EVIDENCE FOR UBIQUITOUS HIGH-EQUIVALENT-WIDTH NEBULAR EMISSION IN z ～ 7 GALAXIES: TOWARD A CLEAN MEASUREMENT OF THE SPECIFIC STAR-FORMATION RATE USING A SAMPLE OF BRIGHT, MAGNIFIED GALAXIES

摘要

Growing observational evidence indicates that nebular line emission has a significant impact on the rest-frame optical fluxes of z ~ 5-7 galaxies. This line emission makes z ~ 5-7 galaxies appear more massive, with lower specific star-formation rates (sSFRs). However, corrections for this line emission have been difficult to perform reliably because of huge uncertainties on the strength of such emission at z 5.5. In this paper, we present the most direct observational evidence thus far for ubiquitous high-equivalent-width (EW) [O III] + Hβ line emission in Lyman-break galaxies at z ~ 7, and we present a strategy for an improved measurement of the sSFR at z ~ 7. We accomplish this through the selection of bright galaxies in the narrow redshift window z ~ 6.6-7.0 where the Spitzer/Infrared Array Camera (IRAC) 4.5?μm flux provides a clean measurement of the stellar continuum light, in contrast with the 3.6?μm flux, which is contaminated by the prominent [O III] + Hβ lines. To ensure a high signal-to-noise ratio for our IRAC flux measurements, we consider only the brightest (H 160 26 mag) magnified galaxies we have identified behind galaxy clusters. It is remarkable that the mean rest-frame optical color for our bright seven-source sample is very blue, [3.6]-[4.5] = –0.9 ± 0.3. Such blue colors cannot be explained by the stellar continuum light and require that the rest-frame EW of [O III] + Hβ is greater than 637 ? for the average source. The four bluest sources from our seven-source sample require an even more extreme EW of 1582 ?. We can also set a robust lower limit of 4 Gyr–1 on the sSFR of our sample based on the mean spectral energy distribution.