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Abstract The construction of an accurate 3D model of the Milky Way center is necessary to understand inflow processes that drive its overall evolution and to compare our Galactic nucleus to other galaxies’ nuclei. A main point of contention is the line-of-sight location of sources observed toward the central 10 pc of the Galaxy, including recent star formation (the Sgr A East supernova remnant and Sgr A HIIregions) and copious gas (the 50 and 20 km s⁻¹molecular clouds, the circumnuclear disk, and the Sgr A West ionized “minispiral” that encircles the central supermassive black hole, Sgr A*). Some models place all of these structures within a radius of 5 pc from Sgr A*, while others place the 20 and 50 km s⁻¹clouds at a distance of at least 30−50 pc away from Sgr A* along the line of sight. We present new radio and millimeter observations of the molecular gas toward the central ∼10 pc, from which we have constructed an alternative 3D model that is consistent with both prior radio observations and orbital gas kinematics. Our model places the 20 km s⁻¹cloud, 50 km s⁻¹cloud, and Sgr A East more than 10 pc in front of Sgr A*. While this model does not conclusively rule out a connection between the 50 and 20 km s⁻¹clouds and the circumnuclear disk, we argue that prior evidence for these connections is tenuous, especially given the complex spatial and kinematic overlap of structures along the line of sight. 

Abstract The nature of the magnetic field structure throughout the Galactic Center (GC) has long been of interest. The recent Far-InfraREd Polarimetric Large-Area Central Molecular Zone (CMZ) Exploration (FIREPLACE) Survey reveals preliminary connections between the seemingly distinct vertical and horizontal magnetic field distributions previously observed in the GC. We use the statistical techniques of the Histogram of Relative Orientation and the Projected Rayleigh Statistic to assess whether the CMZ magnetic field preferentially aligns with the structure of the CMZ molecular clouds or the morphology of the nonthermal emission of the GC nonthermal filament (NTF) population. We find that there is a range of magnetic field orientations throughout the population of CMZ molecular clouds, ranging from parallel to perpendicular orientation. We posit these orientations depend on the prevalence of gravitational shear in the GC, in contrast with what is observed in Galactic Disk star-forming regions. We also compare the magnetic field orientation from dust polarimetry with individual prominent NTFs, finding a preferred perpendicular relative orientation. This perpendicular orientation indicates that the vertical field component found in the FIREPLACE observations is not spatially confined to the NTFs, providing evidence for a more pervasive vertical field in the GC. From dynamical arguments, we estimate an upper limit on the magnetic field strength for this vertical field, findingB≤ 4 mG. A field close to this upper limit would indicate that the NTFs are not local enhancements of a weaker background field and that the locations of the NTFs depend on proximity to sites of cosmic-ray production. 

The central regions of the Milky Way constitute a unique laboratory for a wide swath of astrophysical studies; consequently, the inner ∼400 pc have been the target of numerous large surveys at all accessible wavelengths. In this paper, we present a catalog of sources at 25 and 37μm located within all of the regions observed with the SOFIA/FORCAST instrument in the inner ∼200 pc of the Galaxy. The majority of the observations were obtained as part of the SOFIA Cycle 7 Galactic Center Legacy program survey, which was designed to complement the Spitzer/MIPS 24μm catalog in regions saturated in the MIPS observations. Due to the wide variety of source types captured by our observations at 25 and 37μm, we do not limit the FORCAST source catalog to unresolved point sources, or treat all sources as if they are pointlike sources. The catalog includes all detectable sources in the regions, resulting in a catalog of 950 sources, including point sources, compact sources, and extended sources. We also provide the user with metrics to discriminate between the source types. 

Abstract We present the first estimate of the intrinsic binary fraction of young stars across the central ≈0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 spectroscopically confirmed young stars, using 119 nights of 10″ wide adaptive optics imaging observations taken at W. M. Keck Observatory over 16 yr in the $K\prime$-[2.1μm] andH-[1.6μm] bands. We photometrically detected three binary stars, all of which are situated more than 1″ (0.04 pc) from the SMBH and one of which, S2-36, is newly reported here with spectroscopic confirmation. All are contact binaries or have photometric variability originating from stellar irradiation. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined the experimental sensitivity through detailed light-curve simulations, incorporating photometric effects of eclipses, irradiation, and tidal distortion in binaries. The simulations assumed a population of young binaries, with stellar ages (4 Myr) and masses matched to the most probable values measured for the GC young star population, and underlying binary system parameters (periods, mass ratios, and eccentricities) similar to those of local massive stars. As might be expected, our experimental sensitivity decreases for eclipses narrower in phase. The detections and simulations imply that the young, massive stars in the GC have a stellar binary fraction ≥71% (68% confidence), or ≥42% (95% confidence). This inferred GC young star binary fraction is consistent with that typically seen in young stellar populations in the solar neighborhood. Furthermore, our measured binary fraction is significantly higher than that recently reported by Chu et al. based on radial velocity measurements for stars ≲1″ of the SMBH. Constrained with these two studies, the probability that the same underlying young star binary fraction extends across the entire region is <1.4%. This tension provides support for a radial dependence of the binary star fraction, and therefore, for the dynamical predictions of binary merger and evaporation events close to the SMBH. 

Abstract The time-variable emission from the accretion flow of Sgr A*, the supermassive black hole at the Galactic center, has long been examined in the radio-to-millimeter, near-infrared (NIR), and X-ray regimes of the electromagnetic spectrum. However, until now, sensitivity and angular resolution have been insufficient in the crucial mid-infrared (MIR) regime. The MIRI instrument on JWST has changed that, and we report the first MIR detection of Sgr A*. The detection was during a flare that lasted about 40 minutes, a duration similar to NIR and X-ray flares, and the source's spectral index steepened as the flare ended. The steepening suggests that synchrotron cooling is an important process for Sgr A*'s variability and implies magnetic fields strengths ~ 40–70 G in the emission zone. Observations at 1.3 mm with the Submillimeter Array revealed a counterpart flare lagging the MIR flare by ≈10 minutes. The observations can be self-consistently explained as synchrotron radiation from a single population of gradually cooling high-energy electrons accelerated through (a combination of) magnetic reconnection and/or magnetized turbulence. 

Context.The Galactic Center black hole and the nuclear star cluster are surrounded by a clumpy ring of gas and dust, the circumnuclear disk (CND), that rotates about them at a standoff distance of ≃1.5 pc. The mass and density of individual clumps in the CND are disputed. Aims.We seek to use H₂to characterize the clump size distribution and to investigate the morphology and dynamics of the interface between the ionized interior layer of the CND and the molecular reservoir lying farther out (corresponding to the inner rim of the CND, illuminated in ultraviolet light by the central star cluster). Methods.We have observed two fields of approximately 20″ × 20″ in the CND at near-infrared wavelengths with the OSIRIS spectro-imager at the Keck Observatory. These two fields, located at the approaching and receding nodes of the CND, best display this interface. Our data cover two H₂lines as well as the Brγline (tracing H II). We have developed the tool CubeFit, an original method for extracting maps of continuous physical parameters (such as the velocity field and velocity dispersion) from integral-field spectroscopy data, using regularization to largely preserve spatial resolution in regions of low signal-to-noise ratio. Results.This original method enables us to isolate compact, bright features in the interstellar medium of the CND. Several clumps in the southwestern field assume the appearance of filaments, many of which are parallel to one another. We conclude that these clumps cannot be self-gravitating. 

Abstract We present high-resolution (∼2–3″; ∼0.1 pc) radio observations of the Galactic center cloud M0.10−0.08 using the Very Large Array at K and Ka band (∼25 and 36 GHz). The M0.10−0.08 cloud is located in a complex environment near the Galactic center Radio Arc and the adjacent M0.11−0.11 molecular cloud. From our data, M0.10−0.08 appears to be a compact molecular cloud (∼3 pc) that contains multiple compact molecular cores (5+; <0.4 pc). In this study, we detect a total of 15 molecular transitions in M0.10−0.08 from the following molecules: NH 3 , HC 3 N, CH 3 OH, HC 5 N, CH 3 CN, and OCS. We have identified more than sixty 36 GHz CH 3 OH masers in M0.10−0.08 with brightness temperatures above 400 K and 31 maser candidates with temperatures between 100 and 400 K. We conduct a kinematic analysis of the gas using NH 3 and detect multiple velocity components toward this region of the Galactic center. The bulk of the gas in this region has a velocity of 51.5 km s −1 (M0.10−0.08) with a lower-velocity wing at 37.6 km s −1 . We also detect a relatively faint velocity component at 10.6 km s −1 that we attribute to being an extension of the M0.11−0.11 cloud. Analysis of the gas kinematics, combined with past X-ray fluorescence observations, suggests M0.10−0.08 and M0.11−0.11 are located in the same vicinity of the Galactic center and could be physically interacting. 

Abstract Sgr A* is the variable electromagnetic source associated with accretion onto the Galactic center supermassive black hole. While the near-infrared (NIR) variability of Sgr A* was shown to be consistent over two decades, unprecedented activity in 2019 challenges existing statistical models. We investigate the origin of this activity by recalibrating and reanalyzing all of our Keck Observatory Sgr A* imaging observations from 2005–2022. We present light curves from 69 observation epochs using the NIRC2 imager at 2.12μm with laser-guide star adaptive optics. These observations reveal that the mean luminosity of Sgr A* increased by a factor of ∼3 in 2019, and the 2019 light curves had higher variance than in all time periods we examined. We find that the 2020–2022 flux distribution is statistically consistent with the historical sample and model predictions, but with fewer bright measurements above 0.6 mJy at the ∼2σlevel. Since 2019, we have observed a maximumK_s(2.2μm) flux of 0.9 mJy, compared to the highest pre-2019 flux of 2.0 mJy and highest 2019 flux of 5.6 mJy. Our results suggest that the 2019 activity was caused by a temporary accretion increase onto Sgr A*, possibly due to delayed accretion of tidally stripped gas from the gaseous object G2 in 2014. We also examine faint Sgr A* fluxes over a long time baseline to search for a quasi-steady quiescent state. We find that Sgr A* displays flux variations over a factor of ∼500, with no evidence for a quiescent state in the NIR. 

Abstract We present new absolute proper-motion measurements for the Arches and Quintuplet clusters, two young massive star clusters near the Galactic center. Using multiepoch HST observations, we construct proper-motion catalogs for the Arches (∼35,000 stars) and Quintuplet (∼40,000 stars) fields in ICRF coordinates established using stars in common with the Gaia EDR3 catalog. The bulk proper motions of the clusters are measured to be (μ_α*,μ_δ) = (−0.80 ± 0.032, −1.89 ± 0.021) mas yr⁻¹for the Arches and (μ_α*,μ_δ) = (−0.96 ± 0.032, −2.29 ± 0.023) mas yr⁻¹for the Quintuplet, achieving ≳5× higher precision than past measurements. We place the first constraints on the properties of the cluster orbits that incorporate the uncertainty in their current line-of-sight distances. The clusters will not approach closer than ∼25 pc to Sgr A*, making it unlikely that they will inspiral into the nuclear star cluster within their lifetime. Further, the cluster orbits are not consistent with being circular; the average value ofr_apo/r_periis ∼1.9 (equivalent to an eccentricity of ∼0.31) for both clusters. Lastly, we find that the clusters do not share a common orbit, challenging one proposed formation scenario in which the clusters formed from molecular clouds on the open stream orbit derived by Kruijssen et al. Meanwhile, our constraints on the birth location and velocity of the clusters offer mild support for a scenario in which the clusters formed via collisions between gas clouds on thex₁andx₂bar orbit families. 

Abstract Analyzing longitude–velocity diagrams (LVDs) in the CS (J = 2–1) and H$$^{13}$$CN (J = 1–0) molecular lines from the internal release data of the ALMA Central Molecular Zone (CMZ) Exploration Survey and in the $$^{13}$$CO (J = 1–0) line from the Nobeyama Galactic Center (GC) survey, we identify six GC arms as prominent straight LV ridges. In addition to the currently known Arms I to IV, we identify a new inner arm, Arm V, and further highlight the circumnuclear disk (CND) as Arm VI. Integrated intensity maps of the arms on the sky suggest that most of the arms compose ring-like structures inclined from the Galactic plane. We determine the radii (curvatures) of the arms using the velocity-gradient ($dv/dl$) method, assuming that the arms are rotating in circular orbits at a constant velocity of $$\sim$$150 km s$$^{-1}$$. We show that Arms I and II compose the main ring structure of the CMZ with radii $$\sim$$100–120 pc; Arm III is a dense arm 42 pc from the GC; Arm IV is a clear and narrow arm 20 pc from the GC; and Arm V is a faint, long arm of 8.2 pc radius. We show that the CND comprises the sixth arm, Arm VI, of radius $$\sim$$2.3 pc associated with bifurcated spiral fins. We also discuss the association of the 20 and 50 km s$$^{-1}$$ clouds with these arms. The radii of the arms fall on an empirical relation $$R\sim 630 (2/5)^N$$ for $N=1$ (Arm I) to 6 (VI), suggesting discrete rings or a logarithmic spiral with pitch angle $${\sim }22^{\circ }$$. The vertical full extent of the arm increases with radius and is represented by $$z\sim 0.7 (R/1 {\rm \ pc} )^{0.7}$$ pc. The tilt angle of the arms from the Galactic plane, or the warping, increases rapidly toward the GC.