Radial Metallicity Gradient Research Articles

Metallicity calibrations based on auroral lines from PHANGS–MUSE data

We present a chemical analysis of selected H II regions from the PHANGS-MUSE nebular catalogue. Our intent is to empirically re-calibrate strong-line diagnostics of gas-phase metallicity, applicable across a wide range of metallicities within nearby star-forming galaxies. To ensure reliable measurements of auroral line fluxes, we carried out a new spectral fitting procedure whereby only restricted wavelength regions around the emission lines of interest are taken into account: this assures a better fit for the stellar continuum. No prior cuts to nebulae luminosity were applied to limit biases in auroral line detections. Ionic abundances of O+, O2+, N+, S+, and S2+ were estimated by applying the direct method. We integrated the selected PHANGS-MUSE sample with other existing auroral line catalogues, appropriately re-analysed to obtain a homogeneous dataset. This was used to derive strong-line diagnostic calibrations that span from 12 + log(O/H) = 7.5 to 8.8. We investigate their dependence on the ionisation parameter and conclude that it is likely the primary cause of the significant scatter observed in these diagnostics. We apply our newly calibrated strong-line diagnostics to the total sample of H II regions from the PHANGS-MUSE nebular catalogue, and we exploit these indirect metallicity estimates to study the radial metallicity gradient within each of the 19 galaxies of the sample. We compare our results with the literature and find good agreement, validating our procedure and findings. With this paper, we release the full catalogue of auroral and nebular line fluxes for the selected H II regions from the PHANGS-MUSE nebular catalogue. This is the first catalogue of direct chemical abundance measurements carried out with PHANGS-MUSE data.

Feedback in Emerging Extragalactic Star Clusters, FEAST: The Relation between 3.3 μm Polycyclic Aromatic Hydrocarbon Emission and Star Formation Rate Traced by Ionized Gas in NGC 628

We present maps of ionized gas (traced by Paα and Brα) and 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission in the nearby spiral galaxy NGC 628, derived from new JWST/NIRCam data from the Feedback in Emerging extrAgalactic Star clusTers (FEAST) survey. With this data, we investigate and calibrate the relation between 3.3 μm PAH emission and star formation rate (SFR) in and around emerging young star clusters (eYSCs) on a scale of ∼40 pc. We find a tight (correlation coefficient ρ ∼ 0.9) sublinear (power-law exponent α ∼ 0.75) relation between the 3.3 μm PAH luminosity surface density and SFR traced by Brα for compact, cospatial (within 0.″16 or ∼7 pc) peaks in Paα, Brα, and 3.3 μm (eYSC–I). The scatter in the relationship does not correlate well with variations in local interstellar medium metallicity, due to a radial metallicity gradient, but rather is likely due to stochastic sampling of the stellar initial mass function (IMF) and variations in the PAH heating and age of our sources. The deviation from a linear relation may be explained by PAH destruction in more intense ionizing environments, variations in age, and IMF stochasticity at intermediate to low luminosities. We test our results with various continuum subtraction techniques using combinations of NIRCam bands and find that they remain robust with only minor differences in the derived slope and intercept. An unexpected discrepancy is identified between the relations of hydrogen recombination lines (Paα versus Brα; Hα versus Brα).

Gas-phase metallicity for the Seyfert galaxy NGC 7130

Metallicity measurements in galaxies can provide valuable clues about galaxy evolution. One of the mechanisms postulated for metallicity redistribution in galaxies is gas flows induced by active galactic nuclei (AGNs), but the details of this process remain elusive. We report the discovery of a positive radial gradient in the gas-phase metallicity of the narrow-line region of the Seyfert 2 galaxy NGC 7130, which is not found when considering the star-forming (SF) components in the galaxy disc. To determine gas-phase metallicities for each kinematic component, we used both AGN and SF strong-line abundance relations, as well as Baldwin–Phillips–Terlevich diagnostic diagrams. These relations involve sensitive strong emission lines, namely iii lambda 5007 ii lambda 6584, Halpha , Hbeta ii lambda 6716, and ii lambda 6731, observed with the adaptive-optics-assisted mode of the Multi Unit Spectroscopic Explorer at the Very Large Telescope. The presence of a positive radial metallicity gradient in only the ionised AGN component suggests that metals may be transported from central areas of a galaxy to its purlieus by AGN activity.

The CO-to-H2 Conversion Factor in the Barred Spiral Galaxy M83

We analyze the CO-to-H2 conversion factor (α CO) in the nearby barred spiral galaxy M83. We present new H i observations from the VLA and single-dish GBT in the disk of the galaxy, and combine them with maps of CO(1-0) integrated intensity and dust surface density from the literature. α CO and the gas-to-dust ratio (δ GDR) are simultaneously derived in annuli of 2 kpc width from R = 1–7 kpc. We find that α CO and δ GDR both increase radially, by a factor of ∼2–3 from the center to the outskirts of the disk. The luminosity-weighted averages over the disk are α CO = 3.14 (2.06, 4.96) M⊙pc−2[Kkms−1]−1 and δ GDR = 137 (111, 182) at the 68% (1σ) confidence level. These are consistent with the α CO and δ GDR values measured in the Milky Way. In addition to possible variations of α CO due to the radial metallicity gradient, we test the possibility of variations in α CO due to changes in the underlying cloud populations, as a function of galactic radius. Using a truncated power-law molecular cloud CO luminosity function and an empirical power-law relation for cloud mass and luminosity, we show that the changes in the underlying cloud population may account for a factor of ∼1.5–2.0 radial change in α CO.

Mapping the Chemodynamics of the Galactic Disk Using the LAMOST and APOGEE Red Clump Stars

A detailed measurement is made of the metallicity distributions, kinematics, and dynamics of the thin and thick disks across a large disk volume (5.0 ≤ R ≤ 15.0 kpc and ∣Z∣ ≤ 3.0 kpc) by using the LAMOST–APOGEE red clump stars. The metallicity distribution results show that the radial metallicity gradient Δ[Fe/H]/ΔR of the thin disk weakens with ∣Z∣ from −0.06 dex kpc−1 at around ∣Z∣ < 0.25 kpc to −0.02 dex kpc−1 at around ∣Z∣ > 2.75 kpc, while the thick disk displays a global weak positive Δ[Fe/H]/ΔR that is generally weaker than 0.01 dex kpc−1. The vertical metallicity gradient Δ[Fe/H]/Δ∣Z∣ steadily weakened from −0.36 dex kpc−1 at R ∼ 5.5 kpc to −0.05 dex kpc−1 at around R > 11.5 kpc for the thin disk, while the thick disk presents an almost constant value (nearly −0.06∼−0.08 dex kpc−1) for all the R bins. These results indicate the contribution of the radial migration to the disk evolution, and the obvious north–south asymmetry in [Fe/H] may be linked to disk warp and/or disk perturbation events. The oscillations in the corrected Δ[Fe/H]/Δ∣Z∣ with R likely arise from the resonances with the Galactic bar. Our detailed measurements of ΔV ϕ /Δ[Fe/H] indicate an inside-out and upside-down star formation scenario for the thick disk. The results of eccentricity distributions and [α/Fe]–velocity dispersion relations are likely to suggest that thick-disk stars require an obvious contribution from other heating mechanisms, such as mergers and accretion, or are born in the chaotic mergers of gas-rich systems and/or the turbulent interstellar medium.

A study on the metallicity gradients in the galactic disk using open clusters

We study the metallicity distribution and evolution in the galactic disk based on the largest sample of open star clusters in the galaxy. From the catalog of 1,879 open clusters in the range of galactocentric distance (RGC) from 4 to 20 kpc, we investigate the variation in metallicity in the galactic disk as functions of RGC, vertical distance (Z), and ages of the clusters. In the direction perpendicular to the galactic plane, the variation in metallicity is found to follow a stepped linear relation. We estimate a vertical metallicity gradient d[Fe/H]d|Z| of −0.545 ± 0.046 dex kpc−1 for |Z| &amp;lt; 0.487 kpc and −0.075 ± 0.093 dex kpc−1 for 0.487 &amp;lt; |Z| &amp;lt; 1.8 kpc. On average, metallicity variations above and below the galactic plane are found to change at similar rates. The change in metallicity in the radial direction is also found to follow a two-function linear relation. We obtain a radial metallicity gradient d[Fe/H]dRGC of −0.070 ± 0.002 dex kpc−1 for 4.0 ≲ RGC ≲ 12.8 kpc and −0.005 ± 0.018 dex kpc−1 for 12.8 ≲ RGC ≲ 20.5 kpc, which clearly shows a strong variation in the metallicity gradient when moving from the inner to the outer galactic disk. The age–metallicity relation (AMR) is found to follow a steeper negative slope of −0.031 ± 0.006 dex Gyr−1 for clusters older than 240 Myr; however, there is some hint of positive metallicity age gradient for younger clusters.

Chemical Cartography of the Sagittarius Stream with Gaia

The stellar stream connected to the Sagittarius (Sgr) dwarf galaxy is the most massive tidal stream that has been mapped in the Galaxy, and is the dominant contributor to the outer stellar halo of the Milky Way (MW). We present metallicity maps of the Sgr stream, using 34,240 red giant branch stars with inferred metallicities from Gaia BP/RP spectra. This sample is larger than previous samples of Sgr stream members with chemical abundances by an order of magnitude. We measure metallicity gradients with respect to Sgr stream coordinates (Λ, B), and highlight the gradient in metallicity with respect to stream latitude coordinate B, which has not been observed before. Including the core, we find ∇[M/H] = −2.48 ± 0.08 × 10−2 dex deg−1 above the stream track (B > B 0, where B 0 = 1.5° is the latitude of the Sgr remnant) and ∇[M/H] = −2.02 ± 0.08 × 10−2 dex deg−1 below the stream track (B < B 0). By painting metallicity gradients onto a tailored N-body simulation of the Sgr stream, we find that the observed metallicities in the stream are consistent with an initial radial metallicity gradient in the Sgr dwarf galaxy of ∼−0.1 to −0.2 dex kpc−1, well within the range of observed metallicity gradients in Local Group dwarf galaxies. Our results provide novel observational constraints for the internal structure of the dwarf galaxy progenitor of the Sgr stream. Leveraging new large data sets in conjunction with tailored simulations, we can connect the present-day properties of disrupted dwarfs in the MW to their initial conditions.

North–South asymmetries in the Galactic thin disc associated with the vertical phase spiral as seen using LAMOST-Gaia stars

ABSTRACT We select 1052 469 (754 635) thin disc stars from Gaia eDR3 and LAMOST DR7 in the range of Galactocentric radius R (guiding centre radius Rg) from 8 to 11 kpc to investigate the asymmetries between the North and South of the disc mid-plane. More specifically, we analyse the vertical velocity dispersion profiles ($\sigma _{v_{z}}(z$)) in different bins of R (Rg) and [Fe/H]. We find troughs in the profiles of $\sigma _{v_{z}}(z)$ located in both the North (z ∼ 0.7 kpc) and South (z ∼ −0.5 kpc) of the disc at all radial and chemical bins studied. The difference between the Northern and Southern vertical velocity dispersion profiles ($\Delta \sigma _{v_{z}}(|z|)$) shows a shift between curves of different R and Rg. A similar shift exists in these North–South (NS) asymmetry profiles further divided into different [Fe/H] ranges. The sample binned with Rg more clearly displays the features in the velocity dispersion profiles. The shift in the peaks of the $\Delta \sigma _{v_{z}}$ profiles and the variation in the phase spiral shape binned by metallicity indicate the variation of the vertical potential profiles and the radial metallicity gradient. The wave-like signal in NS asymmetry of $\sigma _{v_{z}}(z)$ largely originates from phase spiral; while the NS asymmetry profiles of [Fe/H] only display a weak wave-like feature near solar radius. We perform a test particle simulation to qualitatively reproduce the observed results. A quantitative explanation of the NS asymmetry in the metallicity profile needs careful consideration of the spiral shape and the perturbation model, and we leave this for future work.

Tracing the Galactic disk with planetary nebulae using Gaia DR3

Aims. We study the population of Galactic planetary nebulae (PNe) and their central stars (CSPNe) through the analysis of their distances and Galactic distribution. The PN distances are obtained by means of a revised statistical distance scale, based on an astrometrically-defined sample of their central stars from the third Gaia Data Release (DR3) as calibrators. The new statistical distances, together with the proper motion of the CSPNe (also from DR3) with published PN abundances as well as radial velocities, are used to characterize the PN populations in the Galaxy and to derive the radial metallicity gradient. Methods. The statistical scale was applied to infer the distances of a significant number (∼850) of Galactic PNe, for which we deliver a new catalog of PN distances. By adopting a circular velocity curve of the Galaxy, we also obtained peculiar 3D velocities for a large sample of PNe (∼300). The elemental abundances of the PNe were culled from the literature for an updated catalog, to be used in our analysis and other external applications. Results. The radial chemical gradient of the Galactic disk is traced by PNe with available chemical abundances and distances, and kinematic data of the CSPNe are employed to identify the halo PN population. We date PN progenitors based both on abundances and kinematic properties, finding a confirmation of the first method with the second. For all PNe with at least one oxygen determination in the literature, we find a slope of the radial oxygen gradient equal to Δ log(O/H)/ΔRG = −0.0144 ± 0.00385 [dex kpc−1]. Furthermore, we estimate radial oxygen gradients for the PNe with old (&gt; 7.5 Gyr) and young (&lt; 1 Gyr) progenitors to be Δ log(O/H)/ΔRG = −0.0121 ± 0.00465 and −0.022 ± 0.00758 [dex kpc−1], respectively, thus disclosing a mild steepening of the gradient since Galaxy formation, with a slope change of 0.01 dex. The time evolution is slightly higher (∼0.015 dex) when we select the best available abundances in the literature. This result broadly agrees with previous PN results, but is now based on Gaia DR3 analysis, and it also agrees with what has been traced by most other Galactic probes. We also find a moderate oxygen enrichment when comparing the PNe with young and old progenitors.

The Magellanic Puzzle: origin of the periphery

ABSTRACT In this paper, we analyse the metallicity structure of the Magellanic Clouds using parameters derived from the Gaia Data Release 3 (DR3) low-resolution XP (for Blue/Red Photometer) spectra, astrometry, and photometry. We find that the qualitative behaviour of the radial metallicity gradients in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) is quite similar, with both of them having a metallicity plateau at intermediate radii and a second at larger radii. The LMC has a first metallicity plateau at [M/H] ≈ −0.8 for 3–7°, while the SMC has one at [M/H] ≈ −1.1 for 3–5°. The outer LMC periphery has a fairly constant metallicity of [M/H] ≈ −1.0 (10–18°), while the outer SMC periphery has a value of [M/H] ≈ −1.3 (6–10°). The sharp drop in metallicity in the LMC at ∼8° and the marked difference in age distributions in these two regions suggest that there were two important evolutionary phases in the LMC. In addition, we find that the Magellanic periphery substructures, likely Magellanic debris, are mostly dominated by LMC material stripped off in old interactions with the SMC. This presents a new picture in contrast with the popular belief that the debris around the clouds had been mostly stripped off from the SMC due to having a lower mass. We perform a detailed analysis for each known substructure and identify its potential origin based on metallicities and motions with respect to each galaxy.

IMaNGA: mock MaNGA galaxies based on IllustrisTNG and MaStar SSPs. - III. Stellar metallicity drivers in MaNGA and TNG50

ABSTRACT The iMaNGA project uses a forward-modelling approach to compare the predictions of cosmological simulations with observations from SDSS-IV/MaNGA. We investigate the dependency of age and metallicity radial gradients on galaxy morphology, stellar mass, stellar surface mass density (Σ*), and environment. The key of our analysis is that observational biases affecting the interpretation of MaNGA data are emulated in the theoretical iMaNGA sample. The simulations reproduce the observed global stellar population scaling relations with positive correlations between galaxy mass and age/metallicity quite well and also produce younger stellar populations in late-type in agreement with observations. We do find interesting discrepancies, though, that can inform the physics and further development of the simulations. Ages of spiral galaxies and low-mass ellipticals are overestimated by about 2–4 Gyr. Radial metallicity gradients are steeper in iMaNGA than in MaNGA, a discrepancy most prominent in spiral and lenticular galaxies. Also, the observed steepening of metallicity gradients with increasing galaxy mass is not well matched by the simulations. We find that the theoretical radial profiles of surface mass density Σ* are steeper than in observations except for the most massive galaxies. In both MaNGA and iMaNGA [Z/H] correlates with Σ*, however, the simulations systematically predict lower [Z/H] by almost a factor of 2 at any Σ*. Most interestingly, for galaxies with stellar mass log M* ≤ 10.80 M⊙, the MaNGA data reveal a positive correlation between galaxy radius and [Z/H] at fixed Σ*, which is not recovered in iMaNGA. Finally, the dependence on environmental density is negligible in both the theoretical iMaNGA and the observed MaNGA data.

Spatially resolved Kennicutt–Schmidt relation at z ≈ 7 and its connection with the interstellar medium properties

ABSTRACT We exploit moderately resolved [O $\scriptstyle \rm III$], [C $\scriptstyle \rm II$] and dust continuum ALMA observations to derive the gas density (n), the gas-phase metallicity (Z), and the deviation from the Kennicutt–Schmidt (KS) relation (κs) on $\approx \, \rm sub-kpc$ scales in the interstellar medium (ISM) of five bright Lyman Break Galaxies at the Epoch of Reionization (z ≈ 7). To do so, we use GLAM, a state-of-art, physically motivated Bayesian model that links the [C $\scriptstyle \rm II$]and [O $\scriptstyle \rm III$]surface brightness (Σ[CII], Σ[OIII]) and the SFR surface density (ΣSFR) to n, κs, and Z. All five sources are characterized by a central starbursting region, where the Σgas versus ΣSFR align ≈10 × above the KS relation (κs ≈ 10). This translates into gas depletion times in the range tdep ≈ 80 − 250 Myr. The inner starbursting centres are characterized by higher gas density (log (n/cm−3) ≈ 2.5–3.0) and higher metallicity (log (Z/Z⊙) ≈ −0.5) than the galaxy outskirts. We derive marginally negative radial metallicity gradients (∇log Z ≈ −0.03 ± 0.07 dex/kpc), and a dust temperature (Td ≈ 32 − 38 K) that anticorrelates with the gas depletion time.

Spectroscopic age estimates for APOGEE red-giant stars: Precise spatial and kinematic trends with age in the Galactic disc

Over the last few years, many studies have found an empirical relationship between the abundance of a star and its age. Here we estimate spectroscopic stellar ages for 178 825 red-giant stars observed by the APOGEE survey with a median statistical uncertainty of 17%. To this end, we use the supervised machine learning technique XGBoost, trained on a high-quality dataset of 3060 red-giant and red-clump stars with asteroseismic ages observed by both APOGEE and Kepler. After verifying the obtained age estimates with independent catalogues, we investigate some of the classical chemical, positional, and kinematic relationships of the stars as a function of their age. We find a very clear imprint of the outer-disc flare in the age maps and confirm the recently found split in the local age-metallicity relation. We present new and precise measurements of the Galactic radial metallicity gradient in small age bins between 0.5 and 12 Gyr, confirming a steeper metallicity gradient for ∼2 − 5 Gyr old populations and a subsequent flattening for older populations mostly produced by radial migration. In addition, we analyse the dispersion about the abundance gradient as a function of age. We find a clear power-law trend (with an exponent β ≈ 0.15) for this relation, indicating a relatively smooth radial migration history in the Galactic disc over the past 7 − 9 Gyr. Departures from this power law may possibly be related to the Gaia Enceladus merger and passages of the Sagittarius dSph galaxy. Finally, we confirm previous measurements showing a steepening in the age-velocity dispersion relation at around ∼9 Gyr, but now extending it over a large extent of the Galactic disc (5 kpc &lt; RGal &lt; 13 kpc). To establish whether this steepening is the imprint of a Galactic merger event, however, detailed forward modelling work of our data is necessary. Our catalogue of precise stellar ages and the source code to create it are publicly available.

On the Migration Origin of the Hercules Moving Group with GAIA, LAMOST, APOGEE, and GALAH Surveys

Using Gaia DR3 data and the wavelet transformation technique, we study the substructures of the Hercules moving group (HMG): Hercules 1 (H1) and Hercules 2 (H2). Spectroscopic survey data from LAMOST, APOGEE, and GALAH are used to obtain metallicities and ages of stars belonging to the HMG. Our analysis leads to several key findings as follows: (a) the HMG is on average richer in metallicity than the Galactic disk, with H2 being metal richer than H1; (b) the HMG likely has a radial metallicity gradient distinct from that of the disk; (c) the HMG is on average older than the disk, with H2 being older than H1; (d) the HMG likely has a radial age gradient distinct from that of the disk; and (e) the metallicity and age distributions of the HMG depend mainly on the Galactic radius but show no dependence on the azimuthal velocity. Taken all together, we conclude that the HMG is composed primarily of stars undergoing radial migration. We suggest that the HMG is associated with a higher-order dynamical resonance of the bar of the Galaxy.

A Dynamically Distinct Stellar Population in the Leading Arm of the Sagittarius Stream

We present a chemical and dynamical analysis of the leading arm (LA) and trailing arm (TA) of the Sagittarius (Sgr) stream, as well as for the Sgr dwarf galaxy core (SC), using red giant branch, main-sequence, and RR Lyrae stars from large spectroscopic survey data. The different chemical properties among the LA, TA, and SC generally agree with recent studies and can be understood by a radial metallicity gradient established in the progenitor of the Sgr dwarf, followed by preferential stellar stripping from the outer part of the Sgr progenitor. One striking finding is a relatively larger fraction of low-eccentricity stars (e < 0.4) in the LA than in the TA and SC. The TA and SC exhibit very similar distributions. Considering that a tidal tail stripped off from a dwarf galaxy maintains the orbital properties of its progenitor, we expect that the e-distribution of the LA should be similar to that of the TA and SC. Thus, the disparate behavior of the e-distribution of the LA is of particular interest. Following the analysis of Vasiliev et al., we attempt to explain the different e-distribution by introducing a time-dependent perturbation of the Milky Way by the Large Magellanic Cloud's (LMC) gravitational pull, resulting in substantial evolution of the angular momentum of the LA stars to produce the low-e stars. In addition, we confirm from RR Lyrae stars with high eccentricity (e > 0.6) that the TA stars farther away from the SC are also affected by disturbances from the LMC.

A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE

We present new maps of the Milky Way disk showing the distribution of metallicity ([Fe/H]), α-element abundances ([Mg/Fe]), and stellar age, using a sample of 66,496 red giant stars from the final data release (DR17) of the Apache Point Observatory Galactic Evolution Experiment survey. We measure radial and vertical gradients, quantify the distribution functions for age and metallicity, and explore chemical clock relations across the Milky Way for the low-α disk, high-α disk, and total population independently. The low-α disk exhibits a negative radial metallicity gradient of −0.06 ± 0.001 dex kpc−1, which flattens with distance from the midplane. The high-α disk shows a flat radial gradient in metallicity and age across nearly all locations of the disk. The age and metallicity distribution functions shift from negatively skewed in the inner Galaxy to positively skewed at large radius. Significant bimodality in the [Mg/Fe]–[Fe/H] plane and in the [Mg/Fe]–age relation persist across the entire disk. The age estimates have typical uncertainties of ∼0.15 in log(age) and may be subject to additional systematic errors, which impose limitations on conclusions drawn from this sample. Nevertheless, these results act as critical constraints on galactic evolution models, constraining which physical processes played a dominant role in the formation of the Milky Way disk. We discuss how radial migration predicts many of the observed trends near the solar neighborhood and in the outer disk, but an additional more dramatic evolution history, such as the multi-infall model or a merger event, is needed to explain the chemical and age bimodality elsewhere in the Galaxy.

Spatial metallicity variations of mono-temperature stellar populations revealed by early-type stars in LAMOST

We investigate the radial metallicity gradients and azimuthal metallicity distributions on the Galactocentric X–Y plane using mono-temperature stellar populations selected from the LAMOST-MRS young stellar sample. The estimated radial metallicity gradient ranges from −0.015 dex/kpc to −0.07 dex/kpc, which decreases as the effective temperature decreases (or when the stellar age increases) at 7500 &lt; Teff &lt; 12 500 K (τ &lt; 1.5 Gyr). The azimuthal metallicity excess (the metallicity after subtracting the radial metallicity gradient, Δ [M/H]) distributions exhibit inhomogeneities with dispersions of 0.04 dex to 0.07 dex, which decrease as the effective temperature decreases. We also identify five potential metal-poor substructures with large metallicity excess dispersions. The metallicity excess distributions of these five metal-poor substructures suggest that they contain a larger fraction of metal-poor stars compared to other control samples. These metal-poor substructures may be associated with high-velocity clouds that infall into the Galactic disk from the Galactic halo, which are not quickly well mixed with the pre-existing interstellar medium (ISM) of the Galactic disk. As a result, these high-velocity clouds produce some metal-poor stars and the observed metal-poor substructures. The variations of metallicity inhomogeneities with different stellar populations indicate that high-velocity clouds are not well mixed with the pre-existing Galactic disk ISM within 0.3 Gyr.

GaiaData Release 3

Context.The motion of stars has been used to reveal details of the complex history of the Milky Way, in constant interaction with its environment. Nevertheless, to reconstruct the Galactic history puzzle in its entirety, the chemo-physical characterisation of stars is essential. PreviousGaiadata releases were supported by a smaller, heterogeneous, and spatially biased mixture of chemical data from ground-based observations.Aims.GaiaData Release 3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the Radial Velocity Spectrometer (RVS) and parametrised by the GSP-Spec module. In this work, we aim to demonstrate the scientific quality ofGaia’s Milky Way chemical cartography through a chemo-dynamical analysis of disc and halo populations.Methods.Stellar atmospheric parameters and chemical abundances provided byGaiaDR3 spectroscopy are combined with DR3 radial velocities and EDR3 astrometry to analyse the relationships between chemistry and Milky Way structure, stellar kinematics, and orbital parameters.Results.The all-skyGaiachemical cartography allows a powerful and precise chemo-dynamical view of the Milky Way with unprecedented spatial coverage and statistical robustness. First, it reveals the strong vertical symmetry of the Galaxy and the flared structure of the disc. Second, the observed kinematic disturbances of the disc – seen as phase space correlations – and kinematic or orbital substructures are associated with chemical patterns that favour stars with enhanced metallicities and lower [α/Fe] abundance ratios compared to the median values in the radial distributions. This is detected both for young objects that trace the spiral arms and older populations. Severalα, iron-peak elements and at least one heavy element trace the thin and thick disc properties in the solar cylinder. Third, young disc stars show a recent chemical impoverishment in several elements. Fourth, the largest chemo-dynamical sample of open clusters analysed so far shows a steepening of the radial metallicity gradient with age, which is also observed in the young field population. Finally, theGaiachemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [α/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters.Conclusions.GaiaDR3 chemo-dynamical diagnostics open new horizons before the era of ground-based wide-field spectroscopic surveys. They unveil a complex Milky Way that is the outcome of an eventful evolution, shaping it to the present day.

Chemical cartography with LAMOST and Gaia reveal azimuthal and spiral structure in the Galactic disc

ABSTRACT Chemical Cartography, or mapping, of our Galaxy has the potential to fully transform our view of its structure and formation. In this work, we use chemical cartography to explore the metallicity distribution of OBAF-type disc stars from the LAMOST survey and a complementary sample of disc giant stars from Gaia DR3. We use these samples to constrain the radial and vertical metallicity gradients across the Galactic disc. We also explore whether there are detectable azimuthal variations in the metallicity distribution on top of the radial gradient. For the OBAF-type star sample from LAMOST, we find a radial metallicity gradient of Δ[Fe/H]/ΔR ∼−0.078 ± 0.001 dex kpc−1 in the plane of the disc and a vertical metallicity gradient of Δ[Fe/H]/ΔZ ∼−0.15 ± 0.01 dex kpc−1 in the solar neighbourhood. The radial gradient becomes shallower with increasing vertical height, while the vertical gradient becomes shallower with increasing Galactocentric radius, consistent with other studies. We also find detectable spatially dependent azimuthal variations on top of the radial metallicity gradient at the level of ∼0.10 dex. Interestingly, the azimuthal variations appear be close to the Galactic spiral arms in one data set (Gaia DR3) but not the other (LAMOST). These results suggest that there is azimuthal structure in the Galactic metallicity distribution and that in some cases it is co-located with spiral arms.

ISM metallicity variations across spiral arms in disk galaxies

Chemical abundance variations in the interstellar medium provide important information about the galactic evolution, star-formation, and enrichment histories. Recent observations of disk galaxies suggest that if large-scale azimuthal metallicity variations appear in the ISM, they are linked to the spiral arms. In this work, using a set of chemodynamical simulations of the Milky Way-like spiral galaxies, we quantify the impact of gas radial motions (migration) in the presence of a pre-existing radial metallicity gradient and the local ISM enrichment on both global and local variations of the mean ISM metallicity in the vicinity of the spiral arms. In all the models, we find the scatter of the gas metallicity of ≈0.04 − 0.06 dex at a given galactocentric distance. On large scales, we observe the presence of spiral-like metallicity patterns in the ISM which are more prominent in models with the radial metallicity gradient. However, in our simulations, the morphology of the large-scale ISM metallicity distributions significantly differs from the spiral arm structure in stellar and gas components resulting in both positive and negative residual (after subtraction of the radial gradient) metallicity trends along spiral arms. We discuss the correlations of the residual ISM metallicity values with the star formation rate, gas kinematics and offset to the spiral arms, concluding that the presence of a radial metallicity gradient is essential for the azimuthal variations of metallicity. At the same time, the local enrichment alone is unlikely to drive systematic variations of the metallicity across the spirals.