Spectroscopic detection of C iv λ1548 in a galaxy at<i>z</i> = 7.045: implications for the ionizing spectra of reionization-era galaxies

Abstract

We present Keck/MOSFIRE observations of UV metal lines in four bright\ngravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The\nspectrum of A1703-zd6, a highly-magnified star forming galaxy with a\nLyman-alpha redshift of z=7.045, reveals a confident detection of the nebular\nCIV emission line (unresolved with FWHM &lt; 125 km/s). UV metal lines are not\ndetected in the three other galaxies. At z~2-3, nebular CIV emission is\nobserved in just 1% of UV-selected galaxies. The presence of strong CIV\nemission in one of the small sample of galaxies targeted in this paper may\nindicate hard ionizing spectra are more common at z~7. The total estimated\nequivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3)\nare comparable to measurements of narrow-lined AGNs. Photoionization models\nshow that the nebular CIV line can also be reproduced by a young stellar\npopulation, with very hot metal poor stars dominating the photon flux\nresponsible for triply ionizing carbon. Regardless of the origin of the CIV, we\nshow that the ionizing spectrum of A1703-zd6 is different from that of typical\ngalaxies at z~2, producing more H ionizing photons per unit 1500A luminosity\nand a larger flux density at 30-50 eV. If such extreme radiation fields are\ntypical in UV-selected systems at z&gt;7, it would indicate that reionization-era\ngalaxies are more efficient ionizing agents than previously thought.\nAlternatively, we suggest that the small sample of Lyman-alpha emitters at z&gt;7\nmay trace a rare population with intense radiation fields capable of ionizing\ntheir surrounding hydrogen distribution. Additional constraints on high\nionization emission lines in galaxies with and without Lyman-alpha detections\nwill help clarify whether hard ionizing spectra are common in the reionization\nera.\n