Evidence for a hard ionizing spectrum from a z = 6.11 stellar population

Journal Article · Fri Feb 10 04:00:00 EST 2017 · Astrophysical Journal Letters

DOI:https://doi.org/10.3847/2041-8213/836/1/L14· OSTI ID:22869373

Mainali, Ramesh; Stark, Daniel P. ^[1]; Kollmeier, Juna A.; Newman, Andrew B. ^[2]; Simcoe, Robert A.; Miller, Daniel R. ^[3]; Walth, Gregory ^[4]

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1. Department of Astronomy, Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm N204, Tucson, AZ, 85721 (United States)
2. Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)
3. MIT-Kavli Center for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139 (United States)
4. University of California, Center for Astrophysics and Space Sciences, 9500 Gilman Drive, San Diego, CA 92093 (United States)

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.

• OSTI ID: 22869373
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 836; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CARBON
COSMOLOGY
EXCITATION
GALAXIES
GRAVITATIONAL LENSES
HELIUM
INFRARED SPECTRA
IONIZATION
METALLICITY
MULTIPLETS
OXYGEN
PHOTONS
RECOMBINATION