About
Professor Xin Liu is a faculty member at the University of Illinois Urbana-Champaign, holding positions in both the Astronomy department and the National Center for Supercomputing Applications (NCSA). She earned her B.S. in Physics from Tsinghua University in 2004 and completed her Ph.D. in Astrophysical Sciences at Princeton University in 2010 under the mentorship of Professor Michael A. Strauss. Prior to joining UIUC in 2015, she was a NASA Einstein Fellow at Harvard and a Hubble Fellow at UCLA. Her research interests lie at the intersection of astronomical survey science, data science, multi-messenger and time-domain astrophysics, and the application of artificial intelligence (AI) to scientific problems. Professor Liu's work addresses the challenges and opportunities presented by the large volumes of data generated by modern astronomical surveys. She focuses on advancing machine learning (ML) methodologies within astronomy, moving beyond the use of pre-packaged models to develop physics-informed learning, statistical learning with probabilistic frameworks that enable uncertainty quantification and generative modeling, and transparent, interpretable ML models that emphasize robustness, accuracy, and comprehensibility. Her research aims to integrate ML as an essential component of principled scientific analysis frameworks, reflecting the maturation of the field as it embraces big data and AI techniques. Her academic contributions are complemented by her teaching roles, where she offers courses such as AI and Big Data in Astronomy, The Big Bang, Black Holes, and the End of the Universe, and Extraterrestrial Life. Professor Liu has been recognized as a Norman P. Jones Professorial Scholar and an Excellent Teacher Ranked by Students, and she has been appointed as an NCSA Faculty Fellow. Her office is located in the Astronomy department at UIUC, where she continues to advance research at the forefront of astrophysics and data science.
Research topics
- Computer Science
- Astrophysics
- Astronomy
- Physics
- Chemistry
- Operating system
Selected publications
Photometric Redshifts in JWST Deep Fields: A Pixel-Based Alternative with DeepDISC
The Open Journal of Astrophysics · 2026-02-02 · 1 citations
articleOpen accessPhoto-z algorithms that utilize SED template fitting have matured, and are widely adopted for use on high-redshift near-infrared data that provides a unique window into the early universe. Alternative photo-z methods have been developed, largely within the context of low-redshift optical surveys. Machine learning based approaches have gained footing in this regime, including those that utilize raw pixel information instead of aperture photometry. However, the efficacy of image-based algorithms on high-redshift, near-infrared data remains underexplored. Here, we test the performance of Detection, Instance Segmentation and Classification with Deep Learning (DeepDISC) on photometric redshift estimation with NIRCam images from the JWST Advanced Deep Extragalactic Survey (JADES) program. DeepDISC is designed to produce probabilistic photometric redshift estimates directly from images, after detecting and deblending sources in a scene. Using NIRCam-only images and a compiled catalog of spectroscopic redshifts, we show that DeepDISC produces reliable photo-zs and uncertainties comparable to those estimated from template fitting using HST+JWST filters; DeepDISC even outperforms template fitting (lower scatter/fewer outliers) when the input photometric filters are matched. Compared with template fitting, DeepDISC does not require measured photometry from images, and can produce a catalog of 94000 photo-zs in ~4 minutes on a single NVIDIA A40 GPU. While current spectroscopic training samples are small and incomplete in color-magnitude space, this work demonstrates the potential of DeepDISC for increasingly larger image volumes and spectroscopic samples from ongoing and future programs. We discuss the impact of the training data on applications to broader samples and produce a catalog of photo-zs for all JADES DR2 photometric sources in the GOOD-S field, with quality flags indicating caveats.
VODKA: Complex molecular gas dynamics in a kpc-separation z=2.17 dual quasar with ALMA
ArXiv.org · 2025-02-07
preprintOpen accessIn galaxy mergers, dual quasars - two actively accreting supermassive black holes (SMBHs) - provide a unique opportunity to study the interplay between galaxy dynamics and quasar activity. However, very little is known about their molecular gas, which fuels star formation and quasar activity. In this study, we map the kinematics of the cold molecular gas in J0749+2255, a 3.8 kpc separation dual quasar at z=2.17 using the Atacama Large Millimeter Array (ALMA) Band 4. We detect CO(4-3)650um, which shows remarkably complex morphological and kinematic structures. While the integrated CO map suggested a lens-like ring, this feature disappears with kinematic decomposition. The kinematic analysis with ALMA resolves the ambiguities introduced by previous observations, further supporting the dual quasar interpretation of J0749+2255. We find two kinematically distinct molecular gas components: spatially extended, yet dynamically complex slow-moving gas (FWHM~130 km/s), and a compact, blueshifted, fast-moving, turbulent gas (FWHM~300 km/s). The disturbed kinematics, likely driven by the merger, show hints of rotation but no molecular outflows, suggesting circumnuclear flows. We estimate a large molecular gas reservoir ($M_{H2}\sim10^{10} M_{\odot}$), yet the starburst activity appears to exceed the available fuel. We detect an extended continuum in excess at rest-frame 455 GHz. The kinematic complexity of CO implicates the connection of mergers on the starburst and quasar activity in J0749+2255, yet whether J0749+2255 represents the dual quasar population remains unclear. Targeted kinematic studies of larger dual quasar samples will be essential to disentangling the nature of dual quasars.
2025-11-01
articleThis study proposes a novel no-collimator cascade coincidence imaging technique using dual detectors positioned in close to the target object. By eliminating collimators, the system significantly reduces photon absorption losses, thereby achieving high detection sensitivity compared with the traditional system. The method leverages spatial information from cascade coincidence photon pairs to enable multi-angle field fusion through simultaneous dual-detector acquisition, resulting in improved localization accuracy and superior image quality compared to conventional single-photon imaging. The proposed approach was validated using Monte Carlo simulations in the GATE platform, with point sources positioned at various locations within the field of view. We use the MLEM algorithm to reconstruct the image and evaluate the performance indicators. The results demonstrated notable improvements: the signal-to-noise ratio of the reconstructed image is 4.0031, the resolution (Full Width at Half Maximum, FWHM) is 7.1097. And the sensitivity of the system is 0.19 %. These findings confirm that no-collimator cascade coincidence system can perform highly sensitive imaging, particularly suitable for low-dose imaging scenarios. The technique holds strong potential for applications in pharmaceutical research, therapeutic development, and radiation-sensitive clinical procedures requiring minimal radiation exposure.
ArXiv.org · 2025-05-16
preprintOpen accessSub-parsec (sub-pc) binary supermassive black holes (BSBHs) should be common from galaxy mergers, yet direct evidence has been elusive. We present HST/WFC3IR F160W imaging for a sample of 8 candidate sub-pc BSBHs at redshifts z~0.1--0.5, as well as cross-comparison with a sample of ordinary quasars with archival HST/WFC3 IR F160W images. These 8 candidate sub-pc BSBHs were identified from multi-epoch spectroscopic surveys of quasars (including both typical quasars and those with single-peaked velocity-offset broad lines). whose broad H$β$ lines are significantly offset (by ~< a few hundred km/s) from the systemic redshifts. We directly test the prediction that the host galaxies of BSBHs would have a higher fraction of disturbed morphologies and younger stellar bulges from recent interactions than those of control quasars. After careful subtraction of the central quasar light, our candidate BSBH hosts show a statistically undifferentiated distribution of host asymmetry, indicative of a similar fraction of recent mergers. While a significantly larger sample is needed to place this result on a much firmer statistical ground, it opens questions as to the timescale differences between galaxy merger and BSBH formation, or the efficacy of the radial-velocity-shift--based selection of sub-pc BSBH candidates.
The Astrophysical Journal · 2025-08-07 · 7 citations
articleOpen accessCorrespondingAbstract Dual active galactic nuclei (AGNs), a phase in some galaxy mergers during which both central supermassive black holes (SMBHs) are active, are expected to be a key observable stage leading up to SMBH mergers. Constraining the population of dual AGNs in both the nearby and high- z Universe has proven to be elusive until very recently. We present a multiwavelength follow-up campaign to confirm the nature of a sample of 20 candidate dual AGNs at cosmic noon ( z ∼ 2) from the VODKA sample. Through a combination of Hubble Space Telescope and Very Large Array imaging, we refute the possibility of gravitational lensing in all but one target. We find evidence of dual AGNs in three systems, while seven exhibit a single AGN in galaxy pairs, through either strong radio emission or ancillary emission-line data. The remaining systems are confirmed as either quasar−star superpositions (seven) or nonlensed pairs (two) that require further investigations to establish AGN activity. Among the systems with radio detections, we find a variety of radio spectral slopes and UV/optical colors suggesting that our sample contains a range of AGN properties, from obscured radio-quiet objects to those with powerful synchrotron-emitting jets. This study presents one of the largest dedicated multiwavelength follow-up campaigns to date searching for dual AGNs at high redshift. We confirm several of the highest- z systems at small physical separations, thus representing some of the most evolved dual-AGN systems at the epoch of peak quasar activity known to date.
The Astrophysical Journal · 2025-07-18 · 3 citations
articleOpen accessAbstract High spatial and spectral resolution observations are essential for identifying subarcsecond dual and lensed quasars and confirming their redshifts. We present Gemini/Gemini Multi-Object Spectrograph and Hubble Space Telescope/STIS optical spectra for 27 dual quasar candidates selected based on their variability-induced astrometric noise or double detections in Gaia (the Varstrometry for Off-nucleus and Dual sub-Kpc AGN (VODKA) project). From this follow-up, we spectroscopically identify 11 star superpositions and seven dual/lensed quasars. Among the remaining targets, two are likely dual/lensed quasars based on additional radio imaging, while the rest are quasars with unknown companions. Without prior photometric or spectroscopic selection, we find the star contamination rate to be 41%–67%, while the dual/lensed quasar fraction is ≳26% in the follow-up VODKA sample. However, when combined with existing unresolved spectra and spatially resolved two-band color cuts, the dual/lensed quasar fraction can be increased to ≳67%. Our study highlights the need for high-quality spectral data, including a signal-to-noise ratio of at least 20, spatial resolution that is at least twice finer than the source separation, and a spectral resolution of R ≳ 1000, in order to separate close sources, exclude stellar superpositions, and reliably identify dual quasars.
Aerospace Science and Technology · 2025-10-28 · 1 citations
article1st authorThe Astrophysical Journal · 2025-11-24 · 1 citations
articleOpen accessAbstract We study the black hole mass–host galaxy stellar mass relation, M BH – M * , for a sample of 706 z ≲ 1.5 and i ≲ 24 optically variable active galactic nuclei (AGNs) in three Dark Energy Survey (DES) Deep Fields: C3, X3, E2, which partially cover Chandra Deep Field-South, XMM Large Scale Structure survey, and European Large Area ISO Survey, respectively. The parent sample was identified by optical variability from the DES supernova survey program imaging. Using publicly available spectra and photometric catalogs, we consolidate their spectroscopic redshifts, estimate their black hole masses using broad line widths and luminosities, and obtain improved stellar masses using spectral energy distribution fitting from X-ray to mid-infrared wavelengths. Our results confirm previous work from Hyper-Suprime Camera imaging that variability searches with deep, high-precision photometry can reliably identify AGNs in low-mass galaxies up to z ∼ 1. However, we find that the hosted black holes are more massive than predicted by the local AGN relation, fixing host galaxy stellar mass. Instead, z ∼ 0.1–1.5 variability-selected AGNs lie in between the M BH – M * relation for local inactive early-type galaxies and local active galaxies. This result agrees with most previous studies of the M BH – M * relation for AGNs at similar redshifts, regardless of the selection technique. We demonstrate that studies of variability-selected AGN provide critical insights into the low-mass end of the M BH – M * relation, shedding light on the occupation fraction of that provides constraints on early black hole seeding mechanisms and self-regulated feedback processes during their growth and coevolution with their hosts.
The Astrophysical Journal · 2025-08-08
articleOpen accessCorrespondingAbstract Subparsec binary supermassive black holes (BSBHs) should be common from galaxy mergers, yet direct evidence has been elusive. We present Hubble Space Telescope (HST)/WFC3IR F160W imaging for a sample of eight candidate subparsec BSBHs at redshifts z ∼ 0.1–0.5, as well as cross-comparison with a sample of ordinary quasars with archival HST/WFC3 IR F160W images. These eight candidate subparsec BSBHs were identified from multiepoch spectroscopic surveys of quasars (including both typical quasars and those with single-peaked velocity-offset broad lines), whose broad H β lines are significantly offset (by ≳ a few hundred kilometers per second) from the systemic redshifts. We directly test the prediction that the host galaxies of BSBHs would have a higher fraction of disturbed morphologies and younger stellar bulges from recent interactions than those of control quasars. After careful subtraction of the central quasar light, our candidate BSBH hosts show a statistically undifferentiated distribution of host asymmetry, indicative of a similar fraction of recent mergers. While a significantly larger sample is needed to place this result on a much firmer statistical ground, it opens questions as to the timescale differences between galaxy merger and BSBH formation, or the efficacy of the radial-velocity-shift-based selection of subparsec BSBH candidates.
The Astrophysical Journal · 2025-03-01 · 8 citations
articleOpen accessAbstract The search for dual supermassive black holes (SMBHs) is of immense interest in modern astrophysics. Galaxy mergers may fuel and produce SMBH pairs. Actively accreting SMBH pairs are observed as dual quasars, which are vital probes of SMBH growth. Dual quasars at cosmic noon are not well characterized. Gaia observations have enabled a novel technique to identify dual quasars at kiloparsec scales based on the small jitters of the light centroid as the two quasars vary stochastically. We present the first detailed study of a z = 2.17, 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 46, 3.8 kpc separation dual quasar, J0749+2255, using JWST/NIRSpec integral field unit spectroscopy. Identified by Gaia, J0749+2255 is one of the most distant small-separation dual quasars known. We detect the faint ionized gas of the host galaxy, traced by the narrow H α emission. Line ratios indicate ionization from the two quasars and from intense star formation. Spectral analysis of the two quasars suggests that they have similar black hole properties, hinting at the possible synchronized accretion activity or lensed quasar images. Surprisingly, the ionized gas kinematics suggest a rotating disk rather than the disturbed system expected in a major gas-rich galaxy merger. Numerical simulations show that this is a plausible outcome of a major gas-rich galaxy merger several tens of Myr before coalescence. Whether J0749+2255 reflects an interesting phase of dual quasar evolution or is a lensed quasar remains unclear. Thus, this study underscores the challenges in definitively distinguishing between dual and lensed quasars, with observations supporting either scenario.
Recent grants
NSF · $354k · 2016–2019
NSF · $367k · 2022–2026
NeTS: Small: Learning-Guided Network Resource Allocation: A Closed-Loop Approach
NSF · $479k · 2017–2021
CIF: Small: The Power of Online Learning in Stochastic System Optimization
NSF · $377k · 2014–2018
WoU-MMA: Dwarf AGNs from Variability for the Origins of Seeds (DAVOS)
NSF · $444k · 2023–2027
Frequent coauthors
- 209 shared
Yue Shen
- 65 shared
Yu-Ching Chen
Bloomberg (United States)
- 63 shared
Hengxiao Guo
Shanghai Astronomical Observatory
- 61 shared
Colin J. Burke
- 61 shared
E. Bertin
Orange (France)
- 57 shared
L. N. da Costa
Laboratório Interinstitucional de e-Astronomia
- 56 shared
M. A. G. Maia
Laboratório Interinstitucional de e-Astronomia
- 54 shared
M. Carrasco Kind
Urbana University
Labs
Department of Astronomy, University of Illinois at Urbana-Champaign
Education
- 2010
PhD, Astrophysical Sciences
Princeton University
- 2006
Master of Science, Physics
Tsinghua University
- 2004
Bachelor of Science, Physics
Tsinghua University
Awards & honors
- Norman P. Jones Professorial Scholar (2023-2026)
- Excellent Teacher Ranked by Students (2023)
- NCSA Faculty Fellow (2020 & 2023)
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