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; akio.inoue@oamp.fr

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.

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

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

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.

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-.

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.

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.

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.

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

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

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.

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.