z≳ 7 galaxies with red Spitzer/IRAC [3.6]-[4.5] colors in the full CANDELS data set: The brightest-known galaxies at z~ 7-9 and a probable spectroscopic confirmation at z = 7.48

Abstract

We identify four unusually bright (H ${}_{160,{AB}}$ < 25.5) galaxies from Hubble Space Telescope (HST) and Spitzer CANDELS data with probable redshifts z ~ 7–9. These identifications include the brightest-known galaxies to date at z gsim 7.5. As Y-band observations are not available over the full CANDELS program to perform a standard Lyman-break selection of z > 7 galaxies, we employ an alternate strategy using deep Spitzer/IRAC data. We identify z ~ 7.1–9.1 galaxies by selecting z gsim 6 galaxies from the HST CANDELS data that show quite red IRAC [3.6]−[4.5] colors, indicating strong [O iii]+Hβ lines in the 4.5 μm band. This selection strategy was validated using a modest sample for which we have deep Y-band coverage, and subsequently used to select the brightest z ≥ 7 sources. Applying the IRAC criteria to all HST-selected optical dropout galaxies over the full ~900 arcmin2 of the CANDELS survey revealed four unusually bright z ~ 7.1, 7.6, 7.9, and 8.6 candidates. The median [3.6]−[4.5] color of our selected z ~ 7.1–9.1 sample is consistent with rest-frame [O iii]+Hβ EWs of ~1500 Å in the [4.5] band. Keck/MOSFIRE spectroscopy has been independently reported for two of our selected sources, showing Lyα at redshifts of 7.7302 ± 0.0006 and ${8.683}_{-0.004}^{+0.001}$, respectively. We present similar Keck/MOSFIRE spectroscopy for a third selected galaxy with a probable 4.7σ Lyα line at z spec = 7.4770 ± 0.0008. All three have H160-band magnitudes of ~25 mag and are ~0.5 mag more luminous (M 1600 ~ −22.0) than any previously discovered z ~ 8 galaxy, with important implications for the UV luminosity function (LF). Our three brightest and highest redshift z > 7 galaxies all lie within the CANDELS-EGS field, providing a dramatic illustration of the potential impact of field-to-field variance

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