arXiv subjects

Hiroya Umeda

Publications and source records attributed to Hiroya Umeda.

At least 19 recordsLinked to original sources

arXiv2026

Gas-Phase Metallicity and Nitrogen Abundances in Low-Mass Galaxies Down to $M_\star\simeq10^{5.7}\,M_\odot$ at $z\simeq4.5$--$10.1$ from JWST Lensing Cluster Surveys

We analyze 405 deep JWST/NIRSpec spectra of star-forming galaxies at $z=4.5$--$10.1$ from DREAMS and other lensing-cluster surveys to study chemical enrichment in intrinsically faint, low-mass galaxies. The sample covers $M_{\rm UV}\simeq-12$ to $-22$ and reaches $M_\star\simeq10^{5.7}\,M_\odot$, with 50\% of the sources at $M_{\rm UV}>-17.5$ magnified by $\mu>3$. From individual spectra and mass-binned stacks, we derive the gas-phase metallicity together with nitrogen and carbon abundances using the rest-frame UV and optical lines. \Add{We derive new empirical strong-line metallicity calibrations using direct-method measurements from stellar-mass-binned stacks, reaching a representative stellar mass of $M_\star\simeq10^{6.6}\,M_\odot$. Applying these calibrations, we trace the $z\sim6$ mass--metallicity relation down to $M_\star\simeq10^{6.6}\,M_\odot$, where it reaches $12+\log({\rm O/H})\simeq7.2$, with a low-mass slope slightly steeper than the local relation and in broad agreement with hydrodynamical simulations.} In the $M_\star\simeq10^{7.7}\,M_\odot$ stack, N/O from N\,{\sc iv}]\,$\lambda\lambda\,1483,1486$ exceeds that from [N\,{\sc ii}]\,$\lambda6583$ by $\simeq1.4$ dex. The UV--optical difference could indicate a localized, highly ionized N-rich component whose high N/O and subsolar C/O resemble nitrogen-rich globular-cluster populations with $M_\star\lesssim10^6\,M_\odot$. \Add{The combination of these abundance patterns and a He\,{\sc ii}\,\W4686/H$\beta$ ratio of $\simeq0.03$, well above BPASS predictions, suggests that WR stars may contribute both prompt CNO-cycle enrichment and hard ionizing radiation, with the inferred WR population potentially supplying enough nitrogen to account for the excess on globular-cluster scales.

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arXiv2026

A UV-Luminous Galaxy at z=11 with Surprisingly Weak Star Formation Activity

One of the major discoveries by the James Webb Space Telescope (JWST) is the identification of a large population of luminous galaxies at $z>10$, challenging theoretical models for early galaxy formation. The unexpectedly high number density of these systems has triggered intense debate about potential differences in the physical properties of galaxies at such extreme redshifts and those at lower redshift. However, progress has been limited by the lack of rest-frame optical diagnostics, which are critical for constraining the key properties. Here we present deep JWST/MIRI observations of a UV-luminous galaxy at $z=11.04$, CEERS2-588, only 400 Myr after the Big Bang. CEERS2-588 is detected in the MIRI F560W and F770W bands, while deep MIRI/MRS spectroscopy yields no detection of H$\alpha$ or [OIII]$\lambda5007$ line, revealing a prominent Balmer break detected for the first time at $z>10$. Spectral energy distribution (SED) fitting indicates an extended star formation history possibly reaching $z>15$, followed by rapid quenching within the recent $\sim10$ Myr, in stark contrast to other $z>10$ galaxies. The MIRI detections also significantly improve our stellar mass estimate to $\mathrm{log}(M_*/M_\odot)=9.1^{+0.1}_{-0.1}$, making CEERS2-588 the most massive galaxy securely confirmed at $z>10$. Remarkably, the inferred gas-phase metallicity is near solar, exceeding predictions from current theoretical models. These results suggest that efficient starbursts play a key role in producing the abundant luminous galaxy population in the early universe.

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arXiv2026

A New Constraint on the Optical Depth from the Reionization History Independent of CMB Large-Scale E-Mode Polarization

Recent studies report a mild discrepancy between BAO and CMB measurements within the $\Lambda$CDM framework. This discrepancy could be explained if the optical depth $\tau$ inferred from the CMB large-scale E-mode polarization is underestimated, which may be biased by foreground-subtraction or instrumental systematics. In this work, we present a determination of $\tau$ independent of the large-scale E-mode polarization, using the latest measurements of the redshift evolution of the neutral hydrogen fraction $x_\mathrm{HI}(z)$, which is constrained by Lyman-$\alpha$ forest and damping-wing absorption measurements at $z\sim5$--$14$, based on ground-based optical and JWST observations. Combining $x_\mathrm{HI}(z)$ with the Planck CMB power spectra excluding the large-scale E-mode polarization, we obtain $\tau=0.0552^{+0.0019}_{-0.0026}(\mathrm{stat.})^{+0.0075}_{-0.0049}(\mathrm{sys.})$, where the systematic uncertainty accounts for possible absorption-modeling effects in the inference of $x_\mathrm{HI}(z)$. This constraint is consistent with previous CMB results including the large-scale E-mode polarization. With this measurement, we resolve the degeneracy in the $\tau$--$\Omega_{\rm m}$ plane and find a $2.4\sigma$ tension with the DESI DR2 BAO results, thereby confirming the claimed mild discrepancy suggestive of physics beyond $\Lambda$CDM. Finally, we derive an upper limit on the sum of neutrino masses, $\Sigma m_\nu < 0.0550\,(0.0717)\,{\rm eV}$ at the 95\%\,(99\%) confidence level. This limit favors the normal mass ordering and, when combined with the lower limits from neutrino oscillation experiments, yields a further constraint, $\Sigma m_\nu = 0.0594_{-0.0007}^{+0.0113}\,{\rm eV}$. However, the cosmological upper limit and the oscillation-based lower limit show a mild $2.2\sigma$ tension, providing an independent indication of possible physics beyond $\Lambda$CDM.

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arXiv2026

VENUS: Two Faint Little Red Dots Separated by $\sim70\,\mathrm{pc}$ Hidden in a Single Lensed Galaxy at $z\sim7$

We report the identification of a pair of faint little red dots (LRDs), dubbed Red Eyes, in a strongly-lensed galaxy at $z\sim7$ behind the PLCKG004.5-10.5 cluster, identified from the JWST Treasury program VENUS. Red Eyes are spatially resolved on the image plane with distinct colors, while the critical curve lies far north of Red Eyes, clearly requiring two different LRDs rather than a single LRD. Red Eyes is an extremely close pair of LRDs separated by $\sim70\,\mathrm{pc}$ in the source plane with a magnification of $\mu\sim20$, which consistently explains another counter-image detected to the north-west. Red Eyes is hosted in a typical star-forming galaxy with $M_{\mathrm{UV,int}}\sim -19$, but its own UV emission is very faint ($M_{\mathrm{UV,int}} \gtrsim -16$). Moreover, Red Eyes does not reside at the galaxy center but lies at an offset position of approximately one effective radius $R_{\mathrm{e}}$ away from the galaxy center. If observed without lensing, Red Eyes would appear as a typical star-forming galaxy at $z\sim 7$ with $M_{\mathrm{UV}}\sim -19$, showing no apparent LRD signatures in either morphology or SED. These results suggest that multiple off-center LRDs, similar to Red Eyes, may be commonly hidden in a typical high-$z$ star-forming galaxy. In this case, various plausible scenarios may emerge, one of which is that intermediate-mass black holes (IMBHs) with $M_\mathrm{BH}\sim10^{4\text{--}6}\,M_\odot$ may form in star clusters on a stellar disk and contribute to the growth of the central supermassive black hole via mergers, with some IMBHs detectable as luminous LRDs in a sufficiently active and massive phase.

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arXiv2025

DREAMS.II. Galaxy Demographics from Direct Te-Based Metallicities at z~2-10: Tracing the Evolution of the Mass-Metallicity and Fundamental Relations

We present the statistics of line ratios and direct Te-based metallicities from JWST medium-resolution spectra of 292 galaxies at z=2-10, combining DREAMS observations with those of JADES and CEERS. To remove systematics caused by stellar mass (M*) and star formation rate (SFR), we construct stacked spectra binned by redshift within fixed M* and SFR ranges, as well as across the full ranges. We find that the [OII]3727/Hb ratio drops by a factor of five from z~3 to 8 at fixed M* and SFR, in contrast to the nearly constant [OIII]5007/Hb ratio. We derive metallicities via the direct Te method using the [OIII]4363 line, and identify that high-z galaxies lie on the low-metallicity end of the anti-correlation between ionization parameter and metallicity at z~0. Photoionization modeling demonstrates that the redshift evolution, where metallicity decreases and ionization parameter increases, self-consistently explains the observed line ratios. We then examine the mass-metallicity (MZ) and fundamental (MZ-SFR) relations. Including additional galaxies at z~10-12, we find that the MZ relation monotonically decreases from z~3 to 10 at fixed M*, while the MZ-SFR relation shows a significant decline at z>8. Based on the ChemicalUniverseMachine model, this evolutionary trend can be explained by enhanced gas inflow (outflow) by a factor of ~5 (~1.7) at z>8.

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arXiv2025

Little red dot variability over a century reveals black hole envelope via a giant Einstein cross

"Little red dots" (LRDs) represent a new population of astronomical objects uncovered by JWST whose nature remains debated. Although many LRDs are suspected as active galactic nuclei (AGN), they show little variability on days-years timescales. We report the discovery of two gravitationally lensed LRDs at redshift $\sim$4.3 behind the cluster RXCJ2211-0350, one of which (RX1) is quadruply imaged with time delays spanning $\sim$130 years. RX1 exhibits intrinsic color and brightness variations of up to 0.7 magnitude among its images. These changes are consistent with blackbody-temperature variations of a photosphere, indicating long-term variability analogous to Cepheid-like pulsations but in a far more extended ($R \sim 2000$ AU) and massive ($M \gtrsim 10^6 \, M_{\odot}$) systems. These results suggest LRDs as a distinct class of AGN with stellar-like envelopes.

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arXiv2025

A Black-Hole Envelope Interpretation for Cosmological Demographics of Little Red Dots

Little red dots (LRDs) newly discovered with JWST are active galactic nuclei (AGN) that may represent black hole (BH) growth at the earliest cosmic epochs. These sources show puzzling features unlike typical AGNs, including red optical continua, weak hot-dust emission, and a lack of detectable X-rays. Previously, LRDs have often been interpreted as dust-reddened AGNs, leading to severe inconsistencies with the luminosity and BH mass densities inferred for previously known AGNs over $0<z<5$. The BH-envelope (BHE) model has been proposed to explain these characteristics, in which an accreting BH is enshrouded by a dense, optically thick gaseous envelope. In this Letter, we reanalyze the SEDs of $\sim 400$ photometric LRDs in the COSMOS-Web survey using the BHE model and reassess their implications for cosmological BH evolution. We find that the optical-NIR spectra of LRDs are well reproduced by blackbody emission with an effective temperatures of $4000-6000~\K$. Within the BHE framework, the inferred bolometric luminosities decrease by $\gtrsim1-2$ orders of magnitude compared to dust-reddened AGN assumptions. As a result, the revised luminosity function, BH accretion density, and BH mass function become consistent with those of AGNs at $z<5$. The stellar masses of LRD hosts are estimated by attributing the UV excesses to star formation. Although the resulting $M_{\rm BH}/M_\star$ ratio remains higher than the local empirical value, the excess is modest. Overall, the BHE model not only resolves the spectral features of LRDs but also brings their statistical properties into agreement with the broader cosmological BH population.

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arXiv2025

Discovery of a Little Red Dot candidate at $z\gtrsim10$ in COSMOS-Web based on MIRI-NIRCam selection

JWST has revealed a new high-redshift population called little red dots (LRDs). Since LRDs may be in the early phase of black hole growth, identifying them in the early universe is crucial for understanding the formation of the first supermassive black holes. However, no robust LRD candidates have been identified at $z>10$, because commonly-used NIRCam photometry covers wavelengths up to $\sim5\,{\rm \mu m}$ and is insufficient to capture the characteristic V-shaped spectral energy distributions (SEDs) of LRDs. In this study, we present the first search for $z\gtrsim10$ LRD candidates using both NIRCam and MIRI imaging from COSMOS-Web, which provides the largest joint NIRCam-MIRI coverage to date ($0.20\,{\rm deg^2}$). Taking advantage of MIRI/F770W to remove contaminants, we identify one robust candidate, CW-LRD-z10 at $z_{\rm phot}=10.5^{+0.7}_{-0.6}$ with $M_{\rm UV}=-19.9^{+0.1}_{-0.2}\,{\rm mag}$. CW-LRD-z10 exhibits a compact morphology, a distinct V-shaped SED, and a non-detection in F115W, all consistent with being an LRD at $z\sim10$. Based on this discovery, we place the first constraint on the number density of LRDs at $z\sim10$ with $M_{\rm UV}\sim-20$ of $1.2^{+2.7}_{-1.0}\times10^{-6}\,{\rm Mpc^{-3}\,mag^{-1}}$, suggesting that the fraction of LRDs among the overall galaxy population increases with redshift, reaching $\sim3\%$ at $z\sim10$. Although deep spectroscopy is necessary to confirm the redshift and the nature of CW-LRD-z10, our results imply that LRDs may be a common population at $z>10$, playing a key role in the first supermassive black hole formation.

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arXiv2025

An Ultra-Faint, Chemically Primitive Galaxy Forming in the Reionization Era

The formation of the first stars and galaxies marked the onset of chemical enrichment, yet direct observations of such primordial systems remain elusive. Here we present James Webb Space Telescope spectroscopic observations of LAP1-B, an ultra-faint galaxy at redshift z_{spec}=6.625 +/-0.001, corresponding to a cosmic age of 800 million years after the Big Bang, strongly magnified by gravitational lensing. LAP1-B exhibits a gas-phase oxygen abundance of (4.2 +/- 1.8) x 10^{-3} times the solar value, making it the most chemically primitive star-forming galaxy discovered to date. The galaxy displays an exceptionally hard ionizing radiation field, which is inconsistent with chemically enriched stellar populations or accreting black holes but matches theoretical predictions for an exceptionally metal-deficient stellar population. It also shows an elevated carbon-to-oxygen abundance ratio for its metallicity in the interstellar medium, consistent with nucleosynthetic yields from a stellar population formed in the absence of initial metals. The lack of detectable stellar continuum constrains the stellar mass to <3,300 Msun, while the dynamical mass, derived from emission-line kinematics, exceeds the combined stellar and gas mass and indicates a dominant dark matter halo. Our findings establish LAP1-B as a "fossil in the making", a direct high-redshift progenitor of the ancient ultra-faint dwarf galaxies observed in the local Universe, offering a rare window into the earliest stages of galaxy formation.

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arXiv2025

Probing the Cosmic Reionization History with JWST: Gunn-Peterson and Ly$\alpha$ Damping Wing Absorption at $4.5 < z < 13$

We present a statistical analysis of Ly$\alpha$ absorption using 581 galaxies at $z=4.5$--13 observed with multiple JWST/NIRSpec spectroscopy programs, including JADES, UNCOVER, CEERS, and GO/DDT. We carefully construct composite spectra binned by redshift with homogeneous UV properties (UV magnitudes, UV slopes, and Ly$\alpha$ equivalent widths) and identify significant Ly$\alpha$ forest signals in galaxies at $z\sim5$--6, which diminish toward higher redshifts. We also find UV continuum breaks at rest-frame 1216 {\AA} that soften beyond $z\gtrsim6$, confirming the effects of cosmic reionization through a self-consistent transition from Gunn-Peterson to Ly$\alpha$ damping wing absorption in galaxies. Fair comparisons of composite spectra with matched UV magnitudes and slopes across redshift reveal that UV-faint galaxies clearly show stronger Ly$\alpha$ absorption than UV-bright galaxies towards high redshift, providing insights into the topological evolution of reionization. We estimate Ly$\alpha$ transmission at the Gunn-Peterson trough and Ly$\alpha$ damping wing absorption by comparing the galaxy spectra to low-$z$ ($z\sim2$-5) galaxy templates that include galactic and circumgalactic absorption and Ly$\alpha$ emission. Using these measurements together with reionization simulations, we derive volume average neutral hydrogen fractions of $\langle x_{\rm HI} \rangle$ = ${0.00}^{+0.12}_{-0.00}$, ${0.25}^{+0.10}_{-0.20}$, ${0.65}^{+0.27}_{-0.35}$, ${1.00}^{+0.00}_{-0.20}$, and ${1.00}^{+0.00}_{-0.40}$ at $z\sim5$, 6, 7, 9, and 10, respectively. These values broadly align with a reionization history characterized by a rapid transition around $z\sim7$--8, consistent with Ly$\alpha$ emitter observations. While the physical driver of this rapid reionization remains unclear, it may involve the emergence of hidden AGN populations and/or the onset of Lyman-continuum escape from galaxies.

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arXiv2025

Census of Ly$\alpha$ Emission from $\sim 600$ Galaxies at $z=5-14$: Evolution of the Ly$\alpha$ Luminosity Function and a Late Sharp Cosmic Reionization

We present the statistical properties of Ly$\alpha$ emission in 586 galaxies at $z=4.5-14.2$, observed by multiple JWST/NIRSpec spectroscopy projects, including JADES, GLASS, CEERS, and GO/DDT programs. We obtain Ly$\alpha$ equivalent width (EW), Ly$\alpha$ escape fraction, and ionizing photon production efficiency measurements or upper limits for these galaxies, and confirm that the Ly$\alpha$ emitting galaxy fraction decreases towards higher redshifts. We derive Ly$\alpha$ luminosity functions from $z\sim 5$ to $z\sim 10-14$ with the observed Ly$\alpha$ EW distributions and galaxy UV luminosity functions, and find a $\sim3$ dex decrease in number density at $L_\mathrm{Ly\alpha}=10^{42}-10^{43}$ erg s$^{-1}$ over the redshift range. Notably, this study presents the first constraints on the Ly$\alpha$ luminosity function at $z\sim 8-14$. We obtain the neutral hydrogen fractions of $x_\mathrm{HI}=0.17_{-0.16}^{+0.23}$, $0.63_{-0.28}^{+0.18}$, $0.79_{-0.21}^{+0.13}$, and $0.88_{-0.13}^{+0.11}$ at $z\sim6$, $7$, $8-9$, and $10-14$, respectively, via comparisons of the reionization models developed by semi-numerical simulations with 21cmFAST explaining the observations of Ly$\alpha$, UV continuum, and Planck electron optical depth. The high $x_\mathrm{HI}$ values over $z\sim 7-14$ suggest a late and sharp reionization, with the primary reionization process occurring at $z\sim 6-7$. Such a late and sharp reionization is not easily explained by either a clumpy inter-galactic medium or sources of reionization in a classical faint-galaxy or a bright-galaxy/AGN scenario, unless a very high escape fraction or AGN duty cycle is assumed at $z\sim 6-7$.

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arXiv2024

EMPRESS. X. Spatially resolved mass-metallicity relation in extremely metal-poor galaxies: evidence of episodic star-formation fueled by a metal-poor gas infall

Using the Subaru/FOCAS IFU capability, we examine the spatially resolved relationships between gas-phase metallicity, stellar mass, and star-formation rate surface densities (Sigma_* and Sigma_SFR, respectively) in extremely metal-poor galaxies (EMPGs) in the local universe. Our analysis includes 24 EMPGs, comprising 9,177 spaxels, which span a unique parameter space of local metallicity (12+log(O/H) = 6.9 to 7.9) and stellar mass surface density (Sigma_* ~ 10^5 to 10^7 Msun/kpc^2), extending beyond the range of existing large integral-field spectroscopic surveys. Through spatially resolved emission line diagnostics based on the [NII] BPT-diagram, we verify the absence of evolved active galactic nuclei in these EMPGs. Our findings reveal that, while the resolved mass-metallicity relation exhibits significant scatter in the low-mass regime, this scatter is closely correlated with local star-formation surface density. Specifically, metallicity decreases as Sigma_SFR increases for a given Sigma_*. Notably, half of the EMPGs show a distinct metal-poor horizontal branch on the resolved mass-metallicity relation. This feature typically appears at the peak clump with the highest Sigma_* and Sigma_SFR and is surrounded by a relatively metal-enriched ambient region. These findings support a scenario in which metal-poor gas infall fuels episodic star formation in EMPGs, consistent with the kinematic properties observed in these systems. In addition, we identify four EMPGs with exceptionally low central metallicities (12+log(O/H) <~ 7.2), which display only a metal-poor clump without a surrounding metal-rich region. This suggests that such ultra-low metallicity EMPGs, at less than a few percent of the solar metallicity, may serve as valuable analogs for galaxies in the early stages of galaxy evolution.

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arXiv2024

A Galaxy with an Extremely Blue UV Slope $\beta=-3$ at $z=9.25$ Identified by JWST Spectroscopy: Evidence for a Weak Nebular Continuum and Efficient Ionizing Photon Escape?

We investigate UV continuum slopes $\beta$ of 863 galaxies at $z=4-14$ using archival JWST/NIRSpec PRISM spectra obtained from major JWST GTO, ERS, and GO programs, including JADES, CEERS, and UNCOVER. Among these galaxies, we identify a remarkable galaxy at $z=9.25$, dubbed EBG-1, with a significantly blue UV slope $\beta=-2.99\pm0.15$, unlike the rest of the galaxies that exhibit red continua or ambiguous blue continua hindered by large uncertainties. We confirm that the $\beta$ value negligibly changes by the data reduction and fitting wavelength ranges for UV emission/absorption line masking. The extreme blue slope, $\beta=-3.0$, rules out significant contributions from dust extinction or AGN activity. Comparing with stellar and nebular emission models, we find that such a blue UV slope cannot be reproduced solely by stellar models even with very young, metal-poor, or top-heavy contiguous star formation associated with strong nebular continua making the UV slopes red, but with a high ionizing photon escape fraction, $f_\mathrm{esc}^\mathrm{ion} \gtrsim 0.5$, for a weak nebular continuum. While the H$\beta$ emission line is not detected, likely due to the limited sensitivity of the spectrum, we find moderately weak [O III] $\lambda\lambda$4959,5007 emission lines for the given star-formation rate ($3\, \mathrm{M_\odot}$ yr$^{-1}$) and stellar mass ($10^{8.0} \, \mathrm{M_\odot}$) that are about three times weaker than the average emission lines, again suggestive of the high ionizing photon escape fraction, $f_\mathrm{esc}^\mathrm{ion} \sim 0.7$ or more. EBG-1 would provide crucial insights into stellar and nebular continuum emission in high-redshift galaxies, serving as an example of the ionizing photon escaping site at the epoch of reionization.

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arXiv2024

SILVERRUSH. XIV. Lya Luminosity Functions and Angular Correlation Functions from ~20,000 Lya Emitters at z~2.2-7.3 from upto 24 ${\rm deg}^2$ HSC-SSP and CHORUS Surveys: Linking the Post-Reionization Epoch to the Heart of Reionization

We present the luminosity functions (LFs) and angular correlation functions (ACFs) derived from 18,960 Ly$\alpha$ emitters (LAEs) at $z=2.2-7.3$ over a wide survey area of $\lesssim24 {\rm deg^2}$ that are identified in the narrowband data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and the Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) surveys. Confirming the large sample with the 241 spectroscopically identified LAEs, we determine Ly$\alpha$ LFs and ACFs in the brighter luminosity range down to $0.5L_{\star}$, and confirm that our measurements are consistent with previous studies but offer significantly reduced statistical uncertainties. The improved precision of our ACFs allows us to clearly detect one-halo terms at some redshifts, and provides large-scale bias measurements that indicate hosting halo masses of $\sim 10^{11} M_\odot$ over $z\simeq 2-7$. By comparing our Ly$\alpha$ LF (ACF) measurements with reionization models, we estimate the neutral hydrogen fractions in the intergalactic medium to be $x_{\rm \HI}<0.05$ (=${0.06}^{+0.12}_{-0.03}$), $0.15^{+0.10}_{-0.08}$ (${0.21}^{+0.19}_{-0.14}$), $0.18^{+0.14}_{-0.12}$, and $0.75^{+0.09}_{-0.13}$ at $z=5.7$, $6.6$, $7.0$, and $7.3$, respectively. Our findings suggest that the neutral hydrogen fraction remains relatively low, $x_{\rm \HI} \lesssim 0.2$, at $z=5-7$, but increases sharply at $z > 7$, reaching $x_{\rm \HI} \sim 0.9$ by $z \simeq 8-9$, as indicated by recent JWST studies. The combination of our results from LAE observations with recent JWST observations suggests that the major epoch of reionization occurred around $z \sim 7-8$, likely driven by the emergence of massive sources emitting significant ionizing photons.

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arXiv2024

The Physical Origin of Extreme Emission Line Galaxies at High redshifts: Strong {\sc [Oiii]} Emission Lines Produced by Obscured AGNs

We present deep Subaru/FOCAS spectra for two extreme emission line galaxies (EELGs) at $z\sim 1$ with strong {\sc[Oiii]}$\lambda$5007 emission lines, exhibiting equivalent widths (EWs) of $2905^{+946}_{-578}$ \AA\ and $2000^{+188}_{-159}$ \AA, comparable to those of EELGs at high redshifts that are now routinely identified with JWST spectroscopy. Adding a similarly large {\sc [Oiii]} EW ($2508^{+1487}_{-689}$ \AA) EELG found at $z\sim 2$ in the JWST CEERS survey to our sample, we explore for the physical origins of the large {\sc [Oiii]} EWs of these three galaxies with the Subaru spectra and various public data including JWST/NIRSpec, NIRCam, and MIRI data. While there are no clear signatures of AGN identified by the optical line diagnostics, we find that two out of two galaxies covered by the MIRI data show strong near-infrared excess in the spectral energy distributions (SEDs) indicating obscured AGN. Because none of the three galaxies show clear broad H$\beta$ lines, the upper limits on the flux ratios of broad-H$\beta$ to {\sc [Oiii]} lines are small, $\lesssim 0.15$ that are comparable with Seyfert $1.8-2.0$ galaxies. We conduct \texttt{Cloudy} modeling with the stellar and AGN incident spectra, allowing a wide range of parameters including metallicities and ionization parameters. We find that the large {\sc [Oiii]} EWs are not self-consistently reproduced by the spectra of stars or unobscured AGN, but obscured AGN that efficiently produces O$^{++}$ ionizing photons with weak nuclear and stellar continua that are consistent with the SED shapes.

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arXiv2024

JWST, ALMA, and Keck Spectroscopic Constraints on the UV Luminosity Functions at z~7-14: Clumpiness and Compactness of the Brightest Galaxies in the Early Universe

We present the number densities and physical properties of the bright galaxies spectroscopically confirmed at $z\sim7-14$. Our sample is composed of 60 galaxies at $z_\mathrm{spec}\sim7-14$, including recently-confirmed galaxies at $z_\mathrm{spec}=12.34-14.32$ with JWST, as well as new confirmations at $z_\mathrm{spec}=6.583-7.643$ with $-24< M_\mathrm{UV}< -21$ mag using ALMA and Keck. Our JWST/NIRSpec observations have also revealed that very bright galaxy candidates at $z\sim10-13$ identified from ground-based telescope images before JWST are passive galaxies at $z\sim3-4$, emphasizing the necessity of strict screening and spectroscopy in the selection of the brightest galaxies at $z>10$. The UV luminosity functions derived from these spectroscopic results are consistent with a double power-law function, showing tensions with theoretical models at the bright end. To understand the origin of the overabundance of bright galaxies, we investigate their morphologies using JWST/NIRCam high-resolution images obtained in various surveys including PRIMER and COSMOS-Web. We find that $\sim70\%$ of the bright galaxies at $z\sim7$ exhibit clumpy morphologies with multiple sub-components, suggesting merger-induced starburst activity, which is consistent with SED fitting results showing bursty star formation histories. At $z\gtrsim10$, bright galaxies are classified into two types of galaxies; extended ones with weak high-ionization emission lines, and compact ones with strong high-ionization lines including NIV]$\lambda$1486, indicating that at least two different processes (e.g., merger-induced starburst and compact star formation/AGN) are shaping the physical properties of the brightest galaxies at $z\gtrsim10$ and are responsible for their overabundance.

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arXiv2024

Statistics for Galaxy Outflows at $z\sim 6-9$ with Imaging and Spectroscopic Signatures Identified with JWST/NIRCam and NIRSpec Data

We present statistics of $z\sim 6-9$ galaxy outflows indicated by spatially-extended gas emission and broad lines. With a total of 61 spectroscopically confirmed galaxies at $z\sim 6-9$ in the JWST CEERS, GLASS, and ERO data, we find four galaxies with [O{\sc iii}]+H$β$ ionized gas emission significantly extended beyond the kpc-scale stellar components on the basis of the emission line images constructed by the subtraction of NIRCam broadband (line on/off-band) images. By comparison with low-$z$ galaxies, the fraction of galaxies with the spatially extended gas, 4/18, at $z\sim 6-9$ is an order of magnitude higher than those at $z\sim 0-1$, which can be explained by events triggered by frequent major mergers at high redshift. We also investigate medium- and high-resolution NIRSpec spectra of 30 galaxies at $z\sim 6-9$, and identify five galaxies with broad ($140-800$ km s$^{-1}$) lines in the [O{\sc iii}] forbidden line emission, suggestive of galaxy outflows. One galaxy at $z=6.38$ shows both the spatially-extended gas emission and the broad lines, while none of the galaxies with the spatially-extended gas emission or broad lines present a clear signature of AGN either in the line diagnostics or Type 1 AGN line broadening ($>1000$ km s$^{-1}$), which hint outflows mainly driven by stellar feedback. The existence of galaxies with/without spatially-extended gas emission or broad lines may be explained by different viewing angles towards outflows, or that these are galaxies in the early, late, post phases of galaxy outflows at high redshift, where the relatively large fractions of such galaxies indicate the longer-duration and/or more-frequent outflows at the early cosmic epoch.

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arXiv2024

Strong He I Emission Lines in High N/O Galaxies at $z \sim 6$ Identified in JWST Spectra: High He/H Abundance Ratios or High Electron Densities?

We present HeI/H$\beta$-flux and He/H-abundance ratios in three JWST galaxies with significant constraints on N/O-abundance ratios, GS-NDG-9422, RXCJ2248-ID, and GLASS150008 at $z\sim 6$ mostly with the spectroscopic coverage from HeI$\lambda$4471 and HeII$\lambda$4686 to HeI$\lambda$7065, comparing with 68 local-dwarf galaxies. We find that these high-$z$ galaxies present strong HeI emission with HeI/H$\beta$ flux ratios generally larger than those of local-dwarf galaxies. We derive He/H with all of the detected HeI, HeII, and $2-3$ hydrogen Balmer lines in the same manner as the local He/H determination conducted for cosmology studies. These high-$z$ galaxies show He overabundance He/H$\gtrsim 0.10$ or high electron density $n_\mathrm{e}\sim 10^{3-4}$ cm$^{-3}$ much larger than local values at low O/H, $12+\log \mathrm{(O/H)}=7-8$. In contrast, we obtain low He/H and $n_\mathrm{e}$ values for our local-dwarf galaxies by the same technique with the same helium and hydrogen lines, and confirm that the difference between the high-$z$ and local-dwarf galaxies are not mimicked by systematics. While two scenarios of 1) He overabundance and 2) high electron density are not clearly concluded, we find that there is a positive correlation on the He/H-N/O or $n_\mathrm{e}$-N/O plane by the comparison of the high-$z$ and local-dwarf galaxies. The scenario 1) suggests that the overabundant helium and nitrogen are not explained by the standard chemical enrichment of core-collapse supernovae, but the CNO-cycle products and equilibrium ratios, respectively. The scenario 2) indicates that the strong helium lines are originated from the central dense clouds of the high-$z$ galaxies by excessive collisional excitation.

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