The Lyman α signature of the first galaxies

Abstract

We present the Cosmic Lyman-$\\alpha$ Transfer code (COLT), a massively\nparallel Monte-Carlo radiative transfer code, to simulate Lyman-$\\alpha$\n(Ly$\\alpha$) resonant scattering through neutral hydrogen as a probe of the\nfirst galaxies. We explore the interaction of centrally produced Ly$\\alpha$\nradiation with the host galactic environment. Ly$\\alpha$ photons emitted from\nthe luminous starburst region escape with characteristic features in the line\nprofile depending on the density distribution, ionization structure, and bulk\nvelocity fields. For example, anisotropic ionization exhibits a tall peak close\nto line centre with a skewed tail that drops off gradually. Idealized models of\nfirst galaxies explore the effect of mass, anisotropic H II regions, and\nradiation pressure driven winds on Ly$\\alpha$ observables. We employ mesh\nrefinement to resolve critical structures. We also post-process an ab initio\ncosmological simulation and examine images captured at various escape distances\nwithin the 1 Mpc$^3$ comoving volume. Finally, we discuss the emergent spectra\nand surface brightness profiles of these objects in the context of high-$z$\nobservations. The first galaxies will likely be observed through the red\ndamping wing of the Ly$\\alpha$ line. Observations will be biased toward\ngalaxies with an intrinsic red peak located far from line centre that reside in\nextensive H II super bubbles, which allows Hubble flow to sufficiently redshift\nphotons away from line centre and facilitate transmission through the\nintergalactic medium (IGM). Even with gravitational lensing to boost the\nluminosity this preliminary work indicates that Ly$\\alpha$ emission from\nstellar clusters within haloes of $M_{\\rm vir}<10^9~{\\rm M}_\\odot$ is generally\ntoo faint to be detected by the James Webb Space Telescope (JWST).\n