Galactic Center Research Articles

Testing kinematic distances under a realistic Galactic potential. Investigating systematic errors in the kinematic distance method arising from a non-axisymmetric potential

Obtaining reliable distance estimates to gas clouds within the Milky Way is challenging in the absence of certain tracers. The kinematic distance approach has been used as an alternative, and it is derived from the assumption of circular trajectories around the Galactic centre. Consequently, significant errors are expected in regions where gas flow deviates from purely circular motions. We aim to quantify the systematic errors that arise from the kinematic distance method in the presence of a Galactic potential that is non-axisymmetric. We investigated how these errors differ in certain regions of the Galaxy and how they relate to the underlying dynamics. We performed 2D isothermal hydrodynamical simulation of the gas disk with the moving-mesh code Arepo adding the capability of using an external potential provided by the Agama library for galactic dynamics. We introduced a new analytic potential of the Milky Way, taking elements from existing models and adjusting parameters to match recent observational constraints. In line with results of previous studies, we report significant errors in the kinematic distance estimate for gas close to the Sun along sight lines towards the Galactic centre and anti-centre and associated with the Galactic bar. Kinematic distance errors are low within the spiral arms, as gas resides close to local potential minima and the resulting line-of-sight velocity is similar to what is expected for an axisymmetric potential. Interarm regions exhibit large deviations at any given Galactic radius, and this is caused by the gas being sped up or slowed down as it travels into or out of spiral arms. In addition, we identify ‘zones of avoidance’ in the $lv$-diagram, where the kinematic distance method is particularly unreliable and should only be used with caution and we find a power-law relation between the kinematic distance error and the deviation of the projected line-of-sight velocity from circular motion.

Sources and Radiations of the Fermi Bubbles

Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local ambient medium of the galactic halo. A decade or so ago, a phenomenological model of the FBs was suggested as a result of routine star disruptions by the supermassive black hole in the GC which might provide enough energy for large-scale structures, like the FBs. In 2020, analytical and numerical models of the FBs as a process of routine tidal disruption of stars near the GC were developed; these disruption events can provide enough cumulative energy to form and maintain large-scale structures like the FBs. The disruption events are expected to be 10−4∼10−5yr−1, providing an average power of energy release from the GC into the halo of E˙∼3×1041 erg s−1, which is needed to support the FBs. Analysis of the evolution of superbubbles in exponentially stratified disks concluded that the FB envelope would be destroyed by the Rayleigh–Taylor (RT) instabilities at late stages. The shell is composed of swept-up gas of the bubble, whose thickness is much thinner in comparison to the size of the envelope. We assume that hydrodynamic turbulence is excited in the FB envelope by the RT instability. In this case, the universal energy spectrum of turbulence may be developed in the inertial range of wavenumbers of fluctuations (the Kolmogorov–Obukhov spectrum). From our model we suppose the power of the FBs is transformed partly into the energy of hydrodynamic turbulence in the envelope. If so, hydrodynamic turbulence may generate MHD fluctuations, which accelerate cosmic rays there and generate gamma-ray and radio emission from the FBs. We hope that this model may interpret the observed nonthermal emission from the bubbles.

Dynamics of star associations in an SMBH-IMBH system: The case of IRS13 in the Galactic centre

The existence of intermediate-mass black holes (IMBHs) still poses challenges to theoretical and observational astronomers. Several candidates have been proposed, including the one in the IRS13 cluster in the Galactic centre, where the evidence is based on the velocity dispersion of its members; however, none have been confirmed to date. We aim to gain insights into the presence of an IMBH in the Galactic centre through a numerical study of the dynamical interplay between an IMBH and star clusters (SCs) in the vicinity of a supermassive black hole (SMBH). We used high-precision $N$-body models of IRS13-like SCs in the Galactic centre, and of more massive SCs that fall into the centre of the Galaxy from larger distances. We find that at IRS13's physical distance of $0.4\ an IRS13-sized SC cannot remain gravitationally bound even if it contains an IMBH of thousands of Msun . Thus, IRS13 appears to be an incidental present-day clumping of stars. Furthermore, we show that the velocity dispersion of tidally disrupted SCs (the likely origin of IRS13) can be fully accounted for by the tidal forces of the central SMBH; the IMBH's influence is not essential.

Prograde and retrograde stars in nuclear cluster merger. Evolution of the supermassive black hole binary and the host galactic nucleus

Nuclear star cluster (NSC) mergers, involving the fusion of dense stellar clusters near the centres of galaxies, play a pivotal role in shaping galactic structures. The distribution of stellar orbits has significant effects on the formation and characteristics of extreme mass ratio inspirals (EMRIs). In this study, we address the orbital distribution of stars in merging NSCs and the subsequent effects on supermassive black hole binary (SMBHB) evolution. We ran dedicated direct-summation $N$-body simulations with different initial conditions to do a detailed study of the resulting NSC after their progenitors had merged. Our findings reveal that prograde stars form a flattened structure, while retrograde stars have a more spherical distribution. The axial ratios of the prograde component vary based on the presence and mass ratio of the SMBHs. The fraction of prograde and retrograde stars depends on the merger orbital properties and the SMBH mass ratio. The interactions of retrograde stars with the SMBHB affect the eccentricity and separation evolution of the binary. Our analysis reveals a strong correlation between the angular momentum and eccentricity of the SMBH binary. This relationship could serve as a means to infer information about the stellar dynamics surrounding the binary. We find that prograde orbits are particularly close to the binary of SMBHs, a promising fact regarding EMRI production. Moreover, prograde and retrograde stars have different kinematic structures, with the prograde stars typically rotating faster than the retrograde ones. The line-of-sight velocity and velocity dispersion, as well as the velocity anisotropy of each NSC, depend on the initial merger orbital properties and SMBH mass ratios. The prograde and retrograde stars always show different behaviours. The distribution of stellar orbits and the dynamical properties of each kinematic population can potentially be used as a way to tell the properties of the parent nuclei apart, and has an important impact on expected rates of EMRIs, which will be detected by future gravitational wave observatories such as the Laser Interferometer Space Antenna (LISA).

North Polar Spur: Gaseous plume(s) from star-forming regions ~3-5 kpc from the Galactic Center?

We argue that the North Polar Spur (NPS) and many less prominent structures are formed by gaseous metal-rich plumes associated with star-forming regions (SFRs). The SFRs located at the tangent to the 3-5 kpc rings might be particularly relevant to the NPS. A multi-temperature mixture of gaseous components and cosmic rays rises above the Galactic disk under the action of their initial momentum and buoyancy. Eventually, the plume velocity becomes equal to that of the ambient gas, which rotates with different angular speeds than the stars in the disk. As a result, the plumes acquire characteristic bent shapes. An ad hoc model of plumes' trajectories shows an interesting resemblance to the morphology of structures seen in the radio continuum and X-rays.

The Extremely Metal-rich Knot of Stars at the Heart of the Galaxy

We show with Gaia XP spectroscopy that extremely metal-rich (EMR) stars in the Milky Way ([M/H]XP ≳ 0.5) are largely confined to a tight “knot” at the center of the Galaxy. This EMR knot is round in projection, has a fairly abrupt edge near R GC,proj ∼ 1.5 kpc, and is a dynamically hot system. This central knot also contains very metal-rich (VMR; +0.2 ≤ [M/H]XP ≤ +0.4) stars. However, in contrast to EMR stars, the bulk of VMR stars forms an extended, highly flattened distribution in the inner Galaxy (R GC ≲ 5 kpc). We draw on TNG50 simulations of Milky Way analogs for context and find that compact, metal-rich knots confined to ≲1.5 kpc are a universal feature. In typical simulated analogs, the top 5%–10% most metal-rich stars are confined to a central knot; however, in our Milky Way data this fraction is only 0.1%. Dust-penetrating wide-area near-infrared spectroscopy, such as the fifth Sloan Digital Sky Survey, will be needed for a rigorous estimate of the fraction of stars in the Galactic EMR knot. Why in our Milky Way only EMR giants are confined to such a central knot remains to be explained. Remarkably, the central few kiloparsecs of the Milky Way harbor both the highest concentration of metal-poor stars (the “poor old heart”) and almost all EMR stars. This highlights the stellar population diversity at the bottom of galactic potential wells.

Forecasting the Population of Globular Cluster Streams in Milky Way–type Galaxies

Abstract Thin stellar streams originating from globular clusters (GCs) are among the most sensitive tracers of low-mass dark matter subhalos. Joint analysis of the entire population of stellar streams will place the most robust constraints on the dark matter subhalo mass function, and therefore the nature of dark matter. Here we use a hierarchical model of GC formation to forecast the total number, masses, and radial distribution of dissolved GC in Milky Way–like galaxies. Furthermore, we generate mock stellar streams from these progenitors’ orbital histories taking into account the clusters’ formation and accretion times, mass, and metallicity. Out of ∼10,000 clusters more massive than 104 M ⊙, ∼9000 dissolved in the central bulge and are fully phase mixed at the present, while the remaining ∼1000 survive as coherent stellar streams. This suggests that the current census of ∼80 GC streams in the Milky Way is severely incomplete. Beyond 15 kpc from the Galactic center we are missing ∼100 streams, of which the vast majority are from accreted GCs. Deep Rubin photometry (g ≲ 27.5) would be able to detect these streams, even the most distant ones beyond >75 kpc. We also find that M31 will have an abundance of streams at galactocentric radii of 30–100 kpc. We conclude that future surveys will find a multitude of stellar streams from GCs, which can be used for dark matter subhalo searches.

Resonant Friction on Disks in Galactic Nuclei

We argue that resonant friction has a dramatic effect on a disk whose rotation direction is misaligned with that of its host nuclear star cluster. The disk’s gravity causes gravitational perturbation of the cluster that in turn exerts a strong torque back onto the disk. We argue that this torque may be responsible for the observed disruption of the clockwise disk of young stars in the Galactic center, and show in numerical experiments that it produces the observed features in the distribution of the stars’ angular momenta. More generally, we speculate that the rotation of nuclear star clusters has a stabilizing effect on the orientation of transient massive accretion disks around the supermassive black holes residing in their centers, and thus on the directions and magnitudes of the black hole spins.

Estimating Dust Attenuation from Galactic Spectra. III. Radial Variations of Dust Attenuation Scaling Relations in MaNGA Galaxies

We investigate the radial dependence of the scaling relations of dust attenuation in nearby galaxies using integral field spectroscopy data from MaNGA. We identify ionized gas regions of kiloparsec size from MaNGA galaxies, and for each region we estimate both the stellar attenuation E(B − V)star and gas attenuation E(B − V)gas. We then quantify the correlations of 15 regional/global properties with E(B − V)gas and E(B − V)star, using both the feature importance obtained with the Random Forest regression technique and the Spearman correlation coefficients. The importance of stellar mass, metallicity, and nebular velocity dispersion found previously from studies based on the Sloan Digital Sky Survey can be reproduced if our analysis is limited to the central region of galaxies. The scaling relations of both E(B − V)gas and E(B − V)star are found to vary strongly as one goes from the galactic center to outer regions, and from Hα-bright regions to Hα-faint regions. For E(B − V)gas, [N ii]/[S ii] is top-ranked with a much higher correlation coefficient than any other property at 0 < R ≲ R e , while [O iii]/[O ii] outperforms [N ii]/[S ii] as the leading property in the outermost region. For E(B − V)star, stellar age shows the strongest correlation with no/weak dependence on radial distance, although ΣHα and specific star formation rate present similarly strong correlations with E(B − V)star in the galactic center. We find Hα-bright regions to generally show stronger correlations with E(B − V)gas, while Hα-faint regions are more strongly correlated with E(B − V)star, although this depends on individual properties and radial distance. The implications of our results for studies of high-z galaxies are discussed.

Experimental and Computational Study of Ethanolamine Ices under Astrochemical Conditions

Ethanolamine (NH2CH2CH2OH) has recently been identified in the molecular cloud G+0.693-0.027, situated in the SgrB2 complex in the Galactic center. However, its presence in other regions, and in particular in star-forming sites, is still elusive. Given its likely role as a precursor to simple amino acids, understanding its presence in the star-forming region is required. Here, we present the experimentally obtained temperature-dependent spectral features and morphological behavior of pure ethanolamine ices under astrochemical conditions in the 2–12 μm (MIR) and 120–230 nm (VUV) regions for the first time. These features would help in understanding its photochemical behavior. In addition, we present the first chemical models specifically dedicated to ethanolamine. These models include all the discussed chemical routes from the literature, along with the estimated binding energies and activation energies from quantum chemical calculations reported in this work. We have found that surface reactions CH2OH + NH2CH2 → NH2CH2CH2OH and NH2 + C2H4OH → NH2CH2CH2OH in warmer regions (60–90 K) could play a significant role in the formation of ethanolamine. Our modeled abundance of ethanolamine complements the upper limit of ethanolamine column density estimated in earlier observations in hot core/corino regions. Furthermore, we provide a theoretical estimation of the rotational and distortional constants for various species (such as HNCCO, NH2CHCO, and NH2CH2CO) related to ethanolamine that have not been studied in existing literature. This study could be valuable for identifying these species in the future.

Astrometric Jitter as a Detection Diagnostic for Recoiling and Slingshot Supermassive Black Hole Candidates

Supermassive black holes (SMBHs) can be ejected from their galactic centers due to gravitational wave recoil or the slingshot mechanism following a galaxy merger. If an ejected SMBH retains its inner accretion disk, it may be visible as an off-nuclear active galactic nucleus (AGN). At present, only a handful of offset AGNs that are recoil or slingshot candidates have been found, and none have been robustly confirmed. Compiling a large sample of runaway SMBHs would enable us to constrain the mass and spin evolution of binary SMBHs and study feedback effects of displaced AGNs. We adapt the method of varstrometry—which was developed for Gaia observations to identify off-center, dual, and lensed AGNs—in order to quickly identify off-nuclear AGNs in optical survey data by looking for an excess of blue versus red astrometric jitter. We apply this to the Pan-STARRS1 3π Survey and report on five new runaway AGN candidates. We focus on ZTF18aajyzfv: a luminous quasar offset by 6.7 ± 0.2 kpc from an adjacent galaxy at z = 0.224, and conclude after Keck LRIS spectroscopy and comparison to ASTRID simulation analogs that it is likely a dual AGN. This selection method can be easily adapted to work with data from the soon-to-be commissioned Vera C. Rubin Telescope Legacy Survey of Space and Time (LSST). LSST will have a higher cadence and deeper magnitude limit than Pan-STARRS1, and should permit detection of many more runaway SMBH candidates.

Disruption of a massive molecular cloud by a supernova in the Galactic Centre

The Milky Way’s Central Molecular Zone (CMZ) differs dramatically from our local solar neighbourhood, both in the extreme interstellar medium conditions it exhibits (e.g. high gas, stellar, and feedback density) and in the strong dynamics at play (e.g. due to shear and gas influx along the bar). Consequently, it is likely that there are large-scale physical structures within the CMZ that cannot form elsewhere in the Milky Way. In this paper, we present new results from the Atacama Large Millimeter/submillimeter Array (ALMA) large programme ACES (ALMA CMZ Exploration Survey) and conduct a multi-wavelength and kinematic analysis to determine the origin of the M0.8–0.2 ring, a molecular cloud with a distinct ring-like morphology. We estimate the projected inner and outer radii of the M0.8–0.2 ring to be 79″ and 154″, respectively (3.1 pc and 6.1 pc at an assumed Galactic Centre distance of 8.2 kpc) and calculate a mean gas density &gt;104 cm−3, a mass of ~106 M⊙, and an expansion speed of ~20 km s−1, resulting in a high estimated kinetic energy (&gt;1051 erg) and momentum (&gt;107 M⊙ km s−1). We discuss several possible causes for the existence and expansion of the structure, including stellar feedback and large-scale dynamics. We propose that the most likely cause of the M0.8–0.2 ring is a single high-energy hypernova explosion. To viably explain the observed morphology and kinematics, such an explosion would need to have taken place inside a dense, very massive molecular cloud, the remnants of which we now see as the M0.8–0.2 ring. In this case, the structure provides an extreme example of how supernovae can affect molecular clouds.

Discovery of MgS and NaS in the Interstellar Medium and Tentative Detection of CaO

We report the first detection of the metal-bearing molecules sodium sulfide and magnesium sulfide and the tentative detection of calcium monoxide in the interstellar medium toward the Galactic center molecular cloud G+0.693-0.027. The derived column densities are (5.0 ± 1.1) × 1010 cm−2, (6.0 ± 0.6) × 1010 cm−2, and (2.0 ± 0.5) × 1010 cm−2, respectively. This translates into fractional abundances with respect to H2 of (3.7 ± 1.0) × 10−13, (4.4 ± 0.8) × 10−13, and (1.5 ± 0.4) × 10−13, respectively. We have also searched for other Na-, Mg-, and Ca-bearing species toward this source but none of them have been detected and thus we provide upper limits for their abundances. We discuss the possible chemical routes involved in the formation of these molecules containing metals under interstellar conditions. Finally, we compare the ratio between sulfur-bearing and oxygen-bearing molecules with and without metals, finding that metal-bearing sulfur molecules are much more abundant than metal-bearing oxygen ones, in contrast with the general trend found in the ratios between other nonmetal-oxygen- and sulfur-bearing molecules. This further strengthens the idea that sulfur may be a little depleted in G+0.693-0.027 as a result of the low-velocity shocks present in this source sputtering large amounts of material from dust grains.

HOLISMOKES. XI. Evaluation of supervised neural networks for strong-lens searches in ground-based imaging surveys

While supervised neural networks have become state of the art for identifying the rare strong gravitational lenses from large imaging data sets, their selection remains significantly affected by the large number and diversity of non-lens contaminants. This work evaluates and compares systematically the performance of neural networks in order to move towards a rapid selection of galaxy-scale strong lenses with minimal human input in the era of deep, wide-scale surveys. We used multiband images from PDR2 of the Hyper-Suprime Cam (HSC) Wide survey to build test sets mimicking an actual classification experiment, with 189 securely-identified strong lenses from the literature over the HSC footprint and 70\,910 non-lens galaxies in COSMOS covering representative lens-like morphologies. Multiple networks were trained on different sets of realistic strong-lens simulations and non-lens galaxies, with various architectures and data preprocessing, mainly using the deepest $gri$-bands. Most networks reached excellent area under the Receiver Operating Characteristic (ROC) curves on the test set of 71,099 objects, and we determined the ingredients to optimize the true positive rate for a total number of false positives equal to zero or 10 $ and TPR$_ $). The overall performances strongly depend on the construction of the ground-truth training data and they typically, but not systematically, improve using our baseline residual network architecture presented in HOLISMOKES VI. TPR$_ $ tends to be higher for ResNets (simeq 10--40&lt;!PCT!&gt;) compared to AlexNet-like networks or G-CNNs. Improvements are found when (1) applying random shifts to the image centroids, (2) using square-root scaled images to enhance faint arcs, (3) adding $z$-band to the otherwise used $gri$-bands, or (4) using random viewpoints of the original images. In contrast, we find no improvement when adding $g - i$ difference images (where alpha is a tuned constant) to subtract emission from the central galaxy. The most significant gain is obtained with committees of networks trained on different data sets, with a moderate overlap between populations of false positives. Nearly-perfect invariance to image quality can be achieved by using realistic PSF models in our lens simulation pipeline, and by training networks either with large number of bands, or jointly with the PSF and science frames. Overall, we show the possibility to reach a $ as high as 60&lt;!PCT!&gt; for the test sets under consideration, which opens promising perspectives for pure selection of strong lenses without human input using the Rubin Observatory and other forthcoming ground-based surveys.

Electromagnetic signatures of black hole clusters in the center of super-Eddington galaxies

Supermassive black holes (SMBHs) at the centers of active galaxies are fed by accretion disks that radiate from the infrared or optical to the X-ray bands. Several types of objects can orbit SMBHs, including massive stars, neutron stars, clouds from the broad- and narrow-line regions, and X-ray binaries. Isolated black holes with a stellar origin (BHs of ∼10 M⊙) should also be present in large numbers within the central parsec of the galaxies. These BHs are expected to form a cluster around the SMBH as a result of the enhanced star formation rate in the inner galactic region and the BH migration caused by gravitational dynamical friction. However, except for occasional microlensing effects on background stars or gravitational waves from binary BH mergers, the presence of a BH population is hard to verify. In this paper, we explore the possibility of detecting electromagnetic signatures of a central cluster of BHs when the accretion rate onto the central SMBH is greater than the Eddington rate. In these supercritical systems, the accretion disk launches powerful winds that interact with the objects orbiting the SMBH. Isolated BHs can capture matter from this dense wind, leading to the formation of small accretion disks around them. If jets are produced in these single microquasars, they could be sites of particle acceleration to relativistic energies. These particles in turn are expected to cool by various radiative processes. Therefore, the wind of the SMBH might illuminate the BHs through the production of both thermal and nonthermal radiation.

Constraints on the accretion properties of quasi-periodic erupters from GRMHD simulations

Some apparently quiescent supermassive black holes (BHs) at centers of galaxies show quasi-periodic eruptions (QPEs) in the X-ray band, the nature of which is still unknown. A possible origin for the eruptions is an accretion disk. However, the properties of such disks are restricted by the timescales of recurrence and the duration of the flares. In this work, we test the possibility that the temporal properties of known QPEs can be explained by accretion from a compact accretion disk with an outer radius out g and we focus on a particular object, GSN 069. We ran several 3D general relativistic magnetohydrodynamic (GRMHD) simulations with the H-AMR code of thin and thick disks and studied how the initial disk parameters such as thickness, magnetic field configuration, magnetization, and Kerr parameter affect the observational properties of QPEs. We show that accretion onto a slowly rotating BH through a small, moderately thin accretion disk with an initially low plasma beta can explain the observed time between outbursts and the lack of evidence for a variable jet emission. In order to form such a disk, the accreting matter should have a low net angular momentum. A potential source for such low angular momentum matter with a quasi-periodic feeding mechanism might be a tight binary of wind-launching stars. Apart from their primary application, our results can also be useful for general studies of systems with small accretion disks, in which evolution occurs very rapidly so that the disks cannot be considered stationary. For such systems, it is important to understand how the initial conditions affect the results.

Determining the Extents, Geometries, and Kinematics of Narrow-line Region Outflows in Nearby Seyfert Galaxies

Outflowing gas from supermassive black holes in the centers of active galaxies has been postulated as a major contributor to galactic evolution. To explore the interaction between narrow-line region (NLR) outflows and their host galaxies, we use Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) spectra and Wide Field Camera 3 (WFC3) images of 15 nearby (z < 0.02) active galactic nuclei (AGN) to determine the extents and geometries of their NLRs. We combine new HST WFC3 continuum and [O iii] λ5007 images of 11 AGN with four archival AGN to match existing spectra from HST STIS. For the six AGN with suitable long-slit coverage of their NLRs, we use isophotal fitting of ground-based images, continuum-subtracted [O iii] images, and the STIS spectra, to resolve, measure, and de-project the gas kinematics to the plane of the host galaxy disk and distinguish NLR outflows from galaxy rotation and/or kinematically disturbed gas. We find an average [O iii] extent of ∼680 pc with a correlation between gas extent and [O iii] luminosity of R [O III] ∝ L[OIII]0.39 . The measured extents depend strongly on the depth of the [O iii] images, highlighting the importance of adopting uniform thresholds when analyzing scaling relationships. The outflows reach from 39% to 88% of the full NLR extents, and we find that all six of the AGN with STIS coverage of their entire NLRs show strong kinematic evidence for outflows, despite previous uncertainty for these AGN. This suggests that NLR outflows are ubiquitous in moderate-luminosity AGN and that standard criteria for kinematic modeling are essential for identifying outflows.

Rescuing the primordial black holes all-dark matter hypothesis from the fast radio bursts tension

Primordial black holes (PBHs) as an all-dark matter (DM) hypothesis has recently been demotivated by the prediction that these objects would source an excessive rate of fast radio bursts (FRBs). However, these predictions were based on several simplifying assumptions to which this rate is highly sensitive. In this article, we improve previous estimates of this rate arising from the capture of PBHs by neutron stars (NSs), aiming to revitalise this theory. We more accurately compute the velocity distribution functions of PBHs and NSs and also consider an enhancement in the NS and DM density profiles at galactic centers due to the presence of a central supermassive black hole. We find that previous estimates of the rate of FRBs sourced by the capture of PBHs by NSs were 3 orders of magnitude too large, concluding that the PBHs as all DM hypothesis remains a viable theory and that the observed FRB rate can only be entirely explained when considering a central, sufficiently spiky PBH density profile. Published by the American Physical Society 2024

Solving the Puzzle of the Carbonic Acid Vibrational Spectrum - an Anharmonic Story.

Against the general belief that carbonic acid is too unstable for synthesis, it was possible to synthesize the solid[1,2] as well as gas-phase carbonic acid.[3] It was suggested that solid carbonic acid might exist in Earth's upper troposphere and in the harsh environments of other solar bodies,[4] where it undergoes a cycle of synthesis, decomposition, and dimerization.[5] To provide spectroscopic data for probing the existence of extraterrestrial carbonic acid,[2,6] matrix-isolation infrared (MI-IR) spectroscopy has shown to be essential.[3,4,6-8] However, early assignments within the harmonic approximation using scaling factors impeded a full interpretation of the rather complex MI-IR spectrum of H2CO3. Recently, carbonic acid was detected in the Galactic center molecular cloud,[9] triggering new interest in the anharmonic spectrum.[10] In this regard, we substantially reassign our argon MI-IR spectra based on accurate anharmonic calculations. We calculate a four-mode potential energy surface (PES) at the explicitly correlated coupled-cluster theory using up to triple-zeta basis sets, i. e., CCSD(T)-F12/cc-pVTZ-F12. On this PES, we perform vibrational self-consistent field and configuration interaction (VSCF/VCI) calculations to obtain accurate vibrational transition frequencies and resonance analysis of the fundamentals, first overtones, and combination bands. In total, 12 new bands can be assigned, extending the spectral data for carbonic acid and thus simplifying detection in more complex environments. Furthermore, we clarify disputed assignments between the cc- and ct-conformer.

Two stellar populations with different metallicities in the low-mass globular cluster Gran 5

With the increasing number of discoveries of globular clusters in the inner Milky Way, the need for spectroscopic confirmation and further investigation of their stellar populations and chemodynamical properties has become crucial. Gran 5 is a newly reported low-mass globular cluster located close to the Galactic center, and it is thought to be an accreted object associated with the $Gaia$-Enceladus structure. This study aims to investigate the stellar populations of Gran 5 and their detailed chemical properties. We performed high-resolution near-infrared spectroscopy on seven stars in the field of Gran 5 using IGRINS on the Gemini-South telescope. We identified six stars as cluster members and reveal that they are divided into two stellar populations with different metallicities, with mean Fe/H values of $-0.76$ dex and $-0.55$ dex, respectively. In addition, the chemodynamical properties of Gran 5 agree with those of in situ globular clusters. Our findings represent the first detection of two stellar populations with different metallicities in a low-mass globular cluster. This suggests that the metallicity variation in Gran 5 may have arisen from processes different from those in other globular clusters with metallicity variation, or that it may have lost a substantial amount of its initial mass during its evolution.