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First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lya Emitters | Kimihiko Nakajima
; Masami Ouchi
; Kazuhiro Shimasaku
; Takuya Hashimoto
; Yoshiaki Ono
; Janice C. Lee
; | Date: |
16 Aug 2012 | Abstract: | We present results from Keck/NIRSPEC and Magellan/MMIRS follow-up
spectroscopy of Lya emitters (LAEs) at z=2.2 identified in our Subaru
narrowband survey. We successfully detect Ha emission from seven LAEs, and
perform a detailed analysis of six LAEs free from AGN activity, two out of
which, CDFS-3865 and COSMOS-30679, have [OII] and [OIII] line detections. They
are the first [OII]-detected LAEs at high-z, and their [OIII]/[OII] ratios and
R23-indices provide the first simultaneous determinations of ionization
parameter and oxygen abundance for LAEs. CDFS-3865 has a very high ionization
parameter (q_{ion}=2.4^{+2.0}_{-0.9}x10^8 cm s^{-1}) and a low oxygen abundance
(12+log(O/H)=7.84^{+0.24}_{-0.25}) in contrast with moderate values of other
high-z galaxies such as Lyman-break galaxies (LBGs). COSMOS-30679 also
possesses a relatively high ionization parameter (q_{ion}=(3.4-15)x10^7 cm
s^{-1}) and a low oxygen abundance (12+log(O/H)=8.08-8.67), and falls below the
mass-metallicity relation of z~2 LBGs. Similarly, a low metallicity of
12+log(O/H)<8.4 is independently indicated for typical LAEs from a composite
spectrum and the [NII]/Ha index. Such high ionization parameters and low oxygen
abundances can be found in local star-forming galaxies, but this extreme local
population occupies only <0.3% of the SDSS spectroscopic galaxy sample with a
number density >10 times smaller than that of LAEs. With their high ionization
parameters and low oxygen abundances, LAEs would represent an early stage of
galaxy formation dominated by massive stars in compact star-forming regions.
Thus, low-mass galaxies such as LAEs produce ionizing photons efficiently
probably with a high escape fraction achieved by density-bounded HII regions,
which would significantly contribute to cosmic reionization at z>6. (abriged) | Source: | arXiv, 1208.3260 | Services: | Forum | Review | PDF | Favorites |
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