National Optical Astronomy Observatory

Comparison of a small region of the DAWN survey’s narrowband image of the EGS field (upper subpanel), and the corresponding DAWN J-band image (lower subpanel). Emission line sources are circled. The emission line source farthest left (east) is the z = 7.73 Ly-α emitting galaxy first identified by Oesch et al. (2015). The remaining three are intermediate-redshift emission line galaxies.

The Cosmic DAWN Survey is a definitive test for the neutral gas fraction near the epoch of reionization using a survey of Lyman-alpha emitters at z = 7.7. The redshift of reionization, its speed, and its homogeneity are unknowns, with the Gunn-Peterson tests probing the end of reionization at z > 6, while WMAP suggests that this occurred at z ~ 11. Ly-α galaxies provide a powerful tool to probe the central phase of reionization with neutral fractions of gas ≈ 50% because the Ly-α photons scatter resonantly. The observed Ly-α luminosity function (LF) should then show a substantial decline. The Cosmic DAWN investigators pioneered the use of Ly-α emitters to show that the IGM is more than 50% ionized at z = 6.5 (Malhotra & Rhoads 2004). The present survey uses 40 nights of deep narrowband imaging in 4 fields to find Ly-α line emitters. Ly-α are found by differencing NEWFIRM images taken in a 3.5nm wide filter at 1066nm with broad J-band images. The expected yield 40 Ly-α emitters, or 10 in each field, if the IGM is ionized at z = 7.7. The four fields guard against cosmic variance and inhomogenieties in reionization.

Cosmic DAWN at a Glance

• Data Release [FTP link]