Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population

Ramesh Mainali, Juna A. Kollmeier, Daniel P. Stark, Robert A. Simcoe, Gregory Walth, Andrew B. Newman, Daniel R. Miller

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99 Scopus citations


We present the Magellan/FIRE detection of highly ionized C iv λ1550 and O iii]λ1666 in a deep infrared spectrum of the z =6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously known Lyα. No corresponding emission is detected at the expected location of He ii λ1640. The upper limit on He ii, paired with detection of O iii] and C iv, constrains possible ionization scenarios. Production of C iv and O iii] requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (∼0.5 Ryd). As a pure recombination line, He ii emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher He ii flux. The conclusions based on ratios of metal line detections to helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.

Original languageEnglish (US)
Article numberL14
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Feb 10 2017


  • cosmology: observations
  • galaxies: evolution
  • galaxies: formation
  • galaxies: high-redshift

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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