arXiv subjects

Andrea Ghez

Publications and source records attributed to Andrea Ghez.

At least 19 recordsLinked to original sources

arXiv2026

A Kinematic Study of Wolf-Rayet Stars at the Galactic Center I: Binary Candidates and Constraints on the Binary Fraction

We report the binary fraction of Wolf-Rayet (WR) 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 is 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 suggesting the presence of a companion for five out of 27 WR 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 WR stars in the Galactic center to be 0.56$\pm$0.18. This is consistent with previous photometric studies of the young stars in the Galactic center, and with the binary fraction of field WR stars. When our results are combined with the results of previous photometric work, we find a binary fraction of 0.69$\pm$0.17 for the WR stars in the Galactic center.

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arXiv2026

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

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.

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arXiv2025

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

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arXiv2024

Thirty Meter Telescope International Observatory Detailed Science Case 2024

The Thirty Meter Telescope (TMT) International Observatory (TIO) will be a revolutionary leap forward in astronomical observing capabilities, enabling us to address some of the most profound questions about the universe. From unraveling the mysteries of dark matter and dark energy to exploring the origins of stars and planets, TMT will transform our understanding of the cosmos. The TIO Detailed Science Case (DSC) presents science goals that inform the top-level requirements for the observatory's design and operations, including the telescope, enclosure, instruments, and adaptive optics system.

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arXiv2023

Constraining a companion of the galactic center black hole, Sgr A*

We use 23 years of astrometric and radial velocity data on the orbit of the star S0-2 to constrain a hypothetical intermediate-mass black hole orbiting the massive black hole Sgr A* at the Galactic center. The data place upper limits on variations of the orientation of the stellar orbit (inclination, nodal angle, and pericenter) at levels between 0.02 and 0.07 degrees per year. We use a combination of analytic estimates and full numerical integrations of the orbit of S0-2 in the presence of a black-hole binary. For a companion IMBH whose semi-major axis $a_c$ is larger than that of S0-2 (1020 a.u.), we find that in the region between 1000 and 4000 a.u., a companion black hole with mass $m_c$ between $10^3$ and $10^5 M_\odot$ is excluded, with a boundary behaving as $a_c \sim m_c^{1/3}$. For a companion with $a_c < 1020$ a.u., we find that a black hole with mass between $10^3$ and $10^5 \, M_\odot$ is again excluded, with a boundary behaving as $a_c \sim m_c^{-1/2}$. These bounds arise from quadrupolar perturbations of the orbit of S0-2. However, significantly stronger bounds on the mass of an inner companion arise from the fact that the location of S0-2 is measured relative to the bright emission of Sgr A*. As a consequence, that separation is perturbed by the ``wobble'' of Sgr A* about the center of mass between it and the companion, leading to ``apparent'' perturbations of S0-2's orbit that also include a dipole component. The result is a set of bounds as small as $400 \, M_\odot$ at 200 a.u.; the numerical simulations suggest a bound from these effects varying as $a_c \sim m_c^{-1}$. We compare and contrast our results with those from a recent analysis by the GRAVITY collaboration.

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arXiv2023

High-Resolution, Mid-Infrared Color Temperature Mapping of the Central 10 Arcseconds of the Galaxy

The neighborhood of the Galactic black hole boasts a plethora of extended interstellar gas and dust features as well as populations of compact (unresolved, or marginally resolved) features such as the G objects. Most are well manifested in the infrared. To disentangle and characterize the infrared structure of extended features and identify compact sources, we used 3.8~$μ$m (L' filter) data from the NIRC2 imager at the Keck Observatory and 8.6~$μ$m (PAH1 filter) data from the VISIR imager at the Very Large Telescope (VLT) to produce the highest-resolution mid-IR color-temperature map of the inner half-parsec of the Galactic Center to date. From this map, we compile a catalog of features that stand out from their background. In particular, we identify 33 compact sources that stand out against the local background temperature, 11 of which are newly identified and are candidates for being members of the G objects population. Additionally, we resolve and newly characterize the morphology of several known extended features. These results prepare the way for ongoing and future JWST studies that have access to a greater range of mid-infrared wavelengths, and thus will allow for refined estimation of the trends of dust temperatures.

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arXiv2023

An Updated Reference Frame for the Galactic Inner Parsec

Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 years, we re-analyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in RA and Dec., and the mean proper motion uncertainties are 0.07 mas yr$^{-1}$ and 0.12 mas yr$^{-1}$, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 AU and 6.9 AU and proper motion uncertainties of 2.7 km s$^{-1}$ and 4.6 km s$^{-1}$. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8 $μ$as yr$^{-1}$ (0.30 km s$^{-1}$) in RA and 11 $μ$as yr$^{-1}$ (0.44 km s$^{-1}$) in Dec., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.

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arXiv2023

The Galactic Center with Roman

We advocate for a Galactic center (GC) field to be added to the Galactic Bulge Time Domain Survey (GBTDS). The new field would yield high-cadence photometric and astrometric measurements of an unprecedented ${\sim}$3.3 million stars toward the GC. This would enable a wide range of science cases, such as finding star-compact object binaries that may ultimately merge as LISA-detectable gravitational wave sources, constraining the mass function of stars and compact objects in different environments, detecting populations of microlensing and transiting exoplanets, studying stellar flares and variability in young and old stars, and monitoring accretion onto the central supermassive black hole. In addition, high-precision proper motions and parallaxes would open a new window into the large-scale dynamics of stellar populations at the GC, yielding insights into the formation and evolution of galactic nuclei and their co-evolution with the growth of the supermassive black hole. We discuss the possible trade-offs between the notional GBTDS and the addition of a GC field with either an optimal or minimal cadence. Ultimately, the addition of a GC field to the GBTDS would dramatically increase the science return of Roman and provide a legacy dataset to study the mid-plane and innermost regions of our Galaxy.

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arXiv2023

Evidence of a Decreased Binary Fraction for Massive Stars Within 20 Milliparsecs of the Supermassive Black Hole at the Galactic Center

We present the results of the first systematic search for spectroscopic binaries within the central 2 x 3 arcsec$^2$ around the supermassive black hole at the center of the Milky Way galaxy. This survey is based primarily on over a decade of adaptive optics-fed integral-field spectroscopy (R$\sim$4000), obtained as part of the Galactic Center Orbits Initiative at Keck Observatory, and has a limiting $K$'-band magnitude of 15.8, which is at least 4 magnitudes deeper than previous spectroscopic searches for binaries at larger radii within the central nuclear star cluster. From this primary dataset, over 600 new radial velocities are extracted and reported, increasing by a factor of 3 the number of such measurements. We find no significant periodic signals in our sample of 28 stars, of which 16 are massive, young (main-sequence B) stars and 12 are low-mass, old (M and K giant) stars. Using Monte Carlo simulations, we derive upper limits on the intrinsic binary star fraction for the young star population at 47% (at 95% confidence) located $\sim$20 mpc from the black hole. The young star binary fraction is significantly lower than that observed in the field (70%). This result is consistent with a scenario in which the central supermassive black hole drives nearby stellar binaries to merge or be disrupted and may have important implications for the production of gravitational waves and hypervelocity stars.

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arXiv2022

AIROPA IV: Validating Point Spread Function Reconstruction on Various Science Cases

We present an analysis of six independent on-sky datasets taken with the Keck-II/NIRC2 instrument. Using the off-axis point spread function (PSF) reconstruction software AIROPA, we extract stellar astrometry, photometry, and other fitting metrics in order to characterize the performance of this package. We test the effectiveness of AIROPA to reconstruct the PSF across the field of view in varying atmospheric conditions, number and location of PSF reference stars, stellar crowding and telescope position angle (PA). We compare the astrometric precision and fitting residuals between a static PSF model and a spatially varying PSF model that incorporates instrumental aberrations and atmospheric turbulence during exposures. Most of the fitting residuals we measure show little to no improvement in the variable-PSF mode over the single-PSF mode. For one of the data sets, we find photometric performance is significantly improved (by ${\sim}10\times$) by measuring the trend seen in photometry as a function of off-axis location. For nearly all other metrics we find comparable astrometric and photometric precision across both PSF modes, with a ${\sim}13$% smaller astrometric uncertainty in variable-PSF mode in the best case. We largely confirm that the spatially variable PSF does not significantly improve the astrometric and other PSF fitting residuals over the static PSF for on-sky observations. We attribute this to unaccounted instrumental aberrations that are not characterized through afternoon adaptive optics (AO) bench calibrations.

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arXiv2021

Measuring the alpha-abundance of subsolar-metallicity stars in the Milky Way's central half-parsec: testing globular cluster and dwarf galaxy infall scenarios

While the Milky Way Nuclear star cluster has been studied extensively, how it formed is uncertain. Studies have shown it contains a solar and supersolar metallicity population that may have formed in-situ, along with a subsolar metallicity population that may have formed via mergers of globular clusters and dwarf galaxies. Stellar abundance measurements are critical to differentiate between formation scenarios. We present new measurements of [$M/H$] and $α$-element abundances [$α/Fe$] of two subsolar-metallicity stars in the Galactic Center. These observations were taken with the adaptive-optics assisted high-resolution (R=24,000) spectrograph NIRSPEC in the K-band (1.8 - 2.6 micron). These are the first $α$-element abundance measurements of sub-solar metallicity stars in the Milky Way nuclear star cluster. We measure [$M/H$]=$-0.59\pm 0.11$, [$α/Fe$]=$0.05\pm 0.15$ and [$M/H$]= $-0.81\pm 0.12$, [$α/Fe$]= $0.15\pm 0.16$ for the two stars at the Galactic center; the uncertainties are dominated by systematic uncertainties in the spectral templates. The stars have an [$α/Fe$] in-between the [$α/Fe$] of globular clusters and dwarf galaxies at similar [$M/H$] values. Their abundances are very different than the bulk of the stars in the nuclear star cluster. These results indicate that the sub-solar metallicity population in the Milky Way nuclear star cluster likely originated from infalling dwarf galaxies or globular clusters and are unlikely to have formed in-situ.

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arXiv2021

The InfraRed Imaging Spectrograph (IRIS) for TMT: photometric characterization of anisoplanatic PSFs and testing of PSF-Reconstruction via AIROPA

The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument for the Thirty Meter Telescope (TMT) that will be used to sample the corrected adaptive optics field by the Narrow-Field Infrared Adaptive Optics System (NFIRAOS) with a near-infrared (0.8 - 2.4 $μ$m) imaging camera and integral field spectrograph. To better understand IRIS science specifications we use the IRIS data simulator to characterize relative photometric precision and accuracy across the IRIS imaging camera 34"x34" field of view. Because the Point Spread Function (PSF) varies due to the effects of anisoplanatism, we use the Anisoplanatic and Instrumental Reconstruction of Off-axis PSFs for AO (AIROPA) software package to conduct photometric measurements on simulated frames using PSF-fitting as the PSF varies in single-source, binary, and crowded field use cases. We report photometric performance of the imaging camera as a function of the instrumental noise properties including dark current and read noise. Using the same methods, we conduct comparisons of photometric performance with reconstructed PSFs, in order to test the veracity of the current PSF-Reconstruction algorithms for IRIS/TMT.

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arXiv2020

Keck/NIRC2 $L$'-Band Imaging of Jovian-Mass Accreting Protoplanets around PDS 70

We present $L$'-band imaging of the PDS 70 planetary system with Keck/NIRC2 using the new infrared pyramid wavefront sensor. We detected both PDS 70 b and c in our images, as well as the front rim of the circumstellar disk. After subtracting off a model of the disk, we measured the astrometry and photometry of both planets. Placing priors based on the dynamics of the system, we estimated PDS 70 b to have a semi-major axis of $20^{+3}_{-4}$~au and PDS 70 c to have a semi-major axis of $34^{+12}_{-6}$~au (95\% credible interval). We fit the spectral energy distribution (SED) of both planets. For PDS 70 b, we were able to place better constraints on the red half of its SED than previous studies and inferred the radius of the photosphere to be 2-3~$R_{Jup}$. The SED of PDS 70 c is less well constrained, with a range of total luminosities spanning an order of magnitude. With our inferred radii and luminosities, we used evolutionary models of accreting protoplanets to derive a mass of PDS 70 b between 2 and 4 $M_{\textrm{Jup}}$ and a mean mass accretion rate between $3 \times 10^{-7}$ and $8 \times 10^{-7}~M_{\textrm{Jup}}/\textrm{yr}$. For PDS 70 c, we computed a mass between 1 and 3 $M_{\textrm{Jup}}$ and mean mass accretion rate between $1 \times 10^{-7}$ and $5 \times~10^{-7} M_{\textrm{Jup}}/\textrm{yr}$. The mass accretion rates imply dust accretion timescales short enough to hide strong molecular absorption features in both planets' SEDs.

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arXiv2019

Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole

General Relativity predicts that a star passing close to a supermassive black hole should exhibit a relativistic redshift. We test this using observations of the Galactic center star S0-2. We combine existing spectroscopic and astrometric measurements from 1995-2017, which cover S0-2's 16-year orbit, with measurements in 2018 March to September which cover three events during its closest approach to the black hole. We detect the combination of special relativistic- and gravitational-redshift, quantified using a redshift parameter, $Υ$. Our result, $Υ=0.88 \pm 0.17$, is consistent with General Relativity ($Υ=1$) and excludes a Newtonian model ($Υ=0$ ) with a statistical significance of 5 $σ$.

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arXiv2019

Envisioning the next decade of Galactic Center science: a laboratory for the study of the physics and astrophysics of supermassive black holes

As the closest example of a galactic nucleus, the Galactic center (GC) presents an exquisite laboratory for learning about supermassive black holes (SMBH) and their environment. We describe several exciting new research directions that, over the next 10 years, hold the potential to answer some of the biggest scientific questions raised in recent decades: Is General Relativity (GR) the correct description for supermassive black holes? What is the nature of star formation in extreme environments? How do stars and compact objects dynamically interact with the supermassive black hole? What physical processes drive gas accretion in low-luminosity black holes? We describe how the high sensitivity, angular resolution, and astrometric precision offered by the next generation of large ground-based telescopes with adaptive optics will help us answer these questions. First, it will be possible to obtain precision measurements of stellar orbits in the Galaxy's central potential, providing both tests of GR in the unexplored regime near a SMBH and measurements of the extended dark matter distribution that is predicted to exist at the GC. Second, we will probe stellar populations at the GC to significantly lower masses than are possible today, down to brown dwarfs. Their structure and dynamics will provide an unprecedented view of the stellar cusp around the SMBH and will distinguish between models of star formation in this extreme environment. This increase in depth will also allow us to measure the currently unknown population of compact remnants at the GC by observing their effects on luminous sources. Third, uncertainties on the mass of and distance to the SMBH can be improved by a factor of $\sim$10. Finally, we can also study the near-infrared accretion onto the black hole at unprecedented sensitivity and time resolution, which can reveal the underlying physics of black hole accretion.

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arXiv2019

The Galactic Center: An Improved Astrometric Reference Frame for Stellar Orbits around the Supermassive Black Hole

Precision measurements of the stars in short-period orbits around the supermassive black hole at the Galactic Center are now being used to constrain general relativistic effects, such as the gravitational redshift and periapse precession. One of the largest systematic uncertainties in the measured orbits has been errors in the astrometric reference frame, which is derived from seven infrared-bright stars associated with SiO masers that have extremely accurate radio positions, measured in the Sgr A*-rest frame. We have improved the astrometric reference frame within 14'' of the Galactic Center by a factor of 2.5 in position and a factor of 5 in proper motion. In the new reference frame, Sgr A* is localized to within a position of 0.645 mas and proper motion of 0.03 mas/yr. We have removed a substantial rotation (2.25 degrees per decade), that was present in the previous less-accurate reference frame used to measure stellar orbits in the field. With our improved methods and continued monitoring of the masers, we predict that orbital precession predicted by General Relativity will become detectable in the next ~5 years.

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arXiv2018

Characterizing and Improving the Data Reduction Pipeline for the Keck OSIRIS Integral Field Spectrograph

OSIRIS is a near-infrared (1.0--2.4 $μ$m) integral field spectrograph operating behind the adaptive optics system at Keck Observatory, and is one of the first lenslet-based integral field spectrographs. Since its commissioning in 2005, it has been a productive instrument, producing nearly half the laser guide star adaptive optics (LGS AO) papers on Keck. The complexity of its raw data format necessitated a custom data reduction pipeline (DRP) delivered with the instrument in order to iteratively assign flux in overlapping spectra to the proper spatial and spectral locations in a data cube. Other than bug fixes and updates required for hardware upgrades, the bulk of the DRP has not been updated since initial instrument commissioning. We report on the first major comprehensive characterization of the DRP using on-sky and calibration data. We also detail improvements to the DRP including characterization of the flux assignment algorithm; exploration of spatial rippling in the reduced data cubes; and improvements to several calibration files, including the rectification matrix, the bad pixel mask, and the wavelength solution. We present lessons learned from over a decade of OSIRIS data reduction that are relevant to the next generation of integral field spectrograph hardware and data reduction software design.

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arXiv2018

Super-Solar Metallicity Stars in the Galactic Center Nuclear Star Cluster: Unusual Sc, V, and Y Abundances

We present adaptive-optics assisted near-infrared high-spectral resolution observations of late-type giants in the nuclear star cluster of the Milky Way. The metallicity and elemental abundance measurements of these stars offer us an opportunity to understand the formation and evolution of the nuclear star cluster. In addition, their proximity to the supermassive black hole ($\sim 0.5$ pc) offers a unique probe of the star formation and chemical enrichment in this extreme environment. We observed two stars identified by medium spectral-resolution observations as potentially having very high metallicities. We use spectral-template fitting with the PHOENIX grid and Bayesian inference to simultaneously constrain the overall metallicity, [M/H], alpha-element abundance [$α$/Fe], effective temperature, and surface gravity of these stars. We find that one of the stars has very high metallicity ([M/H] $> 0.6$) and the other is slightly above solar metallicity. Both Galactic center stars have lines from scandium (Sc), vanadium (V), and yttrium (Y) that are much stronger than allowed by the PHOENIX grid. We find, using the spectral synthesis code Spectroscopy Made Easy, that [Sc/Fe] may be an order of magnitude above solar. For comparison, we also observed an empirical calibrator in NGC6791, the highest metallicity cluster known ([M/H] $\sim 0.4$). Most lines are well matched between the calibrator and the Galactic center stars, except for Sc, V, and Y, which confirms that their abundances must be anomalously high in these stars. These unusual abundances, which may be a unique signature of nuclear star clusters, offer an opportunity to test models of chemical enrichment in this region.

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