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

Lyα Emitters (LAEs) 1. introduction Ono+2010 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 SFR/M * z~3z~3  Galaxies with strong Lyα emission, faint UV continua low-mass & star-forming “Efficient star-formation” → building blocks of galaxies in later epochs? → early phase of galaxy evolution? spectroscopic confirmations

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

NIR Spectroscopy ↑ LAE candidates Blue □ : LAEs observed w/ NIR spectroscopy Subaru/Suprime-Cam to select z=2.2 LAEs Keck/NIRSPEC (Time Exchange) Magellan/MMIRS for NIR spectroscopy 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) JHK [OII] 3727 Hβ, [OIII] 5007, 4959 Hα, [NII] 6584, data

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

Implications of high q ion & low Z (1) High q ion is 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 4. discussion Fundamental Metallicity Relation (Mannucci+2010)

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

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

Summary  Keck/NIRSPEC & Magellan/MMIRS NIR spectroscopy of LAEs at z= 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) 4. summary