arXiv subjects

Xiaojing Lin

Publications and source records attributed to Xiaojing Lin.

At least 37 records · Page 2Linked to original sources

arXiv2026

The Cosmic Owl: Twin Active Collisional Ring Galaxies with Starburst Merging Front at $z=1.14$

Galaxy mergers play a critical role in driving galaxy evolution, especially by transforming galaxy morphology, redistributing gas around galaxies, triggering active galactic nuclei (AGN), and stimulating star formation. We present the Cosmic Owl, a galaxy merger at $z=1.14$, identified in the COSMOS field. Deep imaging and spectroscopy from JWST, ALMA, and VLA reveal a complex system of twin collisional ring galaxies, exhibiting nearly identical morphologies. The grism spectra from the JWST COSMOS-3D program confirm that both galaxies host an AGN. A bipolar radio jet from one AGN extends to strike the merging front. In addition, we detect a starburst at the merging front, characterized by luminous extended nebular line emission and a massive cold gas reservoir. This starburst is likely triggered by interstellar shocks induced by galaxy collision and the AGN jet. The twin ring structure of the Cosmic Owl requires further numerical simulations to clarify the precise conditions that lead to the formation of this rare morphology. This system exemplifies how shock-induced star formation, driven by galaxy collision or AGN jet, can act as a crucial mechanism for triggering intense starbursts in the early Universe.

astro-ph.GA↗

arXiv2026

JADES: Discovery of Large Reservoirs of Small Dust Grains in the Circumgalactic Medium of Massive Galaxies at $z\sim3.5$ through Deep JWST/NIRCam Imaging and Grism Spectroscopy

Using JWST NIRCam imaging and grism spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES) Origins Fields, we report spectroscopic redshift measurements of 1,445 emission-line galaxies at $z=0-9$. Within this sample, we identify two prominent galaxy protoclusters at $z = 3.47$ and 3.69, each anchored by massive dusty star-forming galaxies (DSFGs). In the vicinity of these systems, we discover seven background galaxies at $z=3.6 - 6$ that simultaneously exhibit strong rest-frame optical emission lines (e.g., [O III] and H$α$) and unusually reddened UV-to-optical continua. We attribute this reddening to dust extinction arising from the circumgalactic medium (CGM) of the foreground DSFGs at projected separations of 7-30 kpc. We infer a high dust column density ($\gtrsim 10^{-1}$ Msun/kpc^2), substantially exceeding those measured in low-redshift halos and those predicted by hydrodynamical simulations like IllustrisTNG and FIRE-2. The steep extinction curves, comparable to or steeper than that of the SMC, indicate a dominant population of small dust grains in the high-redshift CGM. We conclude that DSFGs at this epoch host large reservoirs of dusty CGM enriched to solar metallicity. These extended dust components are largely invisible to (sub-)millimeter interferometers such as ALMA because of their low surface brightness. We discuss the physical processes in dust transport that might be key to reproducing our observations, including galaxy mergers, cool-phase gas outflows, dust shattering, sputtering and radiation pressure. Finally, we caution that foreground CGM dust extinction may redden background galaxies at intermediate redshifts to mimic Lyman-break galaxies at $z\gtrsim10$.

astro-ph.GA↗

arXiv2026

Abundant Population of Broad H$α$ Emitters in the GOODS-N Field Revealed by CONGRESS, FRESCO, and JADES

We present a spectroscopic search for broad H$α$ emitters at z$\approx$3.7-6.5 in the GOODS-N field, utilizing JWST/NIRCam slitless spectroscopy from FRESCO and CONGRESS, complemented by JADES imaging. We identify 19 broad H$α$ emitters with FWHM$>$1000 km/s at z$\approx$4-5.5, including 9 new sources. The broad H$α$ luminosity function (LF) derived from our sample is consistent with those of other JWST-selected broad-line AGN reported in the literature. The black hole masses and AGN bolometric luminosities, inferred from the broad H$α$ components, indicate that most sources are accreting at ~10% of the Eddington limit. We derive their host stellar masses via SED fitting and find higher $M_{BH}/M_{*}$ ratios relative to the local $M_{BH}-M_{*}$ relations, consistent with previous studies. We find that 42% of the sample do not satisfy the widely-used color selection criteria for Little Red Dots (LRDs), with the majority of these sources lacking the characteristic steep rest-optical red slope, indicating that the LRD selection is highly incomplete when selecting AGN galaxies. A comparison of the average SEDs between our sample and LRDs selected in the same field reveals that the steep red slopes observed in some LRDs are likely due to line-boosting effects as previously suggested. Furthermore, we find that 68% of color-selected LRDs with H$α$ detections in the NIRCam/Grism spectra do not exhibit broad-line features. While the limited sensitivity of the grism spectra may hinder the detection of broad-line components in faint sources, our findings still highlight the enigmatic nature of the LRD population.

astro-ph.GA↗

arXiv2025

Ionizing Photon Production Efficiencies and Chemical Abundances at Cosmic Dawn Revealed by Ultra-Deep Rest-Frame Optical Spectroscopy of JADES-GS-z14-0

JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI. This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at $z > 14$. Most notably, we detect $[\mathrm{OIII}] λλ4959,5007$ at $\approx 11 σ$ and $\mathrm{H}α$ at $\approx 4 σ$ with these ultra-deep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star-formation rate of $\mathrm{SFR} \approx 10 \pm 2\ M_{\odot} / \mathrm{yr}$, star-formation rate surface density of $Σ_{\mathrm{SFR}} \approx 23 \pm 5\ M_{\odot}/\mathrm{yr}/\mathrm{kpc}^{2}$, and ionizing photon production efficiency of $ξ_{\mathrm{ion}} \approx 10^{25.3 \pm 0.1}\ \mathrm{Hz/erg}$. Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of $[\mathrm{O/H}] \approx -1.1 \pm 0.4$ ($\approx 10\%\ Z_{\odot}$), carbon-to-oxygen ratio of $[\mathrm{C/O}] \approx -0.4 \pm 0.4$, ionization parameter of $\mathrm{log}_{10}(U) \gtrsim -2.4$, and density of $n_{\mathrm{H}} \approx 720 \pm 210\ \mathrm{cm}^{-3}$. Using detailed photoionization modeling, we instead derive $[\mathrm{O/H}] \approx -0.3_{-0.4}^{+0.4}$ ($\approx 50\%\ Z_{\odot}$) and $\mathrm{log}_{10}(U) \approx -1.5_{-0.4}^{+0.3}$. The inferred properties of JADES-GS-z14-0 are similar to those measured for similarly luminous galaxies at $z > 10$ with previous MIRI/Spectroscopy, such as GHZ2/GLASSz12, GN-z11, and MACS0647-JD1. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe. [Abridged]

astro-ph.GA↗

arXiv2025

The Luminosity Function and Clustering of H$α$ Emitting Galaxies at $z\approx4-6$ from a Complete NIRCam Grism Redshift Survey

We study the luminosity function (LF) and clustering properties of 888 H$α$ emitters (HAEs) at $3.75 < z < 6$ in the GOODS-N field. The sample, built from JWST CONGRESS and FRESCO NIRCam grism surveys using a novel redshift assignment algorithm, spans $\sim$62 arcmin$^2$ and reaches $L_{\rm Hα} \sim 10^{41.2} {\rm erg s^{-1}}$. We identify two prominent filamentary protoclusters at $z \approx 4.41$ and $z \approx 5.19$, hosting 98 and 144 HAEs, respectively. The observed H$α$ LFs show similar shallow faint-end slopes for both protocluster and field galaxies at $z=3.75-5$, and for the protocluster at $z=5-6$ ($α\approx 1.2$ to $-1.3$). In contrast, the field LF at $z=5-6$ is much steeper ($α=-1.87_{-0.23}^{+0.30}$), suggesting that protocluster galaxies at $z > 5$ are more evolved, resembling those at $z=3.75-5$. The observed star formation rate density from H$α$, integrated down to 0.45 ${\rm M_\odot yr^{-1}}$, is $0.050^{+0.002}_{-0.003}$ and $0.046^{+0.006}_{-0.004} M_\odot {\rm yr}^{-1} {\rm Mpc}^{-3}$ at $z=3.75-5$ and $z=5-6$, with protoclusters contributing $\sim$25% and 55%, respectively. This implies that a large fraction of star formation at $z > 4$ occurs in protoclusters. We conduct the first star-formation-rate-limited 3D clustering analysis at $z > 4$. We find the filamentary protocluster geometry flattens the power-law shape of the HAE auto-correlation functions, with slopes much shallower than typically assumed. The auto-correlation function of field HAEs have correlation lengths of $r_0 = 4.61^{+1.00}_{-0.68} h^{-1}{\rm Mpc}$ at $z \approx 4-5$ and $r_0 = 6.23^{+1.68}_{-1.13} h^{-1}{\rm Mpc}$ at $z=5-6$. Comparing the observed correlation functions with the UniverseMachine simulation, we infer the dark matter (sub-)halo masses of HAEs to be $\log (M_h/M_\odot)=11.0-11.2$ at $z\approx 4-6$, with a scatter of 0.4 dex.

astro-ph.GA↗

arXiv2025

The Large-scale Environments of Low-luminosity AGNs at $3.9 < z < 6$ and Implications for Their Host Dark Matter Halos from a Complete NIRCam Grism Redshift Survey

We study the large-scale environments and clustering properties of 28 low-luminosity AGNs at $z=3.9-6$ in the GOODS-N field. Our sample, identified from the JWST NIRCam Imaging and WFSS data in CONGRESS and FRESCO surveys with either broad H$α$ emission lines or V-shape continua, are compared to 782 H$α$ emitters (HAEs) selected from the same data. These AGNs are located in diverse large-scale environments and do not preferentially reside in denser environments compared to HAEs. Their overdensity field, $δ$, averaged over (15 $h^{-1}$cMpc)$^3$, ranges from $-0.56$ to 10.56, and shows no clear correlation with broad-line luminosity, black hole (BH) masses, or the AGN fraction. It suggests that $> 10$ cMpc structures do not significantly influence BH growth. We measure the two-point cross-correlation function of AGNs with HAEs, finding a comparable amplitude to that of the HAE auto-correlation. This indicates similar bias parameters and host dark matter halo masses for AGNs and HAEs. The correlation length of field AGNs is 4.26 $h^{-1}$cMpc, and 7.66 $h^{-1}$cMpc at $3.9 < z < 5$ and $5 < z < 6$, respectively. We infer a median host dark matter halo mass of $\log (M_h/M_\odot)\approx 11.0-11.2$ and host stellar masses of $\log (M_\star/M_\odot) \approx 8.4-8.6$ by comparing with the UniverseMachine simulation. Our clustering analysis suggests that low-luminosity AGNs at high redshift reside in normal star-forming galaxies with overmassive BHs. They represent an intrinsically distinct population from luminous quasars and could be a common phase in galaxy evolution.

astro-ph.GA↗

arXiv2025

The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes

JWST observations have revealed a population of high-redshift "little red dots" (LRDs) that challenge conventional AGN models. We report the discovery of three local LRDs at $z = 0.1$-$0.2$, initially selected from the SDSS database, with follow-up optical/near-IR spectroscopy and photometry. They exhibit properties fully consistent with those of high-redshift LRDs, including broad hydrogen and helium emission lines, compact morphologies, V-shaped UV-optical SED, declining near-IR continua, and no significant variability. Two sources were targeted but not detected in X-rays with statistical significance. All three sources show blue-shifted He I absorption, while two exhibit H$α$ and Na D absorption lines. We detect full Balmer and Paschen line series in all three objects, along with abundant narrow [Fe II] emission in two. The emission line analyses suggest narrow lines originate from AGN-powered, metal-poor regions with minimal dust; broad lines come from inner regions with exceptionally high density or atypical dust properties; and [Fe II] emission arises from dense gas between broad and narrow-line regions. One of our objects, J1025+1402 (nicknamed $The~Egg$), shows extremely high equivalent width Na D, K I, and Ca II triplet absorption lines, along with other potential low-ionization absorption features, suggesting the presence of a cool ($\sim$5000 K), metal-enriched gas envelope. The optical/near-IR continua of these LRDs are also consistent with theoretical models featuring an atmosphere around black holes. The WISE-detected IR emission is consistent with weak dust emission of $T \sim 10^2-10^3$ K. We propose a conceptual model consisting of a largely thermalized cool-gas envelope surrounding the central black hole and an extended emission line region with high-density outflowing gas to explain the observed properties of these local LRDs.

astro-ph.GA↗

arXiv2025

Extreme Neutral Outflow in a non-AGN Quiescent Galaxy at z~1.3

We report the discovery of a substantial sodium doublet (Na D $λλ$5890, 5896Å)-traced neutral outflow in the quiescent galaxy JADES-GS-206183 at $z=1.317$. Its JWST/NIRSpec-MSA spectrum shows a deep, blueshifted Na D absorption, revealing a neutral outflow with $v_{\rm out}=828^{+79}_{-49}\,\mathrm{km\,s^{-1}}$ and a mass outflow rate of $\log(\dot{M}_{\rm out}/\mathrm{M_{\odot}\,yr^{-1}})=2.40^{+0.11}_{-0.16}$. This outflow rate exceeds that of any neutral outflows identified beyond $z\sim1$ by the same line and is comparable with those in local galaxies with intensive star formation or luminous AGN. JADES-GS-206183 is also a peculiar quiescent galaxy with a spiral$+$bar morphology, high dust attenuation ($A_V=2.27\pm0.23$ mag). Paschen $α$ (Pa$α$) emission from the FRESCO NIRCam grism confirms its low star formation rate ($\mathrm{SFR_{Paα}}=10.78\pm 0.55\,\mathrm{M_{\odot}\,yr^{-1}}$), placing it 0.5 dex below the main-sequence ($\log(\mathrm{sSFR/yr^{-1}})=-10.2$). Despite the systematics introduced by different star formation history (SFH) priors, the SED modeling, combining HST-to-NIRCam photometry with the VLT/MUSE spectrum, suggests that JADES-GS-206183 experienced an older episode of star formation 0.5-2 Gyr ago and a possible rejuvenation within recent $\sim$10 Myr. Moreover, rest-frame optical lines indicate that the current AGN activity of JADES-GS-206183, if present, is also weak. Even though we tentatively detect a broad component of the H$α$ line, it likely traces an ionized outflow rather than an AGN. The results demonstrate that the Na D outflow in JADES-GS-206183 is highly unlikely to be driven by current star formation or nuclear activity. Instead, it may represent a long-lasting fossil outflow from past AGN activity, potentially co-triggered with the early phase of rejuvenation.

astro-ph.GA↗

arXiv2025

Bridging Quasars and Little Red Dots: Insights into Broad-Line AGNs at $z=5-8$ from the First JWST COSMOS-3D Dataset

We report the discovery of 13 broad-line AGNs at $z = 5 - 8$ from the first 10% data of the JWST Cycle 3 Treasury Program COSMOS-3D. These AGNs are identified by their broad H$α$ or H$β$ emission lines through the NIRCam grism wide-field slitless spectroscopy. One object at $z = 7.646$ with broad H$β$ emission has an F444W magnitude of 23.6 mag, making it one of the brightest $z > 7.5$ broad-line AGNs yet known. Among the 13 AGNs, 10 objects have reddened optical continua with slopes $β_{\rm opt}>0$. The remaining three objects have their overall SEDs that resemble those of UV-luminous quasars at similar redshifts, but their $β_{\rm opt}$, though negative, are not as blue as those of unobscured quasars. We also obtain MIRI photometry at 7.7-18 $μ$m for two AGNs and place strong constraints on their rest-frame near-IR SED. We find no significant variability in the rest-frame UV by comparing the COSMOS-3D and COSMOS-Web F115W images taken apart by 60 days in the rest-frame. We compute the H$α$ luminosity functions (LFs) for the broad H$α$ emitters at $z \approx 5-6$ and find a potential redshift evolution when compared with that of the $z \approx 4-5$ sample. We also derive the H$β$ LF at $z\sim8$ for AGNs and galaxies by combining our sample with those from the literature. The broad H$β$ emitters in this work suggest a number density two orders of magnitude higher than that predicted by the quasar LF based on rest-frame UV-selected samples. As a preview, our work showcases the ability of the COSMOS-3D grism survey to provide a complete view of the properties, growth, and evolution of bright broad-line AGNs at $z>5$.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2025

Quasar Radiative Feedback May Suppress Galaxy Growth on Intergalactic Scales at $z = 6.3$

We present observational evidence that intense ionizing radiation from a luminous quasar suppresses nebular emission in nearby galaxies on intergalactic scales at $z=6.3$. Using JWST/NIRCam grism spectroscopy from the SAPPHIRES and EIGER programs, we identify a moderate but statistically significant decline in [O\,\textsc{iii}]\,$\lambda5008$ luminosity relative to the UV continuum ($L_{5008}/L_{1500}$) among galaxies within $\sim$ 7 comoving Mpc (cMpc) of the quasar J0100$+$2802, the most UV-luminous quasar known at this epoch ($M_{1450}=-29.26$). While $L_{1500}$ remains roughly constant with transverse distance, $L_{5008}$ increases significantly, suggesting suppression of very recent star formation toward the quasar. The effect persists after controlling for completeness, local density, and UV luminosity, and correlates with the projected photoionization-rate profile $Γ_{\mathrm{qso}}$. A weaker but directionally consistent suppression in $L_{5008}/L_{1500}$ is also observed along the line of sight. The transverse suppression radius ($\sim$ 7 cMpc) implies a recent radiative episode with a cumulative duration $\sim$ 3.1 Myr, shorter than required for thermal photoheating to dominate and thus more naturally explained by rapid H$_2$ photodissociation and related radiative processes. Environmental effects alone appear insufficient to explain the signal. Our results provide direct, geometry-based constraints on large-scale quasar radiative feedback and recent quasar lifetimes.

astro-ph.GA↗

arXiv2025

A Metal-Free Galaxy at $z = 3.19$? Evidence of Late Population III Star Formation at Cosmic Noon

Star formation from metal-free gas, the hallmark of the first generation of Population III stars, was long assumed to occur only in the very early Universe. We report the discovery of MPG-CR3 (Metal-Pristine Galaxy COSMOS Redshift 3; hereafter CR3), an extremely metal-poor galaxy at redshift $z= 3.193\pm0.016$. From JWST, VLT, and Subaru observations, CR3 exhibits exceptionally strong Ly$α$, H$α$, and He I $λ$10830 emission. We measure rest-frame equivalent widths of EW$_0$(Ly$α$) $=822\pm101$ Angstrom and EW$_0$(H$α$) $=2814\pm327$ Angstrom, among the highest seen in star-forming systems. No metal lines, e.g. [O III] $λ\lambda4959,5007$, C IV $λ\lambda1548,1550$, have statistically significant detections, placing a 2-$σ$ upper limit on the gas-phase metallicity of 12+log(O/H) < 6.52 ($Z < 7\times10^{-3}\ Z_\odot$) with strong-line calibration established by JWST, making it the most metal-poor galaxy known at cosmic noon. Considering systematic uncertainties of $\gtrsim 0.3$ dex in the calibrations, the most conservative 2-$σ$ upper limit is set to 12+log(O/H) < 6.95. The observed Ly$α$/H$α$ flux ratio is $13.9\pm2.5$, indicating negligible dust attenuation. Spectral energy distribution modeling with Pop III stellar templates indicates a very young ($\sim2$ Myr), low-mass ($M_* \approx 6.1\times 10^5 M_\odot$) stellar population. Further, the photometric redshifts reveal that CR3 could reside in a slightly underdense environment ($δ\approx -0.12$). CR3 provides evidence that first-generation star formation could persist well after the epoch of reionization, challenging the conventional view that pristine star formation ended by $z\gtrsim6$.

astro-ph.GA↗

arXiv2025

Disturbed cold gas in galaxy and structure formation

Cold and cool gas (T $\leq 10^4$ K) in the circumgalactic medium (CGM) and its interaction with galaxies remain poorly understood. Simulations predict that cold gas flows into galaxies through cosmic filaments, determining the disk formation and galaxy evolution. The cold gas accretion modes in the CGM and their dependence on dark matter halo mass and redshift remain puzzling. Resolving the kiloparsec-scale kinematics and dynamics of cold gas interacting with the disk, dust, and metals in different environments is particularly lacking at z > 2. Here we report two disturbed cold gas structures traced by ultra-strong MgII absorbers (rest-frame equivalent width Wr > 2 Å) exhibiting high kinematic velocities (> 500 km/s) and their environments at z ~ 4.9 and z ~ 2.6. Observations were conducted with VLT/MUSE, JWST/NIRCam, and ALMA to detect Lya and nebular emission lines, as well as dust continuum emission in the vicinity of these two absorbing gas structures. We identify two Lya emitters associated with a strong MgII absorber pair separated by ~1000 km/s at z~ 4.87. The pair exhibits relative differences in metallicity, dust content, and ionization states, suggesting internal metal and dust exchange within the ultra-large cold gas structure. For the strong MgII absorber at z = 2.5652, we detect a dusty star-forming galaxy at a projected distance of $D = 38$ kpc. This galaxy exhibits prominent HeI, [SIII], and Paschen$γ$ lines, along with significant dust continuum. It has a star formation rate of ~ 121 +/- 33 $M_{\odot}$/yr and likely harbors a rotating disk. These findings tentatively suggest that cold gas at high redshifts plays a critical role in driving disk formation and actively participates in the transfer of metals and dust within the overdense regions of the CGM.

astro-ph.GA↗

arXiv2025

JWST lens model for A370: A very low dark matter fraction for a brightest cluster galaxy and lensing properties for the Dragon arc

We present a new lens model for the $z=0.375$ galaxy cluster Abell 370 based on previously spectroscopically confirmed lensed galaxies and new lensed systems identified in JWST data, including recent data from the MAGNIF program. Based on the best models able to reproduce two radial arcs near the BCGs, we compare the stellar mass to the total mass from the lens model and find that the fraction of dark matter in the south BCG is consistent with $Λ$CDM while in the north BCG we find a very small amount of dark matter, more consistent with alternative models to $Λ$CDM. We discuss possible causes for this and conclude that additional data is needed to clarify the situation. We study the lensing properties, magnification, time delay and strength of the critical curve, along the Dragon arc, where previous studies have reported tens of alleged microlensing events from supergiant stars at $z=0.7251$. The new lens model is able to reproduce the distribution of microlensing events with great accuracy. Some of the microlensing events may be reinterpreted as long-period Cepheid in future observations. We consider this possibility and study in more detail the challenges for such detection from intervening microlenses.

astro-ph.CO↗

arXiv2025

No Luminous Little Red Dots: A Sharp Cutoff in Their Luminosity Function

One of the most surprising results of early James Webb Space Telescope (JWST) observations is the discovery of an abundance of red, compact, broad-line objects dubbed "little red dots" (LRDs) at $z>4$. Their spatial density ($\sim10^{-4}$-$10^{-5}\,\mathrm{cMpc^{-3}}$) is 100 times more abundant than UV-selected quasars at those redshift if one extrapolates the quasar luminosity function (QLF) down to the LRD regime. However, whether LRDs dominate black hole accretion at quasar-like luminosities ($L_\mathrm{bol}\gtrsim 10^{45-46}\,\mathrm{erg\,s^{-1}}$) remains unanswered, as probing the bright end of the LRD luminosity function requires a much larger area than those able to be surveyed by JWST. In this work, we present our search for the brightest LRDs ($K<23.7$) at $4.5<z<4.9$ using wide-area multiwavelength imaging surveys from the near-UV to the infrared bands. With over 15 square degrees of sky coverage, we only identify one single LRD candidate at $z_\mathrm{phot}\approx4.6$, which translates into a spatial density of $n(M_{5100}<-23.5)\approx10^{-8}\,\mathrm{cMpc^{-3}}$ -- this is nearly 10 times less abundant than the UV-selected quasars at similar optical luminosity. When combined with the LRD sample identified by JWST at the same redshift range, we find a sharp cutoff in the optical luminosity function at $λL_{5100}\approx2.5\times10^{44}\,\mathrm{erg\,s^{-1}}$, while the QLF turnover occurs at $\gtrsim20$ times higher luminosity. We therefore confirm the exclusively low-luminosity nature of LRDs, ruling out that LRDs are the counter parts of quasars. Furthermore, we speculate that, if the shape of the luminosity function holds up, it points to LRDs being powered by low-mass black holes with a narrow range of Eddington-level accretion rates.

astro-ph.GA↗

arXiv2025

A 13-Billion-Year View of Galaxy Growth: Metallicity Gradient Evolution from the Local Universe to $z=9$ with JWST and Archival Surveys

The galaxy gas-phase metallicity gradients have been extensively studied over the past four decades, both in the local and high-redshift universe, as they trace the baryon cycle and growth of galaxies. With the unprecedented spatial resolution and sensitivity of JWST, it is now possible to measure metallicity and its radial gradients out to redshifts as high as $z = 9$. Here, we present a sample of 455 spectroscopically confirmed galaxies from redshifts $1.7 \lesssim z \lesssim 9$ that are spatially resolved on sub-kiloparsec (kpc) scales by deep JWST NIRCam or NIRISS Wide Field Slitless Spectroscopy (WFSS). Synthesizing these new JWST observations with legacy observations from the literature, we observe that at redshift $z > 5$, galaxy centers are more metal-rich, exhibiting negative metallicity gradients of $\sim-0.4$ dex kpc$^{-1}$. These gradients flatten over time, reaching near-zero around $z \approx 2$, coinciding with the peak of the cosmic star formation rate. Beyond this point, the gradients become negative again at lower redshifts approaching $z=0$. This evolution likely reflects transitions in galaxy formation modes: an inside-out growth phase dominated by intense central star formation with inefficient feedback and limited gas mixing during ``cosmic dawn", enhanced gas mixing due to feedback-driven wind and gas accretion at ``cosmic noon", and a later phase of slow evolution and reduced feedback toward the present day. These physical processes, including gas accretion and feedback, not only regulate star and galaxy formation on a cosmic scale but also shape the evolutionary pathways of individual galaxies over cosmic time.

astro-ph.GA↗

arXiv2025

The First Photometric Evidence of a Transient/Variable Source at z>5 with JWST

The James Webb Space Telescope (JWST) discovered 79 transients out to $z$$\sim$4.8 through the JADES Transient Survey (JTS), but the JTS did not find any $z$$>$5 transients. Here, we present the first photometric evidence of a $z$$>$5 transient/variable source with JWST. The source, AT 2023adya, resides in a $z_{\mathrm{spec}}$$=$5.274 galaxy in GOODS-N, which dimmed from $m_{\rm F356W}$$=$26.05$\pm$0.02 mag to 26.24$\pm$0.02 mag in the rest-frame optical over approximately two rest-frame months, producing a clear residual signal in the difference image ($m_{\rm F356W}$$=$28.01$\pm$0.17 mag; SN$_\mathrm{var}$$=$6.09) at the galaxy center. Shorter-wavelength bands (F090W/F115W) show no rest-frame ultraviolet brightness change. Based on its rest-frame V-band absolute magnitude of M$_\mathrm{V}$$=$$-$18.48 mag, AT 2023adya could be any core-collapse supernova (SN) subtype or an SN Ia. However, due to low SN Ia rates at high redshift, the SN Ia scenario is unlikely. Alternatively, AT 2023adya may be a variable active galactic nucleus (AGN). However, the JWST NIRCam/Grism spectrum shows no broad H$α$ emission line (FWHM$=$130$\pm$26 km s$^{-1}$), disfavoring the variable AGN scenario. It is also unlikely that AT 2023adya is a tidal disruption event (TDE) because the TDE models matching the observed brightness changes have low event rates. Although it is not possible to determine AT 2023adya's nature based on the two-epoch single-band photometry alone, this discovery indicates that JWST can push the frontier of transient/variable science past $z$$=$5 and towards the epoch of reionization.

astro-ph.HE↗

arXiv2025

Discovery of a Pair of Galaxies with Both Hosting X-ray Binary Candidates at $z=2.544$

Among high-redshift galaxies, aside from active galactic nuclei (AGNs), X-ray binaries (XRBs) can be significant sources of X-ray emission. XRBs play a crucial role in galaxy evolution, reflecting the stellar populations of galaxies and regulating star formation through feedback, thereby shaping galaxy structure. In this study, we report a spectroscopically confirmed X-ray emitting galaxy pair (UDF3 and UDF3-2) at $z = 2.544$. By combining multi-wavelength observations from JWST/NIRSpec MSA spectra, JWST/NIRCam and MIRI imaging, Chandra, HST, VLT, ALMA, and VLA, we analyze the ionized emission lines, which are primarily driven by H II region-like processes. Additionally, we find that the mid-infrared radiation can be fully attributed to dust emission from galaxy themselves. Our results indicate that the X-ray emission from these two galaxies is dominated by high-mass XRBs, with luminosities of $L_X= (1.43\pm0.40) \times 10^{42} \, \text{erg} \, \text{s}^{-1}$ for UDF3, and $(0.40\pm0.12) \times 10^{42} \, \text{erg} \, \text{s}^{-1}$ for UDF3-2. Furthermore, we measure the star formation rate (SFR) of $529_{-88}^{+64}$ $M_\odot$ yr$^{-1}$ for UDF3, placing it $\approx$ 0.5 dex below the $L_X$/SFR-$z$ relation. This offset reflects the redshift-dependent enhancement of $L_X$/SFR-$z$ relation, which is influenced by metallicity and serves as a key observable for XRB evolution. In contrast, UDF3-2, with the SFR of $34_{-6}^{+6}$ $M_\odot$ yr$^{-1}$, aligns well with the $L_X$/SFR-$z$ relation. This galaxy pair represents the highest-redshift non-AGN-dominated galaxies with individual X-ray detections reported to date. This finding suggests that the contribution of XRBs to galaxy X-ray emission at high redshift may be underestimated.

astro-ph.GA↗