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


Graduate Students

Paul Zivick

Paul is a fourth 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 research focuses on studying the proper motions of Milky Way halo objects, in particular the Small Magellanic Cloud using data from HST and Gaia. CV

Sean Linden

Sean is a fifth 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.


Hannah Richstein

Hannah is a first year graduate student in the Department of Astronomy at the University of Virginia. She received a B.S. in Astronomy and Physics from Texas Christian University. She worked with Kat Barger on interacting galaxies in the SDSS MaNGA Survey. She has also done research involving dark matter halos and the cluster Abell 586. Her research focuses on studying the kinematics of ultra-faint dwarf galaxies in the Local Group.


Undergraduate Students


Sam Welch                     

Sam is a 4th year undergraduate student at the University of Virginia. He is pursuing his B.A. in Astronomy, and his B.A. in Computer Science. His research focuses on analyzing clusters in the Small Magellanic Cloud using proper motions and other data from Gaia.


Previous Members

            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. He worked with the group for four years and has now moved on to a Fellowship at the IAC in the Canary Islands. CV

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.

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

Recent Research Projects and Papers


The Proper Motion Field Along the Magellanic Bridge: a New Probe of the LMC-SMC Interaction

In Zivick, Kallivayalil, Besla, et al. 2019 we present the first detailed kinematic analysis of the proper motions (PMs) of stars in the Magellanic Bridge, from both the Gaia Data Release 2 catalog and from Hubble Space Telescope Advanced Camera for Surveys data. For the Gaia data, we identify and select two populations of stars in the Bridge region, young main sequence (MS) and red giant stars. The spatial locations of the stars are compared against the known HI gas structure, finding a correlation between the MS stars and the HI gas. In the Hubble Space Telescope fields our signal comes mainly from an older MS and turn-off population, and the proper motion baselines range between ∼4 and 13 years. The PMs of these different populations are found to be consistent with each other, as well as across the two telescopes. When the absolute motion of the Small Magellanic Cloud is subtracted out, the residual Bridge motions display a general pattern of pointing away from the Small Magellanic Cloud towards the Large Magellanic Cloud. We compare in detail the kinematics of the stellar samples against numerical simulations of the interactions between the Small and Large Magellanic Clouds, and find general agreement between the kinematics of the observed populations and a simulation in which the Clouds have undergone a recent direct collision.


The Missing Satellites of the Magellanic Clouds? Gaia Proper Motions of the Recently Discovered Ultra-Faint Galaxies

In Kallivayalil, Sales, Zivick, Fritz et al. 2018 we present proper motion measurements for 13 of the 32 newly discovered dwarf galaxy candidates using Gaia data release 2. All 13 also have radial velocity measurements. We compare the measured 3D velocities of these dwarfs to those expected at the corresponding distance and location for the debris of an LMC analog in a numerical simulation. We conclude that 4 of these galaxies (Hor1, Car2, Car3 and Hyd1) have come in with the Magellanic Cloud system, constituting the first confirmation of the type of satellite infall predicted by LCDM. Ret2, Tuc2 and Gru1 have some velocity components that are not consistent within 3 sigma of our predictions and are therefore less favorable. Hyd2 and Dra2 could be associated with the LMC and merit further attention. We rule out Tuc3, Cra2, Tri2 and Aqu2 as potential members. Of the dwarfs without measured PMs, 6 of them are deemed unlikely on the basis of their positions and distances alone which put them too far from the orbital plane expected for LMC debris (Eri2, Ind2, Cet2, Tri2, Cet3 and Vir1). For the remaining sample, we use the simulation to predict proper motions and radial velocities, finding that Phx2 has an overdensity of stars in DR2 consistent with this PM prediction. If its radial velocity is confirmed at ∼−15 km s-1, it is also likely a member.


The Proper Motion Field of the Small Magellanic Cloud: Kinematic Evidence for its Tidal Disruption

In Zivick, Kallivayalil, van der Marel et al. 2018 we present a new measurement of the systemic proper motion of the Small Magellanic Cloud (SMC), based on an expanded set of 30 fields containing background quasars and spanning a ∼3 year baseline, using the Hubble Space Telescope Wide Field Camera 3 (HST WFC3). Combining this data with our previous 5 HST fields, and an additional 8 measurements from the Gaia-Tycho Astrometric Solution Catalog, brings us to a total of 43 SMC fields. We measure a systemic motion of μW = −0.82 ± 0.02 (random) ± 0.10 (systematic) mas yr-1 and μN = −1.21 ± 0.01 (random) ± 0.03 (systematic) mas yr-1. After subtraction of the systemic motion, we find little evidence for rotation, but find an ordered mean motion radially away from the SMC in the outer regions of the galaxy, indicating that the SMC is in the process of tidal disruption. We model the past interactions of the Clouds with each other based on the measured present-day relative velocity between them of 103±26 km s-1. We find that in 97% of our considered cases, the Clouds experienced a direct collision 147±33 Myr ago, with a mean impact parameter of 7.5±2.5 kpc.


The Orbit and Origin of the Ultra-faint Dwarf Galaxy Segue 1

In Fritz, Lokken, Kallivayalil et al. 2018 we present the first proper motion measurement for an ultra-faint dwarf spheroidal galaxy, Segue 1, using SDSS and LBC data as the first and second epochs separated by a baseline of ∼10 years. We obtain a motion of μαcos(δ)=−0.37±0.57 mas yr-1 and μδ=−3.39±0.58 mas yr-1. Combining this with the known line-of-sight velocity, this corresponds to a Galactocentric Vrad=84±9 and Vtan=164+66−55 km s-1. Applying Milky Way halo masses between 0.8 to 1.6×1012 M results in an apocenter at 33.9+21.7−7.4 kpc and pericenter at 15.4+10.1−9.0 kpc from the Galactic center, indicating Segue~1 is rather tightly bound to the Milky Way. Since neither the orbital pole of Segue 1 nor its distance to the Milky Way is similar to the more massive classical dwarfs, it is very unlikely that Segue 1 was once a satellite of a massive known galaxy. Using cosmological zoom-in simulations of Milky Way-mass galaxies, we identify subhalos on similar orbits as Segue 1, which imply the following orbital properties: a median first infall 8.1+3.6−4.3 Gyrs ago, a median of 4 pericentric passages since then and a pericenter of 22.8+4.7−4.8 kpc. This is slightly larger than the pericenter derived directly from Segue 1 and Milky Way parameters, because galaxies with a small pericenter are more likely to be destroyed. Of the surviving subhalo analogs only 27% were previously a satellite of a more massive dwarf galaxy (that is now destroyed), thus Segue 1 is more likely to have been accreted on its own.


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. Our program's third class (pictured below) featured Kimara Pruitt (left) and Katelyn Jackson (right).

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. (Presentation)

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



Downloadable copy



Previous Students:


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.

Zaniyah Dock (Presentation)

Temi Olatinwo (Presentation)



Our debut class of 2016 Summer Researchers included Mayla McCray (first row, second from left) and Colleen Smith-Patikas (first row, leftmost), who are both Physics & Engineering double majors at Spelman.

Mayla McCray (Presentation)

Colleen Smith-Patikas (Presentation)

Jordan Glisan (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 (Daniel Moon, Taylor Tolbert, Charley Inman, and Madison Markham) are studying globular clusters using data from the Gaia Space Telescope.

Past students: Ivy Wheaton (2017), Jeff Przybylek (2017), Andrew Childers (2016), Noah Baumgartner (2016), Harrison Fields (2016)