In Search of the Dark Ages – an Experimental Challenge

Most direct source detections beyond z ∼ 7 are likely to arise from wide-field narrowband surveys of Lyα emission in the J band. For this to be true, the Lyα emission must somehow escape from compact star-forming regions (CSR) presumably associated with massive haloes. Since the Lyα escape fraction is . 10% from an emitting region of size ∼ 1 kpc, these objects will be difficult to find and hard to detect, requiring ∼ 30 – 100 hours at each telescope pointing on 8 – 10 m telescopes. For CSR sources, existing large-format IR arrays are close to ideal in terms of their noise characteristics for conducting wide-field narrowband surveys where pixel sizes are 0.1 or larger. However, we stress that Lyα can also arise from external large-scale shocks (ELS) due to starburst winds, powered by CSRs, ploughing into gas actively accreting onto the dark halo. The winds effectively carry energy from the dense, dusty environment of a starburst into lower density regions, where the escape probability for Lyα photons is greater. ELS emission is expected to be considerably more clumpy (. 100 pc) than CSR emission. For ELS sources, IR arrays will need 1 − 2 orders of magnitude improvement in dark current in order to detect dispersed clumpy emission within the environments of massive haloes. These sources require an IR camera with small pixels (∼ 0.01) and adaptive optics correction (e.g., GSAOI on Gemini), and will therefore require a targetted observation of a Dark Age source identified by a wide-field survey. For either targetted or wide-field surveys, the deepest observations will be those where the pixel sampling is well matched to the size of the emitting regions. We show that there are only 3 – 4 J-band windows [z = 7.72, (8.22),8.79,9.37] suitable for observing the Dark Ages in Lyα; we summarize their cosmological properties in Table 1. Subject headings: cosmology; first stars; intergalactic medium; galaxies — high redshift