About

Gurtina Besla is an Associate Professor in the Department of Astronomy and an Associate Astronomer at Steward Observatory. She earned her Ph.D. in 2011 from Harvard University. Her research focuses on the formation and evolution of low mass dwarf galaxies, which are the most common class of galaxies found in the universe at any epoch. Through numerical simulations, she explores the impact of gravitational interactions on the observed properties of low mass galaxies in various environments. Dr. Besla is a world expert in the study of the closest example of an interacting pair of dwarf galaxies, the Large and Small Magellanic Clouds. Her research on these galaxies has overturned conventional wisdom, demonstrating that the Magellanic Clouds are recent interlopers in our neighborhood rather than long-term companions to the Milky Way. Her work encompasses areas such as galaxy formation, dwarf galaxies, Local Group dynamics, dark matter, and structure formation.

Research topics

• Astrophysics
• Astronomy
• Physics

Selected publications

• Mapping the Distorted Dark Matter Distribution of the LMC-SMC System Prior to Milky Way Infall with Basis Function Expansions

  ArXiv.org · 2026-01-02

  articleOpen access

  The SMC orbits within the LMC's dark matter (DM) halo in a $\sim$1:10 mass-ratio encounter. The LMC:Milky Way (MW) interaction is also $\sim$1:10, and is expected to perturb the MW's DM distribution. However, no framework exists to quantify the severity of these perturbations over multiple pericenters and longer periods of time, such as the LMC-SMC interaction history. We construct basis function expansions of a high-resolution N-body simulation of the Clouds interacting in isolation and analyze their DM distributions at an epoch approximating the time of infall to the MW. Our goal is to quantify how the Clouds distort each other's DM distributions without the MW. The LMC halo's response to the SMC includes a $\sim 20$ kpc-long dynamical friction wake and the displacement of the LMC's density center during each SMC pericenter, which produces two overdensities in the LMC halo (at $\sim$60 and $\sim$100 kpc) at MW infall. The SMC's tidal radius at infall is just $\sim4$ kpc, at which point the SMC has lost two-thirds of its initial DM mass to the LMC. The distortions to the Clouds' halos produce a highly asymmetric acceleration field. Accurate orbit integration in the LMC-SMC system must account for the time-dependent shapes of both halos. The SMC-induced perturbations in the LMC DM halo resemble the MW-LMC system, and persist over multiple SMC pericenters. We conclude that 1:10 satellite:host encounters induce characteristic deformations in both DM halos across all host mass scales, with implications for merger rates and tests of DM models.

  PublisherOA PDF

• Mapping the Distorted Dark Matter Distribution of the LMC–SMC System Prior to Milky Way Infall with Basis Function Expansions

  The Astrophysical Journal · 2026-04-02

  articleOpen access

  Abstract The SMC orbits within the LMC’s dark matter (DM) halo in a ∼1:10 mass-ratio encounter. The LMC–Milky Way (MW) interaction is also ∼1:10, and is expected to perturb the MW’s DM distribution. However, no framework exists to quantify the severity of these perturbations over multiple pericenters and longer periods of time, such as the LMC–SMC interaction history. We construct basis function expansions of a high-resolution N -body simulation of the Clouds interacting in isolation and analyze their DM distributions at an epoch approximating the time of their infall to the MW. Our goal is to quantify how the Clouds distort each other’s DM distributions without the MW. The LMC halo’s response to the SMC includes a ∼20 kpc long dynamical friction wake and the displacement of the LMC’s density center during each SMC pericenter, which produces two overdensities in the LMC halo (at ∼60 and ∼100 kpc) at MW infall. The SMC’s tidal radius at infall is just ∼4 kpc, at which point the SMC has lost two-thirds of its initial DM mass to the LMC. The distortions to the Clouds’ halos produce a highly asymmetric acceleration field. Accurate orbit integration in the LMC–SMC system must account for the time-dependent shapes of both halos. The SMC-induced perturbations in the LMC DM halo resemble the MW–LMC system, and persist over multiple SMC pericenters. We conclude that 1:10 satellite–host encounters induce characteristic deformations in both DM halos across host-mass scales, with implications for merger rates and tests of DM models.

  PublisherDOI

• Mapping the Distorted Dark Matter Distribution of the LMC-SMC System Prior to Milky Way Infall with Basis Function Expansions

  arXiv (Cornell University) · 2026-01-02

  preprintOpen access

  The SMC orbits within the LMC's dark matter (DM) halo in a $\sim$1:10 mass-ratio encounter. The LMC:Milky Way (MW) interaction is also $\sim$1:10, and is expected to perturb the MW's DM distribution. However, no framework exists to quantify the severity of these perturbations over multiple pericenters and longer periods of time, such as the LMC-SMC interaction history. We construct basis function expansions of a high-resolution \textit{N}-body simulation of the Clouds interacting in isolation and analyze their DM distributions at an epoch approximating the time of their infall to the MW. Our goal is to quantify how the Clouds distort each other's DM distributions \textit{without} the MW. The LMC halo's response to the SMC includes a $\sim 20$ kpc long dynamical friction wake and the displacement of the LMC's density center during each SMC pericenter, which produces two overdensities in the LMC halo (at $\sim$60 and $\sim$100 kpc) at MW infall. The SMC's tidal radius at infall is just $\sim4$ kpc, at which point the SMC has lost two-thirds of its initial DM mass to the LMC. The distortions to the Clouds' halos produce a highly asymmetric acceleration field. Accurate orbit integration in the LMC-SMC system must account for the time-dependent shapes of both halos. The SMC-induced perturbations in the LMC DM halo resemble the MW-LMC system, and persist over multiple SMC pericenters. We conclude that 1:10 satellite-host encounters induce characteristic deformations in both DM halos across host-mass scales, with implications for merger rates and tests of DM models.

  PublisherDOI

• TiNy Titans H <scp>I</scp> : Discovering Satellites via H <scp>I</scp> Gas in an Isolated, Compact Group of Dwarf Galaxies

  The Astrophysical Journal · 2026-01-19

  articleOpen access

  Abstract We report on the H I content of an isolated, compact group of six dwarf galaxies at a distance of 145 Mpc. The distribution and kinematics of the H ​​​​​​ I , including multiple gaseous bridges, indicate the group is a gravitationally bound system. The H I maps further reveal two newly discovered dwarf satellites easily identified by their gas but only barely visible in optical images. The four dwarf group members previously identified in the Sloan Digital Sky Survey have 9.06 &lt; log( M * / M ⊙ ) &lt; 9.43 and 9.42 &lt; log( M H I / M ⊙ ) &lt; 9.73. The two newly discovered dwarf satellites have log( M * / M ⊙ ) = 6.10 with log(M H I / M ⊙ ) = 8.71 and log( M * / M ⊙ ) = 7.07 with log( M H I / M ⊙ ) = 9.18. New Gemini optical spectra link the H I detections and their optical counterparts. The group’s 3D velocity dispersion (188 km s −1 ), mass-to-light ratio ( M / L B ∼ 44), dynamical-to-baryonic mass ratio ( M dyn / M bar ∼ 21), size (69 kpc), and gas fraction (0.56) are all consistent with the compact dwarf groups in the TNG50 simulation. The group has a top-heavy satellite mass function that is inconsistent with predictions for LMC-sized hosts and may instead be two or more groups coming together. A Voronoi tessellation reveals the group resides in a tendril outside the intersection of two filaments. These intermediate-density environments within the large-scale structure provide the conditions needed for groups of star-forming, gas-rich dwarf galaxies to form and eventually merge. Our results further show that it is possible to uncover fainter dwarf satellites around dwarf galaxy hosts via H I maps.

  PublisherDOI

• Response of the LMC’s Bar to a Recent SMC Collision and Implications for the SMC’s Dark Matter Profile

  The Astrophysical Journal · 2025-07-15 · 4 citations

  articleOpen access

  Abstract The LMC’s stellar bar is offset from the outer disk center, tilted from the disk plane, and does not drive gas inflows. These properties are atypical of bars in gas-rich galaxies, yet the LMC bar’s strength and radius are similar to typical barred galaxies. Using N -body hydrodynamic simulations, we show that the LMC’s unusual bar is explainable if there was a recent collision (impact parameter ≈2 kpc) between the LMC and SMC. Pre-collision, the simulated bar is centered and coplanar. Post-collision, the simulated bar is offset (≈1.5 kpc) and tilted (≈8 <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> 6). The simulated bar offset reduces with time, and comparing with the observed offset (≈0.8 kpc) suggests the timing of the true collision to be 150–200 Myr ago. Then, 150 Myr post-collision, the LMC’s bar is centered with its dark matter (DM) halo, whereas the outer disk center is separated from the DM center by ≈1 kpc. The SMC collision produces a tilted-ring structure for the simulated LMC, consistent with observations. Post-collision, the simulated LMC bar’s pattern speed decreases by a factor of 2. We also provide a generalizable framework to quantitatively compare the LMC’s central gas distribution in different LMC–SMC interaction scenarios. We demonstrate that the SMC’s torques on the LMC’s bar during the collision are sufficient to explain the observed bar tilt, provided the SMC’s total mass within 2 kpc was (0.8–2.4) × 10 9 M ⊙ . Therefore, the LMC bar’s tilt constrains the SMC’s pre-collision DM profile, and requires the SMC to be a DM-dominated galaxy.

  PublisherDOI

• The Distant Milky Way Halo from the Southern Hemisphere: Characterization of the LMC-induced Dynamical Friction Wake*

  The Astrophysical Journal · 2025-04-09 · 7 citations

  articleOpen access

  Abstract The infall of the Large Magellanic Cloud (LMC) into the Milky Way’s halo impacts the distribution of stars and dark matter (DM) in our Galaxy. Mapping the observational consequences of this encounter can inform us about the properties of both galaxies, details of their interaction, and possibly distinguish between different DM models. N -body simulations predict a localized overdensity trailing the LMC’s orbit both in baryonic and DM, known as the wake. We collected wide-field, deep near-infrared, and optical photometry using VIRCAM and DECam across four fields along the expected wake, covering the sky region expected to span most of its predicted density contrast. We identify over 400 stars comprising two different tracers, near main-sequence turnoff stars and red giants, which map the halo between 60 and 100 kpc, deriving stellar halo densities as a function of sky position and Galactocentric radius. We detect (1) a break in the halo radial density profile at 70 kpc not seen in northern halo studies and (2) a clear halo overdensity starting also at 70 kpc, with density contrast increasing steadily toward the expected current location of the wake. If this overdensity is the LMC wake, its peak density contrast is as pronounced as the most massive LMC model considered. Contamination from unidentified substructures may bias our wake detections, so wider-area surveys with similar depth are needed for confirmation.

  PublisherDOI

• The Hubble Space Telescope Survey of M31 Satellite Galaxies IV. Survey Overview and Lifetime Star Formation Histories

  ArXiv.org · 2025-01-22

  preprintOpen access

  From $&gt;1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within $1.8$ Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. iii) $\sim50$\% of our M31 satellites show prominent ancient star formation ($&gt;12$ Gyr ago) followed by delayed quenching ($8-10$ Gyr ago), which is not commonly observed among the MW satellites. iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts show that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST.

  PublisherOA PDFDOI

• Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf

  The Astrophysical Journal · 2025-01-24 · 2 citations

  articleOpen access

  Abstract Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2 σ width) with the CPS. This interaction enables constraints on Segue 2’s mass and density profile at larger radii ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math> kpc) than are probed by its stars ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>10</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math> pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2’s mass within ∼ 6 kpc is 5 × 10 9 M ⊙ , the CPS's velocity dispersion will be ∼ 40 km s −1 larger at ϕ 1 &gt; 20° than at ϕ 1 &lt; 0°. If no such heating is detected, Segue 2’s mass cannot exceed 10 9 M ⊙ within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2’s density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations.

  PublisherDOI

• Understanding Stellar Mass–Metallicity and Size Relations in Simulated Ultrafaint Dwarf Galaxies

  The Astrophysical Journal · 2025-06-19 · 3 citations

  preprintOpen access

  Abstract Reproducing the physical characteristics of ultrafaint dwarf galaxies (UFDs) in cosmological simulations is challenging, particularly with respect to stellar metallicity and galaxy size. To investigate these difficulties in detail, we conduct high-resolution simulations ( M gas ∼ 60 M ⊙ , M DM ∼ 300 M ⊙ ) on six UFD analogs ( M vir ∼ 10 8 –10 9 M ⊙ , M ⋆ ∼ 10 3 –2.1 × 10 4 M ⊙ at z = 0). Our findings reveal that the stellar properties of the UFD analogs are shaped by diverse star-forming environments from multiple progenitor halos in the early Universe. Notably, our UFD analogs exhibit a better match to the observed mass–metallicity relation, showing higher average metallicity compared to other theoretical models, though our results remain 0.5–1 dex lower than for observed UFDs. The metallicity distribution functions (MDFs) of our simulated UFDs lack high-metallicity stars ([Fe/H]≥ −2.0) while containing low-metallicity stars ([Fe/H] &lt; −4.0). Excluding these low-metallicity stars, our results align well with the MDFs of observed UFDs. However, forming stars with higher metallicity (−2.0 ≤ [Fe/H] max ≤ −1.5) remains a challenge, due to the difficulty of sustaining metal enrichment during the brief star formation period before cosmic reionization. Additionally, our simulations show extended outer structures in UFDs, similar to recent Milky Way UFD observations, resulting from dry mergers between progenitor halos. To ensure consistency, we adopt the same fitting method commonly used in observations to derive the half-light radius. We find that this method tends to produce lower values compared to direct calculations and struggles to accurately describe the extended outer structures.

  PublisherDOI

• Response of the LMC's Bar to a Recent SMC Collision and Implications for the SMC's Dark Matter Profile

  arXiv (Cornell University) · 2025-04-22

  preprintOpen access

  The LMC's stellar bar is offset from the outer disk center, tilted from the disk plane, and does not drive gas inflows. These properties are atypical of bars in gas-rich galaxies, yet the LMC bar's strength and radius are similar to typical barred galaxies. Using N-body hydrodynamic simulations, we show that the LMC's unusual bar is explainable if there was a recent collision (impact parameter $\approx$2 kpc) between the LMC and SMC. Pre-collision, the simulated bar is centered and co-planar. Post-collision, the simulated bar is offset ($\approx$1.5 kpc) and tilted ($\approx8.6^\circ$). The simulated bar offset reduces with time, and comparing with the observed offset ($\approx0.8$ kpc) suggests the timing of the true collision to be 150-200 Myr ago. 150 Myr post-collision, the LMC's bar is centered with its dark matter halo, whereas the outer disk center is separated from the dark matter center by $\approx1$ kpc. The SMC collision produces a tilted-ring structure for the simulated LMC, consistent with observations. Post-collision, the simulated LMC bar's pattern speed decreases by a factor of two. We also provide a generalizable framework to quantitatively compare the LMC's central gas distribution in different LMC-SMC interaction scenarios. We demonstrate that the SMC's torques on the LMC's bar during the collision are sufficient to explain the observed bar tilt, provided the SMC's total mass within 2 kpc was $(0.8-2.4) \times 10^9$ M$_\odot$. Therefore, the LMC bar's tilt constrains the SMC's pre-collision dark matter profile, and requires the SMC to be a dark matter-dominated galaxy.

  PublisherDOI

Recent grants

• Collaborative Research: The Hierarchical Mergers of Low Mass Galaxies

  NSF · $320k · 2017–2021

• CAREER: Constraining the Nature and Distribution of Dark Matter in the Era of High-Precision Astrometry

  NSF · $805k · 2020–2026

Frequent coauthors

• Roeland P. van der Marel

  109 shared

• Nitya Kallivayalil

  90 shared

• Sangmo Tony Sohn

  51 shared

• David L. Nidever

  Montana State University

  46 shared

• Ekta Patel

  University of California, San Diego

  45 shared

• Nicolas F. Martin

  Centre National de la Recherche Scientifique

  43 shared

• Carme Gallart

  37 shared

• Yumi Choi

  36 shared

Awards & honors

• Kenneth G. Gibbs Doctoral Fellowship in Astronomy and Astrop…