[論文レビュー] UNCOVER: A NIRSpec Census of Lensed Galaxies at z=8.50-13.08 Probing a High AGN Fraction and Ionized Bubbles in the Shadow

Seiji Fujimoto, Bingjie Wang+34 more|arXiv (Cornell University)|Aug 22, 2023

Galaxies: Formation, Evolution, Phenomena被引用数 18

ひとこと要約

JWST/NIRSpec prism 分光観測を用いた Abell 2744 の背後のレンズ星系における z≥8.5 の tengalaxies の確認、非無視可能な AGN 比の証拠の明示、z=8.5 での巨大イオン化バブルの同定、AGN が再電離に寄与する可能性を示唆。

ABSTRACT

We present JWST NIRSpec prism spectroscopy of gravitationally lensed galaxies at $z\gtrsim9$ found behind the massive galaxy cluster Abell 2744 in the UNCOVER Cycle 1 Treasury Program. We confirm the source redshift via emission lines and/or the Ly$α$ break feature for ten galaxies at z=8.50-13.08 down to $M_{ m UV}=-17.3$. We achieve a high confirmation rate of 100\% for $z>9$ candidates reported in Atek et al. (2023). Using six sources with multiple emission line detections, we find that the offset of the redshift estimates between the lines and the Ly$α$ break alone with prism can be as large as $\pm0.2$, raising caution in designing future follow-up spectroscopy for the break-only sources with ALMA. With spec-$z$ confirmed sources in UNCOVER and the literature, we derive lower limits on the rest-frame ultraviolet (UV) luminosity function (LF) at $z\simeq9$-12 and find these lower limits to be consistent with recent photometric measurements. We identify at least two unambiguous and several possible active galactic nucleus (AGN) systems based on X-ray emission, broad line (BL) H$β$, high ionization line (e.g., NIV]1487, CIV1549) detections, and excess in UVLF. This requires the AGN LFs at $z\simeq$ 9-10 to be comparable or even higher than the X-ray AGN LF estimated at $z\sim6$ and indicates a plausible cause of the high abundance of $z>9$ galaxies claimed in recent photometric studies may be AGNs. One UV-luminous source is confirmed at the same redshift as a dusty BL AGN at $z=8.50$ with a physical separation of 380 kpc in the source plane. These two sources show blueward Ly$α$ line or continuum emission, suggesting that they reside in the same ionized bubble with a radius of $7.69\pm0.18$ pMpc. Our results imply that AGNs have a non-negligible contribution to cosmic reionization.

研究の動機と目的

• UNCOVER behind Abell 2744 で z≥8.5 の銀河候補の redshift を JWST NIRSpec prism spectroscopy を用いて Confirm する。
• スペクトロスコピー赤方偏移を含む z≈9–12 の UV 光度関数を、レンズ効果で補正された faint sources を含めて評価する。
• AGN の兆候（広い線、高イオン化線、X 線関連）を識別し、再電離時の電離予算への影響を評価する。
• Lyα break の挙動と線スペクトルから推測される高赤方偏の源の周囲に存在する ionized bubble の存在と性質を調査する。

提案手法

• UNCOVER の JWST NIRCam イメージを用いて lensing magnification を伴う z≥9 の銀河候補を選択する。
• 7 つの MSA マスクで NIRSpec prism MOS 観測を実施し、複数の高-z 候補の低分解能スペクトルを得る。
• JWST パイプラインと msaexp でデータを還元し、カスタム補正を適用し、逆分散重み付けを用いて1Dスペクトルを抽出する。
• z_spec を決定するために z=0.1–20 で eazy ベースのテンプレートフィッティングを行い、赤方偏移依存の SFH、塵減衰、IGM ダンピングウィングを組み込む。
• Lyα break を識別し、SNR ≥ 2.5 の faint emission lines を特定して redshift を確保する。複数行源の z_line と z_break を比較する。

Figure 1: NIRCam RGB (R: F444W, G: F356W, B: F277W) map of A2744 taken in UNCOVER (Bezanson et al. 2022 ) . The white-shaded region indicates the highly magnified area with magnifications of $\geq 2$ (Furtak et al. 2023c ) . The yellow squares show the positions of the 10 sources that are spectrosco

実験結果

リサーチクエスチョン

• RQ1 UNCOVER で Abell 2744 の背後にある z≥8.5 銀河のスペクトロスコピック redshift 分布はどのようになるか？
• RQ2 スペクトロスコピーで確認されたソースを含む、lensed, faint galaxies を含めた z≈9–12 での UV 光度関数はどうなるか？
• RQ3 高赤方偏源は AGN 活動の兆候を示すか、再電離時のイオン化予算にどのような影響を与えるか？
• RQ4 close-proximity な高-z 源の周囲に巨大な ionized bubbles の証拠はあるか、それはそれらのイオン化源について何を意味するか？

主な発見

• Ten galaxies are spectroscopically confirmed at z≥8.5, with UV magnitudes down to M_UV≈−17.3 after lensing correction.
• At least two unambiguous AGN systems are identified based on X-ray, broad Hβ, and high-ionization lines (e.g., N IV], C IV, He II), suggesting AGN LFs at z≈9–10 may be comparable to or higher than the z∼6 X-ray AGN LF.
• One UV-luminous source at z=8.50 is physically associated with a broad-line AGN at z=8.50 with a 380 kpc separation in the source plane, and they exhibit emission blueward of Lyα, indicating a giant ionized bubble with radius 7.69±0.18 pMpc.
• Several sources show unusually high [O III] 5008/Hβ ratios (e.g., >10, up to 16.3+21.8−5.9) that may indicate AGN activity or strong radiation fields; some results persist under various background subtraction methods.
• The z_phot–z_spec comparison for four A23 sources shows agreement within ~1–2σ, and z_phot estimates generally do not overestimate redshifts as strongly as some prior studies; breaks and lines yield consistent redshift constraints for most sources.
• Redshift offsets between z_line and z_break for multi-line sources are small (Δz′ ≈ −0.2 to +0.2), implying Lyα break-based redshift estimates can have ~±0.2 uncertainty

Figure 2: Zoom-in $3\mbox{\kern 0.55974pt.\kern-4.09024pt\arcsec\kern-0.43057pt}6\times 3\mbox{\kern 0.55974pt.\kern-4.09024pt\arcsec\kern-0.43057pt}6$ NIRCam RGB (R: F444W, G: F356W, B: F277W) cutouts of the 10 sources whose spec- $z$ are successfully confirmed. The rectangles show the shutter conf