Upload
philippa-barber
View
223
Download
3
Tags:
Embed Size (px)
Citation preview
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