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 ≳ 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 ≳ 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.