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First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lyα Emitters

First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lyα Emitters. ( arXiv:1208.3260, ApJ submitted ). Kimihiko Nakajima (U. Tokyo) in collaboration with … M. Ouchi, K. Shimasaku, T. Hashimoto, Y. Ono (U. Tokyo), and J. C. Lee ( STScI ).

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First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lyα Emitters

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  1. First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lyα Emitters ( arXiv:1208.3260, ApJ submitted ) Kimihiko Nakajima (U. Tokyo) in collaboration with … M. Ouchi, K. Shimasaku, T. Hashimoto, Y. Ono (U. Tokyo), and J. C. Lee (STScI) subaru UM @NAOJ on 16/Jan/2013

  2. Lyα Emitters (LAEs) 1. introduction z~3 • Galaxies with strong Lyα emission, faint UV continua LAE • low-mass & star-forming • “Efficient star-formation” • → building blocks of galaxies • in later epochs? • → early phase of galaxy • evolution? LBG SMG DRG SFR/M* Selection limit LAEs: Ono+10a, Finkelstein+09, Pirzkal+07, Lai+07, Gawiser+06 LBGs: Shapley+03, Papovich+01, Iwata+05 DRGs: van Dokkum+05 SMGs: Borys+05, Chapman+05 Ono+2010 spectroscopic confirmations

  3. 1. introduction our work ~LAEs’ spectroscopic properties • Near-Infrared (NIR) spectroscopy of z~2 LAEs • → rest-frame optical nebular emission lines (e.g., [OII], [OIII], Balmer lines) higher qion Metallicity 12+log(O/H) = lower Z • Ionization parameter ( qion) • Sion/n • sensitive to [OIII]/[OII] • Metallicity ( Z ) • 12+log(O/H) • sensitive to R23-index Ionization parameter SDSS • High qion suggests … • young star-forming galaxies? (+ low-Z) • high escape fraction of ionizing photons? =([OII]+[OIII]5007,4959)/Hβ

  4. 2. data J H K [OII]3727 Hβ, [OIII]5007, 4959 Hα, [NII]6584, 6548 NIR Spectroscopy • Subaru/Suprime-Cam to select z=2.2 LAEs • Keck/NIRSPEC (Time Exchange) • Magellan/MMIRS for NIR spectroscopy ↑ LAE candidates • 7 LAEs with Hα detection (>5σ) • (doubling prev. number of LAEs w/ NIR spec.) • 2 LAEs with [OII] & [OIII] • (first [OII] detection from high-z LAEs) Blue □ : LAEs observed w/ NIR spectroscopy

  5. Ionization parameter & Metallicity in LAEs 3. results + very high qion (x10 higher than local) (x3 higher than LBGs at z~2) + low metallicity (Z~0.14 Zsun) Metallicity 12+log(O/H) = Ionization parameter + relatively high qion (comparable to LBGs) + low metallicity (Z=0.25—0.95 Zsun) =([OII]+[OIII]5007,4959)/Hβ

  6. Implications of high qion & low Z (1) 4. discussion Fundamental Metallicity Relation (Mannucci+2010) • High qionis suggestive of • hard ionizing photon spectrum • → dominated with massive stars • in small HII regions • Both LAEs fall below • the mass-metallicity relation • of LBGs at z~2 • → less chemically-enriched LAEs can be very young galaxies representing early stage of galaxy evolution

  7. Implications of high qion & low Z (2) 4. discussion • Local galaxies typically have • much lower qion • Among local low-Z galaxies, • some have similar qion & Z • (e.g. Green Pea galaxies; Cardamone+09) • → likely analogs of high-z LAEs • GPs occupy only 0.07% of SDSS • galaxy sample (~10-5Mpc-3) • → ~2 orders of magnitude smaller • density than LAEs at z~2 GPs low-Z typical GPs elliptical Cardamone+09 GPs g-r High-qion galaxies may be much more abundant at high-z while rarely seen in local universe r-i

  8. Implications of high qion & low Z (3) 4. discussion Robertson+10 • z~7 galaxy surveys suggestpossible shortage of ionizing photons for Cosmic Re-ionization • (e.g., Ouchi+09, Robertson+10) • assuming no evolution of properties (e.g., fesc) • from z=3 to z~7 • Escape fraction of ionizing photons (fesc) is higher with higher qion • (achieved by density-bounded HII regions?) • LAEs’ optically thin conditions are suggested • observationally (Hashimoto+2013) / theoretically (Yajima+2012) Ouchi+09 100% 20% fesc=5% Yajima+2012 High-qion galaxies (LAEs) could produce ionizing photons efficiently, significantly contribute to Cosmic Re-ionization

  9. 4. summary Summary • Keck/NIRSPEC & Magellan/MMIRS NIR spectroscopy of LAEs at z=2.2 • 7 LAEs with Hα detection • 2 LAEs with [OII] & [OIII] detection • Very high ionization parameter and • low metallicity for at least one LAE • (both LAEs fall below the M-Z relation of z~2 LBGs) • → LAEs could … • be very young population • be similar to Green Peas, but the abundance • 　 is ~ x100 higher at z~2 • produce ionizing photons efficiently, • significantly contribute to Re-ionization (z>6)

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