arXiv subjects

Mengyuan Xiao

Publications and source records attributed to Mengyuan Xiao.

At least 19 recordsLinked to original sources

arXiv2026

PANORAMIC: The Dawn of Massive Quiescent Galaxies I. Number Density and Cosmic Variance from 1000 arcmin$^2$ NIRCam Imaging

We measure the number density and field-to-field variance of massive quiescent galaxies at $z\sim3$ - 8 using the JWST/NIRCam pure-parallel imaging survey PANORAMIC together with archival observations, covering an area of 0.28 deg$^2$ ($\sim1000$ arcmin$^2$) in at least six filters. We identify quiescent galaxy candidates at $z\gtrsim3$ with $M_\ast \gtrsim 10^{10}\,M_\odot$, comprising 101 galaxies in a gold sample of high-confidence candidates and 137 in a more inclusive silver sample. We measure their evolving comoving number density, finding $(1.5$ vs. $3.1)\times10^{-5}\,\mathrm{Mpc}^{-3}$ at $z=3$ - 4 for the gold and silver samples, respectively, and a decline by more than a factor of 20 by $z\sim6$. Comparisons with empirical models and cosmological simulations show that widely used frameworks underpredict the abundance of massive quiescent galaxies at $z\gtrsim4$ by $\gtrsim1$ dex, indicating that current implementations of early star formation, feedback, and quenching do not produce enough early quenched systems. With 34 independent sightlines, we present the first direct empirical measurement of field-to-field variance for quiescent galaxies at $z>3$, finding a high cosmic variance of $σ_{\rm CV}\approx0.7\pm0.3$. This exceeds predictions from abundance-matched mock catalogs, suggesting that early quiescent galaxies are more strongly clustered, and more likely to be found near one another or in more biased regions, than expected in current galaxy-formation models. Any successful model for the emergence of early massive quiescent galaxies must reproduce both their abundance evolution and their imprint on the large-scale distribution.

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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

Lyman-alpha emission at the end of reionization: line strengths and profiles from MMT and JWST observations at z~5-6

With JWST, it is now possible to use Lyman-Alpha (Ly$α$) emission from galaxies beyond z>8 to trace neutral hydrogen in the intergalactic medium (IGM) as the Universe became reionized. However, observed Ly$α$ emission is scattered by neutral hydrogen in the IGM and the interstellar and circum-galactic medium, necessitating `baseline' models of Ly$α$ properties in the ionized IGM to disentangle their impacts. In this work, we characterize Ly$α$ properties at the end of reionization, z~5-6, providing a baseline that can be applied to z>6 observations. We targeted GOODS-N with MMT/Binospec, obtaining R~4360 rest-frame UV spectra of 236 galaxies at z~5-6, selected from HST/CANDELS, finding 62 Ly$α$ detections. We use JWST observations from JADES and FRESCO for a subset of our sources to characterize Ly$α$ properties as a function of UV continuum and H$α$ emission. We present the first statistical measurements of the Ly$α$ FWHM distribution at z~5-6, and produce empirical baseline models of Ly$α$ equivalent width (EWLy$α$) and escape fraction (f$_{esc}^{Lyα}$) conditional on UV magnitude and slope. We find our EWLy$α$ and f$_{esc}^{Lyα}$ models depend on UV magnitude, and infer 45$\pm$5$\%$ and <62$\pm$8$\%$ of MUV=-19.5 galaxies have EWLy$α$>25$Å$ and f$_{esc}^{Lyα}$>0.2, respectively. We find a mean Ly$α$ FWHM of 245km/s and median Ly$α$ velocity offset of 258km/s, both correlating with higher UV luminosity. Our median observed Ly$α$ line profile is broader and has higher velocity offset compared to pre-JWST models based on z~2 lines, which may reflect resonant scattering by residual neutral hydrogen in the IGM at z~5-6 and increasing ISM/CGM densities. Our median line profile predicts higher Ly$α$ transmission in a fully neutral IGM, providing insight into recent z>10 Ly$α$ detections.

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arXiv2026

From Starburst to Quenching: Physical Properties of Extremely Compact Starbursts at z$\sim$0.1

The compaction phase plays a crucial role in galaxy evolution, as it is strongly linked to star formation activities and structural transformation. We have identified a sample of extremely compact starburst galaxies (eCSBs) at low redshift~(z$\sim$0.1), which represent this critical evolutionary stage. These eCSBs are massive outliers with intense star formation and high infrared luminosities comparable to (U)LIRGs, while their structure already resembles quiescent galaxies. To investigate their molecular gas properties, we conducted IRAM 30m observations of $^{12}$CO J = 1--0 and $^{12}$CO J = 2--1 emission lines. Our results indicate that eCSBs exhibit a notably low molecular gas fraction~($\sim3\%$), and short gas depletion time~($\sim$ 20 Myr), suggesting that these galaxies are rapidly exhausting their remaining gas reservoir. Compared to normal (U)LIRGs, eCSBs show systematically lower $^{12}$CO(2-1)/$^{12}$CO(1-0) ratio~($R_{21} \sim 0.65 \pm 0.06$), similar to main sequence galaxies. The relatively low CO excitation may be associated with their high central stellar mass densities. These findings provide new insight into the molecular gas properties of galaxies during the compaction phase, highlighting their unique condition and rapid evolution toward quiescence.

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arXiv2026

A first systematic study of [OIII] 88$μ$m at $z>8$: two luminous oxygen lines and a powerful ionized outflow in the first 600 million years

We present deep ALMA Band 7 observations of the [OIII] $88μ$m line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification $μ= 1.4-3.8$), JWST-selected galaxies at $z = 8.5 - 10.3$, with observed magnitudes $-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5$. [OIII] $88μ$m is confidently detected in UNCOVER-10646 at $z=8.5080 \pm 0.0011$ ($15σ$) and DHZ1 at $z=9.3113 \pm 0.0006$ ($6σ$), with both being intrinsically luminous systems [$L_\text{[OIII]} = (1.1 - 1.6) \times 10^9\,L_\odot$] that follow the local [OIII]-SFR relation. [OIII] $88μ$m remains undetected in the two $z>10$ targets, including in the $z=10.07$ X-ray AGN UHZ1, where we obtain a deep limit of $L_\text{[OIII]} < 6 \times 10^7\,L_\odot$. Dust emission is not detected in any individual source nor in a stack ($<3σ$). The high S/N [OIII] $88μ$m detection in UNCOVER-10646 uniquely reveals an additional broad component ($\mathrm{FWHM} = 1366_{-329}^{+473}\,\mathrm{km/s}$; $Δ\mathrm{BIC}\approx20$) indicative of an ionized outflow. We infer a high outflow rate of $\dot{M}_\mathrm{out} = 128_{-46}^{+80}\,M_\odot\,\mathrm{yr}^{-1}$, corresponding to a mass loading factor $η= \dot{M}_\mathrm{out}/\mathrm{SFR} = 2.9_{-1.0}^{+1.8}$ that matches or exceeds theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm and spatially resolve the outflow, this first systematic study at $z>8$ highlights the unique diagnostic power of [OIII] $88μ$m in characterizing galaxies in the early Universe.

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arXiv2026

Spatial decomposition of Little Red Dots with JWST/NIRSpec IFU into broad-line red cores and narrow-line blue host galaxies

Little Red Dots (LRDs) are a population of compact red sources discovered by the James Webb Space Telescope (JWST). Imaging and spectroscopy have shown that LRDs exhibit a complex spectrum with a ``V-shaped" continuum, broad Balmer emission lines, and in some cases Balmer absorption. While the physical origin of these components remains debated, recent studies propose that they arise from a compact central engine likely hosting a rapidly growing black hole embedded within a more extended host galaxy. We test this central engine + host galaxy model using JWST/NIRSpec integral field unit (IFU) spectroscopy to spectrally decompose the observed continuum, narrow and broad emission lines, and absorption. We spatially map each component for five broad Ha-selected LRDs at z~5 observed with both the prism and high-resolution G395H grating. We find that the blue continuum emission is co-spatial with the narrow emission line region, while the red continuum arises from a compact core co-spatial with the broad Balmer emission and absorption. Spatial maps of the [OIII] equivalent width reveal a pronounced decrease in the central core. Our work provides further evidence that the LRD emission is produced by at least two distinct physical components arising from a red central engine embedded within a blue host galaxy.

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arXiv2026

How galaxies acquire their stellar mass at high redshift: High star formation efficiencies and the relative roles of dust and initial mass function

JWST has measured an unprecedented abundance of galaxies above $z\gtrsim 4-5$, whose formation and evolution are still difficult to reconcile within traditional galaxy evolution models in a $Λ$CDM framework. Here, we present a study on the star formation histories of these high-redshift galaxies between $z\simeq5-12$ via a data-driven semi-empirical model that uses the observed UV LFs as input to retrieve SFRs, naturally bypassing any uncertain modelling of cooling, feedback and/or stochastic processes. Galaxy stellar masses are progressively built in time by integrating their SFRs assigned along their progenitor haloes via the SFR-halo accretion rate relation, derived from abundance matching between the input observed UV LFs with the dark matter halo accretion rate distributions at each redshift. This makes the SFEs a full prediction of the model rather than a tuned input, serving as a natural baseline to test burstiness, dust attenuation, or IMF variations. Our approach naturally reproduces the total stellar mass function, the large-scale clustering, and the star-forming main sequence. We find that massive galaxies grew their stellar mass with a bursty star formation at $z\sim9-10$, broadly in agreement with the star formation histories inferred from spectral energy distribution fitting, with the SFE reaching high peaks of $0.8-0.9$ at $z>9$ and lowering to standard values of $0.2-0.3$ below $z\lesssim9$. We find that the presence of dust could enhance the predicted SFRs at $z\lesssim8$, better reproducing the observed SFRs of massive dusty galaxies, and increase the SFEs to values close to or even above unity at $z \gtrsim 8$. Finally, switching to top-heavy IMFs reduces the SFEs by a factor of $2-3$, highlighting the need for a variable IMF as an inevitable ingredient in the evolution of galaxies at high redshifts to avoid unphysical SFEs, especially in the presence of dust.

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arXiv2026

Little Red Dots as Intermediate Mass, Super-Eddington Engines: Insights from Type IIn Supernovae and The 1837-1856 Great Eruption of $η$ Carinae

JWST's Little Red Dots (LRDs) display a unique constellation of features that do not occur simultaneously in any other class of galaxies or AGN. Here we observe that many of these features find parallels in the 19th century Great Eruption (GE) of $η$ Carinae and a sub-class of supernovae (Type IIn). Drawing on these stellar phenomena -- outflows trapped by dense circumstellar gas envelopes -- we sketch a possible scenario for LRDs. Outflows from the central engine produce an enshrouding envelope of gas that may be thought of as a slow wind. This dense wind and its enormous extent produce an opacity so high that a pseudo-photosphere forms within the wind, obscuring the central engine and manifesting as a blackbody-like continuum. Radiation from the buried engine powers the system. The engine may also launch fast winds that crash into the existing envelope to generate shocks. Lines form within the wind above the photosphere -- electron scattering and absorption in the clumpy (ionized + neutral) medium account for broad wings and P-Cygni cores. A key implication is that inferences of ``overmassive black holes" may be interpreting this wind-like physics as a virial broad-line region. We propose an escape velocity argument to constrain the mass of the engine, which yields $M<10^{5} M_\odot$ for the typical LRD. The lack of variability and low surface gravity of the photosphere provide further support for intermediate mass ($M\approx10^{3-6} M_\odot$), but very luminous super-Eddington ($L_{\rm{bol}}/L_{\rm{edd}}\gtrsim5$) systems harboring a supermassive star or intermediate mass black hole. Paralleling the evolution of IIn SNe, dust production in the envelope may mark the beginnings of classical AGN. This paper explores a possible self-consistent explanation for the entire life-cycle of LRDs, from their enshrouding in dense gas to their fates as seeds of massive black holes.

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arXiv2026

JWST spectroscopy of galaxies at $z>10$: Damped Ly$α$ absorbers reveal efficient star formation and hidden redshift biases

Recent observations with JWST have revealed a remarkable population of surprisingly luminous galaxies at redshifts $z>10$. Their abundance exceed predictions from simulations and empirical extrapolations from lower redshifts, suggesting a transition in the physical conditions under which the first stars formed. Here we investigate the physical conditions of a select sample of 25 galaxies with robust redshift measurements at $z_{\rm spec}\geq 10$ observed with JWST/NIRSpec Prism. We characterize their star-formation efficiency, `burstiness', and presence of strong rest-frame UV nebular lines in relation to the density of the local neutral atomic hydrogen (HI) gas reservoirs they are embedded in. We find that the prominence of strong rest-UV lines are correlated with the burstiness of the galaxies, defined as ${\rm SFR_{10\,Myr} / SFR_{100\,Myr}}$. In contrast, there are no strong connections between the HI gas column density derived from the damped Ly$α$ absorption (DLA) and the $M_{\rm UV}$ brightness, ${\rm SFR_{10\,Myr} / SFR_{100\,Myr}}$, and prominence of rest-UV lines. The most bursty galaxies show a large variation in star-formation efficiencies and HI gas surface densities, though typically with very short depletion timescales, $t_{\rm dep} \lesssim 20$\,Myr. This necessites rapid gas depletion times and external replenishment from infalling, pristine gas, powering starburst episodes on equally short timescales. We further quantify the impact of strong DLAs in galaxy spectra on photometric and Ly$α$-break redshift-inferences, finding average redshift biases of $\langle z \rangle =0.39$ and $0.14$, respectively, when not incorporating DLAs on the emergent spectra. We show the effect of this bias on new measurements of the cosmic UV luminosity density, $ρ_{\rm UV}$, derived here at $z>10$, finding that this has a marginal impact on the UV luminosity function.

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arXiv2026

A Census of the 200 Most Massive Galaxies Spectroscopically Observed with JWST at zspec $\sim$3-15

Massive galaxies provide strong tests of galaxy formation models, yet a comprehensive spectroscopic view of their properties and demographics in the early Universe has remained elusive. Here we present a JWST spectroscopic census of the 200 most massive galaxies at zspec~3-15, selected using an evolving stellar-mass threshold motivated by the halo mass function and anchored at log(Mstar)>10 at z~5. These galaxies represent the top 3% most massive systems among all publicly available prism observations. We derive their physical properties through joint SED fitting of spectroscopy and photometry, and construct a clean massive galaxy sample after removing LRDs and broad-line AGN contaminants. We find that the massive galaxy population evolves strongly with redshift: normal SFGs (Av<1 mag) dominate at z>~6, while dusty SFGs (Av>1 mag) and QGs become more common toward lower redshift. Dust attenuation decreases systematically toward higher redshift. We identify 29 massive QGs, including a population of recently quenched systems whose star formation declined rapidly within the past ~100 Myr. We further show that both the traditional UVJ and recently proposed (ugi)s selections suffer substantial inconsistency with the most massive galaxies at z>3, motivating a revised (ugi)s criterion calibrated using our spectroscopic sample. The inferred formation histories suggest at least two pathways toward quiescence: a dust-enriched pathway linking normal SFGs, dusty SFGs, and QGs, and a more direct pathway connecting normal SFGs and QGs. Massive normal SFGs appear to grow through both relatively gradual and rapid assembly modes. Together, these results suggest that rapid stellar-mass assembly, dust enrichment, and quenching were already shaping the evolutionary pathways of the most massive galaxies within the first billion years after the Big Bang.

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arXiv2026

Ram-pressure stripping caught in action in a young cluster at $z = 2.51$

Galaxy clusters in the local Universe are dominated by massive quiescent galaxies with old ages, formed at high redshifts. Whether their quenching is driven by internal processes or environmental effects is a matter of debate that has been challenging to resolve due to the lack of observations during their peak formation epoch. Here we report clear evidence from the Atacama Large Millimeter/submillimeter Array of extended and elongated gas tails in five galaxies in a forming cluster at z = 2.51. The single-tailed gas distributions, which extend notably beyond the stellar emission probed by JWST in galaxies that are relatively isolated and lack signatures of mergers or interactions (features that are very uncommon in the field), provide evidence of ram-pressure stripping. These very distant confirmed cases of ram-pressure stripping highlight the critical role of environmental effects in gas removal at high redshifts, an often-overlooked quenching pathway.

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arXiv2026

Black Hole Stars Across the Universe: Identifying Central Engine Dominated Little Red Dots at $z\sim1.5-9.5$

Photometric selections of Little Red Dots (LRDs) largely rely on identifying their ``V-shaped'' spectral energy distribution (SED). Recent work suggests this V-shape stems from a combination of a central engine -- also referred to as a Black Hole Star (BH*) -- and a star-forming host galaxy. We present a new and highly complementary photometric selection that is based on incorporating BH* templates in the \texttt{eazy} redshift fitting code. Selecting compact sources where a BH* template contributes $>80$\% to the best fitting SED in the rest-optical, we compile a sample of 241 BH*-dominated candidates from $\sim1000\,{\rm arcmin}^2$ of legacy and pure parallel JWST imaging. Our selection does not require a blue UV-component, and it successfully identifies objects that resemble the paradigmatic sources ``MoM-BH*-1'' and ``The Cliff''. We find that BH*-dominated sources exist across a wide range of redshifts ($z\sim1.7-9.3$) and optical luminosities (log$(L_{5100}/{\rm erg}\,{\rm s}^{-1})\sim42-44.5$), and we measure a median Balmer break strength of $\sim3$, with some breaks reaching values $>10$. We estimate bolometric luminosities in the range log$(L_{\rm bol}/{\rm erg}\,{\rm s}^{-1})\sim42-45$, which, assuming accretion at the Eddington-limit, would translate to black hole masses of $M_{\rm BH}\sim10^4-10^7{\rm M_\odot}$, spanning the intermediate mass black hole to the quasar regime. The number density of BH*-dominated candidates peaks at $z\sim5-6$ ($\sim10^{-5}\,{\rm Mpc}^{-3}$) and it declines by an order of magnitude down to $z\sim2$. Tentatively, comparing to V-shaped LRD samples suggests that the fraction of BH*-dominated sources among the broader LRD population does not decrease towards lower redshift. Crucially, our work demonstrates that BH*-dominated sources are not merely an early-Universe phenomenon but rather persist at least until cosmic noon.

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arXiv2026

Exploring Cosmic Dawn with PANORAMIC I: The Bright End of the UVLF at $z\sim9 -17$

In its first two years of operation, the James Webb Space Telescope has enabled the discovery of a surprising number of UV-bright galaxies at $z\sim10-14$. Their number density is still relatively uncertain, due to cosmic variance effects, and the limited survey area with deep imaging. Here, we combine pure parallel imaging from the PANORAMIC survey with data from legacy fields to constrain the bright end (M$_{\rm UV}<-18.5$) of the UV luminosity function (UVLF) over $0.28\,$deg$^2$ of NIRCam imaging in 6 or more filters, and along 35 independent lines of sight. Using conservative color selections, we compile robust dropout samples at $z\sim10$, $z\sim13$, and $z\sim17$, and identify 16 new candidates from PANORAMIC. Our inferred UVLFs at $z\sim10$ are consistent with literature results and we confirm the high abundance of galaxies at the bright end (M$_{\rm UV}\lesssim-21$) with better number statistics. We find somewhat lower number densities at $z\sim13$ compared to previous studies, and no robust candidates at $z\sim17$, indicating a rapid evolution of the galaxy population from $z\sim10-17$. The improved upper limits at $z\sim17$ imply that the cosmic UV luminosity density drops by at least a factor $\sim50$ from $z\sim10$ to $z\sim17$. Comparing our results to models proposed to explain the abundance of UV-bright galaxies at $z\gtrsim10$, we conclude that a modest increase in the star formation efficiency, or in the burstiness of star formation, a more top-heavy initial mass function, a lack of dust attenuation, or a combination of these effects at $z\gtrsim10$, is sufficient to match our observational constraints.

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arXiv2026

A PANORAMIC of UV-optical morphologies of "Little Red Dots": Two groups of LRDs distinguished by UV half-light radius

Among the most remarkable results from JWST is the discovery of abundant, compact, and very red sources in the early Universe known as "Little Red Dots" (LRDs). The relative degree to which starlight and active galactic nuclei (AGN) drive the rest-frame UV and optical emission from LRDs remains unclear. With a large sample of LRDs selected photometrically from the pure-parallel PANORAMIC survey, we study their morphology as a function of rest-wavelength and find that the rest-UV light is typically more extended than the rest-optical. This result holds both when measuring LRD sizes with a single Sérsic profile and when comparing the fraction of light from a point source via joint PSF+Sérsic modeling. A shift occurs at the Balmer break, with LRDs becoming highly compact and unresolved ($R_{50,\rm{opt}}\lesssim100\;\rm{pc}$) in the rest-optical relative to the rest-UV. When splitting the sample at the Balmer break into those that are resolved and unresolved, a stacking analysis demonstrates that the latter are compact ($R_{50}\lesssim100\;\rm{pc}$) on average across the full rest-UV-optical spectrum. Conversely, those LRDs resolved at the break show extended UV emission ($R_{50,\rm{UV}}>200\;\rm{pc}$) on average. We find a similar dichotomy when repeating with a spectroscopic sample. Altogether, these results are consistent with the rest-UV emission driven by a combination of emission from starlight and a dense, dust-poor cloud of hydrogen gas enveloping an AGN. Differences between LRDs in the relative contribution from the AGN and starlight could reflect an ensemble of black hole seed masses, where a heavier seed produces an LRD of smaller $R_{50,\rm{UV}}$.

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arXiv2026

The Engine and its Flows: Little Red Dot spectra are shaped by the column densities of their gas envelopes

JWST data have enabled the abundant identification of compact broad Balmer line sources nicknamed the Little Red Dots. While they share broad lines with active galactic nuclei, they are unusually X-ray and infrared weak. We investigate the origin of the Balmer line profiles based on an empirical analysis of 18 broad H$α$-selected sources with high quality spectra at $z\approx3-7$. The H$α$ line profiles vary systematically with Balmer break strength: sources with blue UV to optical colors show a narrow core profile, redder sources with Balmer breaks a blue shifted absorption (P Cygni shape), and the reddest sources display absorption-dominated cores. All H$α$ lines have symmetric exponential wings, which are more dominant and slightly broader in red sources. Balmer absorption is present in $\sim60$ % of the sample, with H$β$ showing relatively stronger absorption. Drawing upon empirical analogies with stellar phenomena, we interpret these trends as being due to radiative processes that depend on variations in the optical depth, ionisation state and column density of a clumpy, partially ionised envelope. We unveil a correlation between the absorber velocity and Balmer break strength, with the densest absorbers inflowing and bluer sources having faster outflows. This indicates viewing angle or evolutionary effects where optically thick gas is inflowing, as suggested in models of super-Eddington accretion, and the engine can more easily drive outflows in directions with lower column densities. This new understanding of Balmer line profiles as tracing gas properties rather than dynamical broadening helps resolve tensions associated with high inferred black hole masses from standard virial calibrations, and reveals the complex gas environment around the hot central engine.

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arXiv2026

JWST+ALMA reveal the build up of stellar mass in the cores of dusty star-forming galaxies at Cosmic Noon

Dusty star-forming galaxies have long been suspected to serve as the missing evolutionary bridge between the star-forming and quiescent phases of massive galaxy evolution. With the combined power of JWST and ALMA, it is now possible to use high resolution imaging at rest-frame ultraviolet (UV), optical, near-infrared (NIR), and sub-mm wavelengths to study the multi-wavelength morphologies tracing both the stellar populations and dust during this key phase. We present the joint analysis of JWST/NIRCam imaging in GOODS-S and mm dust emission traced by ALMA for a sample of 33 galaxies at $z=1.5$ to $z=5.5$ selected from the 1.1mm GOODS-ALMA 2.0 survey, and compare the morphologies of this population to mass- and redshift-selected samples of field star-forming and quiescent galaxies. The 1.1mm-selected sample is morphologically distinct from other similarly massive star-forming galaxies; we find a steeper size-wavelength gradient from 1.5-4.4$μ$m, with a more dramatic decrease in size towards longer wavelengths. While the rest-NIR surface brightness profiles of the 1.1mm-selected galaxies are brighter in the inner regions relative to the field star-forming population, they are remarkably similar to the quiescent population. These morphological differences could suggest that dusty star-forming galaxies, unlike more typical star-forming galaxies, have already built up stellar mass in a severely dust-obscured core, leading to extended and clumpy morphologies at rest-UV and rest-optical wavelengths and more compact emission in the rest-NIR that is co-spatial with dust. If the bulge is already established, we speculate that mm-selected galaxies may imminently evolve to join their quiescent descendants.

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arXiv2026

Diversity and Evolution of Dust Attenuation Curves from Redshift z ~ 1 to 9

The UV-optical dust attenuation curve is key to interpreting the intrinsic properties of galaxies and provides insights into the nature of dust grains and their geometry relative to stars. In this work, we constrain the UV-optical slope of the stellar attenuation curve using a spectroscopic-redshift sample of ~3800 galaxies at z~1-9, to characterize the diversity and redshift evolution of stellar attenuation curves and to gain insight into dust production and evolution at high redshifts. The sample is constructed from three JWST/NIRCam grism surveys in GOODS and A2744 fields, with a wealth of JWST/NIRCam and HST photometry. With constraints from spectroscopic redshifts and emission line fluxes, we use the Prospector SED fitting code with a flexible dust model. We find that the attenuation curve slope varies strongly with Av at all redshifts, becoming flatter at higher attenuation. We find no strong correlation between attenuation curve slope and size or axis ratio, and the trends with stellar mass and star-formation rate are largely driven by their correlation with Av. We find strong evidence that at fixed Av, the curve becomes flatter with increasing redshift. On average, the attenuation curves derived here are shallower than those at z~0 and than the SMC curve. The highest redshift galaxies at z=7-9 (124 galaxies, a significantly larger sample than in previous studies) show slopes even flatter than the Calzetti curve, implying reduced UV obscuration and lower IR luminosities than expected from an SMC dust curve, by as large as an order of magnitude. Hydrodynamical simulations that couple dust growth to gas chemical enrichment successfully reproduce the different loci of high- and low-redshift galaxies in the slope-Av diagram, suggesting that dust in high-redshift galaxies is increasingly dominated by large grains produced in supernova ejecta with limited ISM processing at early times.

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arXiv2026

Direct pathway to the Early Supermassive Black Holes: A Red Super-Eddington Quasar in a Massive Starburst Host at $z=7.2$

We present a panchromatic optical-mm characterization of GNz7q, a recently identified X-ray weak, rapidly growing red quasar embedded within a dusty starburst galaxy at $z=7.1899$, using the full suite of JWST/NIRCam, NIRSpec, MIRI, and archival NOEMA observations. Our deep NIRSpec/G395M spectroscopy reveals unambiguous broad Balmer emission (FWHM $=2221\pm20$kms$^{-1}$), confirming a super-Eddington accreting black hole ($λ_{\rm Edd}=2.7\pm0.4$) with a mass of $\log(M_{\rm BH}/M_{\odot})=7.55\pm0.34$, using accretion-rate corrected BH mass estimators. After subtracting the point source, we robustly detect stellar emission from the host galaxy across multiple NIRCam and MIRI filters. Out joint morphological-spectral analysis yields a stellar mass of $\log (M_*/M_\odot)=10.5\pm0.4$ and an intense star formation rate of ${\rm SFR}=330\pm97\,M_\odot\,\rm yr^{-1}$, confirming the host as a massive, dusty starburst galaxy. We find that GNz7q lies on the local $M_{\rm BH}$-$M_*$ relation ($M_{\rm BH}/M_*\simeq 0.001$) and is well positioned to evolve into the locus of massive SDSS quasars with $\log (M_{\rm BH}/M_\odot)\approx 9$ and $M_*\approx 10^{11}\,M_\odot$ at $z\sim 6$, owing to its remarkably rapid growth in both the black hole and its host galaxy. This stands in stark contrast to many recently reported JWST AGN populations at similar redshifts, including the little red dots (LRDs), whose weak or undetected star formation makes it difficult for them to grow into the massive galaxies hosting SDSS-like quasars. These results suggest that GNz7q marks as a rare, pivotal phase of early BH-galaxy co-eolution, plausibly providing a crucial direct pathway to the supermassive black hole systems within the first billion years of the Universe.

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