Abstract The outer Galaxy (Galactocentric distance ≳13.5 kpc) serves as an excellent laboratory for investigating the chemical complexity in low-metallicity environments. Here we present the chemical analyses for the outer Galactic hot core Sh 2-283-1a SMM1 ( D GC = 15.7 kpc and Z ∼ 0.3 Z ⊙ ), recently detected by T. Ikeda et al. using the Atacama Large Millimeter/submillimeter Array. Toward this source, a variety of molecular species, including complex organic molecules (COMs; CH 3 OH, 13 CH 3 OH, CH 2 DOH, and CH 3 OCH 3 ), are detected. The molecular abundances relative to CH 3 OH are similar to those of another outer Galactic hot core, demonstrating that chemically rich hot cores exist in different regions of the outer Galaxy. We also compared molecular abundances among hot cores in the inner Galaxy, outer Galaxy, and Magellanic Clouds. This comparison revealed that the metallicity-corrected N (SO 2 )/ N (H 2 ) ratios of outer Galactic hot cores are significantly lower than those of the inner Galactic ones, while their N (CH 3 OH)/ N (H 2 ) ratios are similar. The Magellanic hot cores show different trends despite having metallicities similar to those of the outer Galaxy, indicating that the chemical complexity of hot cores is governed by environmental conditions (e.g., cosmic-ray intensity and dust temperature) rather than simple metallicity scaling. These environmental differences would also affect the production efficiency of COMs derived from CH 3 OH, as the N (CH 3 OCH 3 )/ N (CH 3 OH) and N (C 2 H 5 OH)/ N (CH 3 OH) ratios in the outer Galactic sources are moderately lower than those of inner Galactic sources. The N (CH 2 DOH)/ N (CH 3 OH) ratio of Sh 2-283-1a SMM1 is 1.5 − 1.2 + 3.9 %, comparable to that of inner Galactic high-mass sources.

We envision a minimalist way to explain a number of astronomical facts associated with the unsolved missing mass problem by considering a new phenomenological paradigm. In this model, no new exotic particles need to be added, and the gravity is not modified; it is the perception that we have of a purely Newtonian (or purely Einsteinian) Universe, dubbed the Newton basis or Einstein basis (actually “viewed through a pinhole” which is “optically” distorted in some manner by a so-called magnifying effect). The κ model is not a theory but rather an exploratory technique that assumes that the sizes of the astronomical objects (galaxies and galaxy clusters or fluctuations in the CMB) are not commensurable with respect to our usual standard measurement. To address this problem, we propose a rescaling of the lengths when these are larger than some critical values, say >100 pc - 1 kpc for the galaxies and ∼1 Mpc for the galaxy clusters. At the scale of the solar system or of a binary star system, the κ effect is not suspected, and the undistorted Newtonian metric fully prevails. A key point of an ontological nature rising from the κ model is the distinction which is made between the distances depending on how they are obtained: (1) distances deduced from luminosity measurements (i.e. the real distances as potentially measured in the Newton basis, which are currently used in the standard cosmological model) and (2) even though it is not technically possible to deduce them, the distances which would be deduced by trigonometry. Those “trigonometric” distances are, in our model, altered by the kappa effect, except in the solar environment where they are obviously accurate. In outer galaxies, the determination of distances (by parallax measurement) cannot be carried out, and it is difficult to validate or falsify the kappa model with this method. On the other hand, it is not the same within the Milky Way, for which we have valuable trigonometric data (from the Gaia satellite). Interestingly, it turns out that for this particular object, there is strong tension between the results of different works regarding the rotation curve of the galaxy. At the present time, when the dark matter concept seems to be more and more illusive, it is important to explore new ideas, even the seemingly incredibly odd ones, with an open mind. The approach taken here is, however, different from that adopted in previous papers. The analysis is first carried out in a space called the Newton basis with pure Newtonian gravity (the gravity is not modified) and in the absence of dark matter-type exotic particles. Then, the results (velocity fields) are transported into the leaves of a bundle (observer space) using a universal transformation associated with the average mass density expressed in the Newton basis. This approach will make it much easier to deal with situations where matter is not distributed centrosymmetrically around a center of maximum density. As examples, we can cite the interaction of two galaxies or the case of the collision between two galaxy clusters in the bullet cluster. These few examples are difficult to treat directly in the bundle, especially since we would include time-based monitoring (with an evolving κ effect in the bundle). We will return to these questions later, as well as the concept of average mass density at a point. The relationship between this density and the coefficient κ must also be precisely defined.

We present the first detailed multi-tracer observation of a 5-pc long outer Galaxy filament, G183, and the massive young stellar object (YSO) IRAS,5480+2545 associated with it. Using the IRAM 30-m telescope at łambda = 1.4 and 3,mm, we probed the molecular gas distribution at angular resolutions of ∼ 12 -28 (0.1--0.3,pc at d = 2.1,kpc). The velocity-resolved observations conclusively show a main filament with a skeleton of ridges. The main filament is a 5,pc long velocity-coherent structure with a continuous and quiescent velocity field along its length up to the star-forming hub that accretes mass from the filament. The internal gas kinematics of most of the G183 filament is dominated by thermal motions (σ_̊m NT/c_s∼ 1) and large-scale velocity gradients arising due to outflows and accretion of matter in the massive YSO. The dispersion-size relation almost up to 1,pc is consistent with Larson's law, suggesting that the origin of the filament is a turbulence cascade. The massive YSO, S1, with no corresponding radio continuum detection is characterized as a high-mass protostellar object with a mass of 156, $, ), respectively. In comparison to the inner Galaxy high-mass star-forming filaments forming massive stars, G183 has a lower column density; however, the accretion and outflow rates in S1 are similar. The detection of hydrocarbons such as ̧hthrcn and and an M/L ratio of 0.04. We identify a kinematic signature of the accretion of material from the filament onto the YSO, S1. The rates of molecular gas accretion and entrainment in S1 are estimated to be 8.6 and 2.6 (in units of 10 -4 -1 indicates the presence of hot-core chemistry in S1. These results highlight the universality of physical processes involved in massive star formation across a range of Galactic environments.

We present a study of the CO line emission anisotropy across the Milky Way’s disk to examine the effect of stellar feedback and Galactic dynamics on the distribution of the dense interstellar medium. We used the Hessian matrix method to characterize the 12 CO(1–0) line emission distribution and identify the preferential orientation across line-of-sight velocity channels in the Dame et al. (2001, ApJ, 547, 792) composite Galactic plane survey, which covers the Galactic latitude range | b | < 5 ° . The structures sampled with this tracer are predominantly parallel to the Galactic plane toward the inner Galaxy, in clear contrast to the predominantly perpendicular orientation of the structures traced by neutral atomic hydrogen (HI) emission toward the same regions. The analysis of the Galactic plane portions sampled at higher angular resolution with other surveys reveals that the alignment with the Galactic plane is also prevalent at smaller scales. We find no preferential orientation in the CO emission toward the outer Galaxy, in contrast to the preferential alignment with the Galactic plane displayed by HI in that portion of the Milky Way. We interpret these results as the combined effect of the decrease in midplane pressure with increasing Galactocentric radius and SN feedback, which lifts diffuse gas more efficiently than dense gas off the Galactic plane.

We investigate the formation and evolutionary trajectory of the Milky Way's inner and outer galactic regions using stars from open clusters in the Gaia -ESO OC survey. Using numerical simulations from Chempy, we leveraged Markov chain Monte Carlo sampling techniques to derive galactic evolutionary parameters for each open cluster by fitting measured abundances of elements C, N, O, Mg, Si, Ca, Ti, Fe, Mn, Zn, Y, and Ba. We find differing evolutionary histories between the inner and outer regions of the Milky Way that align with variations in the slope of the initial mass function, the rate of Type Ia supernovae, and the galactic metallicity gradient traced by open clusters. Our results support established galactic formation and evolutionary theories, highlighting that the inner Galaxy had a short and intense early star formation epoch followed by reduced activity. In contrast, the outer Galaxy maintained a more sustained star formation history.

Abstract Molecular clouds (MCs) are cradles of star and planet formation, thereby playing an important role in the evolution of galaxies. Based on the unbiased Milky Way Imaging Scroll Painting survey data of 12CO, 13CO, and C18O (J = 1–0) line emission in two regions toward the inner and outer Galaxy, i.e., the G50 (44 . ° 75 ≤ l ≤ 60 . ° 25) and G120 (119 . ° 75 ≤ l ≤ 130 . ° 25) regions, the distribution of molecular gas is studied. Both regions have Galactic latitudes of ∣b∣≤ 5 . ° 25. A catalog containing 24,724 MCs is constructed from the data. In our proximity, several molecular structures with large angular scales and small velocity dispersions are discovered, resembling curtains of mist. Beyond the nearby molecular gas, a clear aggregation of MCs along coherent structures in the Galactic plane is visible, sketching spiral arm structures. Nevertheless, the aggregation of MCs is also detected in the inter-arm region between the Perseus and Outer arms in the G50 region. The Galactic molecular disk in this inter-arm region is found to be thinner than that in the adjacent spiral arm region. In addition, the thickness of the Galactic molecular disk examined here is found to be correlated with the warp of it, indicating their homologous origins. The molecular disk has a typical thickness of ∼220 pc in the inner Galaxy. Moreover, the dispersion of the MC systemic velocity decreases with increasing galactocentric radius, resulting in lower kinematic distance uncertainties at larger radii. However, the Perseus arm segment in the G120 region exhibits a relatively large cloud-to-cloud velocity dispersion and split components in its MC velocity distribution.

Abstract Star formation is a fundamental, yet poorly understood, process of the Universe. It is important to study how star formation occurs in different galactic environments. Thus, here, in the first of a series of papers, we introduce the Low-metallicity Star Formation (LZ-STAR) survey of the Sh2-284 (hereafter S284) region, which, at Z ∼ 0.3–0.5 Z ⊙ , is one of the lowest-metallicity star-forming regions of our Galaxy. LZ-STAR is a multifacility survey, including observations with JWST, the Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space Telescope, Chandra, and Gemini. As a starting point, we report JWST and ALMA observations of one of the most massive protostars in the region, S284p1. The observations of shock-excited molecular hydrogen reveal a symmetric, bipolar outflow originating from the protostar, spanning several parsecs, and fully covered by the JWST field of view and ALMA observations of CO(2–1) emission. These allow us to infer that the protostar has maintained a relatively stable orientation of disk accretion over its formation history. The JWST near-infrared continuum observations detect a centrally illuminated bipolar outflow cavity around the protostar, as well as a surrounding cluster of low-mass young stars. We develop new radiative transfer models of massive protostars designed for the low metallicity of S284. Fitting these models to the protostar’s spectral energy distribution implies a current protostellar mass of ∼10 M ⊙ has formed from an initial ∼100 M ⊙ core over the last ∼3 × 10 5 yr. Overall, these results indicate that massive stars can form in an ordered manner in low-metallicity, protocluster environments.

Identification of Outer Galaxy Cluster Members Using Gaia DR3 and Multidimensional Simulation

Context. Gaia data have revealed vertically asymmetric phase-space structures in the Milky Way (MW) disc, such as phase spirals, indicating vertical oscillations. These oscillations exhibit two distinct modes, the bending mode and the breathing mode, associated with one-arm and two-arm phase spirals, respectively. The mechanisms driving these modes remain debated, with both external and internal origins proposed. Aims. With this study, we aim to explore the excitation mechanisms of the bending and breathing modes and their subsequent evolution in the MW disc, focusing on the interplay between direct perturbations from the Sagittarius dwarf galaxy and indirect contributions from tidally induced spiral arms. Methods. We performed high-resolution N-body simulations with five billion particles to model the interaction between an MW-like disc galaxy and a Sagittarius dwarf-like satellite. These simulations resolve fine phase-space structures, enabling analysis of the bending and breathing modes at both macroscopic (global bending and breathing waves) and microscopic (local phase spirals) scales. Results. Our simulations demonstrate that the satellite’s perturbation directly excites the bending mode and induces spiral arms in the Galactic disc. These spiral arms, in turn, excite the breathing mode, making it an indirect consequence of the satellite interaction. Initially, the bending mode dominates, but it rapidly decays due to horizontal mixing. In contrast, the breathing mode persists for a longer duration, sustained by the spiral arms, leading to a transition from a bending-dominated to a breathing-dominated state. This transition progresses faster in the inner galaxy than in the outer galaxy. The simulations successfully reproduce the one-arm phase spiral observed in the solar neighbourhood and reveal two-arm phase spirals, particularly in the inner galaxy, associated with spiral arm-induced breathing modes. The two-arm phase spirals emerge approximately 200–250 Myr after the bending-to-breathing transition. Conclusions. Our findings highlight the combined effects of direct satellite perturbations and indirect spiral arm dynamics in shaping the vertical structure of the MW disc. The emergence of the two-arm phase spiral after the bending-to-breathing transition suggests that the MW disc experienced a significant perturbation more than ∼ 400 Myr ago, likely caused by the Sagittarius dwarf galaxy. This study underscores the importance of considering the dynamic interplay between direct and indirect mechanisms in understanding the vertical dynamics of the MW disc.

Abstract We present the first detection of spatially resolved protostellar outflows and jets in the outer Galaxy. We observed five star-forming regions in the outer Galaxy (Sh 2-283 and NOMF05-16/19/23/63; galactocentric distance = 15.7–17.4 kpc) with the Atacama Large Millimeter/submillimeter Array. Toward Sh 2-283, we have detected distinct outflow (∼5–50 km s−1) and jet components (∼50–100 km s−1) associated with the protostar in CO(3–2) emission. The outflows and jets are well collimated, with the jets exhibiting multiple bullet structures. The position–velocity diagram along the CO flow axis shows two characteristic structures: (a) the flow velocity, which linearly increases with the position offset from the core center (the Hubble-like flow); and (b) the continuous velocity components of the periodical flows (spine-like structures), which may indicate episodic mass ejection events. The time intervals of the mass ejection events are estimated to be 900–4000 yr, based on the slopes of these spine-like structures. These characteristics align with those of nearby protostellar systems, indicating that early star formation in low-metallicity environments, such as the outer Galaxy, resembles that in the inner Galaxy. In contrast to the physical similarities, the N(SiO)/N(CO) ratio in the jet bullet appears to be lower than that measured in the low-mass protostellar sources in the inner Galaxy. This may indicate a different shock chemistry or different dust composition in the outer Galaxy source, although non–local thermodynamic equilibrium effects could also affect the observed low N(SiO)/N(CO) ratio. We also report the new detection of four other outflow sources in the outer Galaxy.

Context. Although current observations indicate that there are two distinct sequences of disk stars in the [α/M] versus [M/H] parameter space, further complexity is evident in the chemical makeup of the Milky Way and consequently suggests a complicated evolutionary history. Aims. We developed two-infall galactic chemical evolution (GCE) models consistent with the Galactic chemical map. Methods. We obtained new GCE models simulating the chemical evolution of the Milky Way, as constrained by a golden sample of 394 000 stellar abundances of the Milky Way Mapper survey from data release 19 of SDSS-V. The separation between the chemical thin and thick disks was defined using [Mg/M]. We used the chemical evolution environment OMEGA+ combined with Levenberg-Marquardt (LM) and bootstrapping algorithms for the optimization and error estimation. We simulated the entire Galactic disk and considered six galactocentric regions, allowing for a more detailed analysis of the formation of the inner, middle, and outer Galaxy. We investigated the evolution of α, odd-Z, and iron-peak elements, covering 15 species altogether. Results. The chemical thin and thick disks are separated by Mg observations, which the other α-elements show similar trends with, while odd-Z species demonstrate different patterns as functions of metallicity. In the inner Galactic disk regions, the locus of the low-Mg sequence is gradually shifted toward higher metallicity, while the high-Mg phase is less populated. The best-fit GCE models show a well-defined peak in the rate of the infalling matter as a function of the Galactic age, confirming a merger event about 10 Gyr ago. We show that the timescale of gas accretion, the exact time of the second infall and the ratio between the surface mass densities associated with the second infall event and the formation event vary with the distance from the Galactic center. According to the models, the disk was assembled within a timescale of (0.32±0.02) Gyr during a primary formation phase, followed by an increasing accretion rate over a (0.55±0.06) Gyr-timescale and a relaxation phase that lasted (2.86±0.70) Gyr, with a second peak seen for the infall rate at (4.13±0.19) Gyr. Conclusions. Our best Galaxy evolution models are consistent with an inside-out formation scenario of the Milky Way disk and in agreement with the findings of recent chemodynamical simulations.

The intrinsic alignment of galaxies is a major astrophysical contaminant to weak gravitational lensing measurements, and the study of its dependence on galaxy properties helps provide meaningful physical priors that aid cosmological analyses. This work studied for the first time the dependence of intrinsic alignments on galaxy structural parameters. We measured the intrinsic alignments of bright galaxies, selected on apparent r-band magnitude r<20, in the Kilo-Degree Survey (KiDS). Machine-learning-based photometric redshift estimates are available for this galaxy sample that helped us obtain a clean measurement of its intrinsic alignment signal. We supplemented this sample with a catalogue of structural parameters from Sérsic profile fits to the surface-brightness profiles of the galaxies. We split the sample on galaxy intrinsic colour, luminosity, and Sérsic index, and we fitted the non-linear linear alignment model to galaxy position–shape projected correlation function measurements on large scales. We observe a power-law luminosity dependence of the large-scale intrinsic alignment amplitude, AIA, for both the red and high-Sérsic-index (ns>2.5) samples, and find no significant difference between the two. We measure an ∼1.5σ lower AIA for red galaxies that also have a Sérsic index of ns<4 compared to the expected amplitude predicted using the sample's luminosity. We also probe the intrinsic alignment of red galaxies as a function of galaxy scale by varying the radial weight employed in the shape measurement. On large scales (above 6 Mpc/h), we do not detect a significant difference in the alignment. On smaller scales, we observe that alignments increase with galaxy scale, with outer galaxy regions showing stronger alignments than inner regions. Finally, for intrinsically blue galaxies, we find AIA=−0.67±1.00, which is consistent with previous works, and we find alignments to be consistent with zero for the low-Sérsic-index (ns<2.5) sample.

Open clusters have long been used as tracers of Galactic structure. However, without a selection function to describe the completeness of the cluster census, it is difficult to quantitatively interpret their distribution. We create a method to empirically determine the selection function of a Galactic cluster catalogue. We test it by investigating the completeness of the cluster census in the outer Milky Way, where old and young clusters exhibit different spatial distributions. We develop a method to generate realistic mock clusters as a function of their parameters, in addition to accounting for Gaia 's selection function and astrometric errors. We then inject mock clusters into Gaia DR3 data, and attempt to recover them in a blind search using HDBSCAN. We find that the main parameters influencing cluster detectability are mass, extinction, and distance. Age also plays an important role, making older clusters harder to detect due to their fainter luminosity function. High proper motions also improve detectability. After correcting for these selection effects, we find that old clusters are $2.97±0.11$ times more common at a Galactocentric radius of 13 kpc than in the solar neighbourhood -- despite positive detection biases in their favour, such as hotter orbits or a higher scale height. The larger fraction of older clusters in the outer Galaxy cannot be explained by an observational bias, and must be a physical property of the Milky Way: young outer-disc clusters are not forming in the outer Galaxy, or at least not with sufficient masses to be identified as clusters in Gaia DR3. We predict that in this region, more old clusters than young ones remain to be discovered. The current presence of old, massive outer-disc clusters could be explained by radial heating and migration, or alternatively by a lower cluster destruction rate in the anticentre.

Abstract Observations of the J Ka, Kc = 41,4 → 30,3, 40,4 → 31,3, and 22,0 → 11,1 rotational lines of c-C3H2 and the J = 3/2 → 1/2, Ω = 1/2 transition of NO (X2Πr) were conducted at 2 mm toward 20 Galactic edge clouds with R GC = 10.8–23.5 kpc using the Arizona Radio Observatory 12 m telescope. The c-C3H2 molecule was detected in all 20 objects in the sample, based typically on 2–3 transitions. NO, which exhibits a distinct pattern of lambda doubling and hyperfine splitting in the measured line, was identified in 16 clouds, with distances as far as R GC ∼ 23.5 kpc. While a 3 mm transition of c-C3H2 had been observed before in some of the sample clouds, NO had not previously been detected at such large distances from the Galactic center. These new identifications double the number of molecular clouds known to contain NO. From a radiative transfer analysis, fractional abundances, relative to H2, were determined to be f(c-C3H2) ∼ 0.5–39.8 × 10−10 and f(NO) ∼ 0.2–21.2 × 10−8. These abundances are comparable to values observed in molecular clouds in the inner Galaxy (R GC < 12 kpc). The abundances therefore appear to remain relatively constant with increasing galactocentric distance. These results suggest that elemental abundance gradients at R GC ≥ 15 kpc in C, N, and O are not as severe as predicted. They also indicate that gas-phase chemistry in the outer Galaxy is quite robust, strengthening the case for widening the extent of the Galactic habitable zone.

ABSTRACT The Outer Galaxy High-Resolution Survey (OGHReS) covers 100 square degrees ($180^{\circ }< \ell < 280^{\circ }$) in the (2–1) transitions of three CO-isotopologues. We use the spectra to refine the velocities and physical properties to 6706 Hi-GAL clumps located in the OGHReS region. In a previous paper, we analysed 3584 clumps between $\ell = 250^{\circ }$ and $280^{\circ }$. Here, we cover a further 3122 clumps ($180^{\circ }< \ell < 250^{\circ }$) and determine reliable velocities for 2781 of these, finding good agreement with the previously assigned velocities ($\sim$80 per cent within 5 km s$^{-1}$). We update velocities for 288 clumps and provide new values for an additional 411. Combining these with the previous results, we have velocities and physical properties for 6193 clumps (92.3 per cent). The 422 non-detections are low surface density clumps or likely contamination by evolved stars and galaxies. Key findings: (i) improved correlation between clumps and spiral arm loci and the discovery of clumps beyond the outer arm supports the existence of a new spiral structure; (ii) decreasing trend in the $L/M$-ratio consistent with less high-mass star formation in the outer Galaxy; (iii) increase in the star formation fraction in the outer Galaxy, suggesting that more clumps are forming stars despite their lower mass; (iv) discrepancies in velocity assignments across different surveys that could affect $\sim$10000 clumps, especially in the fourth quadrant.

Radial metallicity gradients are fundamental to understanding galaxy formation and evolution. In our high-resolution simulation of a NIHAO-UHD Milky Way analogue, we analyze the linearity, scatter, spatial coherence, and age-related variations of metallicity gradients using young stars and gas. While a global linear model generally captures the gradient, it ever so slightly overestimates metallicity in the inner galaxy and underestimates it in the outer regions of our simulated galaxy. Both a quadratic model, showing an initially steeper gradient that smoothly flattens outward, and a piecewise linear model with a break radius around 9.3-11.5~kpc (2.4-3.0 effective radii) fit the data equally better. The spread of [Fe/H] of young stars in the simulation increases by tenfold from the innermost to the outer galaxy at a radius of 20 kpc. We find that stars born at similar times along radial spirals drive this spread in the outer galaxy, with a chemical under- and over-enhancement of up to 0.1 dex at leading and trailing regions of such spirals, respectively. This localised chemical variance highlights the need to examine radial and azimuthal selection effects for both Galactic and extragalactic observational studies. The arguably idealised but volume-complete simulations suggest that future studies should not only test linear and piecewise linear gradients, but also non-linear functions such as quadratic ones to test for a smooth gradient rather than one with a break radius. Either finding would help to determine the importance of different enrichment or mixing pathways and thus our understanding of galaxy formation and evolution scenarios.

Abstract HCN and HCO+ are the most common dense gas tracers used both in the Milky Way and external galaxies. The luminosity of HCN and HCO+ J = 1 → 0 lines are converted to a dense gas mass by the conversion factor, α Q . Traditionally, this α Q has been considered constant throughout the Galaxy and in other galaxies, regardless of the environment. We analyzed 17 outer Galaxy clouds and five inner Galaxy clouds with metallicities ranging from 0.38 Z ⊙ to 1.29 Z ⊙. Our analysis indicates that α Q is not constant; instead, it varies with metallicity. The metallicity-corrected α Q derived from the HCN luminosity of the entire cloud is almost three times higher in the outer Galaxy than in the inner Galaxy. In contrast, HCO+ seems less sensitive to metallicity. We recommend using the metallicity-corrected dense gas conversion factors α tot, gas ′ ( HCN ) = 19 . 5 − 4.4 + 5.6 Z ( − 1.53 ± 0.59 ) and α tot, gas ′ ( HCO + ) = 21 . 4 − 4.4 + 5.5 Z ( − 1.32 ± 0.55 ) for extragalactic studies. Radiation from nearby stars has an effect on the conversion factor of a similar magnitude as that of the metallicity. If we extend the metallicity-corrected scaling relation for HCN to the Central Molecular Zone (CMZ), the value of α ( HCN ) becomes one-third to one-half of the local values. This effect could partially account for the low star formation rate per dense gas mass observed in the CMZ.

Abstract Stellar streams are one of the most promising tracers of low-mass dark-matter subhalos. Existing frameworks for modeling stream perturbations rely on restrictive assumptions for the Milky Way potential (e.g., static, axisymmetric) or are computationally inefficient in generating many realizations of subhalo impacts. We present StreamSculptor, a GPU accelerated code that combines differentiable simulations and Hamiltonian perturbation theory to model the leading-order effect of dark-matter subhalos on stellar streams. Our model works in two stages: First, a base stream is generated in a Milky Way potential, including the effects of nonlinear time-dependent sources like the rotating Galactic bar and a massive satellite galaxy. Then, linear perturbation theory is applied to the base stream, allowing us to rapidly superimpose the effects of different subhalo impacts without having to carry out additional simulations. Subhalo masses and scale-radii can be rescaled as a postprocessing step. We demonstrate how this framework can be used to model subhalo impacts on stellar streams under realistic Milky Way conditions, specifically for an inner Galaxy stream like Palomar 5 and an outer Galaxy stream like Orphan–Chenab. We find that simultaneously modeling subhalo impacts and other time-dependent components of the Galactic gravitational potential is crucial for an unbiased inference of dark-matter substructure.

Context. Models predict that atomic carbon occurs at the surface and in the process of the formation of molecular clouds. This makes its fine-structure transitions a diagnostic of cloud formation. Aims. We study the distribution of atomic carbon in a small inconspicuous region toward the outer Galaxy that might be representative for a large fraction of the molecular gas of the Milky Way that is not directly affected by star formation. Methods. We observed a small strip of 5 arcminutes in the so-called Forgotten Quadrant, the third quadrant of the Milky Way, with the APEX telescope in the 3P1−3P0 [C I] transition of atomic carbon and the J = 2−1 transition of the three most abundant CO isotopologs. We compared their distribution with existing measurements of the gas column density and of ionized carbon. Results. The atomic carbon shows a very smooth distribution with the smallest gradient along the strip compared to the other lines. It is always brighter than 13CO, and in one velocity component, it is even brighter than CO. In contrast to observations of many star-forming regions, the [C I] emission seems to extend beyond the molecular gas. This is in line with models of photon-dominated regions (PDRs). However, a standard PDR model fit to the observations fails because the models either predict more molecular gas than observed, traced through C18O, or more diffuse gas than observed, traced through [C II]. The carbon budget in the gas phase does not add up to the same column seen through dust emission. Conclusions. To understand the [C I] emission from galaxies, it is necessary to obtain the full statistics for the quiescent gas outside of the star-forming regions that behaves significantly different from dense gas that is exposed to high-ultraviolet fields.

Abstract Based on a large and homogeneous sample of 299 open clusters (OCs) from Gaia DR3 and large sky area multiobject fiber spectroscopic telescope DR11, we studied the abundance gradients of the α-elements Mg and Si, and the iron-peak elements Fe and Ni to explore the chemical evolution of the Galactic disk. A noticeable downward trend in metallicities, with a slope of −0.048 ± 0.008 dex kpc−1, is observed as the Galactocentric distance (R GC) increases. The abundance gradients of both α-elements and iron-peak elements in our sample exhibit no apparent symmetry between the regions above and below the Galactic plane. The metallicities in the sample, scaled to the Galactocentric distance of the Sun, show an age-related increase of 0.017 ± 0.016 dex Gyr−1. [El/H] exhibits distinct downward trends toward the outer Galaxy. In the inner Galaxy, younger OCs have lower [El/H] values than older clusters, whereas in the outer Galaxy, this trend is reversed. For OCs aged between 0.5 and 2.5 Gyr, the number of clusters migrating inward is approximately equal to those migrating outward. However, the outward migration distances are notably greater than the inward ones. Analyzing the impact of kinematic orbits on the radial and vertical abundance gradients of the four elements, we found that kinematic orbits exert minimal influence on the radial abundance gradients for both α-elements and iron-peak elements. In contrast, they have a pronounced effect on the vertical abundance gradients.