arXiv subjects

Shunta Shimizu

Publications and source records attributed to Shunta Shimizu.

8 recordsLinked to original sources

arXiv2026

H I Absorbers as Beacons of Hidden Structure at $z \sim 3$: Multi-Component, Metal-Rich Absorption System near a Protocluster

H I gas traces the large-scale structure and provides the primary fuel for star formation. High-$z$ protoclusters are ideal laboratories to study how H I gas is accreted and consumed during the formation of the most massive structures in the Universe. However, much remains unknown about the distribution and physical state of their H I gas. We examine a rare configuration in which a protocluster candidate is located in front of a quasar at $z=3.09$. Our spectroscopic campaign confirms a protocluster at $z=3.079$: however, no corresponding strong H I absorption is found in the background quasar spectrum. Instead, we serendipitously discover a prominent H I absorption feature at $z\sim 3.01$, offset by $\sim 60$ cMpc from the centre of the protocluster. Spanning an exceptionally broad velocity range of $\sim 2000$ km s$^{-1}$ ($\sim 40$ cMpc), this absorption is decoupled from the confirmed member galaxies. Detailed kinematic modelling reveals this absorption comprises five distinct components rather than a single cloud. Moreover, one of these components exhibits a super-solar metallicity ($[\mathrm{O/H}] = +1.19^{+0.91}_{-0.78}$). We propose two physical scenarios for this unique system: (1) an additional, hidden massive protocluster along the line of sight, and/or (2) metal-rich outflows and metal-poor inflows driven by a single massive galaxy. The discovery highlights that while protoclusters are not universally associated with strong H I absorption, targeting the strong H I absorbers may serve as a beacon for uncovering massive, metal-rich protoclusters or complex gas kinematics in the early Universe.

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arXiv2026

Subaru meets JWST: A Direct Measurement of Ly$\boldsymbol{\alpha}$ Escape Fraction at $\boldsymbol{z\simeq6.2}$ with Dual Narrow-Band Imaging

We present a direct measurement of the Ly$\alpha$ escape fraction, $f^{\rm Ly\alpha}_{\rm esc}$, for H$\alpha$ emitters (HAEs) at $z\simeq6.2$ in the JWST CEERS field by combining JWST/NIRCam F470N imaging with Subaru/HSC NB872 imaging. This unique pair of narrow-band filters enables the simultaneous measurement of Ly$\alpha$ and H$\alpha$ fluxes from galaxies during the epoch of reionization (EoR). We select 84 HAEs from F470N excesses, among which 56 have reliable NB872 photometry and 19 are detected in Ly$\alpha$ at $>2\sigma$ significance. The completeness-weighted stack of the HAE sample yields a median $f^{\rm Ly\alpha}_{\rm esc}$ at $z\simeq6.2$ of $0.106^{+0.066}_{-0.044}$, which is in good agreement with recent measurements at similar redshifts. We further find no significant dependence of the stacked $f_{\rm esc}^{\rm Ly\alpha}$ on the lower limit of H$\alpha$ luminosity over the luminosity range probed by our sample. If Ly$\alpha$ escape traces Lyman continuum leakage, this may suggest that relatively luminous HAEs, rather than only the faintest galaxies, can provide an important contribution to the ionizing photon budget during the EoR. For individual galaxies, $f^{\rm Ly\alpha}_{\rm esc}$ positively correlates with Ly$\alpha$ equivalent width and negatively correlates with the UV continuum slope $\beta$ and the rest-frame UV size, while no significant correlation is found with SED-derived $E(B-V)$, or rest-frame optical size, although these trends are based on a limited sample. These results suggest that the galaxy-to-galaxy variation in $f_{\rm esc}^{\rm Ly\alpha}$ is more closely linked to compact, low-attenuation star-forming components traced by the UV continuum than to global dust attenuation or the overall stellar structure.

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arXiv2026

Discovery of a $z\simeq 4.9$ Lyman-$\alpha$ Emitter Protocluster: Wavelength-Dependent Environmental Effects on Galaxy Structure

We report the discovery of a Lyman-alpha emitter (LAE) protocluster at z = 4.90 in the COSMOS field, comprising four distinct overdensity peaks spanning ~65 x 36 cMpc$^2$, with the primary concentration exhibiting a 4-fold surface density enhancement relative to the field within a 1.5 proper Mpc (pMpc) radius. Using SILVERRUSH narrowband survey data combined with JWST COSMOS-Web imaging, we perform a first systematic rest-frame optical and UV morphological comparison of protocluster versus field LAEs at this redshift using JWST NIRCam rest-frame UV (F150W, ~2540 Angstrom) and optical (F277W, ~4700 Angstrom) imaging. Sersic profile fitting for 16 protocluster members and 23 field LAEs reveals a size difference: protocluster LAEs are $\sim$40% larger in rest-optical (median $R_e = 0.81_{-0.04}^{+0.26}$ kpc vs. $0.58_{-0.04}^{+0.11}$ kpc, $p = 0.041$) with no significant difference in rest-UV ($p = 0.51$) or Sersic index. At fixed stellar mass, protocluster LAEs are offset by $+0.12$~dex ($\simeq$31%) in rest-optical size from the field size-mass relation (68% CI: $[+0.08, +0.21]$; Mann-Whitney $p = 0.033$), with 75% exhibiting positive size residuals compared to 44% of field LAEs. This wavelength-dependent environmental signature suggests that protocluster environments at $z \simeq 5$ preferentially affect extended stellar populations, possibly through tidal interactions, with no significant environmental difference detected in rest-UV sizes, providing observational evidence for environmental influences on the structure of LAEs during the early build-up phase of cosmic star formation.

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arXiv2026

Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs). XXV. Large-scale environments of low-luminosity quasars at $z\sim6$ traced by Ly$\alpha$ emitters

High-$z$ quasars are believed to reside in massive dark matter haloes (DMHs), suggesting that they reside in galaxy overdense regions. However, previous observations have shown a range of environments around them. These fields have been limited to luminous quasars ($M_{1450}\lesssim-25$), for which photoevaporation may hinder galaxy formation in their vicinity. Here, we present Subaru/Hyper-Suprime Cam observations of the environments of four low-luminosity quasars ($-24<M_{1450}<-22$) at $z\sim6.18$, which are expected to have a smaller photoevaporation effect. We detect Lyman $\alpha$ emitters (LAEs) with narrowband NB872 imaging, and measure the local LAE overdensity. One quasar (J0844$-$0132) resides in an overdense region ($\delta_\mathrm{LAE}=1.97\pm0.40$), whereas the other three fields are consistent with no overdensity. These results hold over the proximity zone of each quasar, suggesting that the diverse environment around quasars is independent of photoevaporation. We find no significant correlation between the LAE overdensities and the characteristics of host galaxies and supermassive black holes. Our quasars have host stellar mass measurements from JWST, allowing us to compare them with the LAE overdensity around galaxies without quasar activity with comparable stellar masses. We find that the LAE overdensity in the J0844$-$0132 field is stronger than that of galaxies with similar stellar mass at $z\sim6$.

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arXiv2025

Age dependence of Ly\alpha\ escape fraction of Ly\alpha\ emitters and their significant role in cosmic reionization

We study the Ly$\alpha$ escape fraction, $f_{\mathrm{esc}}^{\mathrm{Ly\alpha}}$ of Ly$\alpha$ emitters (LAEs) identified by Subaru/HSC narrowband imaging at $z = 2.2-6.6$, using publicly available deep imaging data from HST and JWST. We perform SED fitting for 127 LAEs at $0.4 - 5.0\, \mathrm{\mu m}$ to estimate their physical properties robustly, and confirm that two distinct LAE populations exist: young LAEs (< 100 Myr) and old LAEs (> 100 Myr). Young LAEs are characterized by burst-like star formation activity and low dust content, significantly differing from Lyman-break galaxies (LBGs) at the same stellar mass, while old LAEs show similar star formation activity to LBGs, yet with lower dust content and more compact morphology in rest-UV/optical than LBGs. The $f_{\mathrm{esc}}^{\mathrm{Ly\alpha}}$ of LAEs is anticorrelated with stellar mass, and this correlation is found to depend on the age of LAEs, such that old LAEs show a weaker anticorrelation than young LAEs, and tend to exhibit higher $f_{\mathrm{esc}}^{\mathrm{Ly\alpha}}$ than young LAEs at a given stellar mass. This implies that Ly$\alpha$ photons escape more efficiently from old LAEs, possibly through the low-density channels of HI and dust created by outflows. The average $f_{\mathrm{esc}}^{\mathrm{Ly\alpha}}$ of young LAEs remains nearly constant at $\sim40$% at $z = 2.2-6.6$, suggesting that the previously observed evolution of global $f_{\mathrm{esc}}^{\mathrm{Ly\alpha}}$ of star-forming galaxies (SFGs) is due to the changes in the LAE fraction among the SFGs. Converting $f_{\mathrm{esc}}^{\mathrm{Ly\alpha}}$ to Lyman continuum escape fraction using empirical relations, we demonstrate that LAEs alone can supply the ionizing photons necessary for reionization at $z\sim6$, causing rapid and late reionization.

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arXiv2025

Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs). XXIII. The powering mechanisms of the Ly$\alpha$ haloes around high-$z$ quasars probed by slit spectroscopy

We present the analysis of Ly$\alpha$ haloes around faint quasars at $z\sim4$ and $z\sim6$. We use 20 and 162 quasars at $z\sim4$ and $z\sim6$, taken by slit spectroscopy, and detect Ly$\alpha$ haloes around 12 and 26 of these quasars, respectively. The average absolute magnitudes of the detected quasars are $\langle M_{1450} \rangle = -23.84$ mag at $z\sim4$ and $\langle M_{1450} \rangle = -23.68$ mag at $z\sim6$, which are comparable at $z\sim4$ and 3 mag fainter at $z\sim6$ than those of previous studies. The median surface brightness profiles are found to be consistent with an exponential curve, showing a hint of flattening within a radius of 5 kpc. The Ly$\alpha$ haloes around these faint quasars are systematically fainter than those around bright quasars in the previous studies. We confirm the previous results that the Ly$\alpha$ halo luminosity depends on both the ionizing and Ly$\alpha$ peak luminosities of quasars at $z\sim4$, and also find that a similar correlation holds even at $z\sim6$. While the observed Ly$\alpha$ halo luminosity is overall attributed to recombination emission from the optically thin gas clouds in the CGM, its luminosity dependences can be consistently explained by the partial contributions of recombination radiation from the optically thick clouds, which is thought to originate from the CGM centre, and the scattered Ly$\alpha$ photons, which is resonantly trapped at the CGM centre and escaping outside of the haloes.

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arXiv2024

Predicting Ly$\alpha$ Emission from Distant Galaxies with Neural Network Architecture

The Ly$\alpha$ emission line is a characteristic feature found in high-$z$ galaxies, serving as a probe of cosmic reionization. While previous works present various correlations between Ly$\alpha$ emission and physical properties of host galaxies, it is still unclear which characteristics predominantly determine the Ly$\alpha$ emission. In this study, we introduce a neural network approach to simultaneously handle multiple properties of galaxies. The neural-network-based prediction model that identifies Ly$\alpha$ emitters (LAEs) from six physical properties: star formation rate (SFR), stellar mass, UV absolute magnitude $M_\mathrm{UV}$, age, UV slope $\beta$, and dust attenuation $E(B-V)$, obtained by the SED fitting. The network is trained with galaxy samples from the VANDELS and MUSE spectroscopic surveys and achieves the performance of 77% true positive rate and 14% false positive rate. The permutation feature importance method shows that $\beta$, $M_\mathrm{UV}$, and $M_*$ are important for the prediction of LAEs. As an independent validation, we find that 91% of LAEs spectroscopically confirmed by the James Webb Space Telescope (JWST) have a probability of LAE higher than 70% in this model. This prediction model enables the efficient construction of a large LAE sample in a wide and continuous redshift space using only photometric data. We apply the prediction model to the JWST photometric galaxy sample and obtain Ly$\alpha$ fraction consistent with previous studies. Moreover, we demonstrate that the difference between the distributions of LAEs predicted by the model and the spectroscopically identified LAEs provides a strong constraint on the HII bubble size.

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arXiv2024

The nature of low-luminosity AGNs discovered by JWST based on clustering analysis: progenitors of low-$z$ quasars?

James Webb Space Telescope (JWST) has discovered many faint AGNs at high-$z$ by detecting their broad Balmer lines. However, their high number density, lack of X-ray emission, and overly high black hole masses with respect to their host stellar masses suggest that they are a distinct population from general type-1 quasars. Here, we present clustering analysis of 27 low-luminosity broad-line AGNs found by JWST (JWST AGNs) at $5<z<6$ based on cross-correlation analysis with 679 photometrically-selected galaxies to characterize their host dark matter halo (DMH) masses. From the angular and projected cross-correlation functions, we find that their typical DMH mass is $\log (M_\mathrm{halo}/h^{-1}\mathrm{M}_\odot) = 11.46_{-0.25}^{+0.19},$ and $11.53_{-0.20}^{+0.15}$, respectively. This result implies that the host DMHs of these AGNs are $\sim1$ dex smaller than those of luminous quasars. The DMHs of the JWST AGNs at $5<z<6$ are predicted to grow to $10^{12\unicode{x2013}13}\,h^{-1}\mathrm{M}_\odot$ at $z\lesssim3$, which is comparable to that of a more luminous quasar at the same epoch. Applying the empirical stellar-to-halo mass ratio to the measured DMH mass, we evaluate their host stellar mass as $\log(M_*/\mathrm{M}_\odot)=9.48_{-0.41}^{+0.31},$ and $9.60_{-0.33}^{+0.24}$, which are higher than some of those estimated by the SED fitting. We also evaluate their duty cycle as $f_\mathrm{duty}=0.37_{-0.15}^{+0.19}$ per cent, corresponding to $\sim4\times10^6$ yr as the lifetime of the JWST AGNs. While we cannot exclude the possibility that the JWST AGNs are simply low-mass type-1 quasars, these results suggest that the JWST AGNs are a different population from type-1 quasars and the progenitors of quasars at $z\lesssim3$.

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