arXiv subjects

Shuqi Fu

Publications and source records attributed to Shuqi Fu.

10 recordsLinked to original sources

arXiv2026

Environmental dependence of the Mass-Metallicity Star Formation Relations at z=4-10 with JWST

We study how environment affects the mass-metallicity relation (MZR) at $z=4$-$10$ using deep imaging and spectroscopy from the James Webb Space Telescope (JWST). Combining CEERS and JADES, we compile a sample of 225 galaxies with stellar masses, star-formation rates, and gas-phase metallicities. We characterize environment using the projected fifth-nearest-neighbour surface density, $\Sigma_{5}$, within $\Delta z=\pm0.25$. At $4.5<z<7$, we find that galaxies in dense regions are more metal-rich at fixed $M_\star$ by $\sim0.1$-0.2 dex, while the slopes of the MZR remain similar across environments. Including SFR increases the separation, suggesting more efficient chemical enrichment in overdense regions. Compared to the local $T_e$-based FMR, our full sample lies $\simeq0.2$-0.3 dex below the $z\sim0$ relation, with a smaller deficit in overdense environments. We also examine how metallicity relates to galaxy size using NIRCam-based effective radii. Metallicity increases weakly with size up to $R_e\sim1$ kpc and then flattens, with only a modest residual trend at fixed $M_\star$ and little environmental dependence. Using mass-weighted stellar ages at $5<z<10$, we find a positive age-metallicity relation in both environments, steeper in the field. Finally, we find that the star-formation rate density is higher in overdense regions at $z\simeq6$-9 by a factor of $\sim2$-3. Overall, our results suggest that environment accelerates both star formation and chemical enrichment during the epoch of reionization. Future wide-area JWST spectroscopy, combined with ALMA and Euclid, will better constrain the role of environment in early galaxy evolution.

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arXiv2026

COSMOS-3D: Black Hole Mass Estimators and Luminosity Functions of Paschen-line AGNs

Near-IR Paschen lines are potentially an excellent tracer of Type 1 AGNs that is hardly affected by dust extinction. JWST allows us, for the first time, to explore Paschen-line objects at redshift z>1. Here we present a study of 62 AGNs with broad Pa$\alpha$ and Pa$\beta$ lines at 1<z<3 using data from the JWST COSMOS-3D program. These AGNs are efficiently selected and identified using NIRCam imaging and grism slitless spectroscopic data. We separate the AGN-host emission with image decomposition and quantify dust attenuation with multi-band data. We construct a calibration sample with optical spectroscopy and use single-epoch, Mg II-based black hole masses ($M_{\mathrm{BH}}$) as an anchor to derive new, Paschen-based $M_{\mathrm{BH}}$ estimators. We obtain three sets of $M_{\mathrm{BH}}$ estimators based on Paschen line luminosities and AGN continuum luminosities at 1 and 2 $\mu$m, respectively. After dust corrections, they are well consistent with each other, and also broadly agree with previous results. With this AGN sample, we further construct the first Pa$\alpha$ and Pa$\beta$ luminosity functions (LFs) of Type 1 AGNs. The derived LFs are 3-5 times higher than those of UV/optical-selected AGNs, indicating that Paschen-selected Type 1 AGNs are more complete. In addition, the intrinsic properties of our AGNs show no dependence on dust reddening, suggesting that the observed reddening is unrelated to the central engine and is thus likely caused by line-of-sight obscuration.

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arXiv2025

COSMOS-3D: Dense Circumnuclear Gas across Black Hole Growth Phases at z ~ 3

We report the discovery of two broad-line X-ray AGNs (cid_414 and cid_947) at z ~ 3 identified in the JWST Cycle 3 COSMOS-3D program using NIRCam F444W grism spectroscopy. Both exhibit prominent HeI+Pa$\gamma$ emission and absorption, indicative of circumnuclear dense gas that is traced in these systems. Complementary UV and optical spectroscopy in the COSMOS field provides Ly$\alpha$, Si IV, and C IV measurements. Both sources are detected in MIRI F1000W, and cid_414 is also detected in F2100W, indicating hot dust emission. The two AGNs show distinct black hole and obscuration properties. The source cid_414 displays little red dots (LRD)-like V-shape spectra energy distribution (SED) shape with a turnover near the Balmer 4000 {\AA} break, and a narrow Ly$\alpha$ line with $\log L_{\rm Ly\alpha}=42.49\pm0.01~\mathrm{erg\ s^{-1}}$, with no additional metal lines detected. In contrast, cid_947 exhibits a higher HeI absorption column density, larger X-ray--inferred $N_{\rm H}$, lower intrinsic 2--10 keV luminosity, and strong blueshifted features in He I, Si IV, and C IV absorption with velocity offsets exceeding $5000~\mathrm{km\ s^{-1}}$. Photoionization modeling implies gas densities of $\sim10^{9-10} \mathrm{cm^{-3}}$ and sizes comparable to the broad-line region, consistent with dense gas envelopes predicted for LRDs. Together with previous detections of HeI absorption in compact little red dots, these results suggest that dense circumnuclear gas is likely prevalent at high redshift and may regulate obscuration and black hole--host co-evolution across AGN types.

astro-ph.GA↗

arXiv2025

Discovery of two little red dots transitioning into quasars

James Webb Space Telescope (JWST) has uncovered a new population of compact objects that show a unique V-shaped spectral energy distribution (SED) in the UV and optical wavelength range. These so-called "little red dots" (LRDs) often exhibit broad Balmer emission lines, indicative of the presence of active galactic nuclei (AGNs). They generally lack detection of X-ray, radio, and mid-IR radiation, which is fundamentally different from typical AGNs. Various models, including super-Eddington-accreting black holes enshrouded in dense and dust-poor gas, have been proposed to explain these features. However, the nature of LRDs remains debated, and their evolutionary fate is unclear. Here we report two unusual LRDs at redshift z = 2.868 and 2.925 that are in a transitional phase towards typical AGNs. Their V-shaped SEDs, compact optical morphology, and broad emission lines satisfy the defining criteria of LRDs. On the other hand, they exhibit intense X-ray, radio, and mid-IR radiation that is much stronger than previously known LRDs. These hybrid properties suggest that the dense gas envelope around their central black holes is dispersing, allowing high-energy photons and radio emission to escape. Meanwhile, a dust torus is forming. This finding provides a direct insight into the nature of LRDs and indicates that at least some LRDs will evolve into AGNs or quasars at later times.

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arXiv2025

Medium-band Astrophysics with the Grism of NIRCam In Frontier fields (MAGNIF): Spectroscopic Census of H$\alpha$ Luminosity Functions and Cosmic Star Formation at $z\sim 4.5$ and 6.3

We measure H$\alpha$ luminosity functions (LFs) at redshifts $z \sim 4.5$ and 6.3 using the JWST MAGNIF (Medium-band Astrophysics with the Grism of NIRCam In Frontier fields) survey. MAGNIF obtained NIRCam grism spectra with the F360M and F480M filters in four Frontier Fields. We identify 248 H$\alpha$ emitters based on the grism spectra and photometric redshifts from combined HST and JWST imaging data. The numbers of the H$\alpha$ emitters show a large field-to-field variation, highlighting the necessity of multiple fields to mitigate cosmic variance. We calculate both observed and dust-corrected H$\alpha$ LFs in the two redshift bins. Thanks to the gravitational lensing, the measured H$\alpha$ LFs span three orders of magnitude in luminosity, and the faint-end luminosity reaches $L_{\mathrm{H}\alpha} \sim 10^{40.3} \mathrm{erg} \mathrm{s}^{-1}$ at $z \sim 4.5$ and $10^{41.5} \mathrm{erg} \mathrm{s}^{-1}$ at $z \sim 6.3$, corresponding to star-formation rates (SFRs) of $\sim$ 0.1 and 1.7 $\mathrm{M}_\odot \mathrm{yr}^{-1}$. We conclude no or weak redshift evolution of the faint-end slope of H$\alpha$ LF across $z\simeq0.4-6.3$, and the comparison with the faint-end slopes of UV LF indicates stochastic star formation history among low-mass H$\alpha$ emitters. The derived cosmic SFR densities are $0.058^{+0.008}_{-0.006}\ \ M_\odot\ \mathrm{yr}^{-1}\ \mathrm{Mpc}^{-3}$ at $z \sim 4.5$ and $0.025^{+0.009}_{-0.007}\ \ M_\odot\ \mathrm{yr}^{-1}\ \mathrm{Mpc}^{-3}$ at $z \sim 6.3$. These are approximately 2.2 times higher than previous estimates based on dust-corrected UV LFs, but consistent with recent measurements from infrared surveys. We discuss uncertainties in the H$\alpha$ LF measurements, including those propagate from the lens models, cosmic variance, and AGN contribution.

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arXiv2025

AGN ruled out as the dominant source of cosmic reionization

Cosmic reionization represents the latest phase transition in the Universe, when the Lyman continuum (LyC) photons turned the intergalactic medium (IGM) from neutral to highly ionized. It has long been debated whether galaxies or active galactic nuclei (AGNs) are the major source of LyC photons responsible for reionization. Previous observations slightly favored galaxies as the major ionizing source. However, the James Webb Space Telescope (JWST) recently discovered an unexpectedly high density of AGN candidates at high redshift, which has largely enhanced the influence of AGNs. Here we derive a definitive upper bound on the AGN contribution to reionization using the latest JWST data, and conclusively rule out AGNs as the dominant ionizing source during the peak epoch of reionization (EoR). We build a sample of galaxies and AGNs in a specific redshift range $7.15 \leq z \leq 7.75$ with a high completeness. Each object is then decomposed into a point-source component and an extended component in their rest-frame far-UV JWST images. We assume all point-source components are AGNs. Our fiducial AGN sample reaches an unprecedentedly low luminosity of $M_{\rm UV} \approx -15$ mag. Based on this sample, AGNs can contribute at most one third of the LyC photons budget required at $z\sim7.5$. Our result implies that galaxies dominate the ionizing source during the EoR.

astro-ph.GA↗

arXiv2024

Constraints on the variation of the fine-structure constant at 3<z<10 with JWST emission-line galaxies

We present constraints on the spacetime variation of the fine-structure constant $\alpha$ at redshifts $2.5\le z<9.5$ using JWST emission-line galaxies. The galaxy sample consists of 621 high-quality spectra with strong and narrow [O III] $\lambda\lambda$4959,5007 doublet emission lines from 578 galaxies, including 232 spectra at $z>5$. The [O III] doublet lines are arguably the best emission lines to probe the variation in $\alpha$. We divide our sample into six subsamples based on redshift and calculate the relative variation $\Delta\alpha/\alpha$ for the individual subsamples. The calculated $\Delta\alpha/\alpha$ values are consistent with zero within $1\sigma$ at all redshifts, suggesting no time variation in $\alpha$ above a level of $(1-2) \times10^{-4}$ ($1\sigma$) in the past 13.2 billion years. When the whole sample is combined, the constraint is improved to be $\Delta\alpha/\alpha = (0.2\pm0.7) \times10^{-4}$. We further test the spatial variation in $\alpha$ using four subsamples of galaxies in four different directions on the sky. The measured $\Delta\alpha/\alpha$ values are consistent with zero at a $1\sigma$ level of $\sim 2\times10^{-4}$. While the constraints in this work are not as stringent as those from lower-redshift quasar absorption lines in previous studies, this work uses an independent tracer and provides the first constraints on $\Delta\alpha/\alpha$ at the highest redshifts. With the growing number of emission-line galaxies from JWST, we expect to achieve stronger constraints in the future.

astro-ph.CO↗

arXiv2024

The Magellan M2FS spectroscopic survey of high-redshift galaxies: the brightest Lyman-break galaxies at $z \sim 6$

We present a study of a sample of 45 spectroscopically confirmed, UV luminous galaxies at $z\sim 6$. They were selected as bright Lyman-break galaxies (LBGs) using deep multi-band optical images in more than 2 deg$^2$ of the sky, and subsequently identified via their strong Ly$\alpha$ emission. The majority of these LBGs span an absolute UV magnitude range from $-22.0$ to $-20.5$ mag with Ly$\alpha$ equivalent width (EW) between $\sim$10 and $\sim$200 \AA, representing the most luminous galaxies at $z\sim 6$ in terms of both UV continuum emission and Ly$\alpha$ line emission. We model the SEDs of 10 LBGs that have deep infrared observations from HST, JWST, and/or Spitzer, and find that they have a wide range of stellar masses and ages. They also have high star-formation rates ranging from a few tens to a few hundreds of Solar mass per year. Five of the LBGs have JWST or HST images and four of them show compact morphology in these images, including one that is roughly consistent with a point source, suggesting that UV luminous galaxies at this redshift are generally compact. The fraction of our photometrically selected LBGs with strong Ly$\alpha$ emission ($\mathrm{EW}>25$ \AA) is about $0.2$, which is consistent with previous results and supports a moderate evolution of the IGM opacity at the end of cosmic reionization. Using deep X-ray images, we do not find evidence of strong AGN activity in these galaxies, but our constraint is loose and we are not able to rule out the possibility of any weak AGN activity.

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arXiv2022

An H{\alpha} Impression of Ly{\alpha} Galaxies at $z\simeq6$ with Deep JWST/NIRCam Imaging

We present a study of seven spectroscopically confirmed (Ly$\alpha$ emitting) galaxies at redshift $z\simeq6$ using the $JWST$/NIRCam imaging data. These galaxies, with a wide range of Ly$\alpha$ luminosities, were recently observed in a series of NIRCam broad- and medium-bands. We constrain the rest-frame UV/optical continua and measure the H$\alpha$ line emission of the galaxies using the combination of the $JWST$/NIRCam and archival $HST$/WFC3 infrared photometry. We further estimate their escape fractions of Ly$\alpha$ photons ($f_{\rm esc}^{\rm Ly\alpha}$) and the production efficiency of ionizing photons ($\xi_{\rm ion}$). Among the sample, 6/7 galaxies have Ly$\alpha$ escape fractions of ${\lesssim}10\%$, which might be the status for most of star-forming galaxies at $z\simeq6$. One UV-faint Ly$\alpha$ galaxy with an extremely blue UV slope owns a large value of $f_{\rm esc}^{\rm Ly\alpha}$ reaching ${\simeq}50\%$. These galaxies spread a broad range of $\xi_{\rm ion}$ over log$_{10}$ $\xi_{\rm ion, 0}$ (Hz erg$^{-1}$) $\sim25.0-26.5$. We find that UV-fainter galaxies with bluer UV continuum slopes likely have higher escape fractions of Ly$\alpha$ photons. We also find that galaxies with higher Ly$\alpha$ line emission tend to produce ionizing photons more efficiently. The most Ly$\alpha$-luminous galaxy in the sample has a very high $\xi_{\rm ion, 0}$ of log$_{10}$ $\xi_{\rm ion, 0}$ (Hz erg$^{-1}$) $>26$. Our results support that Ly$\alpha$ galaxies may have served as an important contributor to the cosmic reionization. Blue and bright Ly$\alpha$ galaxies are excellent targets for $JWST$ follow-up spectroscopic observations.

astro-ph.GA↗

arXiv2022

The Nature of Luminous Quasars with Very Large C IV Equivalent Widths

We report results for a complete sample of ten luminous radio-quiet quasars with large C IV equivalent widths (EW > 150 A). For 8/10 we performed Chandra snapshot observations. We find that, in addition to the enhanced C IV line EW, their He II and Mg II lines are enhanced, but the C III] line is not. Their X-ray emission is substantially stronger than expected from their ultraviolet luminosity. Additionally, these large C IV EW quasars show small C IV blueshifts and possibly low Eddington ratios, suggesting they are "extreme low Eigenvector 1 (EV1)" quasars. The mean excess He II EW is well-matched by Radiation Pressure Compression (RPC) photoionization models, with the harder aox ionizing spectrum. However, these results do not reproduce well the enhancement pattern of the C IV, Mg II, and C III] EWs, or the observed high C IV/Mg II ratio. RPC calculations indicate that the C IV/Mg II line ratio is an effective metallicity indicator, and models with sub-Solar metallicity gas and a hard ionizing continuum reproduce well the enhancement pattern of all four ultraviolet lines. We find that the C IV/Mg II line ratio in quasars is generally correlated with the excess X-ray emission. Extremely high EV1 quasars are characterized by high metallicity and suppressed X-ray emission. The underlying mechanism relating gas metallicity and X-ray emission is not clear, but may be related to radiation-pressure driven disk winds, which are enhanced at high metallicity, and consequent mass loading reducing coronal X-ray emission.

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