LYMAN ALPHA EMISSION FROM THICK CLOUDS PHOTOIONIZED BY THE METAGALACTIC RADIATION

We present calculations of the expected Lyalpha surface brightness and emergent spectral line profiles from thick (N(H) much greater than 10(20) cm-2), Massive (greater than or similar to 10(10) M.) clouds at high redshift (z greater than or similar to 3) photoionized by the metagalactic background radiation. Our results directly apply to the damped Lyalpha systems and to the ''pancakes'' that are expected to form at large redshift in the Sunyaev-Zel'dovich theory of cluster formation. We consider both quasar-dominated and stellar-dominated metagalactic fluxes. For the quasar-dominated metagalactic flux, we use two energy distributions: a power law with index -1, which is representative of a ''raw'' (unfiltered) quasar energy distribution, and a distribution from Miralda-Escude and Ostriker [ApJ, 350, 1 (1990)] in which the ionizing background arises from quasar emission filtered by dust in intervening galaxies and by Lyalpha forest clouds. For the stellar-dominated metagalactic flux, we use a distribution from Miralda-Escude and Ostriker (1990) in which the ionizing background arises predominantly from star-forming regions in young galaxies which are filtered by Lyalpha forest clouds. In addition to recombination from direct photoionization (pure recombination), we include collisional excitation/ionization involving both thermal and suprathermal electrons in the production of Lyalpha. The suprathermal electrons are important if there is a significant x-ray component to the metagalactic flux. We include dust absorption and scattering in the transfer of Lyalpha (Appendix A). If quasars dominate the metagalactic flux, we find that collisional production of Lyalpha exceeds production from pure recombination case B by a factor of 2-4, provided the cloud is fully ionization bounded and metal poor. For such distributions, we establish a relation between the Lyalpha surface brightness and the cloud metallicity at constant value of the ionization parameter (= U =ionizing photon density/total hydrogen density). On the other hand, if stars dominate the metagalactic flux, recombination dominates the Lyalpha production. We find that a metal poor (less than or similar 0.03 cosmic abundance) thick cloud at z almost-equal-to 3.5 which is ionized by a quasar-dominated metagalactic flux would have a Lyalpha surface brightness of approximately 1% of sky brightness across a 10 angstrom filter centered at 1216(1+z) angstrom. It should thus already be possible to detect Lyalpha emission from metagalactic flux reprocessing in these systems. In the case of He II Lyalpha at 304 angstrom, we estimate its brightness becomes marginally significant only at rather high ionization parameters (U much-greater-than 0.001) and only for the hard ''unfiltered'' power law continuum. Using a quasar-dominated background, the emergent (H) Lyalpha spectral line profile is characterized by two narrow peaks which are separated by approximately 0.3(1 +z) angstrom, symmetrically placed about line center, together with two broad shoulders that extend for several angstrom on both sides of line center. The strength and breadth of the extended shoulders depend upon the dust content and the ionization parameter in the clouds. The narrow peaks should be resolvable using a detector with 1 angstrom resolution for an emitter at z greater than or similar to 3. The significant emission contained in the extended shoulders argues for using narrow band imaging to detect Lyalpha emission from massive, thick clouds at high redshift. Using two different methods, we estimate the Lyalpha emissivity at high redshift arising from the reprocessing of the metagalactic ionizing flux by optically thick (N(H0) > 10(17) cm-2) clouds. We then estimate the contribution to the night sky brightness by the redshifted Lyalpha photons and use the observational constraints to obtain J0(-21) less than or similar to 30 at z approximately 3, where J(nu0)(z) = 10(-21) J0(-21)(z) ergs s-1 cm-2 Hz-1 sr-1 is the mean intensity of the metagalactic ionizing background at the Lyman limit.