Near-Field Cosmology

Overview

Welcome to the Near-Field Cosmology Research Group at UVa Astronomy. A primary goal of this group is to test the concordance Lambda (dark energy) + cold dark matter (LCDM) theory on small scales where tension between theory and data are mounting, i.e., in the Local Group, consisting of the Milky Way, Andromeda, and their satellite populations.

At the 2019 PECASE award ceremony, with Dr. Kelvin Droegemeier, Director of the White House Office of Science and Technology Policy (Photo credit: Donica Payne).

Personnel

Faculty

Nitya Kallivayalil

Professor
Nitya is a professor and Dean's Research Fellow in Astronomy. She graduated from Mount Holyoke College where she majored in Physics, and earned a Ph.D in Astronomy from Harvard University. Before coming to UVa in 2013, she was a Pappalardo Fellow at MIT and a Prize Fellow at the Yale Center for Astronomy & Astrophysics. CV

Post-Doctoral Fellows

	Andrew Pace GECO Fellow Andrew’s research centers on near-field cosmology, using the structure and kinematics of Milky Way and Local Group dwarf galaxies, star clusters, and stellar streams to study dark matter and galaxy formation. His work combines dynamical modeling, proper motions, stellar populations, and tidal/orbital analyses to infer the mass profiles and evolutionary histories of low-mass galaxies and their relation to the Milky Way halo.
Image	Niusha Ahvazi GECO Fellow Niusha’s research focuses on the formation and evolution of low-mass galaxies, especially Milky Way dwarf satellites, using semi-analytic models and cosmological simulations to connect galaxy properties to their underlying dark matter halos. Her work emphasizes faint dwarf galaxies, environmental effects, baryonic-physics uncertainties, and how small-scale galaxy populations can be used to test dark matter and galaxy-formation models.
Image	Xiaowei Ou GECO Fellow Xiaowei’s research uses stellar kinematics and chemistry to study the Milky Way’s formation history and dark matter distribution, combining large survey data, high-resolution spectroscopy, and cosmological simulations. Her work connects dynamical modeling of the Galactic potential with chemo-dynamical studies of accreted stellar populations and dwarf galaxies, with an emphasis on using the Milky Way as a near-field laboratory for galaxy formation and dark matter physics.
	Mairéad Heiger GECO Fellow Mairéad's research focuses on galaxy evolution and galactic archaeology, with an emphasis on chemical evolution in dwarf galaxies. Mairéad will use simulation-based inference to constrain the chemical evolution of galaxies. Mairéad brings expertise in observational galaxy formation that will connect the galaxy formation models, likelihood free inference, and cutting-edge observations.

Graduate Students

Andrés Almeida

Andrés is a sixth year graduate student in the Department of Astronomy at UVA. He works with Steve Majewski. His research focuses on the Magellanic Clouds and Local Group dwarf-galaxy dynamics, using Gaia astrometry together with APOGEE radial velocities and chemical abundances to study stellar populations, tidal structure, and galaxy interactions. His work on the Small Magellanic Cloud interprets distance and kinematic substructure as evidence of tidal perturbation by the Large Magellanic Cloud, connecting resolved stellar data to the dynamical evolution of interacting dwarf galaxies.

Kaia Atzberger

Kaia is a third year graduate student in the Department of Astronomy at UVA. She received a B.S. in Astronomy & Astrophysics from the Ohio State University and is currently an NSF Graduate Research Fellow. Her research focuses on using metallicity-sensitive DECam photometry and high-resolution Magellan spectroscopy to study diffuse Milky Way satellites and their faint stellar streams.

Yanbo Pan

Yanbo is a second year graduate student in the Department of Astronomy at UVA. He works with Steve Majewski. He received a B.S. in Astronomy and Statistics from University of Michigan. His current research focuses on resolved stellar populations, binary stars, and statistical techniques in Astrophysics.

Undergraduate Students

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Previous Members

	Jack Warfield Jack defended his PhD at the University of Virginia in the Spring of 2026. He is now a postdoctoral researcher at the University of California Berkeley working with Dan Weisz. His research focused on using data from the Hubble Space Telescope to study the dynamics of Milky Way and Andromeda satellites. He received a B.S. in Physics and Astronomy & Astrophysics from the Ohio State University where he worked on asteroseismology using data from the APOGEE survey.
Image	Chris Garling Chris Garling was a Near-Field Cosmology Prize Research Associate at the University of Virginia. He received his Bachelor's degree from Haverford College where he worked with Beth Willman on variable stars in the Hercules ultra-faint dwarf galaxy. He subsequently earned his PhD from Ohio State, where he worked with Annika Peter on observational and theoretical aspects of near-field cosmology, focusing on low-density environments. Chris is interested in what low-mass dwarf galaxy systems can teach us about fundamental astrophysics like star formation (for example, the stellar-mass-halo-mass relation) and cosmology (for example, the halo and subhalo mass functions) using data from HST and JWST in conjunction with theoretical interpretation using semi-analytic models and cosmological simulations.
Image	Hannah Richstein Hannah defended her PhD at the University of Virginia in the Spring of 2024. Her research focused on using deep HST photometry to study the stellar populations of ultra-faint dwarf galaxies in the Local Group. She received a B.S. in Astronomy and Physics from Texas Christian University where she worked on interacting galaxies with data from the SDSS MaNGA survey.
Image	Paul Zivick Paul defended his PhD at the University of Virginia in the Spring of 2020. For his dissertation, Paul focused primarily on characterizing and building models to describe the kinematic structure of the Small Magellanic Cloud using a combination of space-based data from HST and the Gaia Space Telescope and ground-based spectroscopic data from the APOGEE survey. He was then a postdoctoral research associate at Texas A&M University working with Louis Strigari and Jennifer Marshall on studying the internal kinematics of nearby dwarf galaxies. Paul is now in the field of data science.
Image	Sean Linden Sean defended his PhD at the University of Virginia in the Spring of 2020. He is now a postdoctoral researcher at the University of Massachusetts Amherst working with Daniela Calzetti on studies of nearby galaxies. At UVa Sean primarily studied the star formation properties of Luminous and Ultra-Luminous Infrared Galaxies (U/LIRGs) at high-resolution with HST, VLA, and soon JWST (Hopefully!). Sean has also worked to develop a Source Extractor+PSFeX python-based codes to produce model PSF grids and photometry for any ground-based AO system. `
Image	Tobias Fritz Tobias was a postdoc and the PI of the Gemini Large program GS-2014B-LP-2. In addition he used other facilities like the LBT to obtain proper motions of globular clusters and dwarf galaxies. He worked with the group for four years, then went on to a Fellowship at the IAC in the Canary Islands.
Image	Madeline Gyllenhoff Madeline graduated from the University of Virginia in Spring 2022, receiving her B.S. in Astronomy-Physics with a minor in Computer Science. Her research focused on computational analysis of the morphology of ultra-faint dwarf galaxies using data from the Hubble Space Telescope.
Image	Hannah Woodward Hannah graduated from the University of Virginia in Spring 2022 with a B.S. in Astrophysics. She worked with the Satellites Around Galactic Analogs (SAGA) team to characterize hosts in nearby satellite systems.
Image	Molly Childers Molly graduated from the University of Virginia in Spring 2021, receiving degrees in Astronomy and Computer Science. For her senior thesis, she studied the morphologies of ultra-faint dwarf galaxies as a window into the dark matter universe and galaxy formation.
Image	Martine Lokken Martine Lokken graduated from the University of Virginia in Spring 2018 with a B.S. in Astronomy and Physics, she hopes to pursue a career in research, potentially in the field of cosmology. She is now attending the University of Toronto for graduate school
Image	Sam Welch Sam graduated from the University of Virginia with his B.A. in Astronomy and his B.A. in Computer Science. His research focused on analyzing clusters in the Small Magellanic Cloud using proper motions and other data from Gaia.

Past students:

Leslie Kim, Dylan Angell, Alex Bixel, Melanie Grierson, Lucas Beane, Joy Skipper, Danny Devlin

News

University of Virginia Art Professor William Wylie and Professor of Astronomy Nitya Kallivayalil transform Sloan Digital Sky Survey data into a striking visual experience at the Contemplative Sciences Center:
- Firmament: New Exhibition Blends Art and Astrophysics Using 19th-Century Cyanotype and Modern Star Plates
Astronomers at the University of Virginia have identified one of the most iron-poor stars ever observed outside the Milky Way, a discovery that offers rare insight into how the very first stars lived and died:
- Stellar Fossil Offers Insight into the Early Universe
UVA researchers, including Nitya Kallivayalil, will co-host the NSF-Simons AI Institute for Cosmic Origins, a new, multi-institutional initiative that will address the challenge of using artificial intelligence to accelerate the pace of astronomical discovery:
- UVA to Co-Host AI-Powered Astronomy Institute
We're involved in an Early Release Science (ERS) program with JWST! Here's an update on how that's going:
- How the NIRCam Flux Calibration is Improving

Recent Projects

From observing strategies to velocity dispersion bias: forward modeling unresolved binaries in ultra-faint dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) in the Milky Way are ideal probes of low-mass galaxy formation and dark matter because they are highly dark-matter dominated. In the lowest-mass systems, however, unresolved binary orbital motion complicates dynamical mass estimates by inflating measured velocity dispersions and biasing the interpretation of the inferred masses. We develop a flexible forward-modeling pipeline, the Binary Observation Simulator, to generate mock multi-epoch radial-velocity observations of binary populations in dwarf galaxies and to quantify how binary-induced biases depend on both galaxy properties and observing strategy. We generate mock samples spanning true velocity dispersion of $σ_{t r u e} \sim 0.75$ - $3.5$ km/sec, binary fractions of $0.1$ - $0.9$ , and multi-epoch baselines of up to $10$ yr. We find that, while multi-epoch monitoring reduces the binary-induced bias, residual contamination remains significant for low-mass halos: even a $10$ -yr baseline can leave $\sim 10$ - $120 %$ relative bias for systems with $σ_{t r u e} ≲ 1$ km/sec. We also note that small sample sizes ( $≲ 20$ ) introduce substantial stochastic scatter in the recovered dispersions, sometimes masking the expected improvement from longer observational baselines. Applying the framework to the real $\sim 17$ -yr Bootes I observing record reproduces the literature velocity dispersions, yields an empirical correction conditioned on that cadence, and shows that redistributing the same observations onto half the number of stars would have removed roughly half of the residual binary bias. These results emphasize that robust dispersion estimates for the faintest dwarfs require forward modeling of both binary populations and survey cadence.

Research Papers

Ou, Pace, Kallivayalil et al. 2026

The DECam MAGIC Survey: Uncovering the Tidal Tails of the Crater II Dwarf Galaxy

Crater II (CraII), a large and low-density dwarf spheroidal galaxy, has unusual observed properties that are difficult to reproduce in cold dark matter simulations. Ongoing tidal disruption may help explain the discrepancies, as evidenced by the recent discovery of tidal tails. Here we present metallicity-sensitive narrowband photometry of the Ca II H and K lines from the Dark Energy Camera, covering 128 deg² across the center and identified tidal tails of CraII as part of the Mapping the Ancient Galaxy in CaHK survey. Our combined photometric metallicity, color─magnitude, proper motion, and parallax selections identify 162 CraII candidates. Of these, 37 candidates are located in the tidal tails, which extend at least 7° (∼95 kpc) from the center of CraII, suggesting it has lost ≳25% of its initial stellar mass. We confirm low contamination rates with dedicated control fields and highlight the extremely low surface brightness stellar features that can be uncovered with CaHK data, as faint as ∼36 mag arcsec⁻². We also make the first detection of a metallicity gradient (−0.34 ± 0.17 dex deg⁻¹) in the center of the galaxy and infer a stream width of w ∼ 0 .° 8, roughly 50% larger than the CraII half-light radius. The detection of candidates in the most distant CraII pointings from its center implies that the tidal tails extend beyond our footprint. We compare the CraII stream to N-body models with "cored" and "cuspy" dark matter halo progenitors, determining that CraII's density profile is still ambiguous and warrants further modeling.

Research Papers

Atzberger, Pace, Kallivayalil et al. 2026

The abundance and properties of the lowest luminosity dwarf galaxies around the Milky Way: Insights from Semi-Analytic Models

We investigate the formation and observable properties of faint satellite galaxies (M $_{V} > - 3$ ) in Milky Way-like halos using the semi-analytic galaxy formation model Galacticus. The ability of the smallest dark matter halos to form stars depends sensitively on the balance between gas cooling and reionization heating. To quantify how this balance shapes the abundance and properties of the faintest galaxies, we compare two model variants: a fiducial model that includes molecular hydrogen (H $_{2}$ ) cooling and UV background radiation, and a No-H $_{2}$ model with atomic cooling only. Both models reproduce the structural properties of brighter Milky Way satellites, but they diverge at the lowest luminosities in the hyper-faint regime. The fiducial model predicts a substantially larger population of such systems that are on average hosted in halos with lower peak masses and quenched earlier. Many of these predicted systems lie below current observational thresholds but are within reach of next-generation deep imaging surveys. The predicted size-luminosity distributions of both models overlap with the region occupied by recently discovered "ambiguous" systems, whose classification as galaxies or star clusters remains uncertain. Specifically, we find that hyper-faint satellites have line-of-sight velocity dispersions of $σ_{l o s} \sim 1 - 3$ km/s in the fiducial model, nearly an order of magnitude higher than expected for purely self-gravitating stellar systems of the same stellar mass. This distinction underscores the diagnostic power of precise kinematic measurements for determining whether ambiguous objects are dark matter dominated dwarf galaxies or star clusters, and highlights the importance of upcoming spectroscopic campaigns in resolving the nature of the faintest satellites.

Research Papers

Ahvazi, Pace, Garling et al. 2025

Older Projects

SciX Library

Outreach and Summer Research Programs

UVA-Spelman Summer Research Program

We are proud to partner with the Spelman College Physics department to build a pipeline for training future STEM leaders.

Previous Students:

2019:

Our program's fourth class featured Nina Warren and Chloe Merriweather. Nina and Chloe used data from the Gaia Space Telescope to examine the globular clusters NGC 7089 and NGC 7099. With the cutting-edge astrometric information from Gaia, they measured the mass of the clusters and searched for stars that originally belonged to the clusters but have since been stripped away by the Milky Way.

2018:

Our program's third class Pruitt and Katelyn Jackson.

Kimara studied the properties of dust scattering in protoplanetary disks. Her primary accomplishment was the development and testing of a module to include dust scattering and absorption in an existing Monte Carlo radiation transfer code.

Katelyn studied the orbital dynamics of extrasolar planets. (Presentation)

2017:

Our program's second class included Zaniyah Dock and Temi Olatinwo. Their work focused on determining the membership of Milky Way globular clusters using spectroscopic data, and they presented their results at the 2017 Leadership Alliance National Symposium.

2016:

Our debut class of 2016 Summer Researchers included Mayla McCray and Colleen Smith-Patikas, who are both Physics & Engineering double majors at Spelman.

UVA-Central Virginia Governor's School

In Fall 2016, we began a mentoring project with the Central Virginia Governor's School in Lynchburg, VA to help high school juniors design, develop, and carry out astronomy related research projects.

Past students: Daniel Moon (2018), Taylor Tolbert (2018), Charley Inman (2018), Madison Markham (2018), Ivy Wheaton (2017), Jeff Przybylek (2017), Andrew Childers (2016), Noah Baumgartner (2016), Harrison Fields (2016)