arXiv subjects

Yongda Zhu

Publications and source records attributed to Yongda Zhu.

At least 73 records · Page 4Linked to original sources

arXiv2025

The $z=7.08$ quasar ULAS J1120+0641 May Never Reach a "Normal" Black Hole to Stellar Mass Ratio

JWST observations of quasars in the Epoch of Reionization have revealed that many lie in host galaxies that are severely undermassive relative to the supermassive black holes. It is unclear how these systems will evolve to the tight local relation between stellar mass and black hole mass. We search for companions around the z=7.08 quasar ULAS J1120+0641 using JWST/NIRCam narrow, medium, and wide-band photometry to identify [O III] emitters at the quasar redshift, and explore the potential for growth of the host galaxy through future mergers. We find 22 sources near the quasar's redshift across our two 4.4 arcmin$^2$ fields, indicating that environment of ULAS J1120+0641 is strongly overdense in z~7.1 galaxies relative to the field. We estimate the potential future mass budget of the quasar host galaxy by summing the current stellar and gas masses of the quasar host and surrounding galaxies, correcting for incompleteness and selection effects. With no further black hole growth, ULAS J1120+0641 is unlikely to reach a $M_{\mathrm{BH}}/M_*$ ratio less than ~2.5% at z=0, still much higher than typical for local galaxies. However, such systems -- a quiescent black hole in a low-luminosity galaxy -- may have escaped detection locally if they are sufficiently distant.

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arXiv2025

A Promise for the JWST era: Massive black holes directly collapsed from wave dark matter haloes, and Star formation in and around their accretion flows

There are several puzzling phenomena from recent JWST observations, which seem to push the standard ΛCDM cosmology over the edge. All those puzzles can be understood in a coherent way if we assume that first massive black holes (MBHs, the "heavy eggs") formed by directly collapsing from wave CDM haloes, which can be even earlier than the formation of first galaxies (the "chickens"). We elucidate two false obstacles that have been believed to prevent wave CDM haloes from collapsing into black holes (namely "maximum stable mass" and "bosenova") and point out that general-relativistic instabilities (e.g., the non-axisymmetric instability numerically found in spinning scalar-field boson stars) could serve as the concrete mechanisms for direct-collapse black holes (DCBHs) born from wave dark matter. Once the MBHs formed, star formation bursts in and around the accretion flows, characteristic of a special mode in compact regions of high gravitational accelerations.

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arXiv2025

Ly$α$ Emission from [OIII] Emitters Near Reionization: The role of environment in galaxy Ly$α$ detection

Using galaxy Ly$α$ emission to probe reionization relies on establishing baseline expectations for its detectability in the absence of attenuation by neutral gas in the IGM. Towards this end, the growing numbers of $z \sim 5$--6 star-forming galaxies spectroscopically selected by JWST provide an ideal sample for determining how Ly$α$ emission depends on galaxy properties and environment after reionization has largely completed. In this study, we use Keck LRIS to measure the Ly$α$ emission of 46 JWST-selected [OIII]-emitting galaxies over $5.3 \lesssim z \lesssim 6.2$ in the foreground of the ultra-luminous quasar J0100+2802. Overall, we find that the fraction of galaxies detected in Ly$α$ emission is consistent with previous works; however, the fraction also varies with environment. Most notably, we find an apparent deficit of Ly$α$ in the largest group in our sample, at $z \simeq 6.19$, which falls within the redshift range of the quasar's highly ionized proximity zone. We speculate that the Ly$α$ emission from this group may be partly scattered by a foreground neutral island. In contrast, we detect a high rate of Ly$α$ emission in two groups at $z \simeq 5.73$ and $z \simeq 5.78$. These groups may be part of a structure that is extended along the line of sight, enhancing the transmission of Ly$α$ emission. While our sample size is limited, our results suggest that environment may play a significant role in the detectability of galaxy Ly$α$ emission even as late as $z \sim 6$.

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arXiv2025

The Importance of Dust Distribution in Ionizing-photon Escape: NIRCam and MIRI Imaging of a Lyman Continuum-emitting Galaxy at z ~ 3.8

We present deep JWST/NIRCam and MIRI imaging of Ion1, a previously confirmed Lyman Continuum (LyC)-emitting galaxy at $z_{spec}=3.794$. Together with existing HST imaging, these new observations from the JADES program enable a joint analysis of Ion1's LyC, rest-frame UV, stellar, and dust emission with unprecedented detail. We report the first detection of dust emission at rest-frame $\sim3 μ$m in a high-redshift LyC-emitting galaxy using MIRI/F1500W. Our analysis suggests a porous distribution of dust in Ion1, with regions exhibiting evidence of dust deficit coinciding both with LyC-emitting regions and with the peak of H$α$ emission. Furthermore, multi-band NIRCam imaging reveals a strong FUV-to-optical color gradient, where LyC-emitting regions appear significantly bluer than the rest of Ion1. Spatially resolved SED fitting confirms that this color gradient is primarily driven by spatially varying dust attenuation. Together, these findings suggest that Ion1's LyC emission originates from a compact star-forming complex near its stellar-light centroid, where stellar feedback carves out low HI column density channels, facilitating LyC escape. However, only a fraction of these LyC photons - specifically those along sightlines with minimal HI obscuration - ultimately escape and reach observers. This work underscores the critical role of dust and neutral gas geometry in shaping LyC escape in galaxies at high redshifts. Anisotropic LyC escape may be a common feature in the early Universe, which must be properly incorporated to constrain the Epoch of Reionization.

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arXiv2025

JADES and SAPPHIRES: Galaxy Metamorphosis Amidst a Huge, Luminous Emission-line Region

We report the discovery of a remarkably large and luminous line-emitting nebula extending on either side of the Balmer-break galaxy JADES-GS-518794 at z=5.89, detected with JADES JWST/NIRCam imaging in [O III]$λλ$4959,5007 and H$α$ and spectroscopically confirmed with NIRCam/WFSS thanks to the pure-parallel SAPPHIRES programme. The end-to-end velocity offset is $Δv=830\pm130$ km s$^{-1}$. Nebulae with such large size and high luminosity (25-pkpc diameter, L[O III] = $1.2\times 10^{10}$ L$_\odot$) are routinely observed around bright quasars, unlike JADES-GS-518794. With a stellar mass of $10^{10.1}$ M$_\odot$, this galaxy is at the knee of the mass function at z=6. Its star-formation rate declined for some time (10-100 Myr prior to observation), followed by a recent (10 Myr) upturn. This system is part of a candidate large-scale galaxy overdensity, with an excess of Balmer-break galaxies compared to the field (3 $σ$). We discuss the possible origin of this nebula as material from a merger or gas expelled by an active galactic nucleus (AGN). The symmetry of the nebula, its bubble-like morphology, kinematics, high luminosity, and the extremely high equivalent width of [OIII] together favour the AGN interpretation. Intriguingly, there may be a physical connection between the presence of such a large, luminous nebula and the possible metamorphosis of the central galaxy towards quenching.

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arXiv2025

Enhanced Merger Fractions in a Reionization-Era Protocluster

Mergers play a critical role in galaxy evolution, but their relationship to their surrounding environments is unexplored at high redshift. We investigate the galaxy merger rate among 124 [OIII] emitters at $5.3 3σ$ enhancement of the merger fraction amongst all three overdense structures compared to the field. Eleven galaxies are classified as ``active mergers" satisfying both merger criteria, all of which occur within the overdensity samples. We conclude that environment affects the merger rates of galaxies at $z>6$, leading to increased specific star formation at the 4$σ$ level.

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arXiv2025

SAPPHIRES: Extremely Metal-Poor Galaxy Candidates with $12+{\rm log(O/H)}<7.0$ at $z\sim5-7$ from Deep JWST/NIRCam Grism Observations

Population III stars, the hypothetical first generation metal-free stars, have yet to be discovered. Even after three years of successful JWST operations, studies have shown that most galaxies identified to date at $z > 5$ exhibit a metallicity floor of $Z\gtrsim2\%\,Z_{\odot}$, possibly due to unknown selection biases toward bright galaxies or rapid metal enrichment. To address this question, we search for galaxies with low R3$=$[OIII]$\,λ$5008/H$β$ ratios as part of the JWST Cycle-3 large treasury program, the Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (SAPPHIRES). Using deep NIRCam Wide-Field Slitless Spectrscopy (WFSS) data, we report the discovery of seven extremely metal-poor galaxy candidates in the SAPPHIRES Early Data Release field, with estimated $12+{\rm log(O/H)}<7.0$ at $z\sim5-7$, including two sources with $Z<1\%\,Z_{\odot}$, significantly breaking the metallicity floor observed both locally and at high redshift. These candidates appear extremely faint ($\sim28-30\,$ F200W AB mag) and low-mass (${\rm log}(M_{*}/M_{\odot})\sim6.8-7.8$), as expected from the mass-metallicity relation. They also exhibit very blue UV slopes ($-2.6\lesssimβ\lesssim-2.0$), likely due to low dust content $A_{V}\lesssim0.2\,{\rm mag}$ or young stellar ages $\sim5-20\,{\rm Myr}$. Compared to galaxies at similar redshift, they appear exceptionally bursty in their star formation activity. Our results highlight the power of NIRCam/WFSS in identifying extremely metal-poor galaxies, from just a single pointing, with more data to come in SAPPHIRES. This underscores the potential of pure-parallel programs towards achieving JWST's primary science goal: discovering the first pristine stars and galaxies. Deep JWST/NIRSpec follow-up observations will also be essential to confirm their nature and perform detailed chemical abundance analyses.

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arXiv2025

A Systematic Search for Galaxies with Extended Emission Line and Potential Outflows in JADES Medium-Band Images

For the first time, we present a systematic search for galaxies with extended emission line and potential outflow features using \textit{JWST} medium-band images in the GOODS-S field. This is done by comparing the morphology in medium-band images to adjacent continuum and UV bands. We look for galaxies that have a maximum extent 50\% larger, an excess area 30\% greater, or an axis ratio difference of more than 0.3 in the medium band compared to the reference bands. After visual inspection, we find 326 candidate galaxies at $1.4 < z < 8.4$, with a peak in the population near cosmic noon, benefiting from the good coverage of the medium-band filters. By fitting their SEDs, we find that the candidate galaxies are at least 20\% more bursty in their star-forming activity and have 50\% more young stellar populations compared to a control sample selected based on the continuum band flux. Additionally, these candidates exhibit a significantly higher production rate of ionizing photons. We further find that candidates hosting known AGN produce extended emission that is more anisotropic compared to non-AGN candidates. A few of our candidates have been spectroscopically confirmed to have prominent outflow signatures through NIRSpec observations, showcasing the robustness of the photometric selection. Future spectroscopic follow-up will better help verify and characterize the kinematics and chemical properties of these systems.

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arXiv2025

SMILES: Potentially Higher Ionizing Photon Production Efficiency in Overdense Regions

The topology of reionization and the environments where galaxies efficiently produce ionizing photons are key open questions. For the first time, we investigate the trend between ionizing photon production efficiency, $ξ_{\rm ion}$, and galaxy overdensity, $\log(1+δ)$. We analyze the ionizing properties of 79 galaxies between $1.0 < z < 5.2$ using JWST NIRSpec medium-resolution spectra from the Systematic Mid-infrared Instrument (MIRI) Legacy Extragalactic Survey (SMILES) program. Among these, 67 galaxies have H$α$ coverage, spanning $1.0 < z < 3.1$. The galaxy overdensity, $\log(1+δ)$, is measured using the JADES photometric catalog, which covers the SMILES footprint. For the subset with H$α$ coverage, we find that $\logξ_{\rm ion}$ is positively correlated with $\log(1+δ)$, with a slope of $0.94_{-0.46}^{+0.46}$. Additionally, the mean $ξ_{\rm ion}$ for galaxies in overdense regions ($\log(1+δ) > 0.1$) is 2.43 times that of galaxies in lower density regions ($\log(1+δ) < 0.1$). This strong trend is found to be independent of redshift evolution. Furthermore, our results confirm the robust correlations between $ξ_{\rm ion}$ and the rest-frame equivalent widths of the [O III] or H$α$ emission lines. Our results suggest that galaxies in high-density regions are efficient producers of ionizing photons.

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arXiv2025

Deciphering the Nature of Virgil: An Obscured AGN Lurking Within an Apparently Normal Lyman-α Emitter During Cosmic Reionization

We present a comprehensive analysis of the MIRI Extremely Red Object Virgil, a Lyman-$α$ emitter at $z_{spec} = 6.6379 \pm 0.0035$ with the photometric properties of a Little Red Dot. Leveraging new JWST/MIRI imaging from the MIDIS and PAHSPECS programs, we confirm Virgil's extraordinary nature among galaxies in JADES/GOODS-South, exhibiting a strikingly red NIRCam-to-MIRI color (F444W $-$ F1500W = $2.84\pm0.04$ mag). Deep NIRSpec/PRISM spectroscopy from the OASIS program offers key insights into the host galaxy, revealing properties of an average star-forming galaxy during Cosmic Reionization, such as a subsolar metallicity, low-to-moderate dust content, and a relatively high ionization parameter and electron temperature. By estimating the star formation rate of Virgil from UV and H$α$, we find evidence that the galaxy is either entering or fading out of a bursty episode. Although line-ratio diagnostics employed at high-$z$ would classify Virgil as an Active Galactic Nucleus (AGN), this classification becomes ambiguous once redshift evolution is considered. Nonetheless, Virgil occupies the same parameter space as recently confirmed AGNs at similar redshifts. The new deep MIRI data at 15 $μ$m reinforce the AGN nature of Virgil, as inferred from multiple spectral energy distribution (SED) fitting codes. Virgil's rising infrared SED and UV excess resemble those of Dust-Obscured Galaxies (DOGs) studied with Spitzer at Cosmic Noon, particularly blue-excess HotDOGs. Our results highlight the need for a multi-wavelength approach incorporating MIRI to uncover such extreme sources at $z\gtrsim6$ and to shed light on the interplay between galaxy evolution and early black hole growth during Cosmic Reionization.

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arXiv2025

Witnessing the onset of reionisation via Lyman-$α$ emission at redshift 13

$\require{mediawiki-texvc}$Cosmic Reionisation commenced when ultraviolet (UV) radiation produced in the first galaxies began illuminating the cold, neutral gas that filled the primordial Universe. Recent James Webb Space Telescope (JWST) observations have shown that surprisingly UV-bright galaxies were in place beyond redshift $z = 14$, when the Universe was less than $300 \, \mathrm{Myr}$ old. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of Lyman-$α$ (Ly$α$), the principal hydrogen transition. However, spectral signatures encoding crucial properties of these sources, such as their emergent radiation field, largely remain elusive. Here we report spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES) of a galaxy at redshift $z = 13.0$ that reveal a singular, bright emission line unambiguously identified as Ly$α$, in addition to a smooth turnover. We observe an equivalent width of $\text{EW}_\mathrm{Lyα} > 40 \, Å$ (rest frame), previously only seen at $z < 9$ where the intervening intergalactic medium (IGM) becomes increasingly ionised. Together with an extremely blue UV continuum, the unexpected Ly$α$ emission indicates the galaxy is a prolific producer and leaker of ionising photons. This suggests massive, hot stars or an active galactic nucleus (AGN) have created an early reionised region to prevent complete extinction of Ly$α$, thus shedding new light on the nature of the earliest galaxies and the onset of Reionisation only $330 \, \mathrm{Myr}$ after the Big Bang.

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arXiv2025

Zapped then Napped? A rapidly quenched remnant leaker candidate with a steep spectroscopic $β_{UV}$ slope at z=8.5

We use NIRSpec MSA spectroscopy and NIRCam Photometry to explore the properties of JADES-GS8-RL-1, a rapidly quenched, $z=8.5$ galaxy with a stellar mass of $10^{8.9}M_\odot$, a steep blue UV slope, a Balmer break, and no sign of strong emission lines. With a $β_{UV}$=-2.8$\pm 0.2$, as measured from the NIRSpec spectrum, JADES-GS8-RL-1 is consistent with negligible dust attenuation and little to no contribution from the nebular continuum alongside a probable high escape fraction. The $β_{UV}$ slope measured from photometry varies from -3.0 in the central regions to -2.2 at the outskirts suggesting possible regional differences in the escape fraction. There are no high-ionisation emission lines, only a tentative 2.9\sig detection of [OII]. Using photometry, this emission appears to be extended, possibly corresponding to weakly ionised gas expelled during or after the quenching process. JADES-GS8-RL-1 is spatially resolved with a half-light radius of 240 pc and has an exponential, disc-like morphology. It appears to have formed all its stars in a short burst within the past 100 Myr with a formation time of $\approx$70 Myr and a quenching time of $\approx$30 Myr. This quenching would have occurred rapidly, making it a more distant example of the kind of low-mass "mini-quenched" galaxies previously observed at high-z. Due to the extremely blue $β_{UV}$ slope, our best-fit model predicts a high value for \fesc of >10\%, consistent with the value derived from the $β_{UV}$ slope, which when combined with our extraordinarily low O32 upper limit suggests JADES-GS8-RL-1 is a fascinating example of a high-z "remnant leaker" in one of its earliest phases, deep in the epoch of reionisation.

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arXiv2025

The abundance and nature of high-redshift quiescent galaxies from JADES spectroscopy and the FLAMINGO simulations

We use NIRSpec/MSA spectroscopy and NIRCam imaging to study a sample of 18 massive ($\log\; M_{*}/M_{\odot} \gt 10\;$dex), central quiescent galaxies at $2\leq z \leq 5$ in the GOODS fields, to investigate their number density, star-formation histories, quenching timescales, and incidence of AGN. The data depth reaches $\log M_*/M_\odot \approx 9\;$dex, yet the least-massive central quiescent galaxy found has $\log M_*/M_\odot \gt 10\;$dex, suggesting that quenching is regulated by a physical quantity that scales with $M_*$. With spectroscopy, we assess the completeness and purity of photometric samples, finding number densities 10 times higher than predicted by galaxy formation models, confirming earlier photometric studies. We compare our number densities to predictions from FLAMINGO, the largest-box full-hydro simulation suite to date. We rule out cosmic variance at the 3-$σ$ level, providing spectroscopic confirmation that galaxy formation models do not match observations at $z>3$. Using FLAMINGO, we find that the vast majority of quiescent galaxies' stars formed in situ, with these galaxies not having undergone multiple major dry mergers. This is in agreement with the compact observed size of these systems and suggests that major mergers are not a viable channel for quenching most massive galaxies. Several of our observed galaxies are old, with four galaxies displaying 4000-Å breaks with formation and quenching redshifts of $z\geq8$ and $z\geq6$. Using tracers, we find that 8 galaxies host AGN, including old systems suggesting a high AGN duty cycle with a continuing trickle of gas to fuel accretion.

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arXiv2025

Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (SAPPHIRES): Early Data Release of Deep JWST/NIRCam Images and Spectra in MACS J0416 Parallel Field

We present the early data release (EDR) of SAPPHIRES, a JWST Cycle-3 Treasury imaging and spectroscopic survey using the powerful NIRCam wide-field slitless spectroscopic (WFSS) mode in pure parallel. SAPPHIRES will obtain NIRCam imaging and WFSS data in many cosmological deep fields totaling a telescope charged time of 709 hours (557-hour exposures). In this EDR, we present NIRCam imaging and WFSS data obtained in parallel to the Frontier Field galaxy cluster MACS J0416.1-2403, which are attached to primary observations JWST-GO-4750. With a total dual-channel exposure time of 47.2 hours, we obtain deep NIRCam imaging in 13 bands at 0.6--5.0 micron and deep WFSS at 3.1-5.0 micron through the F356W and F444W filters with grisms in orthogonal dispersion directions. We release reduced NIRCam images, photometric catalogs of 22107 sources and WFSS spectra of 1060 sources with confirmed redshifts ($z\simeq0-8.5$). Preliminary value-added catalogs including photometric redshifts, spectroscopic redshifts and physical properties (mass, star-formation rate, etc.) are also made available. We also characterize the data quality and demonstrate scientific applications, including (1) galaxy candidates at the redshift frontier ($z\gtrsim10$), (2) the ionized gas kinematics of a galaxy reconstructed from $R\sim1500$ grism spectra at orthogonal dispersion directions, (3) massive emission-line galaxies and active galactic nuclei (AGN) around the Epoch of Reionization.

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arXiv2025

SAPPHIRES: A Galaxy Over-Density in the Heart of Cosmic Reionization at $z=8.47$

We report the discovery of a galaxy proto-cluster candidate (dubbed MACS0416-OD-z8p5) at a spectroscopic redshift of $z\sim8.47$, dating back to $\sim550$Myr after the Big Bang. The observations are part of the JWST Cycle-3 treasury program, Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (SAPPHIRES) with NIRCam-grism. Using wide field slitless spectroscopy (WFSS) obtained in the MACS0416 parallel field, we robustly confirm nine galaxies at $z_{\rm spec}\sim8.47$ via emission line detections of [OIII]5008A (with $>5\,σ$) and tentatively confirm one additional galaxy (at $\sim3\,σ$). This discovery represents the highest-redshift, spectroscopically confirmed galaxy over-density known to date, which is $\sim6$--$8$ times more dense than the average volume density of galaxies at the same redshift. Furthermore, a galaxy hosting a low-mass active galactic nucleus (``Little-Red-Dot'') is found as a member, suggesting an early emergence of active, massive black holes and feedback between these black holes and their surrounding environments. We also discuss the spatial structures connecting the galaxy over-density to nearby massive star-forming galaxies (separated by $\sim 5$pMpc, including MACS0416-Y1 and MACS0416-JD. This finding of a massive dark matter halo hosting a galaxy over-density at $z\sim8.5$ is surprising given that our survey covered only a small, random field ($16.5\,{\rm arcmin^2}$) as part of a pure parallel observation. The comparison with cosmological simulations shows that the likelihood of finding such a large-scale structure is $<5\,\%$ under the current galaxy formation scenario and the observed survey volume. Our results demonstrate the power of WFSS observations to build a complete line-emitter sample and suggest an important role for over-densities in enhancing galaxy formation by funneling large-scale gas supplies into small cosmological volumes.

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arXiv2025

The $M_{*}-M_{\rm BH}$ Relation Evolution from z $\sim$ 6 to the Present Epoch

The ratio between the stellar mass of a galaxy, $M_{*}$, and that of its central supermassive black hole (SMBH), $M_\bullet$, the ``Magorrian'' relationship, traces their coevolution. JWST observations have suggested significant evolution in $M_\bullet/M_{*}$ relative to local scaling relationships both in low-mass galaxies and in quasars at z $\ge$ 4. We test this possibility by (1) determining the preferred $M_\bullet/M_{*}$ scaling relation among those proposed locally; and (2) providing uniform host galaxy stellar mass estimates. These steps reduce the prominence of the reported evolution. We then apply Monte Carlo simulations to account for observational biases. We still find a significant increase over the local scaling relation in $M_\bullet/M_{*}$ for z $\ge$ 4 SMBHs in very low-mass galaxies ($\log(M_*/M_{\odot})<10$). However, similarly high values of $M_\bullet/M_{*}$ are also found in low mass galaxies at $z \sim$ 0.5 to 3 that may be common at cosmic noon. Nonetheless, galaxies with similar behavior are rare locally and not accounted for in the local scaling relations. In contrast, z $\sim$ 6 quasars can have $M_\bullet/M_{*}$ well above the local relation value, but they can be explained as extreme cases still within the scaling relation for their higher mass host galaxies. Black holes in some of them and in the low-mass systems may be undergoing very high accretion episodes that result in high $M_\bullet/M_{*}$ but that will be followed by quiescent periods when growth of the host drives the systems toward more typical $M_\bullet/M_{*}$ values.

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arXiv2025

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

We measure H$α$ 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$α$ emitters based on the grism spectra and photometric redshifts from combined HST and JWST imaging data. The numbers of the H$α$ 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$α$ LFs in the two redshift bins. Thanks to the gravitational lensing, the measured H$α$ LFs span three orders of magnitude in luminosity, and the faint-end luminosity reaches $L_{\mathrm{H}α} \sim 10^{40.3} \mathrm{erg} \mathrm{s}^{-1}$ 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$α$ 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$α$ 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$α$ LF measurements, including those propagate from the lens models, cosmic variance, and AGN contribution.

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arXiv2025

A massive HI-absorption-selected galaxy at $z\approx2.356$

We use the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) to detect $\rm CO(1-0)$, $\rm CO(3-2)$, and rest-frame 349-GHz continuum emission from an HI-selected galaxy, DLA1020+2733g, at $z\approx2.3568$ in the field of the $z=2.3553$ damped Lyman-$α$ absorber (DLA) towards QSO J1020+2733. The VLA $\rm CO(1-0)$ detection yields a molecular gas mass of $(2.84\pm0.42)\times10^{11}\times(α_{\rm CO}/4.36)\,\rm{M_\odot}$, the largest ever measured in an HI-selected galaxy. The DLA metallicity is $+0.28 \pm 0.16$, from the ZnII$\, \lambda2026$Å absorption line detected in a Keck Echellette Spectrograph and Imager spectrum. This continues the trend of high-metallicity DLAs being frequently associated with massive galaxies. We obtain a star-formation rate (SFR) of $\lesssim400~$M$_\odot~$yr$^{-1}$ from the rest-frame 349-GHz continuum emission, and a relatively long molecular gas depletion timescale of $\gtrsim0.6~$Gyr. The excitation of the J=3 rotational level is sub-thermal, with $r_{31}\equiv{L'_{\rm{CO(3-2)}}/L'_{\rm{CO(1-0)}}}=0.513\pm0.081$, suggesting that DLA1020+2733g has a low SFR surface density. The large velocity spread of the CO lines, $\approx500~\rm km~s^{-1}$, and the long molecular gas depletion timescale suggest that DLA1020+2733g is likely to be a cold rotating-disk galaxy.

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