3 Evidence for Ubiquitous High-EW Nebular Emission in z (cid:24) 7 Galaxies: towards a Clean Measurement of the Specific Star Formation Rate Using a Sample of Bright, Magnified Galaxies

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 (sSFR). However, corrections for this line emission have been difficult to perform reliably due to huge uncertainties on the strength of such emission at z > ∼ 5.5. In this chapter, we present the most direct observational evidence yet for ubiquitous high-equivalent width (EW) [OIII]+Hβ line emission in Lymanbreak galaxies at z ∼ 7, while also presenting 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/IRAC 4.5 micron flux provides a clean measurement of the stellar continuum light, in contrast with the 3.6 micron flux which is contaminated by the prominent [OIII]+Hβ lines. To ensure a high S/N for our IRAC flux measurements, we consider only the brightest (H160 < 26mag) magnified galaxies we have identified behind galaxy clusters. Remarkably, 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 [OIII]+Hβ is greater than 637Å for the average source. The bluest four sources from our seven-source sample require an even more extreme EW of 1582Å. We can also set a robust lower limit of > ∼ 4 Gyr−1 on the sSFR of our sample based on the mean SED.