arXiv subjects

Sangmo Tony Sohn

Publications and source records attributed to Sangmo Tony Sohn.

At least 19 recordsLinked to original sources

arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper IV. Key Scientific Mission: Solar-System Small Bodies and Planetary Defense

The baseline 0.2--1.5 $\mu$m observatory provides rapid-response astrometry, visible and near-infrared taxonomy, rotation and phase curves, recovery, and long-arc orbit improvement for near-Earth objects and other small bodies. The instrument study also evaluates calibrated throughput to 2.70 $\mu$m with a 3.0 $\mu$m operational band-edge goal. A reduction to 2.5 $\mu$m remains the formal engineering off-ramp if thermal, detector, cooling, mass, power, or cost constraints require it. The 3.5-meter Segmented-Mirror Robotic Space Telescope does not carry a mid-infrared channel. Coordinated ground-based mid-infrared telescopes provide the thermal fluxes required to infer diameter and albedo, while the space mission supplies contemporaneous reflected-light measurements and observing geometry. The program combines recovery, physical characterization, orbit refinement, and covariance-based hazard assessment. Its CODES dynamics system and OGFinder-to-OpenOrb processing path connect measured astrometry to reproducible orbit solutions and close-approach predictions.

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arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper V. Key Scientific Mission: Compact-Object Time-Domain Science

An isolated compact object retains the point-source resolving power of the space-based slitless spectrograph. The baseline wavelength range is 0.2--1.5 $\mu$m. The planning baseline uses $R \simeq 1000$ for broad and faint transient spectra and reserves selectable bands at $R \simeq 5000$ for accretion-disk profiles, velocity structure, and precision line ratios. Broad features can be measured after binning the native $R \simeq 5000$ data to lower resolution. Rapid-response spectroscopy follows gravitational-wave counterparts and kilonovae from hours to days. Repeated spectra of dwarf novae and compact binaries trace accretion state and orbital phase, while uninterrupted imaging of white dwarfs measures pulsation frequencies. The program combines mission-based monitoring with external alerts, including KGMT transient detections. The instrument study must preserve calibrated throughput to 2.70 $\mu$m and evaluate a 3.0 $\mu$m operational band edge, with 2.5 $\mu$m retained as the formal engineering off-ramp. Mid-infrared imaging is not part of the adopted compact-object baseline.

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arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper I. Overall Architecture and Scientific Mission

A 3.5-meter segmented-mirror robotic space telescope is under study as a space-based observatory for precision astrophysical observations and rapid-response transient astronomy in the 0.2-1.5 micron wavelength range. The telescope adopts a Cassegrain optical configuration optimized to deliver diffraction-limited performance across a wide, flat focal plane, achieving a Strehl ratio greater than 0.8 at 633 nm. The proposed scientific payload includes a Wide-field Camera (WC), a spectroscopic instrument, and an optional Exoplanet Imaging Coronagraph. The Wide-field Camera (WC) provides multi-wavelength imaging and high-cadence time-series photometry over a field of view ranging from 10'X10' to 30'X30'. The spectroscopic configuration and resolving power remain under study to accommodate the requirements of the principal science programs. An optional Exoplanet Imaging Coronagraph is being investigated for high-contrast imaging of nearby planetary systems, with performance goals extending toward raw contrasts of approximately 10^(-8) and improved post-processed performance. Candidate orbital configurations, including Earth orbit and the Sun-Earth L2 region, are currently being evaluated. Planned investigations include gravitational-wave counterparts, rapidly evolving transients, Type Ia supernova cosmology, direct imaging of exoplanets, and exoplanet atmospheric spectroscopy. Although driven by these core scientific objectives, the observatory is conceived as a general-purpose facility providing open-access observing time to the international scientific community. This paper presents the preliminary architecture, performance goals, and scientific mission of the proposed 3.5-meter space telescope.

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arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper II. Key Scientific Mission: Wide-Field Cosmology and Galaxy Evolution

The 3.5-meter Segmented-Mirror Robotic Space Telescope uses an image slicer for all spectroscopic observations. The planning baseline uses $R \simeq 1000$ for the wide survey and retains selectable $R \simeq 5000$ bands for precision line measurements. The central science case is a dense emission-line galaxy redshift survey for baryon acoustic oscillations and redshift-space distortions. Supernova and quasar programs exploit the stability, multiplexing, and repeatability of space operations. The supernova tier measures rest-frame U and near-ultraviolet magnitudes that separate optical twins at subgroup precision to $z \simeq 0.9$--$1.1$ in standard visits and to $z \simeq 1.3$--$1.5$ in ten-hour stacks. Every wide-survey tile receives three spectroscopic orientations, and a joint scene reconstruction uses their different overlap geometries to recover the spectra. The flagship survey covers 100--300 deg$^2$ and targets $10^6$--$3 \times 10^6$ emission-line galaxies. A deep pencil-beam tier and a supernova time-domain tier complement the wide survey. The same observations provide a census of ultra-diffuse and low-surface-brightness galaxies, map intracluster light, and test cold, self-interacting, and fuzzy dark matter through dwarf-galaxy structure and low-mass halo abundance.

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arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper III. Key Scientific Mission: Exoplanet Science with a Coronagraph

This volume defines the exoplanet science program enabled by the dedicated high-contrast coronagraph in the baseline science payload of the 3.5-meter Segmented-Mirror Robotic Space Telescope. The observatory architecture incorporates the optical interfaces, wavefront sensing and control, pointing stability, and operations software required for coronagraphic observations from the outset. The observing strategy gives priority to the nearest stellar systems because they provide the most accessible laboratories for planetary exploration and the most likely destinations of future interstellar missions. The diffraction limit sets a reflected-light horizon of roughly 10--15 pc for planets at 1 AU and roughly 50--80 pc for Jupiter analogs. Within those horizons, the telescope can image nearby giant planets, obtain reflected-light spectra of their atmospheres, survey young systems and circumstellar disks, and support the habitability and biosignature programs that larger future missions will pursue. The wide-field imager complements the coronagraph through transit photometry, occurrence-rate statistics, and long-term monitoring of stellar magnetic activity. A systematic census of the nearest stellar neighbors provides a lasting reference for exoplanet science and future space exploration.

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arXiv2026

Reconstructing the orbits of Milky Way dwarf galaxies: A Large Magellanic Cloud perspective

The orbital histories of the dwarf satellites of the Milky Way (MW) are key to understanding their evolution and placing their present-day properties in a dynamical context. We present the results of the orbit integration of 72 dwarfs in the vicinity of the MW, based on accurate 6D phase-space coordinates from the literature and a suite of six realistic, time-evolving gravitational potentials that account for the mutual interaction of the MW and the Large Magellanic Cloud (LMC). We provide the largest catalogue of orbital parameters for MW dwarfs to date, in terms of both galaxy sample size and range of potentials explored. We also assess the binding status of the dwarfs and estimate their infall times, finding that the majority of them have spent the last 5 Gyr within the MW virial radius. From the reconstructed orbits, we identify ten likely LMC satellites, several of which have experienced very close passages within the LMC stellar disc. For the Small Magellanic Cloud (SMC), we find that its most recent pericentre about the LMC ($\sim$8 kpc, $\sim$170 Myr ago) is consistent with predictions from the direct collision scenario proposed to explain the LMC's offset and tilted bar. We also note a broad temporal coincidence between previous SMC pericentres and star formation rate peaks reported in both Magellanic Clouds, suggesting a causal connection. Finally, we identify Grus II and Tucana IV as possible MW satellites recently captured by the LMC, based on their pronounced orbital deflections and velocities relative to the LMC.

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arXiv2026

A Path to an All-Sky Survey with Roman

A deep, space-based, all-sky near-infrared survey carried out with the Nancy Grace Roman Space Telescope would constitute a foundational astronomical infrastructure for decades to come. In this white paper, we present a concrete and feasible path to imaging the entire sky at $\sim0.1''$ resolution, beginning with high-impact fields in Cycle 1 and scaling to ultra-wide coverage within the nominal mission. This first-epoch survey will reach $\mathrm{H}\sim25.5$ AB mag (5$σ$) and maximize synergies with contemporaneous observatories, while preserving substantial time for other ambitious Roman programs. We outline representative scheduling scenarios and an example Cycle 1 program that triples early Roman-LSST overlap and delivers high-value community data products such as LSST forced photometry, joint \textit{Gaia}-Roman astrometry, and catalogs of Galactic substructure, stong lenses, and other rare systems. The Cycle 1 program will lay the foundation for an eventual all-sky survey, while also delivering high-impact early science. We invite broad community participation in shaping and carrying out both the initial program and the long-term vision of an all-sky Roman survey.

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arXiv2026

Encounters Between M33 and Present-Day M31 Satellites Hint at a Previous Group Accretion

This work investigates whether two known Andromeda (M31) satellites, Pisces (LGS 3) and Andromeda XVI, have interacted with M33, M31's most massive satellite. $Λ$CDM predictions imply a handful of satellite galaxies around M33, yet few M33 satellites have been found and confirmed despite its high mass. We use proper motions combined with backward orbit integration in a semi-analytic potential to constrain plausible interaction scenarios for Pisces and And XVI. Both dwarfs are currently M31 satellites, defined as being inside its virial radius. However, our results show that, in our fiducial mass models, 42% (And XVI) and 60% (Pisces) of dwarf orbits support that they were previously satellites of M33 (i.e., once inside its virial radius). Both dwarfs had fly-by encounters with M33 at relative velocities greater than M33's escape speed within the past 1-2 Gyr. In over 70% of orbits, Pisces and And XVI also had a close approach with each other post-M33 interaction and share an orbital plane, suggesting possible past group accretion. We explore a range of mass combinations for M31 and M33, finding that this primarily regulates the likelihood that the dwarfs were satellites of M33 in the past, while upholding conclusions of recent flybys about M33. These close interactions provide new evidence for past satellite exchange and/or group infall scenarios between M31 and M33. Such interactions also affect comparisons to observational surveys that define satellites primarily by their distance relative to host galaxies.

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arXiv2026

HSTPROMO Internal Proper Motion Kinematics of Dwarf Spheroidal Galaxies: II. Velocity Anisotropy and Dark Matter Cusp Slope of Sculptor

We analyze three epochs of HST imaging over 20 years for the Sculptor dwarf spheroidal galaxy, measuring precise proper motions for 119 stars and combining them with 1760 existing line-of-sight velocities. This catalog yields the first radially-resolved 3D velocity dispersion profiles for Sculptor. We confirm mild oblate rotation, with major-axis velocities reaching $\sim 2$ km s$^{-1}$ beyond 20.0 arcmin. Using a methodology similar to that in the first paper in this series, we solve the Jeans equations in oblate axisymmetric geometry to infer the galaxy's mass profile. Our modeling reveals a significant degeneracy due to the unknown galaxy inclination, which is overlooked under spherical symmetry assumptions. This degeneracy allows acceptable fits across a range of dark matter profiles, from cuspy to cored. While we do not directly constrain the inclination with our Jeans models, higher-order line-of-sight velocity moments provide useful additional constraints: comparisons with scalefree models from de Bruijne et al. (1996) favor highly flattened (more face-on) configurations. Adopting an inclination well consistent with these comparisons ($i = 57.1$ degrees), we find, alongside radial velocity anisotropy, a dark matter density slope of $Γ_{\rm dark} = 0.29^{+0.31}_{-0.41}$ within the radial extent of the 3D velocity data, ruling out a cusp with $Γ_{\rm dark} \leq -1$ at 99.8% confidence. This confidence increases for lower inclinations and decreases drastically for nearly edge-on configurations. The results qualitatively agree with $Λ$CDM, SIDM, and Fuzzy DM scenarios that predict core formation, while our specific measurements provide quantitative constraints on the prescriptions of feedback, cross sections, or particle masses required by these models, respectively.

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arXiv2025

The Proper Motion of Draco II with HST using Multiple Reference Frames and Methodologies

We present proper motion (PM) measurements for Draco II, an ultra-faint dwarf satellite of the Milky Way. These PMs are measured using two epochs of Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) imaging separated by a 7 year time baseline. Measuring PMs of low-luminosity systems is difficult due to the low number of member stars, requiring a precise inertial reference frame. We construct reference frames using three different sets of external sources: 1) stars with Gaia DR3 data, 2) stationary background galaxies, and 3) a combination of the two. We show that all three reference frames give consistent PM results. We find that for this sparse, low-luminosity regime including background galaxies into the reference frame improves our measurement by up to $\sim2\times$ versus using only Gaia astrometric data. Using 301 background galaxies as a reference frame, we find that Draco II's systemic PM is $(μ_α^*, μ_δ) = (1.043\pm0.029, 0.879\pm0.028)$ mas/yr, which is the most precise measurement of the three we present in this paper.

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arXiv2025

The Orbits of Isolated Dwarfs in the Local Group from New 3D Kinematics: Constraints on First Infall, Backsplash, and Quenching Mechanisms

It is commonly supposed that quenched field dwarfs near the edge of the Local Group (LG) are backsplash galaxies, having previously orbited within the Milky Way (MW) or M31's virial radius, whereas galaxies on first infall should still have gas and star formation. We measured proper motions (PMs) for six dwarf galaxies located 400-1000 kpc from the MW using the Hubble Space Telescope. For four galaxies (Aquarius, Cetus, Pisces, Tucana), we report the first PMs. For the remaining two (Leo T and Pegasus), we measure PMs with order-of-magnitude improvement. We compute orbital histories to assess whether any of the six are backsplash galaxies. While some have non-zero likelihoods of past interaction with the MW or M31, these are weak and typically occur at large distances (e.g., $>$ 2Rvir). The properties of Aquarius, Leo T, Pisces, and Pegasus are consistent with first passage through a massive halo. Cetus, which shows a low probability (~4-6%) of interacting with the MW or M31 in the last 6 Gyr, is more likely a backsplash galaxy resulting from an interaction with M31 over 6 Gyr ago, in the same regime where rigid orbital models become less reliable. Tucana has been thought to be a backsplash galaxy, but our orbits indicate it cannot have interacted with a massive LG host. Our results highlight the diversity of evolutionary pathways for isolated, intermediate-mass dwarfs ($M_* \approx 10^5-10^7 M_{\odot}$) and the need to reassess quenching mechanisms beyond environmental interactions with massive hosts.

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arXiv2025

JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e

TRAPPIST-1 e is one of the very few rocky exoplanets that is both amenable to atmospheric characterization and that resides in the habitable zone of its star -- located at a distance from its star such that it might, with the right atmosphere, sustain liquid water on its surface. Here, we present a set of 4 JWST/NIRSpec PRISM transmission spectra of TRAPPIST-1 e obtained from mid to late 2023. Our transmission spectra exhibit similar levels of stellar contamination as observed in prior works for other planets in the TRAPPIST-1 system (Lim et al, 2023; Radica et al., 2024), but over a wider wavelength range, showcasing the challenge of characterizing the TRAPPIST-1 planets even at relatively long wavelengths (3-5 um). While we show that current stellar modeling frameworks are unable to explain the stellar contamination features in our spectra, we demonstrate that we can marginalize over those features instead using Gaussian Processes, which enables us to perform novel exoplanet atmospheric inferences with our transmission spectra. In particular, we are able to rule out cloudy, primary H$_2$-dominated ($\gtrsim$ 80$\%$ by volume) atmospheres at better than a 3$σ$ level. Constraints on possible secondary atmospheres on TRAPPIST-1 e are presented in a companion paper (Glidden et al., 2025). Our work showcases how JWST is breaking ground into the precisions needed to constrain the atmospheric composition of habitable-zone rocky exoplanets.

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arXiv2025

The HST Legacy Archival Uniform Reduction of Local Group Imaging (LAURELIN). I. Photometry and Star Formation Histories for 36 Ultra-faint Dwarf Galaxies

We present uniformly measured resolved stellar photometry and star formation histories (SFHs) for 36 nearby ($\lesssim$ 400 kpc) ultra-faint dwarf galaxies (UFDs; $-7.1 \le M_V \le +0.0$) from new and archival HST imaging. We measure homogeneous distances to all systems via isochrone fitting and find good agreement ($\le$ 2%) for the 18 UFDs that have literature RR Lyrae distances. From the ensemble of SFHs, we find: (i) an average quenching time (here defined as the lookback time by which 80% of the stellar mass formed, $τ_{80}$) of 12.48 $\pm$ 0.18 Gyr ago ($z = 4.6_{-0.5}^{+0.6}$), which is compatible with reionization-based quenching scenarios; and (ii) modest evidence of a delay ($\lesssim$ 800 Myr) in quenching times of UFDs thought to be satellites of the LMC or on their first infall, relative to long-term Galactic satellites, which is consistent with previous findings. We show that robust SFH measurement via the ancient main sequence turnoff (MSTO) requires a minimum effective luminosity (i.e., luminosity within the observed field of view) of $M_V \leq -2.5$, which corresponds to $\sim$100 stars around the MSTO. We also find that increasing the S/N above $\sim$100 at the MSTO does not improve SFH precision, which remains dominated by stochastic effects associated with the number of available stars. A main challenge driving the precision of UFD SFHs is limitations in the accuracy of foreground dust maps. We make all photometry catalogs public as the first data release of a larger HST archival program targeting all dwarf galaxies within $\sim$1.3 Mpc.

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arXiv2025

Understanding Stellar Mass-Metallicity and Size Relations in Simulated Ultra-Faint Dwarf Galaxies

Reproducing the physical characteristics of ultra-faint 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 ($M_{\rm gas} \sim 60 \, M_{\odot}$, $M_{\rm DM} \sim 370 \, M_{\odot}$ ) on six UFD analogs ($M_{\rm vir} \sim 10^8 - 10^9 \, M_{\odot}$, $M_{\rm \star} \sim 10^3 - 2.1 \times 10^4 \, M_{\odot}$). Our findings reveal that the stellar properties of 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 (MZR), showing higher average metallicity compared to other theoretical models. The metallicity distribution functions (MDFs) of our simulated UFDs lack high-metallicity stars ($[\rm Fe/H] > -2.0$) while containing low-metallicity stars ($[\rm Fe/H] < -4.0$). Excluding these low-metallicity stars, our results align well with the MDFs of observed UFDs. However, forming stars with higher metallicity ($-2.0 \leq [\rm Fe/H]_{\rm max} \leq -1.5$) remains a challenge due to the difficulty of sustaining metal enrichment during their brief star formation period before cosmic reionization. Additionally, our simulations show extended outer structures in UFDs, 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. To address this, we employ a two-component density profile to obtain structural parameters, finding that it better describes the galaxy shape, including both inner and outer structures.

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arXiv2024

JWST-TST DREAMS: Quartz Clouds in the Atmosphere of WASP-17b

Clouds are prevalent in many of the exoplanet atmospheres that have been observed to date. For transiting exoplanets, we know if clouds are present because they mute spectral features and cause wavelength-dependent scattering. While the exact composition of these clouds is largely unknown, this information is vital to understanding the chemistry and energy budget of planetary atmospheres. In this work, we observe one transit of the hot Jupiter WASP-17b with JWST's MIRI LRS and generate a transmission spectrum from 5-12 $\rmμ$m. These wavelengths allow us to probe absorption due to the vibrational modes of various predicted cloud species. Our transmission spectrum shows additional opacity centered at 8.6 $\rmμ$m, and detailed atmospheric modeling and retrievals identify this feature as SiO$_2$(s) (quartz) clouds. The SiO$_2$(s) clouds model is preferred at 3.5-4.2$σ$ versus a cloud-free model and at 2.6$σ$ versus a generic aerosol prescription. We find the SiO$_2$(s) clouds are comprised of small ${\sim}0.01$ $\rmμ$m particles, which extend to high altitudes in the atmosphere. The atmosphere also shows a depletion of H$_2$O, a finding consistent with the formation of high-temperature aerosols from oxygen-rich species. This work is part of a series of studies by our JWST Telescope Scientist Team (JWST-TST), in which we will use Guaranteed Time Observations to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).

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arXiv2024

HSTPROMO Internal Proper Motion Kinematics of Dwarf Spheroidal Galaxies: I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

We analyze four epochs of HST imaging over 18 years for the Draco dwarf spheroidal galaxy. We measure precise proper motions (PMs) for hundreds of stars and combine these with existing line-of-sight (LOS) velocities. This provides the first radially-resolved 3D velocity dispersion profiles for any dwarf galaxy. These constrain the intrinsic velocity anisotropy and resolve the mass-anisotropy degeneracy. We solve the Jeans equations in oblate axisymmetric geometry to infer the mass profile. We find the velocity dispersion to be radially anisotropic along the symmetry axis and tangentially anisotropic in the equatorial plane, with a globally-averaged value $\overline{β_{\mathrm B}}=-0.20^{+ 0.28}_{- 0.53}$, (where $1 - β_{\mathrm B} \equiv \langle v_{\mathrm{ tan}}^2 \rangle / \langle v_{\mathrm{ rad}}^2 \rangle$ in 3D). The logarithmic dark matter (DM) density slope over the observed radial range, $Γ_{\mathrm{ dark}}$, is $-0.83^{+ 0.32}_{- 0.37}$, consistent with the inner cusp predicted in $Λ$CDM cosmology. As expected given Draco's low mass and ancient star formation history, it does not appear to have been dissolved by baryonic processes. We rule out cores larger than 487, 717, 942 pc at respective 1-, 2-, 3-$σ$ confidence, thus imposing important constraints on the self-interacting DM cross-section. Spherical models yield biased estimates for both the velocity anisotropy and the inferred slope. The circular velocity at our outermost data point (900 pc) is $24.19^{+ 6.31}_{- 2.97} \ \mathrm{km~s^{-1}}s$. We infer a dynamical distance of $75.37^{+ 4.73}_{- 4.00}$ kpc, and show that Draco has a modest LOS rotation, with $\left = 0.22 \pm 0.09$. Our results provide a new stringent test of the so-called `cusp-core' problem that can be readily extended to other dwarfs.

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arXiv2024

JWST-TST High Contrast: Achieving direct spectroscopy of faint substellar companions next to bright stars with the NIRSpec IFU

The JWST NIRSpec integral field unit (IFU) presents a unique opportunity to observe directly imaged exoplanets from 3-5 um at moderate spectral resolution (R~2,700) and thereby better constrain the composition, disequilibrium chemistry, and cloud properties of their atmospheres. In this work, we present the first NIRSpec IFU high-contrast observations of a substellar companion that requires starlight suppression techniques. We develop specific data reduction strategies to study faint companions around bright stars, and assess the performance of NIRSpec at high contrast. First, we demonstrate an approach to forward model the companion signal and the starlight directly in the detector images, which mitigates the effects of NIRSpec's spatial undersampling. We demonstrate a sensitivity to planets that are 3e-6 fainter than their stars at 1'', or 3e-5 at 0.3''. Then, we implement a reference star point spread function (PSF) subtraction and a spectral extraction that does not require spatially and spectrally regularly sampled spectral cubes. This allows us to extract a moderate resolution (R~2,700) spectrum of the faint T-dwarf companion HD 19467 B from 2.9-5.2 um with signal-to-noise ratio (S/N)~10 per resolution element. Across this wavelength range, HD~19467~B has a flux ratio varying between 1e-5-1e-4 and a separation relative to its star of 1.6''. A companion paper by Hoch et al. more deeply analyzes the atmospheric properties of this companion based on the extracted spectrum. Using the methods developed here, NIRSpec's sensitivity may enable direct detection and spectral characterization of relatively old (~1 Gyr), cool (~250 K), and closely separated (~3-5 au) exoplanets that are less massive than Jupiter.

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arXiv2024

Satellite group infall into the Milky Way: exploring the Crater-Leo case with new HST proper motions

Context. Within $Λ$ Cold Dark Matter ($Λ$CDM) simulations, Milky Way-like galaxies accrete some of their satellite galaxies in groups of 3-5 members rather than individually. It was also suggested that this might be the reason behind the origin of satellite planes. Objects accreted in groups are expected to share similar specific total energy and angular momentum, and also identical orbital planes and directions. Aims. Looking at observations of Milky Way satellites, the dwarf galaxies Leo II, IV, V, Crater II, and the star cluster Crater 1 were proposed to be a vestige of group infall. The suggested "Crater-Leo group" shows a coherent distance gradient and all these objects align along a great circle on the sky. We use proper motion data to investigate whether the phase-space distribution of the members of the proposed group are indeed consistent with group infall. Methods. To further investigate this possibility, we use Gaia Data Release 3 (DR3) and new Hubble Space Telescope (HST) proper motions, $(μ_{α*}, μ_δ) = (-0.1921 \pm 0.0514, -0.0686 \pm 0.0523)$ mas yr$^{-1}$ for Leo IV and $(μ_{α*}, μ_δ) = (0.1186 \pm 0.1943, -0.1183 \pm 0.1704)$ mas yr$^{-1}$ for Leo V, to derive accurate orbital properties for the proposed group objects. In addition, we explore other possible members of this putative association. Results. Leo II, Leo IV, and Crater 1 show orbital properties consistent with those we predict from assuming group infall. However, our results suggest that Crater II was not accreted with the rest of the objects. If confirmed with increasingly accurate proper motions in the future, the Crater-Leo objects appear to constitute the first identified case of a cosmologically expected, typical group infall event, as opposed to the highly hierarchical Magellanic Cloud system.

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