We investigate the association between galaxies and neutral O i absorption systems at z similar to 6, which trace metal-enriched gas during the epoch of reionization. We identify 40 galaxies across six quasar fields, residing in 15 overdensities within 300 kpc of the background sight lines. Five O i absorption systems are associated with five of these overdensities, yielding a covering fraction of 0.27-0.10+0.13 within 300 kpc. The absorption occurs beyond typical virial radii, indicating that the gas traces extended overdensity environments rather than individual galaxy halos, unlike the z similar to 0 circumgalactic medium (CGM), which is largely bound to halos. These galaxy-associated absorbers account for similar to 35% of all O i systems seen in blind quasar surveys, implying the remainder arise in lower-mass galaxies below our detection threshold or in dense neutral intergalactic medium pockets. The CGM around these galaxies contains greater than or similar to 2 & times; 106 M circle dot of oxygen, comparable to the interstellar medium oxygen mass of the galaxies themselves, suggesting that the surrounding environment holds as much metal mass as the galaxies. All five galaxy-associated systems show significantly higher log(NCII/NOI) ratios than absorbers lacking galaxy associations. Furthermore, relative abundance ratios ([Si/O], [C/O]) reveal that four of the five exhibit enrichment patterns consistent with Population III nucleosynthesis at the outskirts of galaxy overdensities. These rare systems offer a unique window into the role of first-generation stars in shaping the early metal enrichment of galaxies and their environments.

We present a comprehensive analysis of the relationship between galaxies and the intergalactic medium (IGM) during the late stages of cosmic reionization, based on the complete JWST EIGER dataset. Using deep NIRCam 3.5 μ m slitless spectroscopy, we construct a sample of 948 [O iii ] λ 5008-emitting galaxies with −21.4 ≲ M _UV ≲ −17.2 spanning 5.33 < z < 6.97 along six quasar sight lines. We correlate these galaxies with Ly α and Ly β transmission measured from high-resolution quasar spectra across multiple redshift intervals. We find clear redshift evolution in the correlation between galaxy density and transmission: it is suppressed in overdense regions at z < 5.50, while enhanced at 5.70 < z < 6.15. The intermediate range exhibits a transitional behavior. Cross-correlation measurements further reveal excess absorption within ∼8 cMpc of galaxies at low redshifts, and enhanced transmission at intermediate scales (∼5–20 cMpc) at z > 5.70. Statistical tests using mock catalogs with realistic galaxy clustering but no correlation with the transmission field confirm that the observed correlations are unlikely to arise by chance. The evolving signals can be explained by stronger absorption in overdense regions, combined with the competing influences of local radiation fields and the rising background radiation. While local radiation dominates ionization of the surrounding IGM at earlier times, the background becomes increasingly important, eventually surpassing the impact of nearby galaxies. These results support an inside-out progression of reionization, with ionized regions originating around clustered, star-forming galaxies and gradually extending into underdense regions.

We present the COSMOS Spectroscopic Redshift Compilation encompassing 20 years of spectroscopic redshifts within a 10 deg^2 area centered on the 2 deg^2 COSMOS legacy field. This compilation contains 487,666 redshifts of 266,284 unique objects from 138 individual observing programs up to z ∼ 8 with median stellar mass ∼ 10^8.4 to 10^10 M_⊙ (redshift dependent). Rest-frame NUVrJ colors and SFR – stellar mass correlations show the compilation primarily contains low- to intermediate-mass star-forming and massive, quiescent galaxies at z < 1.25 and mostly low-mass bursty star-forming galaxies at z > 2. Sources in the compilation cover a diverse range of environments, including protoclusters such as “Hyperion”. The full compilation is 50% spectroscopically complete by i ∼ 23.4 and K_s ∼ 21.6 mag; however, this is redshift dependent. Spatially, the compilation is >50% (>30%) complete within the central (outer) region limited to i < 24 mag and K_s < 22.5 mag, separately. We demonstrate how the compilation can be used to validate photometric redshifts and investigate calibration metrics. By training self-organizing maps on COSMOS2020/Classic and projecting the compilation onto it, we find key galaxy subpopulations that currently lack spectroscopic coverage including z < 1 intermediate-mass quiescent galaxies and low-/intermediate-mass bursty star-forming galaxies, z ∼ 2 massive quiescent galaxies, and z > 3 massive star-forming galaxies. This highlights how combining self-organizing maps with our compilation can provide guidance for future spectroscopic observations to get a complete spectroscopic view of galaxy populations. Lastly, the compilation will undergo periodic data releases that incorporate new spectroscopic redshift measurements, providing a lasting legacy resource for the community.

We report JWST/NIRCam measurements of quasar host galaxy emissions and supermassive black hole (SMBH) masses for six quasars at 5.9 < z < 7.1 in the Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization (EIGER) project. We obtain deep NIRCam imaging in the F115W, F200W, and F356W bands, as well as F356W grism spectroscopy of the quasars. We use bright unsaturated stars to construct models of the point-spread functions (PSFs) and estimate the errors of these PSFs. We then measure or constrain the fluxes and morphology of the quasar host galaxies by fitting the quasar images as a point source plus an exponential disk. We successfully detect the host galaxies of three quasars, which have host-to-quasar-flux ratios of ∼1%–5%. Spectral energy distribution fitting suggests that these quasar host galaxies have stellar masses of M _* ≳ 10 ^10 M _⊙ . For quasars with host galaxy nondetections, we estimate the upper limits of their stellar masses. We use the grism spectra to measure the H β line profile and the continuum luminosity, then estimate the SMBH masses for the quasars. Our results indicate that the positive relation between SMBH masses and host galaxy stellar masses already exists at redshift z ≳ 6. The quasars in our sample show a high BH-to-stellar-mass ratio of M _BH / M _* ∼ 0.15, which is about ∼2 dex higher than local relations. We find that selection effects only contribute partially to the high M _BH / M _* ratios of high-redshift quasars. This result hints at a possible redshift evolution of the M _BH – M _* relation.

Characterising the prevalence and properties of faint active galactic nuclei (AGN) in the early Universe is key for understanding the formation of supermassive black holes (SMBHs) and determining their role in cosmic reionization. We perform a spectroscopic search for broad H$\alpha$ emitters at $z\approx4-6$ using deep JWST/NIRCam imaging and wide field slitless spectroscopy from the EIGER and FRESCO surveys. We identify 20 H$\alpha$ lines at $z=4.2-5.5$ that have broad components with line widths from $\sim1200-3700$ km s$^{-1}$, contributing $\sim30-90$ % of the total line flux. We interpret these broad components as being powered by accretion onto SMBHs with implied masses $\sim10^{7-8}$ M$_{\odot}$. In the UV luminosity range M$_{\rm UV}=-21$ to $-18$, we measure number densities of $\approx10^{-5}$ cMpc$^{-3}$. This is an order of magnitude higher than expected from extrapolating quasar UV luminosity functions. Yet, such AGN are found in only $<1$ % of star-forming galaxies at $z\sim5$. The SMBH mass function agrees with large cosmological simulations. In two objects we detect narrow red- and blue-shifted H$\alpha$ absorption indicative, respectively, of dense gas fueling SMBH growth and outflows. We may be witnessing early AGN feedback that will clear dust-free pathways through which more massive blue quasars are seen. We uncover a strong correlation between reddening and the fraction of total galaxy luminosity arising from faint AGN. This implies that early SMBH growth is highly obscured and that faint AGN are only minor contributors to cosmic reionization.

ABSTRACT Observationally mapping the relation between galaxies and the intergalactic medium (IGM) is of key interest for studies of cosmic reionization. Diffuse hydrogen gas has typically been observed in H i Lyman-α (Lyα) absorption in the spectra of bright background quasars. However, it is important to extend these measurements to background galaxies as quasars become increasingly rare at high redshift and rarely probe closely separated sight lines. Here, we use deep integral field spectroscopy in the MUSE eXtremely Deep Field to demonstrate the measurement of the Lyα transmission at z ≈ 4 in absorption to a background galaxy at z = 4.77. The H i transmission is consistent with independent quasar sight lines at similar redshifts. Exploiting the high number of spectroscopic redshifts of faint galaxies (500 between z = 4.0–4.7 within a radius of 8 arcmin) that are tracers of the density field, we show that Lyα transmission is inversely correlated with galaxy density, i.e. transparent regions in the Lyα forest mark underdense regions at z ≈ 4. Due to large-scale clustering, galaxies are surrounded by excess H i absorption over the cosmic mean out to 4 cMpc/h70. We also find that redshifts from the peak of the Lyα line are typically offset from the systemic redshift by +170 km s−1. This work extends results from z ≈ 2–3 to higher redshifts and demonstrates the power of deep integral field spectroscopy to simultaneously measure the ionization structure of the IGM and the large-scale density field in the early Universe.

We report new observations of the cool diffuse gas around 29, 2.3 < z < 6.3 galaxies using deep JWST/NIRCam slitless grism spectroscopy around the sight line to the quasar J0100+2802. The galaxies span a stellar mass range of 7.1 <= log M-*/M-circle dot <= 10.7 , and star formation rates (SFRs) of -0.1 < log SFR/M-circle dot yr(-1) < 2.3. We find galaxies for seven Mg ii absorption systems within 300 kpc of the quasar sight line. The Mg II radial absorption profile falls off sharply with radius, with most of the absorption extending out to 2-3 R-200 of the host galaxies. Six out of seven Mg ii absorption systems are detected around galaxies with log M-*/M-circle dot > 9. The Mg ii absorption kinematics are shifted from the systemic redshifts of host galaxies with a median absolute velocity approximate to 135 km s(-1) and standard deviation approximate to 85 km s(-1). The high kinematic offset and large radial separation (R > 1.3 R-200), suggest that five out of the seven Mg ii absorption systems are not gravitationally bound to their host galaxy. In contrast, most of the cool circumgalactic medium at z < 1 is gravitationally bound. The high incidence of unbound Mg II gas in this work suggests that toward the end of reionization, galaxy halos are in a state of remarkable disequilibrium, and are highly efficient in enriching the intergalactic medium. The two strongest Mg II absorption systems are detected at z similar to 4.22 and 4.5, the former associated with a merging galaxy system and the latter associated with three kinematically close galaxies. Both of these galaxies reside in local galaxy overdensities, indicating the presence of cool Mg ii absorption in two "protogroups" at z > 4.

We expect luminous (M-1450 less than or similar to -26.5) high-redshift quasars to trace the highest-density peaks in the early Universe. Here, we present observations of four z greater than or similar to 6 quasar fields using JWST/NIRCam in the imaging and wide-field slitless spectroscopy mode and report a wide range in the number of detected [O iii]-emitting galaxies in the quasars' environments, ranging between a density enhancement of delta approximate to 65 within a 2 cMpc radius-one of the largest protoclusters during the Epoch of Reionization discovered to date-to a density contrast consistent with zero, indicating the presence of a UV-luminous quasar in a region comparable to the average density of the Universe. By measuring the two-point cross-correlation function of quasars and their surrounding galaxies, as well as the galaxy autocorrelation function, we infer a correlation length of quasars at < z > = 6.25 of r(0)(QQ)=22.0(-2.9)(+3.0)cMpch(-1), while we obtain a correlation length of the [O iii]-emitting galaxies of r0GG=4.1 +/- 0.3cMpch-1. By comparing the correlation functions to dark-matter-only simulations we estimate the minimum mass of the quasars' host dark matter halos to be log10(M-halo,M-min/M-circle dot)=12.43-0.15+0.13 (and log10(M-halo,M-min[OIII]/M-circle dot)=10.56(-0.03)(+0.05) for the [O iii] emitters), indicating that (a) luminous quasars do not necessarily reside within the most overdense regions in the early Universe, and that (b) the UV-luminous duty cycle of quasar activity at these redshifts is f(duty) << 1. Such short quasar activity timescales challenge our understanding of early supermassive black hole growth and provide evidence for highly dust-obscured growth phases or episodic, radiatively inefficient accretion rates.

We present emission-line measurements and physical interpretations for a sample of 117 [O iii ] emitting galaxies at z = 5.33–6.93, using the first deep JWST/NIRCam wide-field slitless spectroscopic observations. Our 9.7 hr integration is centered upon the z = 6.3 quasar J0100+2802—the first of six fields targeted by the EIGER survey—and covers λ = 3–4 μ m. We detect 133 [O iii ] doublets, but close pairs motivated by their small scale clustering excess. The galaxies are characterized by a UV luminosity M UV ∼ −19.6 (−17.7 to −22.3), stellar mass ∼10 8 (10 6.8−10.1 ) M ⊙ , H β and [O iii ] 4960+5008 EWs ≈ 850 Å (up to 3000 Å), young ages, a highly excited interstellar medium, and low dust attenuations. These high EWs are very rare in the local universe, but we show they are ubiquitous at z ∼ 6 based on the measured number densities. The stacked spectrum reveals H γ and [O iii ] 4364 , which shows that the galaxies are typically dust- and metal-poor (E ( B − V ) = 0.1, 12 + log ( O / H ) = 7.4 ) with a high electron temperature (2 × 10 4 K) and a production efficiency of ionizing photons ( ξ ion = 10 25.3 Hz erg −1 ). We further show the existence of a strong mass–metallicity relation. The properties of the stars and gas in z ∼ 6 galaxies conspire to maximize the [O iii ] output from galaxies, yielding an [O iii ] luminosity density at z ≈ 6 that is significantly higher than that at z ≈ 2. Thus, [O iii ] emission-line surveys with JWST prove a highly efficient method to trace the galaxy density in the Epoch of Reionization.

In this paper, we estimate the “star formation change parameter,” SFR 79 , which characterizes the current SFR relative to the average during the last 800 Myr, for ∼300,000 galaxies selected from the Sloan Digital Sky Survey (SDSS). The goals are to examine, in a much larger and independent sample, the trends previously reported in a sample of star-forming (SF) MaNGA galaxies and also to search for spectroscopic signatures of ongoing quenching in the so-called “Green Valley,” which is generally believed to contain galaxies that are migrating from the SF population to the quenched population of galaxies. Measuring SFR 79 for our large sample of SDSS galaxies, we first confirm the basic results of SF galaxies published by Wang & Lilly. We then discuss in detail the calibration and meaning of SFR 79 for galaxies that are well below the SFMS and establish the expected statistical signature of systematic ongoing quenching from modeling the z ∼ 0 quenching rate of the SF population. We conclude that it is not possible at present to establish unambiguous observational evidence for systematic ongoing quenching processes, due to limitations both in the noise of the observational data, in particular in the measurements of H δ absorption, and in the calibration of SFR 79 , as well as biases introduced by the necessity of selecting objects with significant H α emission. We do however see plausible indications of ongoing quenching, which are quantitatively consistent with expectations from “growth+quenching” models of galaxy evolution and a typical e-folding timescale for quenching of ∼500 Myr.

The physical conditions of the circumgalactic medium are investigated by means of intervening absorption-line systems in the spectrum of background quasi-stellar objects (QSOs) out to the epoch of cosmic reionization1–4. A correlation between the ionization state of the absorbing gas and the nature of the nearby galaxies has been suggested by the sources detected in either Lyα or [C ii] 158 μm near to, respectively, highly ionized and neutral absorbers5,6. This is also probably linked to the global changes in the incidence of absorption systems of different types and the process of cosmic reionization7–12. Here we report the detection of two [C ii]-emitting galaxies at redshift z ≈ 5.7 that are associated with a complex, high-ionization C iv absorption system. These objects are part of an overdensity of galaxies and have compact sizes (<2.4 kpc) and narrow linewidths (full width at half maximum (FWHM) ≈ 62–64 km s−1). Hydrodynamic simulations predict that similar narrow [C ii] emission may arise from the heating of small (≲3 kpc) clumps of cold neutral medium or a compact photodissociation region13,14. The lack of counterparts in the rest-frame ultraviolet (UV) indicates severe obscuration of the sources that are exciting the [C ii] emission. These results may suggest a connection between the properties of the [C ii] emission, the rare overdensity of galaxies and the unusual high ionization state of the gas in this region. Investigation of the physical conditions of the circumgalactic medium led to detection of two compact [C ii]-emitting galaxies with narrow linewidths at a redshift of 5.7, associated with a complex, high-ionization C iv absorption system.

Hydrodynamic simulations suggest that galactic gas disks form when coplanar gas spirals into the inner regions of the disk. We recently presented a simple "modified accretion disk" model of viscous galactic disks in which star-formation is fed by a radial flow of gas. However, little observational evidence has been presented for such inflows, which are expected to be only a few km s^-1 in the central regions of the disk, i.e. within three disk scale-lengths, but could reach of order 50-100 km s^-1 in the very outer disk. The effects of systematic inflow on the 2-d velocity field are examined and it is shown that these are quite similar to those produced by geometric warps of the disks, with twist distortions of both the kinematic major and minor axes. This makes it potentially difficult to distinguish between these in practice. By comparing the handedness of the observed twisting of the kinematic axes and of the spiral arms for a sample of nearby galaxies, we find (assuming that the spiral arms are generally trailing) that the effects of warps are in fact likely to dominate over the effects of radial inflows. However, the common practice of treating these twist distortions of the kinematic major and minor axes as being due only to warps can lead, for galaxies of low-to-intermediate inclinations, to substantial underestimates of any systematic inflow.

We report new observations of the cool diffuse gas around 29, $2.3$9. MgII absorption kinematics are shifted from the systemic redshift of host galaxies with a median absolute velocity of 135 km/s and standard deviation of 85 km/s. The high kinematic offset and large radial separation ($R> 1.3 R_{200}$), suggest that five out of the seven MgII absorption systems are gravitationally not bound to the galaxies. In contrast, most cool circumgalactic media at $z<1$ are gravitationally bound. The high incidence of unbound MgII gas in this work suggests that towards the end of reionization, galaxy halos are in a state of remarkable disequilibrium, and are highly efficient in enriching the intergalactic medium. Two strongest MgII absorption systems are detected at $z\sim$ 4.22 and 4.5, the former associated with a merging galaxy system and the latter associated with three kinematically close galaxies. Both these galaxies reside in local galaxy over-densities, indicating the presence of cool MgII absorption in two "proto-groups" at $z>4$.

We investigate the disk formation process in the TNG50 simulation, examining the profiles of SFR surface density (Σ SFR ), gas inflow and outflow, and the evolution of the angular momentum of inflowing gas particles. The TNG50 galaxies tend to have larger star-forming disks, and they also show larger deviations from exponential profiles in Σ SFR when compared to real galaxies in the Mapping Nearby Galaxies at APO survey. The stellar surface density of TNG50 galaxies show good exponential profiles, which is found to be the result of strong radial migration of stars over time. However, this strong radial migration of stars in the simulation produces flatter age profiles in TNG50 disks compared to observed galaxies. The star formation in the simulated galaxies is sustained by a net gas inflow, and this gas inflow is the primary driver for the cosmic evolution of star formation, as expected from simple gas-regulator models of galaxies. There is no evidence for any significant loss of angular momentum for the gas particles after they are accreted on to the galaxy, which may account for the large disk sizes in the TNG50 simulation. Adding viscous processes to the disks, such as the magnetic stresses from magnetorotational instability proposed by Wang & Lilly, will likely reduce the sizes of the simulated disks and the tension with the sizes of real galaxies, and this may produce more realistic exponential profiles.

We present a first sample of 117 [O iii ] λλ 4960, 5008–selected star-forming galaxies at 5.33 < z < 6.93 detected in JWST/NIRCam 3.5 μ m slitless spectroscopy of a 6 .′ 5 × 3 .′ 4 field centered on the hyperluminous quasar SDSS J0100+2802, obtained as part of the Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization (EIGER) survey. Three prominent galaxy overdensities are observed, one of them at the redshift of the quasar. Galaxies are found within 200 pkpc and 105 km s −1 of four known metal absorption-line systems. We focus on the role of the galaxies in ionizing the intergalactic medium (IGM) during the later stages of cosmic reionization and construct the mean Ly α and Ly β transmission as a function of distance from the galaxies. At the lowest redshifts in our study, 5.3 < z < 5.7, the IGM transmission rises monotonically with distance from the galaxies, as seen previously at lower redshifts. In contrast, at 5.7 < z < 6.14, the transmission of both Ly α and Ly β first increases with distance but then peaks at a distance of 5 cMpc before declining. Finally, in the region 6.15 < z < 6.26, where the additional ionizing radiation from the quasar dominates, the monotonic increase in transmission with distance is reestablished. This result is interpreted to represent evidence that the transmission of the IGM at z ∼ 5.9 toward J0100+2802 results from the “local” ionizing radiation of galaxies that dominates over the much-reduced cosmic background.

We present the first rest-frame optical spectrum of a high-redshift quasar observed with JWST/NIRCam in Wide Field Slitless mode. The observed quasar, J0100+2802, is the most luminous quasar known at z > 6. We measure the mass of the central supermassive black hole (SMBH) by means of the rest-frame optical H β emission line, and find consistent mass measurements of the quasar’s SMBH of M • ≈ 1010 M ☉ when compared to the estimates based on the properties of rest-frame UV emission lines C iv and Mg ii, which are accessible from ground-based observatories. To this end, we also present a newly reduced rest-frame UV spectrum of the quasar observed with X-Shooter/Very Large Telescope (VLT) and FIRE/Magellan for a total of 16.8 hr. We readdress the question whether this ultraluminous quasar could be effected by strong gravitational lensing making use of the diffraction limited NIRCam images in three different wide band filters (F115W, F200W, F356W), which improves the achieved spatial resolution compared to previous images taken with the Hubble Space Telescope by a factor of 2. We do not find any evidence for a foreground deflecting galaxy, nor for multiple images of the quasar, and determine the probability for magnification due to strong gravitational lensing with image separations below the diffraction limit of Δθ ≲ 0.″05 to be ≲2.2 × 10−3. Our observations therefore confirm that this quasar hosts a 10 billion solar mass black hole less than 1 Gyr after the Big Bang, which is challenging to explain with current black hole formation models.

The Cosmic Evolution Survey (COSMOS) has become a cornerstone of extragalactic astronomy. Since the last public catalog in 2015, a wealth of new imaging and spectroscopic data have been collected in the COSMOS field. This paper describes the collection, processing, and analysis of these new imaging data to produce a new reference photometric redshift catalog. Source detection and multiwavelength photometry are performed for 1.7 million sources across the 2 deg 2 of the COSMOS field, ∼966,000 of which are measured with all available broadband data using both traditional aperture photometric methods and a new profile-fitting photometric extraction tool, The Farmer , which we have developed. A detailed comparison of the two resulting photometric catalogs is presented. Photometric redshifts are computed for all sources in each catalog utilizing two independent photometric redshift codes. Finally, a comparison is made between the performance of the photometric methodologies and of the redshift codes to demonstrate an exceptional degree of self-consistency in the resulting photometric redshifts. The i < 21 sources have subpercent photometric redshift accuracy and even the faintest sources at 25 < i < 27 reach a precision of 5%. Finally, these results are discussed in the context of previous, current, and future surveys in the COSMOS field. Compared to COSMOS2015, it reaches the same photometric redshift precision at almost one magnitude deeper. Both photometric catalogs and their photometric redshift solutions and physical parameters will be made available through the usual astronomical archive systems (ESO Phase 3, IPAC-IRSA, and CDS).

The Multi Object Optical and Near-infrared Spectrograph (MOONS) instrument is the next generation multi-object spectrograph for the Very Large Telescope (VLT). The instrument combines the high multiplexing capability offered by 1000 optical fibres deployed by individual robotic positioners with a novel spectrograph able to provide both low- and high-resolution spectroscopy simultaneously across the wavelength range 0.64μm - 1.8μm. Powered by the collecting area of the 8-m VLT, MOONS will provide the astronomical community with a world-leading facility able to serve a wide range of Galactic, Extragalactic and Cosmological studies. This paper provides an updated overview of the instrument and its construction progress, reporting on the ongoing integration phase.

We measure the relationship between stellar mass and stellar metallicity, the stellar mass--metallicity relation (MZR), for 1336 star-forming galaxies at $1.6\le z\le3.0$ ( =2.2) using rest-frame far-ultraviolet spectra from the zCOSMOS-deep survey. High signal-to-noise composite spectra containing stellar absorption features are fit with population synthesis model spectra of a range of metallicity. We find stellar metallicities, which mostly reflect iron abundances, scaling as $(Z_{Fe,\ast}/Z_{Fe,\odot})=-(0.81\pm0.01)+(0.32+0.03)\log(M_\ast/10^{10}M_\odot)$ across the mass range of $10^9\lesssim M_\ast/M_\odot\lesssim10^{11}$, being $\approx6\times$ lower than seen locally at the same masses. The instantaneous oxygen-to-iron ratio ($\alpha$-enhancement) inferred using the gas-phase oxygen MZRs, is on average found to be [O/Fe]$\approx0.47$, being higher than the local [O/Fe]$\approx0$. The observed changes in [O/Fe] and [Fe/H] are reproduced in simple flow-through gas-regulator models with steady star-formation histories (SFHs) that follow the evolving main sequence. Our models show that the [O/Fe] is determined almost entirely by the instantaneous specific star formation rate alone while being independent of the SFHs, mass, and the gas-regulation characteristics of the systems. We find that the locations of $\sim10^{10}M_\odot$ galaxies at z~2 in the [O/Fe]--metallicity planes are in remarkable agreement with the sequence of low-metallicity thick-disk stars in our Galaxy. This manifests a beautiful concordance between the results of Galactic archaeology and observations of high-redshift Milky Way progenitors. However, there remains a question of how and when the old metal-rich, low-$\alpha$/Fe stars seen in the bulge had formed by z~2 because such a stellar population is not seen in our data and difficult to explain in the context of our models.

Simulations indicate that the inflow of gas of star-forming galaxies is almost coplanar and corotating with the gas disk, and that the outflow of gas driven by stellar winds and/or supernova explosions is preferentially perpendicular to the disk. This indicates that the galactic gas disk can be treated as a modified accretion disk. In this work, we focus on the metal enhancement in galactic disks in this scenario of gas accretion. Assuming that the star formation rate surface density (ΣSFR) is of exponential form, we obtain the analytic solution of gas-phase metallicity with only three free parameters: the scale length of ΣSFR (h R), the metallicity of the inflowing gas, and the mass-loading factor defined as the wind-driven outflow rate surface density per ΣSFR. According to this simple model, the negative gradient of gas-phase metallicity is a natural consequence of the radial inflow of cold gas that is continuously enriched by in situ star formation as it moves toward the disk center. We fit the model to the observed metallicity profiles for six nearby galaxies chosen to have well-measured metallicity profiles extending to very large radii. Our model can well characterize the overall features of the observed metallicity profiles. The observed profiles usually show a floor at the outer regions of the disk, corresponding to the metallicity of inflow gas. Furthermore, we find the h R of ΣSFR inferred from these fits agree well with independent estimates from ΣSFR profiles, supporting the basic model.