Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population
Abstract
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 z=2{--}3 population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2017
- DOI:
- 10.3847/2041-8213/836/1/L14
- arXiv:
- arXiv:1611.07125
- Bibcode:
- 2017ApJ...836L..14M
- Keywords:
-
- cosmology: observations;
- galaxies: evolution;
- galaxies: formation;
- galaxies: high-redshift;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 7 pages, 4 figures, 1 table. Accepted for publication to ApJL