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Article overview
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Evidence for a Hard Ionizing Spectrum from a z=6.11 Stellar Population | Ramesh Mainali
; Juna A. Kollmeier
; Daniel P. Stark
; Robert A. Simcoe
; Greg Walth
; Andrew B. Newman
; Daniel R. Miller
; | Date: |
22 Nov 2016 | Abstract: | We present the Magellan/FIRE detection of highly-ionized CIV 1550 and OIII]
1666 in a deep infrared spectrum of the z=6.11 gravitationally lensed low-mass
galaxy RXC J2248.7-4431-ID3, which has previously-known Lyman-alpha. No
corresponding emission is detected at the expected location of HeII 1640. The
upper limit on HeII paired with detection of OIII] and CIV constrains possible
ionization scenarios. Production of CIV and OIII] 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, HeII 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 HeII 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 AGN. | Source: | arXiv, 1611.7125 | Services: | Forum | Review | PDF | Favorites |
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