The Detectability of High Redshift Lyman Alpha Emission Lines Prior to the Reionization of the Universe

For a source of Ly alpha radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the emission line is strongly suppressed by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough can extend to the red side of the emission line, and erase a significant fraction of the total line flux. However, the transmitted fraction increases with the size of the local cosmological HII region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Ly alpha emitting galaxy at a redshift z=6.56 (Hu et al. 2002), possibly prior to the reionization of the IGM, we revisit the effects of a neutral IGM on the Ly alpha emission line. We show that even for faint sources with little ionizing continuum, the emission line can remain observable. In particular, the line detected by Hu et al. is consistent with a source embedded in a neutral IGM. We provide characterizations of the asymmetry and total transmitted flux of the Ly alpha line as functions of the ionizing emissivity of its source. A statistical sample of Ly alpha emitters extending beyond the reionization redshift can be a useful probe of reionization.Comment: Submitted to ApJL, 4 figures include

Electromagnetic signatures of supermassive black hole binaries resolved by PTAs

Pulsar timing arrays (PTAs) may eventually be able to detect not only the stochastic gravitational-wave (GW) background of SMBH binaries, but also individual, particularly massive binaries whose signals stick out above the background. In this contribution, we discuss the possibility of identifying and studying such `resolved' binaries through their electromagnetic emission. The host galaxies of such binaries are themselves expected to be also very massive and rare, so that out to redshifts z~2 a unique massive galaxy may be identified as the host. At higher redshifts, the PTA error boxes are larger and may contain as many as several hundred massive-galaxy interlopers. In this case, the true counterpart may be identified, if it is accreting gas efficiently, as an active galactic nucleus (AGN) with a peculiar spectrum and variable emission features. Specifically, the binary's tidal torques expel the gas from the inner part of the accretion disk, making it unusually dim in X-ray and UV bands and in broad optical emission lines. The tails of the broad wings of any FeK-alpha emission line may also be `clipped' and missing. The binary's orbital motion, as well as the gas motions it induces, may trigger quasiperiodic variations. These include coherent flux variability, such as luminous, multi-wavelength flares, as well as Doppler shifts of broad emission lines and `see-saw' oscillations in the FeK-alpha line. Additional features, such as evidence for a recent major merger or dual collimated jets, could also corroborate the counterpart. These properties would make resolved PTA sources stand out among AGN with similar overall luminosities and allow their identification.Comment: 18 pages, 5 figures, accepted for publication in a focus issue on PTAs in Classical and Quantum Gravit