arXiv subjects

Kohei Inayoshi

Publications and source records attributed to Kohei Inayoshi.

At least 19 recordsLinked to original sources

arXiv2026

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

We present a direct measurement of the Ly$α$ escape fraction, $f^{\rm Lyα}_{\rm esc}$, for H$α$ 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 H$α$ and Ly$α$ fluxes from galaxies during the epoch of reionization (EoR). We select 84 HAEs from F470N excesses, among which 56 have reliable NB872 photometry and 14 are detected in Ly$α$. The completeness-weighted stack of the HAE sample yields a median $f^{\rm Lyα}_{\rm esc}$ 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 that the stacked $f_{\rm esc}^{\rm Lyα}$ shows no statistically significant change as the H$α$ luminosity threshold is lowered over the range probed by our sample. If Ly$α$ escape traces Lyman continuum leakage, this may suggest that relatively H$α$-luminous HAEs, rather than only the faintest ones, can provide an important contribution to the ionizing photon budget during the EoR. For individual galaxies, $f^{\rm Lyα}_{\rm esc}$ positively correlates with Ly$α$ equivalent width and negatively correlates with the UV continuum slope $β$. We also find a tentative trend toward higher $f^{\rm Lyα}_{\rm esc}$ at smaller rest-frame UV sizes, but no significant correlation with SED-derived $E(B-V)$ or rest-frame optical size. Although based on a limited sample, these results suggest that the galaxy-to-galaxy variation in $f_{\rm esc}^{\rm Lyα}$ may be 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.

astro-ph.GA↗

arXiv2026

EMBER: A Little Red Dot Candidate at $z_{\rm spec}=10.36$ in the Seed Black Hole Era

Little red dots (LRDs) are a newly discovered population that may trace rapidly growing black holes (BHs) and provide a link between seed BHs and luminous active galactic nuclei (AGNs). However, no LRD had previously been spectroscopically confirmed at $z>10$, leaving this earliest phase observationally unexplored. Using JWST/NIRSpec spectroscopy from the EMBER program, we measure a spectroscopic redshift of $z_{\rm spec}=10.361^{+0.003}_{-0.002}$ for \target, the highest-redshift LRD candidate known to date. The redshift is constrained by a prominent Ly$α$ break and multiple tentative emission lines, including C\,{\sc iv}\,$λ\lambda1549,1551$ and [O\,{\sc iii}]\,$\lambda4363$ detected at $S/N=3.9$ and $3.6$, respectively. Its extreme NIRCam-MIRI color of $m_{\rm F444W}-m_{\rm F770W}=1.82\pm0.1$, characteristic V-shaped continuum with a turnover near the Balmer limit, and continuum slopes of $β_{\rm UV}=-2.26^{+0.11}_{-0.11}$ and $β_{\rm opt}=1.17^{+0.74}_{-0.70}$ all support its LRD nature. A possible iron complex at $λ_{\rm rest}\sim0.24$--$0.25\,{\rm μm}$ and a possibly high $\log\left([{\rm O\,III]\,\lambda4363/\rm Hγ}\right)=0.17^{+0.24}_{-0.22}$ further support an AGN-like interpretation, as seen in previously known LRDs. Tentative detection of Ly$α$ emission may indicate enhanced Ly$α$ transmission at $z\sim10$. Our findings extend the observational frontier of BH growth into the seed-BH formation era, and upcoming MIRI spectroscopy will further establish the physical nature of the source.

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arXiv2026

SN Helios: A Multiply Imaged Type II Supernova Opening Time-Delay Cosmography beyond redshift of 3

We report the discovery and spectroscopic confirmation of SN Helios, a strongly lensed, multiply imaged Type II supernova (SN) at $z=3.34$ behind the galaxy cluster RXC J0018.5+1626. The transient was identified as a red F150W-dropout source (F277W = 26.1 mag) in JWST/NIRCam imaging taken on 2026 July 3 for the VENUS survey, with no counterpart in 2024 December imaging. Director's Discretionary Time observations on 2026 August 17 added second-epoch NIRCam imaging and a deep NIRSpec prism spectrum. The spectrum shows a strong, spectrally resolved H$α$ emission line (FWHM $\approx 7900$ km/s) with a P-Cygni absorption blueshifted by 8800 km/s, together with corresponding H$β$ and He I $λ$10830 features, establishing a hydrogen-rich photosphere at $z=3.34$. Template fits classify SN Helios as a Type II SN, and light-curve comparisons with well-studied local SNe II yield a phase of $27.5\pm2.8$ rest-frame days past B-band maximum. Independent cluster lens models magnify the observed image by $μ\approx 10$-20 and predict another image within $\simeq 2$-6 yr, offering a rare opportunity for a time-delay measurement of $H_{0}$ with a source at $z>3$. The host galaxy is undetected in deep rest-frame ultraviolet and optical imaging, in emission lines, and in ALMA 2 mm dust continuum, leaving a heavily obscured counterpart unlikely. Its de-lensed $M_{\rm UV}\gtrsim -13$ places it far below blank-field limits and adds support to an elevated core-collapse rate per unit star formation in ultra-faint galaxies. Without the multi-epoch data, SN Helios would masquerade as a convincing $z\approx 16$ Lyman-break galaxy, with galaxy-template fits strongly disfavoring any low-redshift solution even with multiple medium bands. Its spectrum thus provides an empirical SED template with which high-redshift searches can vet similar dropout candidates.

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arXiv2026

How Little Red Dots Turn Blue: Evolution of Kelvin-Helmholtz Contracting Black Hole Envelopes and Observational Diagnostics

James Webb Space Telescope (JWST) observations have revealed compact, red-colored, broad-line emitters known as little red dots (LRDs), while a bluer counterpart population has been referred to as little blue dots (LBDs). Their physical connection remains unclear: they may represent different evolutionary stages of the same phenomenon, distinct populations, or different phases of the broader evolution of early galaxies and their black holes (BHs). We propose a model in which a rapidly growing BH is embedded in a geometrically thick gaseous envelope accompanied by concurrent nuclear star formation. The model reproduces the observed spectral energy distributions (SEDs) of LRDs and LBDs, and naturally connects the two phases through Kelvin-Helmholtz contraction of the envelope and exposure of the nuclear accretion disk, as the gas supply declines due to external stellar activity or feedback. We predict the corresponding evolutionary tracks in the UV-optical slope ($β_{\rm UV}$-$β_{\rm opt}$) plane which bridge the apparent gap between LRDs and LBDs. These evolutionary tracks start from the LRD regime and consistently exhibit a red excursion that extends toward $(β_{\rm UV},β_{\rm opt})\sim(2,-1)$ before turning toward the LBD regime. This feature is largely insensitive to variations in the model parameters that govern the envelope structure, providing a robust signature of the transitional population. The transition lasts $\sim5$-$50\,\mathrm{Myr}$, depending primarily on the BH feedback efficiency and the gas-retention efficiency of the envelope. The abundance ratio of transitional systems to LRDs therefore provides a constraint on BH feedback and gas retention during the rapid growth of early supermassive BHs.

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arXiv2026

An Attenuation Bump at z=9.3: Probing Dust Diversity at Cosmic Dawn

We report the identification of a UV attenuation bump in the Black Pearl, a galaxy at a spectroscopic redshift of 9.3335 observed with JWST/NIRSpec as part of the ZENITH and EMBER surveys. This is the highest-redshift UV bump reported to date, probing dust properties only approximately 500 Myr after the Big Bang. The feature is directly detected as a broad depression in the rest-frame UV continuum and is well described by a local Drude profile, with a central wavelength of 2132+/-17 Angstroms, an FWHM of 413+/-54 Angstroms, and a peak attenuation of 0.21+/-0.02 mag. Joint spectrophotometric modeling with a flexible attenuation law independently recovers a pronounced UV bump without imposing its presence a priori, regardless of the adopted star-formation history. The modeling indicates a relatively low-mass, actively star-forming galaxy, with a logarithmic stellar mass of 8.35 (+0.09/-0.08) in solar units, a star-formation rate of 2.36 (+0.54/-0.42) solar masses per year, and a visual attenuation of 0.34 (+0.12/-0.11) mag. Rest-frame optical emission-line diagnostics indicate a hard ionizing spectrum and allow for a moderate AGN contribution, but provide no evidence for a powerful AGN capable of dominating the observed UV continuum. The bump is blueshifted by 43+/-17 Angstroms (approximately 2.5 sigma) relative to the canonical Milky Way 2175 Angstrom feature and by 127 +/- 17 Angstroms relative to the few UV bumps currently known at redshift approximately 7, while its width is consistent with the broad distribution measured along Milky Way sightlines. The displaced central wavelength therefore provides the clearest evidence for a nonstandard UV-bump profile at redshift 9.3335, pointing to substantial diversity in the properties of UV-bump carriers at early cosmic times, with both PAHs and small graphite grains remaining plausible candidates.

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arXiv2026

A dense, metal-rich absorber driven by a heavily dust-obscured galaxy: The direct evidence of CGM enrichment at $z\sim7$

We report the serendipitous discovery of a dense, metal-enriched circumgalactic medium (CGM) absorber at z=7.03, together with its host galaxy, identified along the sightline toward a red quasar. The absorber exhibits the largest rest-frame equivalent widths and column densities observed to date at z>5, tracing a metal-enriched gaseous halo. Using deep JWST/NIRSpec and NOEMA observations, we characterize the absorber's physical properties and identify its host, ND1, as a vigorously star-forming galaxy at an exceptionally close impact parameter of ~16kpc, so heavily dust-obscured that it is detected only in the longest-wavelength NIRCam bands. Detections of [OIII]$λ$5007, H$α$, and [CII]$λ158\,μ$m emission anchor the systemic redshift to z~7.0321, revealing a coherent ~50km/s blueshift of the absorbing gas relative to the host, suggesting outflow-driven metal transport into the halo. The outflow velocity, combined with the measured column density, implies an outflow rate of $\dot{M}_{\rm out}=64 M_\odot\,yr^{-1}$ and a mass loading factor $η=3$, indicating an energetic, chemically-enriched wind launched from a dust-obscured starburst. While the overall ionization state and [Mg/Fe] ratio align with empirical cosmic evolutionary trends, the absorber displays an enhanced carbon-to-oxygen ([C/O]) ratio. This abundance pattern suggests the nucleosynthetic yields of Population~III stellar explosions, and aligns with the host's star-formation history extending to $z \gtrsim 8$, potentially offering a rare fossil record of primordial metal enrichment. Our findings demonstrate that intense star-formation feedback can rapidly and efficiently pollute the CGM/IGM well into the Epoch of Reionization. The NIR-dark nature of the host further implies that rest-UV surveys may systematically miss the dusty sources that drive CGM- and IGM-scale metal enrichment in the early Universe.

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arXiv2026

Probing Spatial Variations in IGM Transmission: Higher Ly$α$ Visibility in an H$α$-Selected Galaxy Overdensity at $z\simeq6.2$

The visibility of Ly$α$ emission during the epoch of reionization depends on both galaxy properties and the transmission of Ly$α$ photons through the intergalactic medium (IGM). Using JWST/NIRCam F470N imaging, we identify a prominent projected overdensity of H$α$ emitters (HAEs) at $z\simeq6.2$ in CEERS field. We use Subaru/HSC NB872 imaging to measure their Ly$α$ fluxes and compare the Ly$α$ visibility of galaxies within $2'$ of the density center with that of galaxies outside this region. The Ly$α$-emitter (LAE) fraction is $0.41^{+0.15}_{-0.14}$ in the inner region and $0.27^{+0.19}_{-0.15}$ in the outer region. Stacking the H$α$ and Ly$α$ images gives median Ly$α$ escape fractions of $0.100^{+0.091}_{-0.053}$ and $0.063^{+0.130}_{-0.063}$ in the inner and outer regions, respectively. Both measures indicate higher Ly$α$ visibility in the inner region, although only at modest significance. HAEs closer to the density center are systematically more massive and have redder UV slopes, which instead favor lower Ly$α$ escape. The environmental difference in Ly$α$ visibility is therefore difficult to explain solely by variations in underlying galaxy population. Our results are consistent with reduced Ly$α$ attenuation through a more highly ionized IGM in the overdense region, as expected if large H\,{\sc ii} bubbles preferentially form around such galaxy overdensities. The wide-field LAE distribution provides complementary support for this scenario, as the HAE-defined density center coincides with the strongest LAE overdensity across the $1.4\,{\rm deg}^2$ HSC field. These findings highlight the importance of spatial variations in IGM attenuation in shaping Ly$α$ visibility near the end of reionization.

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arXiv2026

Halo assembly bias in the early Universe: a clustering probe of the origin of the Little Red Dots

The clustering of galaxies encodes key information about the structure and assembly history of their host dark matter (DM) haloes, providing a powerful probe of the origin of extreme high-redshift systems. While halo assembly bias has been extensively studied at low redshift, its behavior in the early Universe remains poorly explored. Using the large-volume, high-resolution Shin-Uchuu cosmological $N$-body simulation, we characterize halo assembly bias associated with formation time, concentration, and angular momentum across a wide range of halo masses and redshifts. We find that the sign and amplitude of assembly bias depend on halo mass for both concentration and spin. High-concentration and low-spin haloes are more strongly clustered below characteristic peak heights of $ν\sim 1.5$ and $\sim 0.75$, respectively, while the trends weaken or reverse at higher masses. Halo age bias persists at all redshifts but decreases toward higher masses and earlier cosmic times. We apply these results to assess whether clustering can distinguish competing formation scenarios for the Little Red Dots (LRDs). We find that the direct-collapse-black-hole (DCBH) scenario predicts the strongest large-scale bias and enhanced pair fractions, the self-interacting-dark-matter (SIDM) core-collapse scenario and low-spin compact-galaxy scenarios yield weaker clustering due to lower characteristic halo masses and spin-related secondary bias, and a primordial-black-hole (PBH) scenario predicts unbiased clustering. Our results demonstrate that halo assembly bias and characteristic host masses provide powerful diagnostics for constraining the physical origin of LRDs, offering testable predictions for upcoming clustering measurements with JWST and future deep surveys.

astro-ph.CO↗

arXiv2026

Free-Free Radio Emission from Little Red Dots as a Probe of Ionized Gas

Recent studies have hypothesized that little red dots (LRDs) are rapidly accreting black holes surrounded by dense gaseous environments. Both neutral and ionized gas play a key role in explaining many of the puzzling spectral features of LRDs, including the emission line profiles in the optical-infrared bands, although the physical conditions required for these gas components remain poorly constrained. We propose that free-free emission provides a robust, independent probe of ionized gas in LRDs. Free-free self-absorption produces a characteristic spectral turnover whose critical frequency and luminosity depend on the electron column density and the characteristic size of the ionized region, such that radio observations simultaneously constrain both quantities. The spectral slope at frequencies below the turnover further probes the radial density distribution on spatial scales far below those achievable by current and future optical-infrared observatories. We also present predicted free-free spectral energy distributions for two representative LRDs in the local and high-redshift universe and show that future radio-to-millimeter observations with ALMA and ngVLA can probe the range of ionized-gas parameters inferred from current observations. Free-free emission therefore offers a unique way to probe the geometry and physical conditions of the ionized gas surrounding LRDs.

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arXiv2026

SHELLQs. Black Hole Mass and Eddington Ratio Distributions of Intermediate-Luminosity Quasars at 6<z<7

We present near-infrared spectroscopy of 21 quasars at 6.07 < z < 6.90 obtained with JWST/NIRSpec and Subaru/MOIRCS. These targets have absolute ultraviolet magnitudes of -25 < M_1450 < -22 and are drawn from a sample of z>6 quasars identified in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project. These quasars occupy the intermediate-luminosity regime between luminous quasars and the faint high-redshift AGNs uncovered by JWST. We detect broad Balmer emission lines and MgII 2798 with underlying continua from the NIRSpec and MOIRCS targets, respectively. The virial black hole masses of this sample span a wide range of 7.2 < log M_BH/M_sun < 9.4, with Eddington ratios of -1.3 < log L_bol/L_Edd < 0.4. Combining these measurements with our previous mass estimates for other SHELLQs quasars, we construct a sample of 27 quasars with M_BH estimates at 6 < z < 7 and derive the distributions of M_BH and L_bol/L_Edd. When compared with luminosity-matched quasars at z ~ 1.3, we find median offsets of delta log M_BH/M_sun = -0.4 and Delta log L_bol/L_Edd = 0.2 in the high-redshift sample. In addition, 11% of the high-redshift quasars are accreting near or above the Eddington limit. As a systematic check, adopting the Eddington-ratio-dependent calibration of single-epoch M_BH estimates increases the inferred super-Eddington fraction to 22%. These results indicate that a representative population of distant supermassive black holes undergo a rapid accretion phase, although not as extreme as that seen in the most luminous quasar population, consistent with the anti-hierarchical growth scenario of supermassive black holes.

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arXiv2026

Hidden in Pixels. I. Discovery of dual "little red dots" indicates excess clustering on kilo-parsec scales

``Little Red Dots'' (LRDs) are an abundant high-redshift population newly discovered by the James Webb Space Telescope (JWST) and considered to be an early growth phase of supermassive black holes (SMBHs). Using a method of pixel-by-pixel color selection and relaxing the compactness criteria, we identify four dual LRD candidates in the COSMOS-Web survey with projected separations of $0.\!\!^{\prime\prime}2$-$1.\!\!^{\prime\prime}2$. A comparison between existing LRD samples and mock data reveals that the projected separations of these dual LRD candidates are unlikely to result from chance projections of objects at different redshifts. Furthermore, two of the four systems are covered by COSMOS-3D slitless spectroscopy, and a single-line detection at the same observed wavelength for each LRD in a pair strongly supports that they are at identical redshifts. Assuming that the detected lines are H$α$ based on their high equivalent width and broad profile, the spectroscopic redshifts of $z=5.822$ and $5.464$ for the two pairs are consistent with their photometric redshifts, yielding projected separations of $1.64$ and $7.36\,{\rm kpc}$. These discoveries suggest that the angular auto-correlation function (ACF) of LRDs exhibits an excess ($\sim20$-$30$ times) on sub-arcsec (kilo-parsec) separations compared to an extrapolation of a power-law ACF of JWST-found AGNs measured over $10^{\prime\prime}$-$100^{\prime\prime}$. Our sample is likely to represent precursors of mergers between LRDs, and such mergers may be one of the mechanisms that can drive the rapid growth of SMBHs in their early evolutionary stages.

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arXiv2026

JWST Reveals a Candidate Supermassive Black Hole Binary at z=4.3 in the Brightest Sub-millimeter Galaxy in COSMOS-Web

We present JWST/NIRSpec PRISM and G395M grating spectroscopy for AzTEC-1, a massive sub-mm bright galaxy at $z=4.34$ in the COSMOS extragalactic field. The PRISM spectrum reveals strong H$α$, a significant Balmer break, and no H$β$ detection, indicating a $100-400$ Myr-old stellar population and high dust attenuation. BPT line ratios indicate the presence of an Active Galactic Nucleus (AGN). Decomposing narrow and broad line components, we recover broad, blueshifted H$α$ with a velocity offset of $1245{\,\rm km\,s^{-1}}$ from the systemic narrow line velocity and with FWHM$\,\sim2500\,{\rm km\,s^{-1}}$. AzTEC-1's smooth morphology and stellar age is suggestive of a past merger-induced starburst period that would have brought in a second supermassive black hole, raising the possibility for a binary supermassive black hole system. In this scenario, we assume that the lower mass black hole hosts a broad line region orbiting a quiescent primary. Evidence for an extended outflow is not found in the 2D spectrum, NIRCam imaging, resolved ALMA observations of dust continuum, or CO, [C II]$_{157\,μ\rm m}$ and [N II]$_{\rm 205\,μm}$ kinematics. AzTEC-1's high central gas mass surface density and dynamically unstable gas disk indicates that massive gas clouds external to the candidate binary SMBH's orbit might have played a role in stalling infall from $\sim10$ Myr to $\sim100$ Myr through dynamical torques, which has been theorized to occur in the nuclei of massive galaxies like AzTEC-1. If the supermassive black hole binary is confirmed, AzTEC-1 would be an excellent laboratory into the astrophysics driving low-frequency gravitational wave detections.

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arXiv2026

Skyfire: A Spectroscopic Census of Little Red Dots and Broad-Line AGN in the CEERS Field

We present the Skyfire program, a 21-hour Cycle 3 JWST/NIRSpec survey with the G395M medium-resolution grating covering five pointings in the Extended Groth Strip. The survey is designed to carry out a systematic census of faint, broad-line AGN candidates with a range of rest-optical colors identified at z > 3 by the Cosmic Evolution Early Release Science (CEERS) Survey. Our primary targets include photometrically-selected Little Red Dots (LRDs), blue extreme emission line galaxies (EELGs), and X-ray-detected AGN. We present spectroscopic redshifts for 178 sources observed by Skyfire, as well as a catalog of 34 sources with broad emission lines in the redshift range 2.7 < z < 6.5. Our broad-line sample includes 18 LRDs, which brings the spectroscopic completeness of LRDs with $β_{\rm opt}>-0.02$ in the CEERS field to 73%. We explore the prevalence of broad emission lines in photometrically-selected LRDs as a function of their rest-frame continuum slope and observed color distributions. We find the broad-line detection fraction in LRDs remains high at relatively blue rest-optical colors and extends smoothly into the bluer regime occupied by Little Blue Dots (LBDs). We discuss the implications of this finding for LRD-LBD unification scenarios. We also find that only 18% (3/17) of EELGs selected primarily for their high-equivalent-width emission lines and compact morphologies exhibit broad emission lines, suggesting these criteria alone are poor predictors of broad-line activity. We present a revised set of LRD selection criteria that captures bluer sources by extending down to $β_{\rm opt}=-0.52$. Using this new threshold, we find that $80.9^{+4.6}_{-7.5}\%$ of photometrically-selected LRDs brighter than 26.5 in F444W show broad emission lines and that LRDs make up 54% of the overall broad-line population identified in the CEERS field.

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arXiv2026

Extended Components of Little Red Dots in the Rest-Frame Optical

Recent JWST observations have revealed a population of red, compact, high-redshift objects called Little Red Dots (LRDs), whose host components have remained largely unconstrained, possibly due to their extreme compactness. Current morphological studies suggest the presence of extended emission in LRDs at rest-frame ultraviolet wavelengths. However, in the rest-frame optical regime, investigations have been limited by small sample sizes and insufficient imaging depth, hindering reliable separation between point-like and potential extended components. Here we perform the image stacking analysis of 217 LRDs in four NIRCam bands, a large and homogeneous sample observed with the COSMOS-Web survey. Our results reveal the detection of faint extended emission in the F444W band, with a typical size of ~200 parsecs and magnitude of ~27.7 AB at z~6.5. We perform four-band photometric spectral energy distribution fitting based on galaxy templates and derive an average stellar mass of log(M*/M_sun) = 9.02 +0.20/-0.18. Given this stellar mass, the host galaxy is compact, that is, ~2.5 times smaller than star-forming galaxies of similar mass at comparable redshifts. This work provides direct observational evidence for the existence of LRD host galaxies at rest-frame optical wavelengths and offers new insights into the stellar buildup of these systems within the first billion years after the Big Bang.

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arXiv2026

Dust in the Average Galaxy: Attenuation, Emission, and Opacity from $0<z<7$

We present constraints on the dust emission and attenuation properties of galaxies across 0 10^{10.5}M_\odot$. We derive empirical relationships for the effective attenuation, dust temperature, fraction of star formation that is unobscured, and dust-to-stellar mass ratio as functions of redshift and stellar mass. We separate the first order effect of star/dust geometry from dust grain properties by combining constraints on the IR SED, UV SED, and dust mass surface density. Importantly, we measure over an order of magnitude decrease in $κ_{UV}/κ_{FIR}$--the ratio of dust mass absorption coefficients in the UV at 1600Å and FIR at 500$μ$m--from z~0 to z~7. A depressed $κ_{UV}/κ_{FIR}$ is consistent with a deficit of small dust grains, possibly attributable to the intense radiation fields of high-$z$ star formation; indeed, we find a redshift-invariant inverse relationship between $κ_{UV}/κ_{FIR}$ and $Σ_{SFR}$. Most evolution in the dust-to-stellar ratio is at $z<1$, the product of mild downward evolution in the dust-to-gas ratio combined with steep evolution in the gas-to-stellar ratio. The significant evolution and dynamic range of $κ_{UV}/κ_{FIR}$ and prevailing disconnect between the UV/optical and FIR regimes emphasize that direct dust constraints are irreplaceable for the majority of star-forming galaxies at z<7, not just the most extreme star-formers.

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arXiv2026

Two Exciting High-redshift Galaxy Candidates Turn Out to Be Two Exciting Ultra-cool Brown Dwarfs

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts $z>10$ and as high as $z=14$. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at $z>15$ may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright $z\sim 14$ galaxies should be visible at redshifts as high as $z\sim 20-30$, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures ${272\mbox{--}351\mbox{K}}$ and ${445\mbox{--}525\mbox{K}}$ (using ATMO2020 and Sonora Elf Owl models) and distances of $\sim 150\mbox{--}650\mbox{pc}$. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken $\sim 1$ year later, we also detect their proper motions of $(49 \pm 8)\,\mbox{mas/yr}$ and $(24 \pm 3)\,\mbox{mas/yr}$, further indicating that at least some F277W and F356W dropouts are sub-stellar cold Milky Way objects such as brown dwarfs.

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arXiv2026

Gravitational Wave Astronomy With TianQin

The opening of the gravitational wave window has significantly enhanced our capacity to explore the universe's most extreme and dynamic sector. In the mHz frequency range, a diverse range of compact objects, from the most massive black holes at the farthest reaches of the Universe to the lightest white dwarfs in our cosmic backyard, generate a complex and dynamic symphony of gravitational wave signals. Once recorded by gravitational wave detectors, these unique fingerprints have the potential to decipher the birth and growth of cosmic structures over a wide range of scales, from stellar binaries and stellar clusters to galaxies and large-scale structures. The TianQin space-borne gravitational wave mission is scheduled for launch in the 2030s, with an operational lifespan of five years. It will facilitate pivotal insights into the history of our universe. This document presents a concise overview of the detectable sources of TianQin, outlining their characteristics, the challenges they present, and the expected impact of the TianQin observatory on our understanding of them.

astro-ph.GA↗

arXiv2026

VENUS: an ultra-faint galaxy hosting the metal-poor type II supernova at $z=5.13$ Witnessing the initial metal enrichment with extremely frequent core-collapse supernovae?

We present the first characterization of the host galaxy of a recently discovered type IIP SN at $z=5.13$ (SN Eos). SN Eos and its host galaxy are gravitationally lensed and multiply imaged. The total magnification $μ\sim53$ enables spatially resolving the system, allowing us to localize the core-collapse supernova (CCSN) position and to characterize its local environment within an early galaxy. Our observation reveals that the host is an ultra-faint ($M_{\rm UV}=-14.4\pm0.3$ mag) Lyman-$α$ emitter with a very high equivalent width. The host galaxy also shows very weak [O iii]4959,5007 lines despite an H$α$ line detection ([O iii]5007/H$β<0.7$ with case B recombination). Assuming that the weak [O iii] is due to low gas-phase metallicity given the low-metallicity of SN Eos itself, SN Eos plausibly marks the formation and explosion of a metal-poor star in an extremely metal-poor environment ($<1\ \%\ Z_\odot$), facilitating the initial stages of the chemical enrichment of the host. Finding the CCSN in such an ultra-faint galaxy at $z=5.13$ also indicates that the SN rate could be considerably higher in high-$z$, metal-poor environments, potentially implying e.g., a $Z$-dependent IMF, $Z$-dependent massive star explodability, or runaway stellar collisions in dense star clusters. Without lensing, only SN Eos would be detectable and the host would be below the detection limit in any NIRCam surveys ever performed. The Eos host galaxy can thus be representative of the origin of {\it hostless} supernovae frequently found in JWST blank field surveys.

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