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Kei Ito

Publications and source records attributed to Kei Ito.

At least 37 records · Page 2Linked to original sources
arXiv2025

ECOGAL I. Project design and the first catalogue

We present ECOology for Galaxies using ALMA archive and Legacy surveys (ECOGAL), an ALMA data mining project. Using the footprints of the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST), we query and uniformly reprocess ALMA data to produce continuum images and two complementary source catalogues: (i) a prior-based catalogue anchored to optical/near-infrared detections, and (ii) a blind catalogue based on significant ALMA detections. Detection thresholds are established through peak SNR statistics and analysis of inverted maps. In this paper, we focus on the ALMA-accessible CANDELS fields (COSMOS, GOODS-S, and UDS), covering ~130,000 optical/NIR-selected galaxies spanning $0.0 < z < 15.0$. We identify 1288 detections (622 unique) with optical/NIR counterparts across the two methods, of which 832 detections (395 unique) appear in both. Among the 622 unique sources, 307 have spectroscopic redshifts from $0.12 < z < 6.85$. ECOGAL expands the parameter space previously explored by $A^3COSMOS$ and $A^3GOODS-S$ by incorporating the UDS field and integrating publicly available JWST datasets from the DAWN JWST Archive (DJA). We also highlight several science cases enabled by ECOGAL, including the evolution of cosmic gas and dust masses, obscured star formation and optically dark systems. ECOGAL provides science-ready data sets for the community and showcases the power of combining over a decade of accumulated public data with other legacy datasets. The data product is made publicly available and accompanies a post demonstrating the usage of the catalogue on the DJA webpage.

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arXiv2025

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

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

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arXiv2025

Exploring over 700 massive quiescent galaxies at z = 2-7: Demographics and stellar mass functions

High-redshift ($z>2$) massive quiescent galaxies are crucial tests of early galaxy formation and evolutionary mechanisms through their cosmic number densities and stellar mass functions (SMFs). We explore a sample of 743 massive ($\rm M_*> 10^{9.5}M_\odot$) quiescent galaxies from $z=2-7$ in over 800 arcmin$^2$ of NIRCam imaging from a compilation of public JWST fields (with a total area $>$ 5 $\times$ previous JWST studies). We compute and report their cosmic number densities, stellar mass functions, and cosmic stellar mass density. We confirm a significant overabundance of massive quiescent galaxies relative to a range of cosmological hydrodynamical simulations and semi-analytic models (SAMs). We find that no simulations or SAMs accurately reproduce the SMF for massive quiescent galaxies at any redshift within the interval $z=2-5$. This shows that none of these models' feedback prescriptions are fully capturing high-z galaxy quenching, challenging the standard formation scenarios. We find a greater abundance of lower-mass ($\rm M_*<10^{10}M_\odot$) quiescent galaxies than previously found, highlighting the importance of sSFR cuts rather than simple colour selection. We show the importance of this selection bias, alongside individual field-to-field variations caused by cosmic variance, in varying the observed quiescent galaxy SMF, especially at higher-z. We also find a steeper increase in the cosmic stellar mass density for massive quiescent galaxies than has been seen previously, with $ρ_*\propto (1+z)^{-7.2\pm0.3}$, indicating the dramatic increase in the importance of galaxy quenching within these epochs.

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arXiv2025

DAWN JWST Archive: Morphology from profile fitting of over 340 000 galaxies in major fields

To better understand how galaxies assemble their structure and evolve over cosmic time, we present a new catalog of morphological measurements for over 340,000 sources spanning $0 < z < 12$, derived from deep JWST NIRCam imaging across four major extragalactic fields (CEERS, PRIMER-UDS, PRIMER-COSMOS, GOODS) compiled in the DAWN JWST Archive (DJA). We perform two-dimensional surface brightness fitting for all galaxies in a uniform, flux-limited sample. Each galaxy is modeled with both a Sérsic profile and a two-component (bulge and disk) decomposition, yielding consistent structural parameters - including effective radius, Sérsic index, axis ratio, and bulge-to-total ratio ($B/T$). To demonstrate the scientific application of our morphology catalogs, we combined these measurements with DJA photometric redshifts, physical parameters and rest-frame colors, and investigated the relation between total, bulge and disk sizes, Sérsic index ($n_S$), star formation activity, and redshift. Bulge-dominated galaxies (high $n_S$ and $B/T$) predominantly occupy the quiescent region of the $UVJ$ diagram, while disk-dominated galaxies are mostly star-forming. A significant bimodality persists, with quiescent disks and compact, bulge-dominated star-forming galaxies observed out to $z > 3$. Quiescent galaxies also show significantly higher stellar mass surface densities, nearly an order of magnitude greater at $z \sim 4$ than at $z \sim 1$. Our results confirm a strong and evolving link between morphology and star formation activity, and support a scenario in which bulge growth and quenching are closely connected. This work is a highly valuable addition to the DJA, adding a morphological dimension to this rich dataset and thus enabling a wider scientific application.

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arXiv2025

Black hole mass of a quiescent galaxy hosting a Type 1 AGN at z=2.09: Implications for black hole - galaxy co-evolution and AGN quenching at high redshift

We report a characterization of an X-ray-detected quiescent galaxy at $z=2.09$, named COS-XQG1, using JWST/NIRCam and NIRSpec data. This galaxy is detected in Chandra imaging, suggesting the presence of an AGN with a high black hole accretion rate of $\dot{M}_{\rm BH}=0.22\pm0.03\, {\rm M_\odot yr^{-1}}$. Using multi-wavelength photometry from X-ray to sub-millimeter, including the latest JWST imaging, we confirm that COS-XQG1 is massive ($M_\star = (1.6\pm0.2)\times10^{11}\, M_\odot$) and quiescent (${\rm sSFR}<10^{-10}\, {\rm yr^{-1}}$) as reported previously, even considering the contribution from AGN emission. Noticeably, COS-XQG1 displays a broad H$β$ and H$α$ emission component with a full width at half maximum of $4365^{+81}_{-81}\, {\rm km\, s^{-1}}$ in its NIRSpec spectrum, which is typical of Type 1 AGNs. The line width and luminosity of the broad H$α$ emission give a black hole mass of $\log{(M_{\rm BH}/M_\odot)} = 8.43\pm0.02\, (\pm 0.5)$. With a stellar velocity dispersion measurement ($σ_\star=238\pm32\, {\rm km\, s^{-1}}$), we find that this galaxy is consistent with the local relations in the $M_{\rm BH} - σ_\star$ and $M_{\rm BH}- M_\star$ planes, which might suggest that massive quiescent galaxies at $z\geq2$ have already been mature in terms of both stellar and black hole masses and will not evolve significantly. In addition, image 2D-decomposition analysis finds that this galaxy comprises disk and point source components. The latter is likely the composition of an AGN and a stellar bulge. Based on a comparison with numerical simulations, we expect that COS-XQG1 will evolve into a typical quiescent galaxy with lower AGN activity by redshift 0. This study shows the usefulness of X-ray-detected quiescent galaxies in investigating the co-evolution between SMBHs and galaxies in the early Universe.

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arXiv2025

A Post-Starburst Pathway for the Formation of Massive Galaxies and Black Holes at z>6

Understanding the rapid formation of supermassive black holes (SMBHs) in the early universe requires insight into stellar mass growth in host galaxies. Here, we present NIRSpec rest-frame optical spectra and NIRCam imaging from JWST of two galaxies at z>6, both hosting moderate-luminosity quasars. These galaxies exhibit Balmer absorption lines, similar to low-redshift post-starburst galaxies. Our analyses of the medium-resolution spectra and multiband photometry show bulk of the stellar mass (log (M_* / M_sun) > 10.6) formed in starburst episodes at redshift 9 and 7. One of the galaxies shows a clear Balmer break and lacks spatially resolved H alpha emission. It falls well below the star formation main sequence at z = 6, indicating quiescence. The other is transitioning to quiescence; together, these massive galaxies are among the most distant post-starburst systems known. The blueshifted wings of the quasar [O III] emission lines suggest quasar-driven outflow possibly influencing star formation. Direct stellar velocity dispersion measurements reveal one galaxy follows the local black hole mass-sigma_* relation while the other is overmassive. The existence of massive post-starburst galaxies hosting billion-solar-mass BHs in short-lived quasar phases suggests SMBHs and host galaxies played a major role in each other's rapid early formation.

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arXiv2025

Novel $z\sim~10$ auroral line measurements extend the gradual offset of the FMR deep into the first Gyr of cosmic time

The mass assembly and chemical enrichment of the first galaxies provide key insights into their star-formation histories and the earliest stellar populations at cosmic dawn. Here we compile and utilize new, high-quality spectroscopic JWST/NIRSpec Prism observations from the JWST archive. We extend the wavelength coverage beyond the standard pipeline cutoff up to 5.5$μ$m, enabling a detailed examination of the rest-frame optical emission-line properties for galaxies at $z\approx 10$. The improved calibration allows us to detect H$β$ and the [OIII]$λλ4959,5007$ doublet and resolve the auroral [OIII]$λ4363$ line for the 11 galaxies in our sample ($z=9.3-10.0$) to obtain direct $T_e$-based metallicity measurements. We find that all galaxies show high ionisation fields and electron temperatures, with derived metallicities in the range $12+\log {\rm (O/H)} = 7.1 - 8.3$, consistent with previous strong-line diagnostics at high-z. We derive an empirical relation for $M_{\rm UV}$ and 12+log(O/H) at $z\approx 10$, useful for future higher-z studies, and show that the sample galaxies are `typical' star-forming galaxies though with relatively high specific star-formation rates and with evidence for bursty star formation. Combining the rest-frame optical line analysis and detailed UV to optical SED modelling, we determine the mass-metallicity relation and the fundamental-metallicity relation of the sample, pushing the redshift frontier of these measurements to $z=10$. These results, together with literature measurements, point to a gradually decreasing MZR at higher redshifts, with a break in the FMR at $z\approx 3$, decreasing to metallicities $\approx 3\times$ lower at $z=10$ than observed during the majority of cosmic time at $z=0-3$, likely caused by massive pristine gas inflows diluting the observed metal abundances during early galaxy assembly at cosmic dawn.

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arXiv2025

Cosmic Himalayas: The Highest Quasar Density Peak Identified in a 10,000 deg$^2$ Sky with Spatial Discrepancies between Galaxies, Quasars, and IGM HI

We report the identification of a quasar overdensity in the BOSSJ0210 field, dubbed Cosmic Himalayas, consisting of 11 quasars at $z=2.16-2.20$, the densest overdensity of quasars ($17σ$) in the $\sim$10,000 deg$^2$ of the Sloan Digital Sky Survey. We present the spatial distributions of galaxies and quasars and an HI absorption map of the intergalactic medium (IGM). On the map of 465 galaxies selected from the MAMMOTH-Subaru survey, we find two galaxy density peaks that do not fall on the quasar overdensity but instead exist at the northwest and southeast sides, approximately 25 $h^{-1}$ comoving-Mpc apart from the quasar overdensity. With a spatial resolution of 15 $h^{-1}$ comoving Mpc in projection, we produce a three-dimensional HI tomography map by the IGM Ly$α$ forest in the spectra of 23 SDSS/eBOSS quasars behind the quasar overdensity. Surprisingly, the quasar overdensity coincides with neither an absorption peak nor a transmission peak of IGM HI but lies near the border separating opaque and transparent volumes, with the more luminous quasars located in an environment with lesser IGM HI. Hence remarkably, the overdensity region traced by the 11 quasars, albeit all in coherently active states, has no clear coincidence with peaks of galaxies or HI absorption densities. Current physical scenarios with mixtures of HI overdensities and quasar photoionization cannot fully interpret the emergence of Cosmic Himalayas, suggesting this peculiar structure is an excellent laboratory to unveil the interplay between galaxies, quasars, and the IGM.

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arXiv2025

Cartan subalgebras of C*-algebras associated with iterated function systems

Let $X$ be a compact Hausdorff space, and let $γ$ be an iterated function system on $X$. Kajiwara and Watatani showed that if $γ$ is self-similar and satisfies the open set condition and some additional technical conditions, $C(X)$ is a maximal abelian subalgebra of the Kajiwara--Watatani algebra $Ø_γ$ associated with $γ$. In this paper, we extend their results by providing sufficient conditions under which $C(X)$ becomes a Cartan subalgebra of $Ø_γ$. Additionally, we present sufficient conditions under which $C(X)$ fails to be a masa of $Ø_γ$.

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arXiv2025

Cartan subalgebras of C*-algebras associated with complex dynamical systems

Let $R$ be a rational function with degree $\geq 2$ and $X$ be its Julia set, its Fatou set, or the Riemann sphere. Suppose that $X$ is not empty. We can regard $R$ as a continuous map from $X$ onto itself. Kajiwara and Watatani showed that in the case that $X$ is the Julia set, $C_0(X)$ is a maximal abelian subalgebra of $\mathcal{O}_R(X)$, where $\mathcal{O}_R(X)$ denotes the C*-algebra associated with the dynamical system $(X,R)$ introduced by them. In this paper, we develop their result and give the equivalent condition for $C_0(X)$ to be a Cartan subalgebra of $\mathcal{O}_R(X)$.

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arXiv2024

Predicting Ly$α$ Emission from Distant Galaxies with Neural Network Architecture

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

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arXiv2024

Mining for Protoclusters at $z\sim4$ from Photometric Datasets with Deep Learning

Protoclusters are high-$z$ overdense regions that will evolve into clusters of galaxies by $z=0$, making them ideal for studying galaxy evolution expected to be accelerated by environmental effects. However, it has been challenging to identify protoclusters beyond $z=3$ only by photometry due to large redshift uncertainties, hindering statistical study. To tackle the issue, we develop a new deep-learning-based protocluster detection model, PCFNet, which considers a protocluster as a point cloud. To detect protoclusters at $z\sim4$ using only optical broad-band photometry, we train and evaluate PCFNet with mock $g$-dropout galaxies based on the N-body simulation with the semi-analytic model. We use the sky distribution, $i$-band magnitude, $(g-i)$ color, and the redshift probability density function surrounding a target galaxy on the sky. PCFNet achieves to detect five times more protocluster member candidates while maintaining high purity (recall $=7.5\pm0.2$%, precision $=44\pm1$%) than conventional methods. Moreover, PCFNet is able to detect more progenitors ($M_\mathrm{halo}^{z=0}=10^{14-14.5}\,M_\odot$) that are less massive than supermassive clusters like the Coma cluster. We apply PCFNet to the observational photometric dataset of the HSC-SSP Deep/UltraDeep layer ($\sim17\mathrm{\,deg^2}$) and detect $121$ protocluster candidates at $z\sim4$. We find the rest-UV luminosities of our protocluster member candidates are brighter than those of field galaxies, which is consistent with previous studies. Additionally, the quenching of satellite galaxies depends on both the core galaxy's halo mass at $z\sim4$ and accumulated mass until $z=0$ in the simulation. PCFNet is very flexible and can find protoclusters at other redshifts or in future extensive surveys by Euclid, LSST, and Roman.

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arXiv2024

Galaxy Size and Mass Build-up in the First 2 Gyrs of Cosmic History from Multi-Wavelength JWST NIRCam Imaging

The evolution of galaxy sizes in different wavelengths provides unique insights on galaxy build-up across cosmic epochs. Such measurements can now finally be done at $z>3$ thanks to the exquisite spatial resolution and multi-wavelength capability of the JWST. With the public data from the CEERS, PRIMER-UDS, and PRIMER-COSMOS surveys, we measure the sizes of $\sim 3500$ star-forming galaxies at $3 \leqslant z<9$, in 7 NIRCam bands using the multi-wavelength model fitting code GalfitM. The size-mass relation is measured in four redshift bins, across all NIRCam bands. We find that, the slope and intrinsic scatter of the rest-optical size-mass relation are constant across this redshift range and consistent with previous HST-based studies at low-z. When comparing the relations across different wavelengths, the average rest-optical and rest-UV relations are consistent with each other up to $z=6$, but the intrinsic scatter is largest in rest-UV wavelengths compared to rest-optical and redder bands. This behaviour is independent of redshift and we speculate that it is driven by bursty star-formation in $z>4$ galaxies. Additionally, for $3\leqslant z<4$ star-forming galaxies at $\rm M_* > 10^{10} M_{\odot}$, we find smaller rest-$\rm 1\rm\,μm$ sizes in comparison to rest-optical (and rest-UV) sizes, suggestive of colour gradients. When comparing to simulations, we find agreement over $\rm M_* \approx 10^{9} - 10^{10} M_{\odot}$ but beyond this mass, the observed size-mass relation is significantly steeper. Our results show the power of JWST/NIRCam to provide new constraints on galaxy formation models.

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arXiv2024

JWST/NIRSpec spectroscopy of intermediate-mass quiescent galaxies at $z \sim 3-4$

We present the analysis of three intermediate-mass quiescent galaxies (QGs) with stellar masses of $\sim10^{10}M_{\rm \odot}$ at redshifts $z\sim 3 - 4$ using NIRSpec low-resolution spectroscopy. Utilising the SED fitting code BAGPIPES, we confirm these target galaxies are consistent with quiescent population, with their specific star formation rates (sSFR) falling below 2-dex the star-forming main sequence at the same redshifts. Additionally, we identify these QGs to be less massive than those discovered in previous works, particularly prior to the JWST era. Two of our target galaxies exhibit the potentially-blended H$α$+[NII] emission line within their spectra with $S/N>5$. We discuss whether this feature comes from an Active Galactic Nucleus (AGN) or star formation although future high-resolution spectroscopy is required to reach a conclusion. One of the target galaxies is covered by JWST/NIRCam imaging of the PRIMER survey. Using the 2D profile fitting code Galfit, we examine its morphology, revealing a disc-like profile with a Sérsic index of $n=1.1 \pm 0.1$. On the size-mass relation, we find a potential distinction between less-massive ($\log_{10}{(M_*/M_\odot)}<10.3$) and massive ($\log_{10}{(M_*/M_\odot)}>10.3$) QGs in their evolutionary pathways. The derived quenching timescales for our targets are less than 1 Gyr. This may result from these galaxies being quenched by AGN feedback, supporting the AGN scenario of the emission line features.

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arXiv2024

Galaxy properties from the outskirts to the core of a protocluster at z=3.699

We present follow-up spectroscopy on a protocluster candidate selected from the wide-field imaging of the Hyper SuprimeCam Subaru Strategic Programme. The target protocluster candidate was identified as a $4.5σ$ overdense region of $g$-dropout galaxies, and the redshifts of $g$-dropout galaxies are determined by detecting their Ly$α$ emission. Thirteen galaxies, at least, are found to be clustering in the narrow redshift range of $Δz<0.05$ at $z=3.699$. This is clear evidence of the presence of a protocluster in the target region. Following the discovery of the protocluster at $z=3.699$, the physical properties and three-dimensional distribution of its member galaxies are investigated. Based on spectroscopically-confirmed $g$-dropout galaxies, we find an overabundance of rest-frame ultraviolet (UV) bright galaxies in the protocluster. The UV brightest protocluster member turns out to be an active galactic nucleus, and the other UV brighter members tend to show smaller Ly$α$ equivalent widths than field counterparts. The member galaxies tend to densely populate near the centre of the protocluster, but the separation from the nearest neighbour rather than the distance from the centre of the protocluster is more tightly correlated to galaxy properties, implying that the protocluster is still in an early phase of cluster formation and only close neighbours have a significant impact on the physical properties of protocluster members. The number density of massive galaxies, selected from an archival photometric-redshift catalogue, is higher near the centre of the protocluster, while dusty starburst galaxies are distributed on the outskirts. These observational results suggest that the protocluster consists of multiple galaxy populations, whose spatial distributions may hint at the developmental phase of the galaxy cluster.

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arXiv2024

A massive quiescent galaxy in a group environment at $z=4.53$

We report on the spectroscopic confirmation of a massive quiescent galaxy at $z_\mathrm{spec}=4.53$ in the COSMOS field. The object was first identified as a galaxy with suppressed star formation at $z_\mathrm{phot}\sim4.65$ from the COSMOS2020 catalog. The follow-up spectroscopy with Keck/MOSFIRE in the $K$-band reveals faint [OII] emission and the Balmer break, indicative of evolved stellar populations. We perform the spectral energy distribution fitting using photometry and spectrum to infer physical properties. The obtained stellar mass is high ($M_*\sim 10^{10.8}\,M_\odot$) and the current star formation rate is more than 1 dex below that of main-sequence galaxies at $z=4.5$. Its star formation history suggests that this galaxy experienced rapid quenching from $z\sim 5$. The galaxy is among the youngest quiescent galaxies confirmed so far at $z_\mathrm{spec}>3$ with $z_\mathrm{form}\sim5.2$ ($200\,\mathrm{Myr}$ ago), which is the epoch when 50\% of total stellar mass was formed. A unique aspect of the galaxy is that it is in an extremely dense region; there are four massive star-forming galaxies at $4.4<z_\mathrm{phot}<4.7$ located within 150 physical kpc from the galaxy. Interestingly, three of them have strongly overlapping virial radii with that of the central quiescent galaxy ($\sim 70\,\mathrm{kpc}$), suggesting that the over-density region is likely the highest redshift candidate of a dense group with a spectroscopically confirmed quiescent galaxy at the center. The group provides us with a unique opportunity to gain insights into the role of the group environment for quenching at $z\sim5$, which corresponds to the formation epoch of massive elliptical galaxies in the local Universe.

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arXiv2024

Size - Stellar Mass Relation and Morphology of Quiescent Galaxies at $z\geq3$ in Public $JWST$ Fields

We present the results of a systematic study of the rest-frame optical morphology of quiescent galaxies at $z \geq 3$ using the Near-Infrared Camera (NIRCam) onboard $JWST$. Based on a sample selected by $UVJ$ color or $NUVUVJ$ color, we focus on 26 quiescent galaxies with $9.8<\log{(M_\star/M_\odot)}<11.4$ at $2.8 10.3$ implies that our size - stellar mass relations are below those at lower redshifts, with the amplitude of $\sim0.6\, {\rm kpc}$ at $M_\star = 5\times 10^{10}\, M_\odot$. This value agrees with the extrapolation from the size evolution of quiescent galaxies at $z<3$ in the literature, implying that the size of quiescent galaxies increases monotonically from $z\sim3-5$. Our sample is mainly composed of galaxies with bulge-like structures according to their median Sérsic index and axis ratio of $n\sim3-4$ and $q\sim0.6-0.8$, respectively. On the other hand, there is a trend of increasing fraction of galaxies with low Sérsic index, suggesting $3<z<5$ might be the epoch of onset of morphological transformation with a fraction of very notable disky quenched galaxies.

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