The [O III] plus H β equivalent width distribution at z ≃ 7: implications for the contribution of galaxies to reionization

We quantify the distribution of [O III] + H beta line strengths at z similar or equal to 7 using a sample of 20 bright (M-UV less than or similar to -21) galaxies. We select these systems over wide-area fields (2.3 deg 2 total) using a new colour-selection that precisely selects galaxies at z similar or equal to 6.63-6.83, a redshift range where blue Spitzer/IRAC [3.6]-[4.5] colours unambiguously indicate strong [O III] + H beta emission. These 20 galaxies suggest a lognormal [O III] + H beta EW distribution with median EW = 759(-113)(+112)angstrom and standard deviation = 0.26(-0.05)(+0.06) dex. We find no evidence for strong variation in this EW distribution with UV luminosity. The typical [0 EW at z similar or equal to 7 implied by our sample is considerably larger than that in massive star-forming galaxies at z similar or equal to 2, consistent with a shift towards larger average sSFR (4.4 Gyr(-1)) and lower metallicities (0.16 Z(circle dot)). We also find evidence for the emergence of a population with yet more extreme nebular emission ([O III] + H beta EW > 1200 angstrom) that is rarely seen at lower redshifts. These objects have extremely large sSFR (>30 Gyr(-1)), as would be expected for systems undergoing a burst or upturn in star formation. While this may be a short-lived phase, our results suggest that 20 per cent of the z similar or equal to 7 population has such extreme nebular emission, implying that galaxies likely undergo intense star formation episodes regularly at z > 6. We argue that this population may be among the most effective ionizing agents in the reionization era, both in terms of photon production efficiency and escape fraction. We furthermore suggest that galaxies passing through this large sSFR phase are likely to be very efficient in forming bound star clusters.

Automated summary

Analyzing a sample of 20 bright galaxies at z similar to 7, we found a distribution of [O III] + H beta line strengths, with some galaxies exhibiting extreme nebular emission. The typical [0 EW value at z similar to 7 is significantly higher than that at z similar to 2 in massive star-forming galaxies.