2015/04/30 by David Sobral, Jorryt Matthee, Behnam Darvish +5 · 3 citations
Physics and Astronomy · #astro-ph.GA #astro-ph.CO
paper · pdf · doi:10.1088/0004-637x/808/2/139
15 pages, 8 figures. Accepted for publication in ApJ
arxiv created 2015/06/04 · arxiv updated 2015/08/06
Faint Lyman-α (Lyα) emitters become increasingly rarer towards the re-ionisation epoch (z~6-7). However, observations from a very large (~5deg2) Lyα survey at z=6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters. Here we present follow-up observations of the two most luminous z~6.6 Lyα candidates in the COSMOS field: `MASOSA' and `CR7'. We used X-SHOOTER, SINFONI and FORS2 (VLT), and DEIMOS (Keck), to confirm both candidates beyond any doubt. We find redshifts of z=6.541 and z=6.604 for MASOSA and CR7, respectively. MASOSA has a strong detection in Lyα with a line width of 386±30 km/s (FWHM) and with high EW0 (>200 Å), but it is undetected in the continuum. CR7, with an observed Lyα luminosity of 1043.93±0.05erg/s is the most luminous Lyα emitter ever found at z>6. CR7 reveals a narrow Lyα line with 266±15 km/s FWHM, being detected in the NIR (rest-frame UV, with β=-2.3±0.1) with an excess in J, and also strongly detected in IRAC/Spitzer. We detect a narrow HeII1640Å emission line (6σ) which explains the excess seen in the J band photometry (EW0~80 Å). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra (HeII/OIII]1663A>3 and HeII/CIII]1908A>2.5). We find that CR7 is best explained by a combination of a PopIII-like population which dominates the rest-frame UV and the nebular emission, and a more normal stellar population which dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Lyα and HeII emission, and two red components (~5 kpc away), which dominate the mass. Our findings are consistent with theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star formation.