Download - HII regions at high redshift
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HII regions at high redshift
Anticipating JWST NIRSpec spectroscopy
Bob Fosbury, Space Telescope - European Coordinating Facility, Garching, Germany
[email protected] www.stecf.org/~rfosbury
European Space Agency
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Example: The Lynx arcz = 3.357
Richness of UV spectrumHigh colour temperature of ionizing source(s)High ionization parameter (cf. PNe)Nebular metal abundance ~ 10-2 SolarStellar abundances ???Huge Lyman continuum luminosityTop-heavy IMF ?
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Time - redshift
Hig
hest
red
shift
qua
sar
Lynx
arc
Ear
th f
orm
s
Now
Lynx
clu
ster
<- µwave background comes from z~1000
A18
35 I
R19
16,
z =
10
z ~
2.5
rad
io g
alax
ies
3C radio galaxiesLBG
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OutlineA primary scientific driver for NIRSpec is a
survey of ~2500 galaxies to measure the redshifted nebular emission lines Known as: “Kennicutt Science”
Use familiar techniques to measure SFR, reddening, element (nebular) abundances, kinematics and presence of AGN
Predicated on the use of the restframe optical spectrum ([OII] 3727 -> [SII] 6725)Gives H to z ~ 6.6 and [OII] to z ~ 12.4
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Expectation is that many galaxies at high z will have emission line spectra representing the higher SFR at earlier epochs
This programme is severely limited from the groundGiving H only to z ~ 2.8
However, progress can be made from the ground using the restframe UV spectrum
What might we expect to measure and what can be deduced about the gas in the early universe in the presence of star formation or AGN?
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Systematics of AGN spectraAGN in the UV are characterised by
resonance and intercombination emission lines from regions of different density
The lines result from recombination and collisional excitation processes
They tell us about the nature of the ionization processes
the physical condition of the gas: Te and ne
the kinematicsand, to some extent, the chemical composition
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Complexities include:Resonance line transfer
The various effects of dust
Accounting for regions of different density and continuum opacity (radiation and matter-bounded clouds)
Photoionization codes can be used to understand the general behaviour of the nebulosities in the UV — but it is more difficult than in the optical
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TXS0211-122 z = 2.340
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Comparison of the kpc-scale ISM data from radio galaxies with the Quasar BLR data discussed by Hamann & Ferland
Quasar BLR
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Spectral sequence
From low-polarization, metal-poor (?) radio galaxies
to
High-polarization, metal-rich (?) ULIRG
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Ly/CIV & NV/CIV vs P(%) correlations
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UV spectra of AGN and Ly-break
Ly
NVSiIVOIV
NIV] HeII
OIII]
CIII]
CIV
NIII]
SiIII]
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Systematics of Ly-break galaxiesSee: Shapley et al. 2003, ApJ, 588, 65
Hot stars, HII regions, dust, outflows
Relationships seen between Ly EW, continuum slope and interstellar kinematics
Likely to be determined by the nature of the outflows (covering factor and velocity) and the metallicities of the HII regions
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Composite Ly-break spectra
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HII regions in the UVLocal HII regions have a UV spectrum
dominated by the continuum from the stellar population
Emission lines are weak since the nebular excitation is quite low — ionizing O stars are typically Teff ~ 40,000K
The presence of dust can render observations difficult
e.g., FOS spectroscopy of M101 HII regions by Rosa & Benvenuti (1994)
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M101 HII regions
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LH - relationTerlevich2 & Melnick 2002
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Expectations for HII galaxies at high z
Low stellar metallicitiesMassive (hot) stars => high ionization nebulæTop-heavy IMF for 1st generation (Pop III)
Low gas metallicitiesbut get rapid pollution by Type II supernovæ
The high stellar Teff means that, for a given bolometric luminosity (which is mostly below the Lyman limit) the UV/optical continuum is relatively weaker (~ T-3
eff in the Rayleigh-Jeans tail)can mean that the SED longward of Ly is dominated by
the 2-photon continuum
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Models: Panagia et al.
Dependence of [OIII] on Z and Teff
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Can we observe sources like this?
Look at the Ly emitters and distinguish between AGN and stellar-ionized nebulæ
Search for sources dominated by emission lines => characteristic loci in colour-colour diagrams
Get lucky -> the Lynx arcMagnified sources: the ‘critical line’ searches
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QuickTime™ and aSorenson Video decompressorare needed to see this picture.
The Lynx arc, z = 3.357Discovered as part
of the ROSAT Deep Cluster
Survey (Holden et al. 2001)
Studied in detail by Fosbury et al.
(2003)
Magnified by ~ x10 by two clusters
at z ~ 0.5
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QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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UV spectrum
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Optical spectrum
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SED
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Lensing modelMarco Lombardi
Lensed by two clusters at z = 0.57 and z = 0.54
Close to a caustic in the source planeTotal magnification between 8 and 16
(but could be some differential effects)
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‘Vital statistics’
Ionizing sourceTBB = 80,000 ± 10,000K
Qion = 1.6 x 1055 ph s-1 (assuming µ(A,B) = 10)
=> 105–6 massive (Pop III - like) stars
Nebulane ≤ 1000 cm-3 (from CIII]), Te ~ 20,000K, U = 0.1
Z/Zsun = 0.05, gas ≈ 30 km s-1 (from CIII] and OIII])
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The nebular continuum (light blue line) is simply scaled from the observed H flux with no reddening
S99 population models are for 107 Msun with a Salpeter IMF ( = 2.35; 1–100Msun) and Z/Zsun = 0.05
They produce ~ 20 x too few ionizing photons
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Can we find these sources in the field?
Colour-redshift loci determined predominantly by the emission lines (Lynx is VERY bright at K due to [OIII])
Will appear as Lyman forest dropouts at high z
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GOODS CDF-S BViz
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GOODS ACS (V1.7) #6746
zAB = 24.43
z = 5.6
r ~ 1kpc
cf. Lynx
F814WAB = 23.1
z = 3.36
m(z)=1.32
Ly
NIV] CIV
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ConclusionsThe very early phase of these massive
starbursts is very brightWe don’t see ANY starlight directly – just
the HII region glowThese are efficient H-ionization enginesThe restrame UV spectra are a lot more
interesting than those of local, metal-rich HII regions – the intercombination lines are good for abundance determinations
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They are considerably more massive than globular clustersWe estimated ~ 109 Msun for Lynx
The ionizing stars are close to our expectation for Pop III
Are these the very early phase of collapse of the galaxies that produce the metals seen in the oldest globular cluster stars (~ -2 dex)?