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New method for probing Lyman continuum galaxies
with high LyC escape fraction
Satoshi Yamanaka (Waseda University)
Akio Inoue (Waseda University)
Toru Yamada (JAXA/ISAS)
Ikuru Iwata (NAOJ),
Erik Zackrisson (Uppsala University)
Genoveva Micheva (AIP)
Mariko Kubo (NAOJ)
Ken Mawatari (ICRR)
Takuya Hashimoto (Waseda University)
Takuya Otsuka (Tohoku University)
Yuki Kimura (Tohoku University)

June 7, 2019
6th Galaxy Evolution Workshop 2019 @ ICRR

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Contents・Introduction➢ Cosmic reionization➢ Escape fraction of ionizing photons➢ EW (Hβ)−β method・Observation➢ Details of spectroscopic observation➢ Target objects

・Analysis
・Results
・Discussions
・Summary3/20

Introduction
Cosmic reionization- Neutral hydrogen in the intergalactic medium (IGM) is ionized- Reionization seems to be finished until z~6- The sources of cosmic reionization is not well understood- One of the key parameter is the escape fraction of ionizing photons

Robertson+10

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Introduction
Escape fraction of ionizing photons- The fraction of ionizing photons ( Lyman continuum; LyC, λrest<912A )which escape from galaxies into the surrounding IGM.
- One of the key parameter for understanding the sources ofcosmic reionization.

fesc,LyC

Neutral Hydrogenin IGM
Zackrisson+13

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Introduction
Escape fraction of ionizing photons- The escape fraction of LyC, fesc,LyC, is measured as the ratio ofobserved to intrinsic number of LyC photons- Due to the foreground IGM absorption by HI clouds,
it is almost impossible to directly measure the escaped LyC photons
for galaxies at z > 5.

ObserverZackrisson+13

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Introduction
Escape fraction of ionizing photons- The escape fraction of LyC, fesc,LyC, is measured as the ratio ofobserved to intrinsic number of LyC photons- Due to the foreground IGM absorption by HI clouds,
it is almost impossible to directly measure the escaped LyC photons
for galaxies at z > 5.

Zackrisson+13

Neutral Hydrogenin IGM

Observer7/20

Introduction
EW(Hβ)−β method- EW(Hβ)−β method is proposed byZackrisson+13- Zackrisson+13 predicts thatgalaxies are separately distributed on
the space of Hβ equivalent width (EW)
and the UV spectral slope βaccording to fesc.
- EW(Hβ) represents the amount of
LyC photons used in the galaxy.
- The UV spectral slope β represents
the stellar population of the galaxy,
and it roughly indicates the intrinsic
production rate of LyC photons

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Introduction
EW(Hβ)−β method- From the JWST observation,
we can indirectly investigate
the fesc,LyC value of star-forming
galaxies at z > 6
by using the EW(Hβ)−β method.
- Before applying this new method
to galaxies at z > 6,
the method needs to be verified
for galaxies for which fesc,LyC values
have already been obtained
through more direct method.

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Observation
Details ofspectroscopic observation- The target field is the SSA22 field.
( Steidel+98, Hayashino+04,Matsuda+04, Yamada+12)- One full-night observation of
the K-band multi-object
spectroscopy with Keck/MOSFIRE- The integration time of Mask1
is 2.45h.

10/20Observation
Details ofspectroscopic observation- Main target is LCG-1 which isthe Lyman Continuum Galaxy (LCG)
at zspec=3.287 reportedby Micheva+17.
- We simultaneously observe
19 Lyman Break Galaxies (LBGs)
at zphot ~ 3 in Mask1.
- Their spectroscopic redshift isdetermined by [OIII] λλ5007/4959.
- Our final goal is to measure
their emission line flux of Hβ λ4861.

11/20Observation
Target objects
4.5

NB359 - R (= LLyC/LUV)

43.5
3
2.5

NB
359,

1σ

Obs. Targets
Hβ det. sample
LCG-1- The color-magnitude diagram ofNB359-R vs. RNB- The mean uncertainties are shownat the top-right.

359,NB

3σ359,

5σ- In this work, we use and discuss
the Hβ-detected objects(green circle).

21.5
ƒesc,abs=0.5

10.502425
R26

12/20Observation
Main target : LCG-1 (Micheva+17)
OriginalSmoothed- The reduced 2-D and 1-D spectrum of LCG-1 is shown- The spectroscopic redshift estimated from [OIII] λ5007
is z[OIII]5007 = 3.2889 and it is consistent with Micheva+17- The observation and the data reduction are OK.

13/20Observation
Main target : LCG-1 (Micheva+17)Hβλ4861[OIII][OIII]λ4959 λ5007- The reduced 2-D and 1-D spectrum of LCG-1 is shown- The spectroscopic redshift estimated from [OIII] λ5007
is z[OIII]5007 = 3.2889 and it is consistent with Micheva+17- The observation and the data reduction are OK.

14/20Analysis
Estimation of UV spectral slope β- The UV spectral slope β is estimated from a least square fitting
for the broad-band filters of Subaru S'cam/R, i', z', and HSC/y.
- The uncertainty in β is the fitting uncertainty.

Estimation of EW(Hβ)- The continuum flux is estimated from the broad-band photometryof UKRIT/K after subtracting the contribution of the [OIII] and Hβ lines.
- The objects detected with UKIRT/K-band at 2σ ( K > 2σ )→ Assuming a flat continuum spectrum, the EW(Hβ) is calculated
from the continuum flux and the Hβ emission line flux.
- The objects not detected with UKIRT/K-band at 2σ ( K< 2σ )→ The confidence range of EW(Hβ) is estimated from the 3σ upper-limitof UKIRT/K and the extrapolation of UV spectral slope β.

15/20Results
Cyan circles or Blue triangle (LCG-1)
with error-bars
Observed UV spectral slope β-1.5LCG-1
K > 2σK < 2σ- The objects are detected with
UKIRT/K-band at 2σ ( K > 2σ ).
- The EW(Hβ) is estimated fromthe continuum flux and Hβ line flux.

-2

Magenta error-bars- The objects are not detected with
UKIRT/K-band at 2σ ( K < 2σ ).

-2.5-3103- The confidence range of EW(Hβ) isshown.
102
101
log10 EWrest(Hβ)

100- For the sake of clarity, the error-barof β is shown at the left edge ofthe error-bar of EW(Hβ).

16/20Discussions
LCG-1
K > 2σK < 2σObserved UV spectral slope β-1.5-2-2.5-3103

102
101
log10 EWrest(Hβ)

100

Assuming the following parameters, the observed NB359-R
indicates fesc,LyC ≲ 50% for all of our targets.
Assumption :intrinsic LLyC/LUV = 0.3, (Steidel+18),
AUV=1.67 (Micheva+17),
IGM_trans=0.4 (Inoue+14)17/20Discussions
Observed UV spectral slope β-1.5

LCG-1
K > 2σK < 2σ-2-2.5

LCG-1-3103

102
101
log10 EWrest(Hβ)

100- LCG-1 is the highest fesc,LyC LCG in our sample, and the fesc,LyC value
is 54% assuming the previous parameters.
- Although there is a large uncertainty in the assumptions, the observed βand EW(Hβ) of LCG-1 is not inconsistent with the prediction.

18/20Discussions
Case of LCG-1
HI

Part of LyC
escape fromgalaxy

HII

Optically
thin hole
Most of LyC photons
cannot escape from galaxy( global fesc,LyC~ a few % )ObserverWe directly observe
LyC photons
and identify it
as high fesc,LyC object

If the optically thin hole is aligned with the line-of-sight,"apparent" escape fraction of Lyman continuum is large.
However, "global" escape fraction of Lyman continuum is small.

19/20Discussions
Example caseHIHII

Observer
Non-ionizing UV photons

Hβ emission line
Half of LyC photons canescape from galaxy

LyC photons
cannot be observed

If there is an above situation, the direct LyC measurement
identifies this object as a fesc,LyC~0% galaxy.
On the other hand, the EW(Hβ)−β method identify this object
as a fesc,LyC~50% galaxy.20/20

Summary- We conduct the K-band multi-object spectroscopy
with Keck/MOSFIRE.
Observed UV spectral slope β-1.5- We detect and measure the emission line flux of[OIII] λλ5007, 4959 and Hβ λ4861 from 8 objects.

LCG-1
K > 2σK < 2σ

- We estimate EW(Hβ) and verify
the EW(Hβ)−β method-2- The fesc,LyC expected from the EW(Hβ)−β method
is consistent with the fesc,LyC expected fromthe NB359-R color.

-2.5-3103

102101
log10 EWrest(Hβ)

100- We need the further investigation for much higher
fesc,LyC LCGs, and it is interesting to apply
this method to star-forming galaxies at z > 6
by using future instruments such as JWST.