of detailed radiative transfer of Lya photons through ultra-high resolution (159pc) large-scale (>30Mpc) adaptive mesh-refinement cosmological hydrodynamic simulations with galaxy formation, this model is shown to be able to, for the first time, reproduce simultaneously the global Lya luminosity function and luminosity-size relation of the observed LABs. Physically, a combination of dust attenuation of Lya photons within galaxies, clustering of galaxies, and complex propagation of Lya photons through circumgalactic and intergalactic medium gives rise to the large sizes and frequently irregular isophotal shapes of LABs that are observed. A generic and unique prediction of this model is that there should be strong far-infrared (FIR) sources within each LAB, with the most luminous FIR source likely representing the gravitational center of the proto-cluster, not necessarily the apparent center of the Lya emission of the LAB or the most luminous optical source. Upcoming ALMA observations should unambiguously test this prediction. If verified, LABs will provide very valuable laboratories for studying formation of galaxies in the most overdense regions of the universe at a time when global star formation is most vigorous.