first spectroscopic evidence for high ionization state and low oxygen abundance in lyα emitters (...
TRANSCRIPT
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