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

Abstract

Growing observational evidence now indicates that nebular line emission has a\nsignificant impact on the rest-frame optical fluxes of z~5-7 galaxies observed\nwith Spitzer. This line emission makes z~5-7 galaxies appear more massive, with\nlower specific star formation rates. However, corrections for this line\nemission have been very difficult to perform reliably due to huge uncertainties\non the overall strength of such emission at z&gt;~5.5. Here, we present the most\ndirect observational evidence yet for ubiquitous high-EW [OIII]+Hbeta line\nemission in Lyman-break galaxies at z~7, while also presenting a strategy for\nan improved measurement of the sSFR at z~7. We accomplish this through the\nselection of bright galaxies in the narrow redshift window z~6.6-7.0 where the\nIRAC 4.5 micron flux provides a clean measurement of the stellar continuum\nlight. Observed 4.5 micron fluxes in this window contrast with the 3.6 micron\nfluxes which are contaminated by the prominent [OIII]+Hbeta lines. To ensure a\nhigh S/N for our IRAC flux measurements, we consider only the brightest\n(H_{160}&lt;26 mag) magnified galaxies we have identified in CLASH and other\nprograms targeting galaxy clusters. Remarkably, the mean rest-frame optical\ncolor for our bright seven-source sample is very blue, [3.6]-[4.5]=-0.9+/-0.3.\nSuch blue colors cannot be explained by the stellar continuum light and require\nthat the rest-frame EW of [OIII]+Hbeta be greater than 637 Angstroms for the\naverage source. The bluest four sources from our seven-source sample require an\neven more extreme EW of 1582 Angstroms. Our derived lower limit for the mean\n[OIII]+Hbeta EW could underestimate the true EW by ~2x based on a simple\nmodeling of the redshift distribution of our sources. We can also set a robust\nlower limit of &gt;~4 Gyr^-1 on the specific star formation rates based on the\nmean SED for our seven-source sample. (abridged)\n