constraints on lyman continuum flux escaping from galaxies at z~3 using vlt narrow-band photometry
DESCRIPTION
Constraints on Lyman continuum flux escaping from galaxies at z~3 using VLT narrow-band photometry. Akio K. INOUE 1 , I. IWATA, J.-M. DEHARVENG, V. BUAT, & D. BURGARELLA 1 Laboratoire d’Astrophysique de Marseille, FRANCE; [email protected]. Ionization history of the universe. - PowerPoint PPT PresentationTRANSCRIPT
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Constraints on Lyman continuum flux escaping from galaxies at z~3 using VLT narrow-band photometryAkio K. INOUE1,
I. IWATA, J.-M. DEHARVENG, V. BUAT, & D. BURGARELLA
1 Laboratoire d’Astrophysique de Marseille, FRANCE; [email protected]
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Ionization history of the universe
Cosmic reionization epoch End at z~6 (Becker et al.2001) Start at z~17 (Kogut et al.2003)
How to proceed? What is the main ionizing source; galaxie
s (stars) or others? Can the ionizing photons escape from gal
axies efficiently? We will discuss the escape of Lyman c
ontinuum from galaxies.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Lyman continuum escape
Steidel et al.(2001) succeeded in detecting the Lyman continuum photons from galaxies at z~3, whereas all the other attempts could not detect.
Spectroscopy was always adopted in the attempts, except for Malkan et al.(2003) who performed a broad-band photometry for z~1 galaxies and succeeded in obtaining the most strict upper limits on the escape fraction of Lyman continuum.
Steidel et al.(2001): the composite spectrum of 29 Lyman break galaxies at z~3.4.
Lyman limit
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Narrow-band photometry close to the Lyman limit (smaller IGM opacity)
very important for high-z galaxies moderate sensitivity (better than spectroscopy) select galaxies with an appropriate redshift
to avoid any contamination of light above the Lyman limit. The redshift criterion
for the OII+44 filter of VLT/FORS is z~3.2.
Our approach
Lyman limit (z=3.2)
Ly
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Observations
After the pre-selection of galaxies in the HDF-South based on the photometric redshift catalog of Labbé et al.(2003), we performed the spectroscopy with the VLT/FORS2 to know the accurate redshifts.
Because of unexpected systematic errors of the photometric redshifts, only two galaxies with an appropriate redshift were left in our sample.
We spent ~11 h exposure (on-source) toward the HDFS field through the narrow-band, OII+44 (central wavelength ~372 nm), filter with the VLT/FORS1.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Observational results
HDFS 85 1825
zsp 3.170 3.275
OII+44 <27.4 <26.6U300 <67.8 <28.1
B450 392 182
V606 699 405
I814 860 504
HDFS85
HDFS1825
Unit of the photometric data is nJy.ID of galaxies and U, B, V, I photometric data are taken from Casertano et al.(2000).Upper limits are 2-.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Definition of escape fractions
Absolute escape fraction:
Relative escape fraction:
intLC
outLC
esc F
Ff
)exp()/(
)/(
/
/ IGMLC
obsLCUV
intLCUVint
UVout
UV
intLC
outLC
relesc, FF
FF
FF
FFf
outLC
intLC
F
F : intrinsic Lyman continuum (LC) flux
: LC flux just outside of the galaxy
intUVF : intrinsic UV flux, out
UVF : UV flux just outside of the galaxy
observed LC flux: observed UV flux:
)exp( IGMLC
outLC
obsLC FF
outUV
obsUV FF
NOTE1: UV wavelength is set to be longer than Ly to be free from the IGM opacity.NOTE2: fesc,rel is not restricted to less than unity.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Comparison of the data at z~3
galaxies zsp (FUV/FLC)obs LCIGM fesc,rel ref.
HDFS85 3.170 >24.4 1.32 <0.46 a
HDFS1825 3.275 >14.6 1.99 <1.5 aLBG composite 3.40 17.7 1.50 0.76 b
DSF2237+116 C2 3.319 >37 1.41 <0.33 c
Q0000-263 D6 2.961 >35 1.04 <0.24 c
FLY99:957 3.367 >3.5 4.63 ... d
FLY99:825 3.369 >18 4.64 ... d
FLY99:824 3.430 >12 4.94 ... da: this work, narrow-band (OII+44) photometry; b: Steidel et al.(2001), spectroscopy; c: Giallongo et al.(2002), spectroscopy; d: broad-band (U300) photometry.NOTE1: (FUV/FLC)int is given by a population synthesis model to calculate fesc,rel.NOTE2: IGM is calculated by a model of the mean IGM opacity.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Advantages of our approach
Narrow-band photometry can put the most strict constraint on the relative escape fraction for normal LBGs. For individual galaxies, we have reached a similar lev
el of FUV/FLC to the composite spectrum of 29 LBGs by Steidel et al.
Higher sensitivity of the broad-band U are lost by heavier IGM absorption because the effective wavelength in the source rest-frame is as short as ~700A.
Giallongo’s LBGs are ~4 times brighter than ours and Steidel’s.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
To estimate absolute escape fraction
Absolute escape fraction is
FLCint and LC
IGM can be estimated by a multi-color SED fitting.
“Physically” based SED fitting method: NOT just a 2 fitting. ISM dust and IGM absorption amounts, age, and SFR
can be estimated from B, V, I, K data, assuming dust attenuation law (e.g. Calzetti’s law), IGM cloud number distribution (only functional shape), metallicity (e.g. 40% of solar), and constant star formation history.
. )exp( IGMLCint
LC
obsLC
esc F
Ff
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Physically based SED fitting
A unique solution in ISMIGM can be found from observed B-I and V-I because the significance of the IGM absorption is different between B and V.
However, to do that, we have to know the intrinsic spectrum which depends on SFH and metallicity.
Assuming a constant SFR, I-K is a good indicator of the age. Metallicity effect is secondary.
dust attenuation
IGM
abs
orpt
ion
B-I
V-Ired: Solar, blue: 1/5 Solar, green: 1/20 Solar metallicity
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Fitting results
U
B V
I
J
H K
U
B
V IJ
HK
OII+44 OII+44
HDFS FOIIobs FOII
int OIIIGM fesc
85 <27.4 752 1.2 <0.12
1825 <26.5 393 1.6 <0.35Unit of flux density is nJy and upper limits are 2-.NOTE: IGM relative to a mean one is estimated by the fitting.
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September 6—10 2004
Starburst 2004 at the Institute of Astronomy, Cambridge
Conclusions
We did not find any significant escape of Lyman continuum from two Lyman break galaxies (LBGs) at z~3 through the narrow-band photometry with VLT.
The two LBGs have a small absolute escape fraction, less than 10—30%.
For the moment, the sample size is too small to conclude that many LBGs have a small escape of Lyman continuum.
Narrow-band photometry can put a stronger constraint on the escape fraction of individual normal LBGs than spectroscopy and broad-band photometry.