About

Andrea Ghez is a member of the UCLA Galactic Center Group, where she is involved in research related to the Galactic Center. Her work focuses on understanding the dynamics and properties of the central region of our galaxy, including the supermassive black hole at its core. As a senior scientist, she contributes to the group's efforts in exploring the complex astrophysical phenomena occurring in this area, utilizing advanced observational techniques and data analysis to uncover insights about the galaxy's core.

Research topics

• Astronomy
• Quantum mechanics
• Physics
• Astrophysics

Selected publications

• First Observation of CO ₂ Foreground Absorption toward the Galactic Center with JWST

  The Astrophysical Journal · 2026-03-20

  articleOpen access

  Abstract 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.

  PublisherDOI

• IRS 9: The Case for a Dynamically-Ejected Star from the Galactic Center

  arXiv (Cornell University) · 2026-02-02

  articleOpen access

  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 HST WFC3-IR observations (2010 - 2023), our measurements have a median precision of 0.038 mas/yr - up to ~100x more precise then 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; v3d = 370 +/- 1.2 km/s). 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 that could 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.

  PublisherOA PDF

• IRS 9: The Case for a Dynamically-Ejected Star from the Galactic Center

  Open MIND · 2026-02-02

  preprint

  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 HST WFC3-IR observations (2010 - 2023), our measurements have a median precision of 0.038 mas/yr - up to ~100x more precise then 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; v3d = 370 +/- 1.2 km/s). 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 that could 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.

  DOI

• Interstellar medium phases and abundances in the central parsec

  Astronomy and Astrophysics · 2025-11-07 · 2 citations

  articleOpen access

  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.

  PublisherDOI

• The HST–Gaia Near-infrared Astrometric Reference Frame Near the Milky Way Galactic Center

  The Astrophysical Journal · 2025-08-05 · 1 citations

  articleOpen accessCorresponding

  Abstract We present the first high-precision proper-motion catalog, tied to the International Celestial Reference System (ICRS), of infrared astrometric reference stars within R ≤ 25″ (1 pc) of the central supermassive black hole at the Galactic center (GC). This catalog contains ∼2900 sources in a highly extinguished region that is inaccessible via Gaia. New astrometric measurements are extracted from Hubble Space Telescope (HST) observations (14 epochs, 2010–2023) and transformed into the ICRS using 40 stars in common with Gaia-DR3. We implement a new method for modeling proper motions via Gaussian processes that accounts for systematic errors, greatly improving measurement accuracy. Proper-motion and position measurements reach precisions of ∼0.03 mas yr −1 and ∼0.11 mas, respectively, representing a factor of ∼20 improvement over previous ICRS proper-motion catalogs in the region. These measurements define a novel HST–Gaia reference frame that is consistent with Gaia-CRF3 to within 0.025 mas yr −1 in proper motion and 0.044 mas in position, making it the first ICRS-based reference frame precise enough to probe the distribution of extended mass within the orbits of stars near SgrA*. In addition, HST-Gaia provides an independent test of the radio measurements of stellar masers that form the basis of current GC reference frames. We find that the HST–Gaia and radio measurements are consistent to within 0.041 mas yr −1 in proper motion and 0.54 mas in position at 99.7% confidence. Gaia-DR4 is expected to reduce the HST–Gaia reference-frame uncertainties by another factor of ∼2, further improving the reference frame for dynamical studies.

  PublisherDOI

• A multiwavelength study of the Galactic center black hole candidate MAXI J1744-294

  ArXiv.org · 2025-01-01

  preprintOpen access

  For the first time in nearly a decade, a bright transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294, or -- as it was known in its previous life -- Swift J174540.2-290037, was discovered in outburst by the MAXI telescope in January 2025. We present the results of a broadband, multi-wavelength study of MAXI J1744-294, including data from the NuSTAR, Chandra, XMM-Newton, Swift, and NICER X-ray telescopes, as well as complementary radio and near-infrared observations. We analyze the changing X-ray emission as the outburst evolved from the high/soft to the low/hard state. Using relativistic reflection features in the data, we estimate a spin of $a>0.92$ and viewing inclination $θ=28^{+3}_{-4}$ deg, consistent with the parameters measured for Swift J174540.2-290037. Based on the spectral and temporal characteristics of MAXI J1744-294, we reaffirm its classification as a candidate black hole (BH) low-mass X-ray binary (LMXB) -- the third candidate BH transient discovered within 20 arcsec of the Galactic supermassive black hole Sgr~A*. This work provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as argued for in previous observational studies and suggested by analytical and theoretical work. Our ongoing multi-wavelength study, involving a complementary range of observatories and spanning different outburst states, can serve as a model for future time domain astrophysics research.

  PublisherOA PDF

• The HST-Gaia Near-Infrared Astrometric Reference Frame near the Milky Way Galactic Center

  ArXiv.org · 2025-06-24

  preprintOpen access

  We present the first high-precision proper motion catalog, tied to the International Celestial Reference System (ICRS), of infrared astrometric reference stars within R $\leq$ 25" (1 pc) of the central supermassive black hole at the Galactic center (GC). This catalog contains $\sim$2,900 sources in a highly extinguished region that is inaccessible via Gaia. New astrometric measurements are extracted from HST observations (14 epochs, 2010 - 2023) and transformed into the ICRS using 40 stars in common with Gaia-DR3. We implement a new method for modeling proper motions via Gaussian Processes that accounts for systematic errors, greatly improving measurement accuracy. Proper motion and position measurements reach precisions of $\sim$0.03 mas/yr and $\sim$0.11 mas, respectively, representing a factor of $\sim$20x improvement over previous ICRS proper motion catalogs in the region. These measurements define a novel HST-Gaia reference frame that is consistent with Gaia-CRF3 to within 0.025 mas/yr in proper motion and 0.044 mas in position, making it the first ICRS-based reference frame precise enough to probe the distribution of extended mass within the orbits of stars near SgrA*. In addition, HST-Gaia provides an independent test of the radio measurements of stellar masers that form the basis of current GC reference frames. We find that the HST-Gaia and radio measurements are consistent to within 0.041 mas/yr in proper motion and 0.54 mas in position at 99.7% confidence. Gaia-DR4 is expected to reduce the HST-Gaia reference frame uncertainties by another factor of $\sim$2x, further improving the reference frame for dynamical studies.

  PublisherOA PDFDOI

• Tidal phenomena in the Galactic Center: The curious case of X7

  Astronomy and Astrophysics · 2025-07-10 · 1 citations

  articleOpen accessSenior author

  Context . Several enigmatic dusty sources have been detected in the central parsec of the Galactic Center. Among them is X7, located at only ~0.02 pc from the central supermassive black hole, Sagittarius A* (Sgr A*). Recent observations have shown that X7 is becoming elongated due to the tidal forces of Sgr A*. X7 is expected to be fully disrupted during its pericenter passage around 2035, which might impact the accretion rate of Sgr A*. However, its origin and nature are still unknown. Aims . We investigated the tidal interaction of X7 with Sgr A* in order to constrain its origin. We tested the hypothesis that X7 was produced by one of the observed stars with constrained dynamical properties in the vicinity of Sgr A*. Methods . We employed a set of test-particle simulations to reproduce the observed structure and dynamics of X7. The initial conditions of the models were obtained by extrapolating the observationally constrained orbits of X7 and the known stars into the past, making it possible to find the time and source of origin by minimizing the three-dimensional separation and velocity difference between them. Results . Our results show that ejecta from the star S33/S0-30, launched in ~1950, can to a large extent replicate the observed dynamics and structure of X7, provided that it is initially elongated with a velocity gradient across it, and with an initial maximum speed of ~600 km s −1 . Conclusions . Our results show that a grazing collision between the star S33/S0-30 and a field object such as a stellar-mass black hole or a Jupiter-mass object is a viable scenario to explain the origin of X7. Despite the uncertainties in the rate of these encounters, recent estimations show that it is plausible for such a scenario to have occurred recently.

  PublisherDOI

• New Evidence for a Flux-independent Spectral Index of Sgr A* in the Near-infrared

  The Astrophysical Journal · 2024-12-01 · 4 citations

  articleOpen access

  Abstract In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the H (1.6 μ m) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> (2.2 μ m) broadband filters in the near-infrared (NIR), sampling over a factor ∼40 in brightness, the largest range probed to date by a factor ∼3. Sgr A*-NIR is highly variable, and studying the spectral index α (with F ν ∝ ν α ) is essential to determine the underlying emission mechanism. For example, variations in α with flux may arise from shifts in the synchrotron cutoff frequency, changes in the distribution of electrons, or multiple concurrent emission mechanisms. We investigate potential variations of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>−</mml:mo> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:mrow> </mml:msub> </mml:math> with flux by analyzing seven epochs (2005–2022) of Keck Observatory imaging observations from the Galactic Center Orbits Initiative. We remove the flux contribution of known sources confused with SgrA*-NIR, which can significantly impact color at faint flux levels. We interpolate between the interleaved H and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> observations using multi-output Gaussian processes. We introduce a flexible empirical model to quantify α variations and probe different scenarios. The observations are best fit by an <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>−</mml:mo> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.50</mml:mn> <mml:mo>±</mml:mo> <mml:msub> <mml:mrow> <mml:mn>0.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>stat</mml:mi> </mml:mrow> </mml:msub> <mml:mo>±</mml:mo> <mml:msub> <mml:mrow> <mml:mn>0.17</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>sys</mml:mi> </mml:mrow> </mml:msub> </mml:math> that is constant from ∼1 mJy to ∼40 mJy (dereddened 2 μ m flux). We find no evidence for a flux dependence of Sgr A*'s intrinsic spectral index. In particular, we rule out a model explaining NIR variability purely by shifts in the synchrotron cutoff frequency. We also constrain the presence of redder, quiescent emission from the black hole, concluding that the dereddened 2 μ m flux contribution must be ≤0.3 mJy at 95% confidence level.

  PublisherDOI

• An Estimate of the Binary Star Fraction Among Young Stars at the Galactic Center: Possible Evidence of a Radial Dependence

  arXiv (Cornell University) · 2024-01-23

  preprintOpen access

  We present the first estimate of the intrinsic binary fraction of young stars across the central $\approx$ 0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 young stars, using 119 nights of 10"-wide adaptive optics imaging observations taken at Keck Observatory over 16 years in the K'- and H-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. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined experiment 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 similar to those of local massive stars. The detections and simulations imply young, massive stars in the GC have a stellar binary fraction $\geq$ 71% (68% confidence), or $\geq$ 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. (2023) based on RV measurements of young stars &lt;~1" of the SMBH. Constrained with these two studies, the probability that the same underlying young binary fraction extends across the entire region is &lt;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.

  PublisherOA PDFDOI

Recent grants

• A Laser Guide Star Adaptive Optics Study of Stellar Dynamics at the Galactic Center: A Laboratory for Understanding Interactions with a Central Supermassive Black Holes

  NSF · $1.1M · 2009–2015

• Probing the Remarkable Neighborhood of the Galactic Center's Supermassive Black Hole with Diffraction-Limited Studies of Stars, Dark Matter, and Accreting Gas

  NSF · $991k · 2004–2010

• New Probes of the Galactic Black Hole and its Environs

  NSF · $1.2M · 2014–2018

• The Galactic Center Orbits Initiative: Probing the Physics & Astrophysics of the Closest Supermassive Black Hole

  NSF · $1.0M · 2019–2023

Frequent coauthors

• M. Morris

  232 shared

• Jessica R. Lu

  University of California, Berkeley

  215 shared

• Tuan Do

  182 shared

• E. E. Becklin

  101 shared

• Gaspard Duchêne

  University of California, Berkeley

  98 shared

• K. Matthews

  84 shared

• Aurélien Hees

  Systèmes de Référence Temps-Espace

  76 shared

• Gunther Witzel

  Max Planck Institute for Radio Astronomy

  75 shared

Labs

• Galactic Center GroupPI

  Not provided

Education

• B.S.

  MIT

  1987

• Ph.D.

  Caltech

  1992

Awards & honors

• Nobel Prize in Physics (2020)
• Crafoord Prize in Astronomy from the Royal Swedish Academy o…
• Bakerian Medal from the Royal Society of London
• MacArthur Fellowship
• election to the National Academy of Sciences