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

Research output: Contribution to journalArticle

39 Citations (Scopus)

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 population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.

Original languageEnglish (US)
Article numberL14
JournalAstrophysical Journal Letters
Volume836
Issue number1
DOIs
StatePublished - Feb 10 2017

Fingerprint

metallicity
photons
FIRE (climatology)
galaxies
helium
recombination
excitation
ionization
infrared spectra
fine structure
metal
gases
gas
metals
energy
detection

Keywords

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

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Mainali, R., Kollmeier, J. A., Stark, D. P., Simcoe, R. A., Walth, G., Newman, A. B., & Miller, D. R. (2017). Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population. Astrophysical Journal Letters, 836(1), [L14]. https://doi.org/10.3847/2041-8213/836/1/L14

Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population. / Mainali, Ramesh; Kollmeier, Juna A.; Stark, Daniel P; Simcoe, Robert A.; Walth, Gregory; Newman, Andrew B.; Miller, Daniel R.

In: Astrophysical Journal Letters, Vol. 836, No. 1, L14, 10.02.2017.

Research output: Contribution to journalArticle

Mainali, R, Kollmeier, JA, Stark, DP, Simcoe, RA, Walth, G, Newman, AB & Miller, DR 2017, 'Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population', Astrophysical Journal Letters, vol. 836, no. 1, L14. https://doi.org/10.3847/2041-8213/836/1/L14
Mainali, Ramesh ; Kollmeier, Juna A. ; Stark, Daniel P ; Simcoe, Robert A. ; Walth, Gregory ; Newman, Andrew B. ; Miller, Daniel R. / Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population. In: Astrophysical Journal Letters. 2017 ; Vol. 836, No. 1.
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