arXiv subjects

Zheng Cai

Publications and source records attributed to Zheng Cai.

At least 145 records · Page 8Linked to original sources

arXiv2022

The PAU Survey: Measurements of the 4000 Å spectral break with narrow-band photometry

The D4000 spectral break index is one of the most important features in the visible spectrum, as it is a proxy for stellar ages and is also used in galaxy classification. However, its direct measurement has always been reserved to spectroscopy. Here, we present a general method to directly measure the D4000 with narrow-band (NB) photometry; it has been validated using realistic simulations, and then evaluated with PAUS NBs, cross-matched with VIPERS spectra ($i_{\rm AB} < 22.5$, $0.562 < z < 0.967$). We also reconstruct the D4000 with the SED-fitting code CIGALE; the use of PAUS NBs instead of broad bands significantly improves the SED fitting results. For D4000$_{\rm n}$, the direct measurement has $\rm \langle SNR \rangle \sim 4$, but we find that for $i_{\rm AB}<21$ all direct D4000 measurements have $\rm SNR>3$. The CIGALE D4000$_{\rm n}$ has $\rm \langle SNR \rangle \sim 20$, but underestimates the error by $>$50\%. Furthermore, the direct method recreates well the D4000-SFR relation, as well as the D4000-mass relation for blue galaxies (for red galaxies, selection effects impact the results). On the other hand, CIGALE accurately classifies galaxies into red and blue populations. We conclude that the direct measurement of D4000 with narrow-band photometry is a promising tool to determine average properties of galaxy samples, with results compatible with spectroscopy.

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arXiv2022

Probing the Diffuse Lyman-alpha Emission on Cosmological Scales: Lyα Emission Intensity Mapping Using the Complete SDSS-IV eBOSS Survey

Based on the Sloan Digital Sky Survey Data Release 16, we have detected the large-scale structure of Ly$α$ emission in the Universe at redshifts $z = 2$--3.5 by cross-correlating quasar positions and Ly$α$ emission imprinted in the residual spectra of luminous red galaxies. We apply an analytical model to fit the corresponding Ly$α$ surface brightness profile and multipoles of the redshift-space quasar-Ly$α$ emission cross-correlation function. The model suggests an average cosmic Ly$α$ luminosity density of ${6.6_{-3.1}^{+3.3}}\times 10^{40} {\rm erg\, s^{-1} cMpc^{-3}}$, a $\sim 2σ$ detection with a median value about 8--9 times those estimated from deep narrowband surveys of Ly$α$ emitters at similar redshifts. Although the low signal-to-noise ratio prevents us from a significant detection of the Ly$α$ forest-Ly$α$ emission cross-correlation, the measurement is consistent with the prediction of our best-fit model from quasar-Ly$α$ emission cross-correlation within current uncertainties. We rule out the scenario that these Ly$α$ photons mainly originate from quasars. We find that Ly$α$ emission from star-forming galaxies, including contributions from that concentrated around the galaxy centers and that in the diffuse Ly$α$ emitting halos, is able to explain the bulk of the the Ly$α$ luminosity density inferred from our measurements. Ongoing and future surveys can further improve the measurements and advance our understanding of the cosmic Ly$α$ emission field.

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arXiv2022

The physical origin for spatially large scatter of IGM opacity at the end of reionization: the IGM Ly$α$ opacity-galaxy density relation

The large opacity fluctuations in the $z > 5.5$ Ly$α$ forest may indicate inhomogeneous progress of reionization. To explain the observed large scatter of the effective Ly$α$ optical depth ($τ_{\rm eff}$) of the intergalactic medium (IGM), fluctuation of UV background ($Γ$ model) or the IGM gas temperature ($T$ model) have been proposed, which predict opposite correlations between $τ_{\rm eff}$ and galaxy density. In order to address which model can explain the large scatter of $τ_{\rm eff}$, we search for Ly$α$ emitters (LAEs) around two (J1137+3549 and J1602+4228) quasar sightlines with $τ_{\rm eff}\sim3$ and J1630+4012 sightline with $τ_{\rm eff}\sim5.5$. Using a narrowband imaging with Subaru/Hyper Suprime-Cam, we draw LAE density maps to explore their spatial distributions. Overdensities are found within 20 $h^{-1}$Mpc of the quasar sightlines in the low $τ_{\rm eff}$ regions, while a deficit of LAEs is found in the high $τ_{\rm eff}$ region. Although the $τ_{\rm eff}$ of the three quasar sightlines are neither high nor low enough to clearly distinguish the two models, these observed $τ_{\rm eff}$-galaxy density relations all consistently support the $Γ$ model rather than the $T$ model in the three fields, along with the previous studies. The observed overdensities near the low $τ_{\rm eff}$ sightlines may suggest that the relic temperature fluctuation does not affect reionization that much. Otherwise, these overdensities could be attributed to other factors besides the reionization process, such as the nature of LAEs as poor tracers of underlying large-scale structures.

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arXiv2022

An Unsupervised Learning Approach for Quasar Continuum Prediction

Modeling quasar spectra is a fundamental task in astrophysics as quasars are the tell-tale sign of cosmic evolution. We introduce a novel unsupervised learning algorithm, Quasar Factor Analysis (QFA), for recovering the intrinsic quasar continua from noisy quasar spectra. QFA assumes that the Ly$α$ forest can be approximated as a Gaussian process, and the continuum can be well described as a latent factor model. We show that QFA can learn, through unsupervised learning and directly from the quasar spectra, the quasar continua and Ly$α$ forest simultaneously. Compared to previous methods, QFA achieves state-of-the-art performance for quasar continuum prediction robustly but without the need for predefined training continua. In addition, the generative and probabilistic nature of QFA paves the way to understanding the evolution of black holes as well as performing out-of-distribution detection and other Bayesian downstream inferences.

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arXiv2022

First Census of Gas-phase Metallicity Gradients of Star-forming Galaxies in Overdense Environments at Cosmic Noon

We report the first spatially resolved measurements of gas-phase metallicity radial gradients in star-forming galaxies in overdense environments at $z\gtrsim2$. The spectroscopic data are acquired by the \mg\ survey, a Hubble Space Telescope (HST) cycle-28 medium program. This program is obtaining 45 orbits of WFC3/IR grism spectroscopy in the density peak regions of three massive galaxy protoclusters (BOSS 1244, BOSS 1542 and BOSS 1441) at $z=2-3$. Our sample in the BOSS 1244 field consists of 20 galaxies with stellar-mass ranging from $10^{9.0}$ to $10^{10.3}$ \Msun\ , star formation rate (SFR) from 10 to 240 \Msun\,yr$^{-1}$, and global gas-phase metallicity (\oh) from 8.2 to 8.6. At $1σ$ confidence level, 2/20 galaxies in our sample show positive (inverted) gradients -- the relative abundance of oxygen increasing with galactocentric radius, opposite the usual trend. Furthermore, 1/20 shows negative gradients and 17/20 are consistent with flat gradients. This high fraction of flat/inverted gradients is uncommon in simulations and previous observations conducted in blank fields at similar redshifts. To understand this, we investigate the correlations among various observed properties of our sample galaxies. We find an anticorrelation between metallicity gradient and global metallicity of our galaxies residing in extreme overdensities, and a marked deficiency of metallicity in our massive galaxies as compared to their coeval field counterparts. We conclude that the cold-mode gas accretion plays an active role in shaping the chemical evolution of galaxies in the protocluster environments, diluting their central chemical abundance, and flattening/inverting their metallicity gradients.

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arXiv2022

Spatially resolved molecular interstellar medium in a $z=6.6$ quasar host galaxy

We present high spatial resolution (0.4", 2.2kpc) observations of the CO(6-5), CO(7-6) and [CI] lines and dust continuum emission from the interstellar medium in the host galaxy of the quasar J0305$-$3150 at $z=6.6$. These, together with archival [CII] data at comparable spatial resolution, enable studies of the spatial distribution and kinematics between the ISM in different phases. When comparing the radial profiles of CO, [CII] and the dust continuum, we find that the CO and dust continuum exhibit similar spatial distributions, both of which are less extended than the [CII], indicating that the CO and dust continuum are tracing the same gas component, while the [CII] is tracing a more extended one. In addition, we derive the radial profiles of the [CII]/CO, [CII]/far-infrared (FIR), CO/FIR, and dust continuum $S_{98.7 \rm GHz}/S_{258.1 \rm GHz}$ ratios. We find a decreasing $S_{98.7 \rm GHz}/S_{258.1 \rm GHz}$ ratio with radius, possibly indicating a decrease of dust optical depth with increasing radius. We also detect some of the ISM lines and continuum emission in the companion galaxies previously discovered in the field around J0305$-$3150. Through comparing the line-to-line and line-to-FIR ratios, we find no significant differences between the quasar and its companion galaxies.

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arXiv2022

Submillimetre galaxies in two massive protoclusters at z = 2.24: witnessing the enrichment of extreme starbursts in the outskirts of HAE density peaks

Submillimetre galaxies represent a rapid growth phase of both star formation and massive galaxies. Mapping SMGs in galaxy protoclusters provides key insights into where and how these extreme starbursts take place in connections with the assembly of the large-scale structure in the early Universe. We search for SMGs at 850$\,μm$ using JCMT/SCUBA-2 in two massive protoclusters at $z=2.24$, BOSS1244 and BOSS1542, and detect 43 and 54 sources with $S_{850}>4\,$mJy at the $4σ$ level within an effective area of 264$\,$arcmin$^2$, respectively. We construct the intrinsic number counts and find that the abundance of SMGs is $2.0\pm0.3$ and $2.1\pm0.2$ times that of the general fields, confirming that BOSS1244 and BOSS1542 contain a higher fraction of dusty galaxies with strongly enhanced star formation. The volume densities of the SMGs are estimated to be $\sim15-$30 times the average, significantly higher than the overdensity factor ($\sim 6$) traced by H$α$ emission-line galaxies (HAEs). More importantly, we discover a prominent offset between the spatial distributions of the two populations in these two protoclusters -- SMGs are mostly located around the high-density regions of HAEs, and few are seen inside these regions. This finding may have revealed for the first time the occurrence of violent star formation enhancement in the outskirts of the HAE density peaks, likely driven by the boosting of gas supplies and/or starburst triggering events. Meanwhile, the lack of SMGs inside the most overdense regions at $z\sim2$ implies a transition to the environment disfavouring extreme starbursts.

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arXiv2022

Deep Learning of DESI Mock Spectra to Find Damped Lyα Systems

We have updated and applied a convolutional neural network (CNN) machine learning model to discover and characterize damped Ly$α$ systems (DLAs) based on Dark Energy Spectroscopic Instrument (DESI) mock spectra. We have optimized the training process and constructed a CNN model that yields a DLA classification accuracy above 99$\%$ for spectra which have signal-to-noise (S/N) above 5 per pixel. Classification accuracy is the rate of correct classifications. This accuracy remains above 97$\%$ for lower signal-to-noise (S/N) $\approx1$ spectra. This CNN model provides estimations for redshift and HI column density with standard deviations of 0.002 and 0.17 dex for spectra with S/N above 3 per pixel. Also, this DLA finder is able to identify overlapping DLAs and sub-DLAs. Further, the impact of different DLA catalogs on the measurement of Baryon Acoustic Oscillation (BAO) is investigated. The cosmological fitting parameter result for BAO has less than $0.61\%$ difference compared to analysis of the mock results with perfect knowledge of DLAs. This difference is lower than the statistical error for the first year estimated from the mock spectra: above $1.7\%$. We also compared the performance of CNN and Gaussian Process (GP) model. Our improved CNN model has moderately 14$\%$ higher purity and 7$\%$ higher completeness than an older version of GP code, for S/N $>$ 3. Both codes provide good DLA redshift estimates, but the GP produces a better column density estimate by $24\%$ less standard deviation. A credible DLA catalog for DESI main survey can be provided by combining these two algorithms.

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arXiv2021

Reply to: Three papers regarding the origin of GN-z11-flash

In Jiang et al., we detected a bright flash (hereafter GN-z11-flash) that appeared as compact continuum emission during our Keck MOSFIRE observations of the galaxy GN-z11 at z ~ 11. We performed a comprehensive analysis of the origin of the flash using all available information and our current understanding of known man-made objects or moving objects in the solar system. We found that GN-z11-flash was likely a rest-frame UV flash associated with a long gamma-ray burst (GRB) from GN-z11. Recently, Steinhardt et al., Michalowski et al., and Nir et al. reported that GN-z11-flash was more likely from a satellite. While one cannot completely rule out the possibility of unknown satellites (or debris), we find that either the chance probabilities of being a satellite estimated by these authors have been largely overestimated or their identified satellites have been ruled out in our original analysis. Our new calculations show that the probability of GN-z11-flash being a satellite is still lower than that of it being a signal originated from GN-z11.

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arXiv2021

From large-scale environment to CGM angular momentum to star forming activities -- I: star-forming galaxies

The connection between halo gas acquisition through the circumgalactic medium (CGM) and galaxy star formation has long been studied. In this series of two papers, we put this interplay within the context of the galaxy environment on large scales (several hundreds of kpc), which, to a certain degree, maps out various paths for galaxy interactions. We use the IllustrisTNG-100 simulation to demonstrate that the large-scale environment modulates the circumgalactic gas angular momentum, resulting in either enhanced (Paper I) or suppressed (Paper II) star formation inside a galaxy. In this paper (Paper I), we show that the large-scale environment around a star-forming galaxy is often responsible for triggering new episodes of star formation. Such an episodic star formation pattern is well synced with a pulsating motion of the circumgalactic gas, which, on the one hand receives angular momentum modulations from the large-scale environment, yielding in-spiralling gas to fuel the star-forming reservoir, while, on the other hand, is affected by the feedback activities from the galaxy centre. As a result, a present-day star-forming galaxy may have gone through several cycles of star-forming and quiescent phases during its evolutionary history, with the circumgalactic gas carrying out a synchronized cadence of "breathing in and out" motions out to $\sim 100$ kpc.

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arXiv2021

A Multiwavelength Study of ELAN Environments (AMUSE$^2$): Ubiquitous dusty star-forming galaxies associated with enormous Ly$α$ nebulae on megaparsec scales

We have been undertaking a systematic survey at 850 $μ$m based on a sample of four prototypical $z\sim2-3$ enormous Ly$α$ nebulae (ELANe) as well as their megaparsec-scale (Mpc-scale) environments to study the physical connections between ELANe and their coeval dusty submillimeter galaxies (SMGs). By analysing the SCUBA-2 data with self-consistent Monte Carlo simulations to construct the number counts, here, we report on the overabundance of 850 $μ$m-selected submillimeter sources around all the four ELANe, by a factor of 3.6$\pm$0.6 (weighted average) compared to the blank fields. This suggests that the excessive number of submillimeter sources are likely to be part of the Mpc-scale environment around the ELANe, corroborating the co-evolution scenario for SMGs and quasars; this is a process which may be more commonly observed in the ELAN fields. If the current form of the underlying count models continues toward the fainter end, our results would suggest an excess of the 850 $μ$m extragalactic background light by a factor of between 2-10, an indication of significant background light fluctuations on the survey scales. Finally, by assuming that all the excessive submillimeter sources are associated with their corresponding ELAN environments, we estimate the SFR densities of each ELAN field, as well as a weighted average of $Σ$SFR=1200$\pm$300 $M_\odot$ yr$^{-1}$ Mpc$^{-3}$, consistent with that found in the vicinity of other quasar systems or proto-clusters at similar redshifts; in addition, it is a factor of about 300 greater than the cosmic mean.

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arXiv2021

Mid-InfraRed Outbursts in Nearby Galaxies (MIRONG). II. Optical Spectroscopic Follow-up

Infrared echo has proven to be an effective means to discover transient accretion events of supermassive black holes (SMBHs), such as tidal disruption events (TDEs) and changing-look active galactic nuclei (AGNs), in dusty circumnuclear environments. To explore the dusty populations of SMBH transient events, we have constructed a large sample of Mid-infrared Outbursts in Nearby Galaxies (MIRONG) and performed multiwavelength observations. Here we present the results of multiepoch spectroscopic follow-up observations of a subsample of 54 objects spanning a time scale of 4 yr. Emission-line variability was detected in 22 of them with either emergence or enhancement of broad Balmer emission lines in comparison with pre-outburst spectra. Coronal lines, HeIIλ4686 and Bowen line NIIIλ4640 appeared in the spectra of nine,seven and two sources, respectively. These results suggest that MIRONG is a mixed bag of different transient sources. We have tentatively classified them into different subclass according to their spectral evolution and light curves. Two sources have been in a steady high broad Hα flux up to the latest observation and might be turn-on AGNs. Broad lines faded out in the remaining sources, indicating a transient ionizing source ignited by TDE or sporadic gas accretion. Thirty-one sources do not show noticeable spectral change with respect to their pre-outburst spectra. They have a statistically redder MIR color and lower MIR luminosity of the outbursts,which are consistent with heavily obscured events.

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arXiv2021

The mass-metallicity relation at cosmic noon in overdense environments: first results from the MAMMOTH-Grism HST slitless spectroscopic survey

The MAMMOTH-Grism slitless spectroscopic survey is a Hubble Space Telescope (HST) cycle-28 medium program, which is obtaining 45 orbits of WFC3/IR grism spectroscopy in the density peak regions of three massive galaxy protoclusters at $z=2-3$ discovered using the MAMMOTH technique. We introduce this survey by presenting the first measurement of the mass-metallicity relation (MZR) at high redshift in overdense environments via grism spectroscopy. From the completed MAMMOTH-Grism observations in the field of the BOSS1244 protocluster at $z=2.24\pm0.02$, We secure a sample of 36 protocluster member galaxies at $z\sim2.24$, showing strong nebular emission lines ([O III], H$β$ and [O II]) in their G141 spectra. Using the multi-wavelength broad-band deep imaging from HST and ground-based telescopes, we measure their stellar masses in the range of $[10^{9},10^{10.4}]M_\odot$, instantaneous star formation rates (SFR) from 10 to 240$M_\odot yr^{-1}$, and global gas-phase metallicities [$\frac{1}{3}$,1] of solar. Compared with similarly selected field galaxy sample at the same redshift, our galaxies show on average increased SFRs by $\sim$0.06dex and $\sim$0.18dex at $\sim$10$^{10.1}M_\odot$ and $\sim$10$^{9.8}M_\odot$, respectively. Using the stacked spectra of our sample galaxies, we derive the MZR in the BOSS1244 protocluster core as $12+\log({\rm O/H})=(0.136\pm0.018)\times\log(M_\ast/M_\odot)+(7.082\pm0.175)$, showing significantly shallower slope than that in the field. This shallow MZR slope is likely caused by the combined effects of efficient recycling of feedback-driven winds and cold-mode gas accretion in protocluster environments. The former effect helps low-mass galaxies residing in overdensities retain their metal production, whereas the latter effect dilutes the metal content of high-mass galaxies, making them more metal poor than their coeval field counterparts.

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arXiv2021

Massive molecular outflow and 100 kpc extended cold halo gas in the enormous Ly$α$ nebula of QSO 1228+3128

The link between the circum-galactic medium (CGM) and the stellar growth of massive galaxies at high-$z$ depends on the properties of the widespread cold molecular gas. As part of the SUPERCOLD-CGM survey (Survey of Protocluster ELANe Revealing CO/\ci\ in the Ly$α$-Detected CGM), we present the radio-loud QSO Q1228+3128 at $z=2.2218$, which is embedded in an enormous Ly$α$ nebula. ALMA+ACA observations of CO(4-3) reveal both a massive molecular outflow, and a more extended molecular gas reservoir across $\sim$100 kpc in the CGM each containing a mass of M$_{\rm H2}$\,$\sim$\,4$-$5\,$\times$\,10$^{10}$ M$_{\odot}$. The outflow and molecular CGM are aligned spatially, along the direction of an inner radio jet. After re-analysis of Ly$α$ data of Q1228+3128 from the Keck Cosmic Web Imager, we found that the velocity of the extended CO agrees with the redshift derived from the Ly$α$ nebula and the bulk velocity of the massive outflow. We propose a scenario where the radio source in Q1228+3128 is driving the molecular outflow and perhaps also enriching or cooling the CGM. In addition, we found that the extended CO emission is nearly perpendicular to the extended Ly$α$ nebula spatially, indicating that the two gas phases are not well mixed, and possibly even represent different phenomena (e.g., outflow vs. infall). Our results provide crucial evidence in support of predicted baryonic recycling processes that drive the early evolution of massive galaxies.

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arXiv2021

From large-scale environment to CGM angular momentum to star forming activities -- II: quenched galaxies

The gas needed to sustain star formation in galaxies is supplied by the circumgalactic medium (CGM), which in turn is affected by accretion from large scales. In a series of two papers, we examine the interplay between a galaxy's ambient CGM and central star formation within the context of the large-scale environment. We use the IllustrisTNG-100 simulation to show that the influence exerted by the large-scale galaxy environment on the CGM gas angular momentum results in either enhanced (Paper I) or suppressed (Paper II, this paper) star formation inside a galaxy. We find that for present-day quenched galaxies, both the large-scale environments and the ambient CGM have always had higher angular momenta throughout their evolutionary history since at least $z=2$, in comparison to those around present-day star-forming disk galaxies, resulting in less efficient gas inflow into the central star-forming gas reservoirs. A sufficiently high CGM angular momentum, as inherited from the larger-scale environment, is thus an important factor in keeping a galaxy quenched, once it is quenched. The process above naturally renders two key observational signatures: (1) a coherent rotation pattern existing across multiple distances from the large-scale galaxy environment, to the circumgalactic gas, to the central stellar disk; and (2) an anti-correlation between galaxy star-formation rates and orbital angular momenta of interacting galaxy pairs or groups.

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arXiv2021

Discovery of a protocluster core associated with an enormous Ly$α$ Nebula at $z = 2.3$

The MAMMOTH-1 nebula at $z=2.317$ is an enormous Ly$α$ nebula (ELAN) extending to a $\sim$440 kpc scale at the center of the extreme galaxy overdensity BOSS 1441. In this paper, we present observations of the $\rm CO(3-2)$ and 250 GHz dust-continuum emission from the MAMMOTH-1 using the IRAM NOrthern Extended Millimeter Array. Our observations show that $\rm CO(3-2)$ emission in this ELAN has not extended widespread emission into the circum- and inter-galactic media. We also find a remarkable concentration of six massive galaxies in $\rm CO(3-2)$ emission in the central $\sim$100 kpc region of the ELAN. Their velocity dispersions suggest a total halo mass of $M_{200c} \sim 10^{13.1} M_{\odot}$, marking a possible protocluster core associated with the ELAN. The peak position of the $\rm CO(3-2)$ line emission from the obscured AGN is consistent with the location of the intensity peak of MAMMOTH-1 in the rest-frame UV band. Its luminosity line ratio between the $\rm CO(3-2)$ and $\rm CO(1-0)$ $r_{3,1}$ is 0.61$\pm$0.17. The other five galaxies have $\rm CO(3-2)$ luminosities in the range of (2.1-7.1)$\times 10^9$ K $\rm km\,s^{-1}$ pc$^2$, with the star-formation rates derived from the 250GHz continuum of ($<$36)-224 $M_{\odot}$yr$^{-1}$. Follow-up spectroscopic observations will further confirm more member galaxies and improve the accuracy of the halo mass estimation.

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arXiv2021

A [C II] 158$μ$m Emitter Associated with an OI Absorber at the End of the Reionization Epoch

The physical and chemical properties of the circumgalactic medium (CGM) at $z\gtrsim6$ have been studied successfully through the absorption in the spectra of background Quasi-Stellar Objects (QSOs). One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers. Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the CGM and the intergalactic medium (IGM) at z=5-6. Deep, dedicated surveys have searched for Ly$α$ emission associated with strong CIV absorbers at $z\approx 6$, but only a few Ly$α$ emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Ly$α$ is a resonant line, raising the need for complementary techniques for detecting absorbers' host galaxies. Here, using Atacama Large Millimeter Array (ALMA), we report a [C II] 158$μ$m emitter associated with a strong low-ionization absorber, OI, at $z=5.978$. The projected impact parameter between OI and [C II] emitter is 20.0 kpc. The measured [C II] luminosity is $7.0\times 10^7$ solar luminosities. Further analysis indicates that strong OI absorbers may reside in the circumgalactic medium of massive halos one to two orders of magnitude more massive than expected values.

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arXiv2021

Improved Lyman Alpha Tomography using Optimized Reconstruction with Constraints on Absorption (ORCA)

In this work, we propose an improved approach to reconstruct the three-dimensional intergalactic medium from observed Lyman-$α$ forest absorption features. We present our new method, the Optimized Reconstruction with Constraints on Absorption (ORCA), which outperforms the current baseline Wiener Filter (WF) when tested on mock Lyman Alpha forest data generated from hydrodynamical simulations. We find that both reconstructed flux errors and cosmic web classification improve substantially with ORCA, equivalent to 30-40\% additional sight-lines with the standard WF. We use this method to identify and classify extremal objects, i.e. voids and (proto)-clusters, and find improved reconstruction across all summary statistics explored. We apply ORCA to existing Lyman Alpha forest data from the COSMOS Lyman Alpha Mapping and Tomography Observations (CLAMATO) Survey and compare it to the WF reconstruction.

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