Tuan Do

We report the first proper motion and acceleration measurements of the infrared (IR) counterpart to Sagittarius A* (Sgr A*-IR), the supermassive black hole at the center of our Galaxy, in the Gaia Celestial Reference Frame 3 (Gaia-CRF3). This reference frame realizes the International Celestial Reference System, which is an absolute reference coordinate system defined by quasars. A combination of Gaia and Hubble Space Telescope data was used to transform Keck adaptive optics observations into Gaia-CRF3. We developed a method for selecting reference stars that minimizes astrometric transformation errors (statistical error = 0.10–0.63 mas) and drift of the coordinate system (systematic error ∼ 0.01 mas yr ⁻¹ ). We find the proper motion of Sgr A*-IR in Gaia-CRF3 to be μ α * = − 3.093 ± 0.085 mas yr ⁻¹ and μ _δ = −5.62 ± 0.13 mas yr ⁻¹ with the initial position at t ₀ = 2016.0 of R.A. = 266 . ° 41680848 ± 0 . ° 00000029 and decl. = −29 . ° 00783947 ± 0 . ° 00000050, which translates to a precision of 1.05 mas in R.A. and 1.79 mas in decl. This is consistent with the astrometric measurements of the radio counterpart to Sgr A* by S. Xu et al. We also place a 2 σ upper constraint of the acceleration of Sgr A*-IR on the sky at 0.061 mas yr ⁻² . This acceleration limit on Sgr A*-IR excludes any intermediate-mass black hole companion with mass ≳ 4 × 10 ⁴ M _⊙ within a distance of ∼0.01 pc, consistent with previous studies. With the release of Gaia Data Release 4, we predict these limits will be improved by at least a factor of 2.

Astrophysical spectroscopy provides a powerful probe of spacetime variations of fundamental constants, as atomic and ionic emission and absorption lines depend sensitively on the fine-structure constant. In particular, coherent temporal oscillations induced by an ultralight scalar background produce characteristic, time-resolved signatures that can be robustly disentangled from intrinsic variability. In the Galactic Center, such scalar backgrounds can be substantially enhanced, either through the formation of dense scalar clouds powered by black hole rotational energy extraction or as ultralight scalar dark matter forming a soliton-like core. These scalar configurations generically induce oscillations of the fine-structure constant, with periods set by the scalar mass and spatial profiles determined by the scalar wavefunction and its coupling to the electromagnetic sector. We show that precise, time-resolved spectroscopy of S-stars orbiting the supermassive black hole Sgr A^* provides a sensitive test of these effects, enabling constraints on quadratic scalar-photon couplings in the exceptionally high boson-density environment of the Galactic Center.

We report the binary fraction of Wolf–Rayet (W-R) stars within 0.5 pc of the Galactic center obtained through the longest time-baseline (1994–2024) kinematic study of this population of stars. The new radial velocity (v_z) data we present are primarily from the W. M. Keck Observatory, with additional v_z measurements from Gemini North Observatory. When combining our new v_z measurements with literature measurements, we find v_z variations that suggest the presence of a companion for five out of 27 W-R stars, of which two are newly identified here (IRS 13E4, S8-181), along with three previously detected binaries (IRS 16SW, IRS 16NE, S4-258). Based on our experimental sensitivity and expected properties of the underlying population, we infer the binary fraction of the W-R stars in the Galactic center to be 0.56 ± 0.18. This is consistent with previous photometric studies of the young stars in the Galactic center and with the binary fraction of field W-R stars. When our results are combined with the results of previous photometric work, we find a binary fraction of 0.69 ± 0.17 for the W-R stars.

We present spatially resolved rest-frame optical emission line maps of four galaxies at z ∼ 2 observed with the Keck/OH-Suppressing Infra-Red Imaging Spectrograph to study the physical conditions of the interstellar medium (ISM) at cosmic noon. Our analysis of strong emission line ratios in these galaxies reveals an offset from the local star-forming locus on the Baldwin–Phillips–Terlevich diagram but agrees with other star-forming galaxies at similar redshifts. Despite the offset toward higher [O iii ] λ 5008/H β and [N ii ] λ 6585/H α , these strong line ratios remain consistent with or below the maximum starburst threshold even in the inner ∼1 kpc region of the galaxies, providing no compelling evidence for central active galactic nucleus activity. The galaxies also exhibit flat radial gas-phase metallicity gradients, consistent with previous studies of z ∼ 2 galaxies and suggesting efficient radial mixing possibly driven by strong outflows from intense star formation. Overall, our results reveal the highly star-forming nature of these galaxies, with the potential to launch outflows that flatten metallicity gradients through significant radial gas mixing. Future observations with JWST/NIRSpec are crucial to detect fainter emission lines at higher spatial resolution to further constrain the physical processes and ionization mechanisms that shape the ISM during cosmic noon.

CO ₂ is an important, stable, and abundant molecule in the Universe, but it is very difficult to detect because it has no observable pure rotational transitions. The unique sensitivity and resolution of the James Webb Space Telescope provide a fresh way to investigate it. CO ₂ is typically found in the solid phase (ice) on grain mantles in dense molecular clouds, but is less commonly detected in the gas phase (compared to common molecules such as CO and H ₂ O) and has mostly been found in protostellar and protoplanetary environments. Here, we report and characterize the first observations of gas-phase CO ₂ absorption toward two IR-bright regions of the Galactic Center, thanks to the high sensitivity of JWST. Using an LTE model, we find a CO ₂ gas excitation temperature between 20 and 50 K, a column density around 2 × 10 ¹⁵ cm ⁻² and a radial velocity with respect to the local standard of rest consistent with 0. We also report: (1) simultaneous detections of C ₂ H ₂ and HCN absorption bands (near 13.7 and 14.0 μ m, respectively), with column density ratios of 1:3 and 3:2 with respect to gas-phase CO ₂ , and (2) CO ₂ ice absorption with an ice-to-gas abundance ratio of 90, consistent with previous findings. We conclude that the absorbing medium is likely in the foreground, most likely from one or more somewhat clumpy cloud(s), located between 0.15 and 4 kpc away from Earth. Additionally, we detected point-like CO ₂ emission likely associated with a Galactic Center evolved star (IRS 11SW), which is also spatially coincident with a previously reported X-ray source, raising the possibility that the system is a symbiotic binary.

Measuring stellar motions at the Milky Way’s Galactic center (GC) provides unique insight into the dynamical processes within galactic nuclei. We present proper-motion measurements for 23 SiO-maser-emitting stars within 45″ of SgrA*, including four previously reported to have velocities exceeding their local escape velocities (i.e., they are “locally unbound” from the GC). Derived from 14 epochs of Hubble Space Telescope WFC3-IR observations (2010–2023), our measurements have a median precision of 0.038 mas yr ⁻¹ —up to ∼100x more precise than previous constraints for some sources. By combining these proper motions with published radial velocities, we derive updated 3D velocities for the masers and find that only one is locally unbound (IRS 9; v _3d = 370 ± 1.2 km s ⁻¹ ). Orbit integrations place the first constraints on the orbit of IRS 9, which is bound to the GC at larger radii with r _peri ≥ 0.100 ± 0.005 pc and r _apo ≥ 5.25 ± 0.18 pc. IRS 9’s high velocity relative to stars at similar radii in the Nuclear Star Cluster makes it a candidate to have experienced a strong dynamical interaction in order to place it on its orbit. We explore the Hills mechanism as a possible origin, but binary evaporation and ejection velocity limits indicate that IRS 9 is unlikely to have experienced such an event in the past 0.4 Myr (the timescale constrained by the orbit integrations). Alternative mechanisms to produce IRS 9 include binary supernova disruption, two-body interactions, and stellar collisions. Identifying additional stars like IRS 9 will be essential for understanding these various dynamical processes.

Accurate redshift estimates are a vital component in understanding galaxy evolution and precision cosmology. In this paper, we explore approaches to increase the applicability of machine learning models for photometric redshift estimation on a broader range of galaxy types. Typical models are trained with ground-truth redshifts from spectroscopy. We test the utility and effectiveness of two approaches for combining spectroscopic redshifts and redshifts derived from multiband (∼35 filters) photometry, which sample different types of galaxies compared to spectroscopic surveys. The two approaches are (1) training on a composite dataset, and (2) transfer learning from one dataset to another. We compile photometric redshifts from the COSMOS2020 catalog (TransferZ) to complement an established spectroscopic redshift dataset (GalaxiesML). We used two architectures, deterministic neural networks (NN) and Bayesian neural networks (BNN), to examine and evaluate their performance with respect to the Legacy Survey of Space and Time (LSST) photo- z science requirements. We also use split conformal prediction for calibrating uncertainty estimates and producing prediction intervals for the BNN and NN, respectively. We find that a NN trained on a composite dataset predicts photo- z 's 4.5 times less biased within the redshift range 0.3 < z < 1.5, 1.1 times less scattered, and has a 1.4 times lower outlier rate than a model trained on only spectroscopic ground truths. We also find that BNNs produce reliable uncertainty estimates, but are sensitive to the different ground truths. This investigation leverages different sources of ground truths to develop models that can accurately predict photo- z 's for a broader galaxy population, which is crucial for surveys such as Euclid and LSST.

Context. The Galactic center (GC) is a unique and extreme astrophysical laboratory for studying the interplay between gas, stars, and a supermassive black hole (SMBH). In particular, the circumnuclear disk (CND) and its central cavity (CC) present two contrasting environments in terms of gas content, density, and stellar activity, making them ideal regions in which to study the multiphase structure and chemical composition of the interstellar medium (ISM). Aims. We aim to determine the properties (temperature, density, abundances, and spatial distribution) of the various phases of the ISM in the central parsec of the GC, with particular attention in this paper to the ionized medium. Methods. We used newly obtained observations from the Mid-Infrared Instrument (MIRI) equipped with the Medium Resolution Spectrometer (MRS) aboard the James Webb Space Telescope (JWST) to extract spectra covering the entire spectral range from 5 to 27 µm in the CND and in the CC. We used the photoionization code CLOUDY to generate synthetic spectra with the same spectral range and resolution, simulating a wide range of gas phases and abundances. We then determined the contribution of each phase to the spectra. Once the abundances and contribution from each phase of the gas were determined, we identified four dominant phases and performed a spatial analysis to determine their contribution to each spaxel of the datacubes. Results. We find that in both the CND and the CC, the bulk of the emission originates from warm ionized gas with temperatures of between 10 ⁴ and 10 ^{4 . 8} K. In the CND, molecular gas contributes significantly to the flux and is spatially structured, while the CC shows minimal molecular gas content, as is expected from these regions. Coronal gas is detected in both regions at the interface between molecular and warm ionized gas. The hottest coronal phase appears faint and patchy in the CC, and has an elongated morphology in the CND. Abundance fitting (in solar-normalized logarithmic units) is primarily constrained by abundances: we measure a robust depletion of Fe relative to α elements with log(Fe /α ) = −0 . 78 ± 0 . 20 (CC) and −0 . 84 ± 0 . 26 (CND), while CNO is only mildly enhanced relative to α , log(CNO /α ) = 0 . 27 ± 0 . 20 (CC) and 0 . 05 ± 0 . 26 (CND). Absolute abundances are supersolar but more degenerate; the best-fitting models yield (log α, log CNO , log Fe) = (1 . 4 , 1 . 4 , 0 . 4) in the CC and (2 . 0 , 1 . 8 , 1 . 2) in the CND. Conclusions. The observed abundance pattern (enhanced CNO and α elements with suppressed Fe) indicates a chemically young environment, recently enriched by core-collapse supernovae and stellar winds, with a limited contribution from older Type Ia supernovae. This favors a scenario of massive, recent star formation rather than cumulative long-term enrichment. Additionally, the projected orientation of the newly identified CND elongated hot coronal feature, perpendicular to the direction toward the SMBH, suggests the action of a large-scale shock possibly resulting from past energetic outflows.

JWST/MIRI observations can place photometric limits on the presence of an intermediate-mass black hole (IMBH) near the Galactic Centre. The stellar complex IRS 13E, a co-moving conglomerate of young and massive stars, is a prime location to study because it has been speculated to be bound by an IMBH. Assuming a standard radiatively inefficient accretion flow (RIAF) and a minimum fractional variability of 10% of the intrinsic luminosity, the wavelength of peak emission in the spectral energy distribution for an IMBH would lie in the mid-infrared (∼5–25 μ m), and the variability would be detectable in MIRI time-series observations. Monitoring fails to detect such variable emission (other than from Sgr A*) in and around the IRS 13E complex, and upper limits on a putative IMBH’s intrinsic variability on timescales of minutes to ∼1 hr are ≲ 1 mJy at 12 μ m and ≲2 mJy at 19 μ m. These translate to luminosities ≲ 25 × 10 ³² erg s ⁻¹ . The resulting limits on the IMBH mass and accretion rate rule out any IMBH with mass ≳10 ³ M _⊙ accreting at ≳10 ⁻⁶ times Eddington rate at the location of IRS 13E. Further, the observations rule out an IMBH anywhere in the central 6″ × 6″ region that is more massive than ≈ 2 × 10 ³ M _⊙ and accreting at ≥10 ⁻⁶ of the Eddington rate. Assuming Bondi accretion scaled to typical RIAF-accretion efficiencies, albeit somewhat uncertain, also allows us to rule out IMBHs moving with typical velocities ∼200 km s ⁻¹ and masses ≳ 2 × 10 ³ M _⊙ . These methods showcase the effectiveness of photometric variability measurements in constraining the presence of accreting black holes in Galactic centre-like environments.