First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance

in Lyα Emitters( arXiv:1208.3260, ApJ submitted )

subaru UM @NAOJ 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

LAE

Ono+2010

Selection limit

LBG

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

SMGDRG

SFR/

M*

z~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 ( qion )

• Sion/n

• sensitive to [OIII]/[OII]

Metallicity ( Z )• 12+log(O/H)• sensitive to R23-index

our work ~ LAEs’ spectroscopic properties1. 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

SDSS

Ionization parameter

Metallicity12+log(O/H) =

higher qion lower Z

High qion 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)

J H K[OII]3727 Hβ, [OIII]5007, 4959 Hα, [NII]6584, 6548

2. data

Ionization parameter & Metallicity in LAEs

=([OII]+[OIII]5007,4959)/Hβ

Metallicity12+log(O/H) =

Ionization parameter

3. results

+ very high qion 　 (x10 higher than local) 　 (x3 higher than LBGs at z~2)

+ low metallicity 　 (Z~0.14 Zsun)

+ relatively high qion 　 (comparable to LBGs)

+ low metallicity 　 (Z=0.25—0.95 Zsun)

Implications of high qion & low Z (1)

• High qion 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 qion & low Z (2)• 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

High-qion galaxies may be much more

abundant at high-z while rarely seen in local universe

typical

low-ZGPs

Cardamone+09

r-i

g-r

GPs

GPs elliptical

Implications of high qion & low Z (3)

High-qion galaxies (LAEs) could produce ionizing photons efficiently,

significantly contribute to Cosmic Re-ionization

Robertson+10

4. 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., 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

fesc=5%

20%

100%

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)

4. summary