Rest-frame UV emission line diagnostics of ionized ISM properties
in a Lyman-alpha emitter at z = 6.1
Kaho Sunaga1, Yoichi Tamura1, Ken Mawatari3, Akio Inoue4, Takuya Hashimoto4, Minju Lee5, Hiroshi Matsuo2, Akio Taniguchi1, Tom Bakx1,2
1) Nagoya University, 2) National Astronomical Observatory of Japan, 3) University of Tokyo, 4) Waseda University, 5) MPE

Abstract
We investigate the photoionization models of the Lyman-alpha emitter at redshift z = 6.11 by modeling rest-frame ultraviolet (UV) emission lines using a public photoionization
code. Our target, RXC J2248-ID3 (ID3), is abundant in observational photometric and spectroscopic data from rest-frame far-infrared to UV. A suite of multi-wavelength data
sets obtained for ID3 allows us to reveal the physical properties of the galaxy in the reionization era. SED modeling with these observations reveals that ID3 has young andmature stellar populations. On the other hand, the best-fit model fails to reproduce the intensities of UV lines, implying that ID3 has extreme ISM properties. To investigate them,we construct a photoionization model that reproduces the observed UV line ratios. We find that ID3 favors density-bounded H II region models with a high ionization parameter
larger than 10-2. Our modeling shows that radiation-bounded models are insufficient to reproduce a bright C IV emission, suggesting the presence of additional ionizing
mechanisms such as AGN or radiative-shock.

1. Introduction

2. Sample : RXC J2248-ID3 (ID3)• It is assumed that the galaxies at redshift z > 6 contributed the reionization, but
the nature of these galaxies and interstellar medium (ISM) are not wellunderstood.
• Atacama Large Millimeter/submillimeter Array (ALMA) observations revealthat the [O III] 88 µm line is a good indicator for z > 6 galaxies.

ISM properties differ fromthose that are often found in
the local universe.

[O III] 88Flux• Recent observations with ground-based telescopes (e.g., VLT) report thatbright UV nebular emission lines from z = 6 - 7 galaxies.
Ø These emission lines contain a variety of information about the physicalconditions in galaxies.

• Gravitationally lensed (magnification factor ~ 5.3) Lyman-α emitter at z = 6.105.
• Detections of C IV λ 1550 Å, O III] λ 1663 Å and [O III] 88 µm• Upper limit of N V λ 1240 Å, He II λ 1640 Å and C III] λλ 1907,1909 Å• Spectral energy distribution (SED) modeling with PANHIT (Mawatari in prep.)suggests that there are two populations; young (age ~ 2 Myr) and mature (age ~640 Myr).
• The best-fit model can not reproduce observed UV lines (bottom-right panel in Fig.1).

• Photoionization models with UV emission lines are well constructed at z = 2 4. (Byler et al. 2018, Nakajima et al. 2018)
Ø The strength of ratios of UV emission lines can be used to constrain the nature ofionizing source and physical properties of the galaxies.

500

700

Photoionization modelingoffer a unique opportunity
to unveil them.

Rest-UV lines
C IV

Combining the observed emission lines, we can explore the physical conditions ofthe galaxies at the epoch of reionization using photoionization models.

C III]

Figure 1. Red line shows the best-fit SED.

O III]
He II

1.5

1

3. Modeling Approach
We construct models of radiation-bounded and density-bounded H II regions using
photoionization code Cloudy (version c17.01; Ferland et al. 2017).

H II Radiation-bounded

Ionizing
photonIonizing photons areabsorbed by ISM.

It has both young (blue) and mature (orange)
stellar components. The gray squares areobserved flux or flux density, and the crosses
are model predictions. Right bottom panel
shows UV lines. Upper left panel shows the[O III] 88 µm line.

4. Results
H II

Density-bounded 
Ionizing photons canescape the H II region.

Radiation bounded model• C IV λ 1550 Å has large
discrepancies between observeddata and model prediction.
• There is less dependence on gas
density.

He IICCIV IVC III]

NVHe IIC IV

Neutral gas

Parameters

Figure 2. UV emission line ratios from• Ionizing source: Binary stellar population using BPASS 2.1 (Eldridge et al. 2017)• Gas density (nH): 102, 103 cm-3
• Ionization parameter (U): 10-3 to 100 with steps of 0.5 dex

radiation-bounded models. Observed line
ratio and those uncertainties are plotted assolid line and gray shadow, respectively.
Dashed line reproduce 2σ upper limit.

#(%)!='()*+ ,- .

/ 0 : The number of ionizing photons
23 : The separation between center source to illuminated face [cm]4 ∶ speed of light [cm/s]• Gas metallicity Z: 0.2 Zsun (obtained by SED model),•C/O, N/O and Si/O abundances are scaled by 0.15 (Berg et al. 2018)• Grains: Orion abundances• We adopted closed, spherical geometry• CMB emission at z = 6.11
• magnetic field 10 µGauss
The calculations are stopped at• (Radiation-bounded) the electron fraction to nH falls below 1 %• (Density-bounded) optical depth at 912 Å (τ912) : log(τ912) = 0.3, 0, -0.3, -0.5

Observed emission line ratios (Mainali et al. 2017) :C IV/O III] = 3.2 ± 1.1, N V/O III] < 0.49, He II/O III] < 0.41, C III]/O III] < 0.98

Goal:
To construct photoionization models to reproduce observed UV line ratios.

5. Multiple ionization sources

Density bounded model• To reproduce C IV/O III] ratio,high ionization parameter andsmall τ912 are preferred.
• The upper limit of C III]/O III]ratio gives a lower limit of theionization parameter;log (U) > -2.

log(τ912)

Figure 3. Circles show UV emission
line ratios from density-bounded models
with a fixed gas density nH = 103 cm-3.
Triangles are radiation-bounded models
with nH = 103 cm-3.

C III]

NV

Density-bounded H II region models with high ionization parameters
reproduce observed data well.

References•

Balestra, I., Vanzella, E., Rosati, P., et al. 2013, A&A, 559, L9•Berg, Danielle A., Erb, Dawn K., Auger, Matthew W., et al.
2018, ApJ, 859, 164•

Byler, N., Dalcanton, J. J., Conroy, C., et al. 2018, ApJ, 863, 14•• assuming power-law radiation fields, nH = 109 cm-3 .
• Stellar models: same as Figure 2 with nH = 103 cm-3.

Eldridge, J. J., Stanway, E. R., Xiao, L., et al. 2017, PASA, 34,e058•

Ferland, G. J., Chatzikos, M., Guzman, F., et al. 2017, RMxAA,53, 385

ü The model with 40 % of AGN contribution to total emission
reproduces a bright C IV emission line.

•Inoue, A. K. 2011, MNRAS, 415, 2920•

Mainali, R., Kollmeier, J. A., Stark, D. P., et al. 2017, ApJ, 836,
L14•

Monna, A., Seitz, S., Greisel, N., et al. 2014, MNRAS, 438,1417•

Nakajima, K., Schaerer, D., Le Fevre, O., et al. 2018, A&A,612, A94•

Schmidt, K. B., Huang, K. H., Treu, T., et al. 2017, ApJ, 839, 17

It is known that AGN and radiative-shock enhance highionization emission lines.
AGN + stellar models• AGN broad line region models:

Figure 4. UV emission line ratios as a function of ionization
parameters. Purple circles show pure stellar (BPASS) models, andred circles show pure AGN BLR models. Blue circles are the stellar(60 %) + AGN (40 %) models.