University of Virginia, College and Graduate School of Arts and Sciences

Near-Field Cosmology


Welcome to the Kallivayalil 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.





Nitya Kallivayalil

Assistant Professor
Nitya is an assistant professor 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



            Tobias Fritz 

Research Associate
Tobias is a postdoc and the PI of the Gemini Large program GS-2014B-LP-2. In addition he uses other facilities like the LBT to obtain proper motions of globular clusters and dwarf galaxies. CV


Graduate Students

Paul Zivick

Paul is a third year graduate student in the Department of Astronomy at the University of Virginia. He received a B.S. in Physics and Astrophysics with Honors and Research Distinction from The Ohio State University in Spring 2015. His previous research focused on using voids in galaxy distributions as cosmological tracers. At UVa he works on measurements of proper motions of Milky Way halo objects.

Sean Linden

Sean is a fourth year graduate student in the Department of Astronomy at the University of Virginia. Sean received his undergraduate BS in Astronomy at Case Western Reserve University working with Chris Mihos on simulating the dynamical history of M101. Here at UVA Sean primarily studies the star formation properties of Luminous and Ultra-Luminous Infrared Galaxies (U/LIRGs) at high-resolution with HST and the VLA. Sean is also developing a Sextractor+PSFeX python-based code to produce model PSF grids for ground-based AO data.


Undergraduate Students

Martine Lokken                      
Martine Lokken is a rising fourth-year undergraduate student. As an Astronomy-Physics major, she hopes to pursue a career in research, potentially in the field of cosmology.


Past students: Dylan Angell, Alex Bixel, Melanie Grierson, Lucas Beane, Joy Skipper

Recent Research Projects and Papers

The Proper Motion of Pyxis: the first use of Adaptive Optics in tandem with HST on a faint halo object:

In Fritz, Linden, Zivick, Kallivayalil et al. 2017 we present a proper motion measurement for the halo globular cluster Pyxis, using HST/ACS data as the first epoch, and GeMS/GSAOI Adaptive Optics data as the second, separated by a baseline of ∼ 5 years. Our inertial reference frame consists of background galaxies. This is both the first measurement of the proper motion of Pyxis, as well as the first calibration and use of Multi-Conjugate Adaptive Optics data to measure an absolute proper motion for a faint, distant halo object. We use the obtained three-dimensional velocity of Pyxis and dynamical modeling to show that Pyxis is not plausibly a progenitor of the ATLAS stream. We use a cosmological numerical simulation of the Milky Way with an LMC analog to show that Pyxis is very unlikely to be associated with the Magellanic system. The eccentric orbit strengthens the case for an extragalactic origin of Pyxis. The metallicity and age of Pyxis points to an origin from a rather massive former host, at least the mass of Leo II. This work was highlighted in a Gemini Press Release.


The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams:

In Bovy, Bahmanyar, Fritz & Kallivayalil 2016 we constrain the shape of the Milky Way's halo by dynamical modeling of the observed phase-space tracks of the Pal 5 and GD-1 tidal streams. We find that the only information about the potential gleaned from the tracks of these streams are precise measurements of the shape of the gravitational potential---the ratio of vertical to radial acceleration---at the location of the streams, with weaker constraints on the radial and vertical accelerations separately. The latter will improve significantly with precise proper-motion measurements from Gaia. We measure that the overall potential flattening is 0.95 +/- 0.04 at the location of GD-1 ([R,z] ~ [12.5,6.7] kpc) and 0.94 +/- 0.05 at the position of Pal 5 ([R,z] ~ [8.4,16.8] kpc). Combined with constraints on the force field near the Galactic disk, we determine that the axis ratio of the dark-matter halo's density distribution is 1.05 +/- 0.14 within the inner 20 kpc, with a hint that the halo becomes more flattened near the edge of this volume. The halo mass within 20 kpc is 1.1 +/- 0.1 x 10^{11} M_sun. A dark-matter halo this close to spherical is in tension with the predictions from numerical simulations of the formation of dark-matter halos.


Identifying true satellites of the Magellanic Clouds:

In Sales, Navarro, Kallivayalil & Frenk 2016 we explore which of the newly found low mass satellites might have been brought in by the Magellanic System. The hierarchical nature of LCDM suggests that the Magellanic Clouds must have been surrounded by a number of satellites before their infall into the Milky Way. Many of those satellites should still be in close proximity to the Clouds, but some could have dispersed ahead/behind the Clouds along their Galactic orbit. Either way, prior association with the Clouds results in strong restrictions on the present-day positions and velocities of candidate Magellanic satellites: they must lie close to the nearly-polar orbital plane of the Magellanic stream, and their distances and radial velocities must follow the latitude dependence expected for a tidal stream with the Clouds at pericenter. We use a cosmological numerical simulation of the disruption of a massive subhalo in a Milky Way-sized LCDM halo to test whether any of the 20 low mass satellites recently-discovered in the DES, SMASH, Pan-STARRS, and ATLAS surveys are truly associated with the Clouds. Of the 6 systems with kinematic data, only Hydra II and Hor 1 have distances and radial velocities consistent with a Magellanic origin. Of the remaining low mass satellites, six (Hor 2, Eri 3, Ret 3, Tuc 4, Tuc 5, and Phx 2) have positions and distances consistent with a Magellanic origin, but kinematic data are needed to substantiate that possibility. Conclusive evidence for association would require proper motions to constrain the orbital angular momentum direction, which, for true Magellanic satellites, must coincide with that of the Clouds. We use this result to predict radial velocities and proper motions for all new low mass satellites. Our results are relatively insensitive to the assumption of first or second pericenter for the Clouds.


Astrometry with MCAO at Gemini and at ELTs:

In Fritz et al. 2016 we present a first analysis of the astrometric error budget of absolute astrometry relative to background galaxies using adaptive optics. We use for this analysis multi-conjugated adaptive optics (MCAO) images obtained with GeMS/GSAOI at Gemini South. We find that it is possible to obtain 0.3 mas reference precision in a random field with 1 hour on source using faint background galaxies. Systematic errors are correctable below that level, such that the overall error is approximately 0.4 mas. Because the reference sources are extended, we find it necessary to correct for the dependency of the PSF centroid on the used aperture size, which would otherwise cause an important bias. This effect needs also to be considered for Extremely Large Telescopes (ELTs). When this effect is corrected, ELTs have the potential to measure proper motions of dwarf galaxies around M31 with 10 km/s accuracy over a baseline of 5 years.


The Proper Motion of Palomar 5:

Fritz & Kallivayalil (2015) - Used UVa’s Large Binocular Telescope in tandem with the Sloan Digital Sky Survey to measure the first CCD (charge coupled device)-based proper motion for the globular cluster Palomar 5 which is being tidally disrupted by the Milky Way. Subsequent modeling of this disruption shows surprising evidence that the Milky Way dark halo is adequately described by a spherical potential, rather than a triaxial one.


Probing the dark halo of the Milky Way with GeMS/GSAOI

We are developing the use of Adapative Optics (AO) methods to measure proper motions (PMs) for a wide variety of tracers in the Local Group, too faint for GAIA astrometry. The main scientific goals of this program are to definitively constrain the dark halo shape, orientation, radial profile and total mass of the Milky Way. PMs are difficult to measure. The size of the subtended motion in the plane of the sky at typical halo distances (50 kpc) is very small compared to the precisions achievable with normal ground-based telescopes. With Hubble Space Telescope (HST) techniques, 10 km/s accuracy has been achieved at such distances. However, the continued use of HST is obviously limited by its lifetime, and only a fraction of Local Group substructure has been investigated. AO techniques are the most promising long-term avenue. The aim here is to develop multi-conjugate AO methods for measuring high accuracy PMs within the context of a recently approved Long/Large Gemini program. This will also serve as an anchor point of HST optical to K-band AO, which can be applied to much of the substantial HST archive, including M31 substructures, with potential for very high accuracy when extended with extremely large telescopes in the future.

Images of Pyxis (left) and Carina (right) taken using the GeMS/GSAOI system on Gemini South.

*If you are interested in using this data for your work, please contact Nitya Kallivayalil at

Outreach and Summer Research Programs

For more on our group's Summer Research Programs, see here.

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 from under-represented groups.

This year our program's second class included Zaniyah Dock (pictured below and to the left) and Temi Olatinwo (below and to the right). 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. Temi's Presentation. Zaniyah's Presentation.


Spelman Info session 2017: We recently spent some time at Spelman College speaking to a range of Physics and Dual Degree Physics and Engineering majors about opportunities at UVa.

Downloadable copy


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


Mayla McCray

Mayla is a third-year undergraduate student at Spelman College. As a Physics : Dual Degree Engineering major, she hopes to use this summer research experience to pursue a career in engineering. Mayla studied the kinematic properties of stars to identify stars belonging to the Orphan stream. Mayla's Research Presentation

Colleen Smith-Patikas

Colleen is a third-year undergraduate student at Spelman College in the Physics : Dual Degree Engineering Program. This summer Colleen studied the chemical properties of stars to identify stars belonging to the Orphan stream. Colleen's Research Presentation


VA-NC Alliance


Jordan Glisan

Jordan is a second year undergraduate at the Virginia Commonwealth University in the Engineering program. This summer he studied the kinematic and chemical properties of stars to identify stars belonging to the M13 globular cluster. Jordan's Research Presentation


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. This year the students (Harrison Fields, Noah Baumgartner, and Andrew Childers, pictured below with Dom Pesce and Paul Zivick) worked on analyzing spectroscopic data of globular clusters from the APOGEE survey.