This presentation is an attempt to lay out, in the context of observational studies of the masses of Milky Way dwarf galaxies, what it is I think we want to do, why we want to do it, and how we go about doing it. They are my own personal opinions, and are presented here in the hope that they will stimulate others to step back and consider what are the most important scientific investigations that we should be doing, given the plethora of facilities and data available that are now available to us.

The Milky Way satellite dwarf galaxy Antlia II is one of the lowest surface brightness galaxies known. It has a size comparable to the Large Magellanic Cloud, but only 106 solar masses of stars. We present kinematic and chemical measurements from the Southern Stellar Stream Spectroscopic Survey using the AAT/2dF that clearly demonstrate that Antlia II is tidally disrupting. The orbit and velocity gradient also clearly shows that the Milky Way has moved in response to the Large Magellanic Cloud. However, Antlia II currently lies on the galaxy mass-metallicity relation, suggesting that it has not lost too much stellar mass. These measurements illustrate the importance of full dynamic models when interpreting the masses of local group galaxies.

We use spectroscopic observations of Antlia B, a distant (d ∼ 1.35 Mpc) faint dwarf galaxy (MV = −9.7, M* =105 M⊙), from MUSE-Faint, to explore alternative dark matter (DM) models to Λ CDM, which are one possible explanation for the small-scale problems in the standard model.

We measure line-of-sight velocities of 127 stars, and combine these with GravSphere, a Jeans modelling code, to place constraints on DM models and derive the first DM density profiles for this object.

We present constraints on the nature of self-interacting DM, in which DM particles can interact with one another in the form of annihilation (we find a core radius of rc ≲ 69 pc) and on scalar field DM, comprised of ultralight bosons that form a Bose-Einstein Condensate (we find a characteristic length scale of the self-interaction of RTF ≲ 180 pc). These results suggest that we can rule out these models as an explanation for the cores in the larger dwarf galaxies in the Local Group.

We discuss recent results from high-resolution Magellan/MIKE spectroscopy of five stars in the outskirts (up to ∼ 1 kpc) of the Tucana II ultra-faint dwarf galaxy (UFD), complemented by prior observations of seven stars closer to the galaxy’s center. These outer stars were identified via their low SkyMapper photometric metallicities and consistent Gaia DR2 proper motions, and their membership was confirmed through follow-up medium-resolution spectroscopy. The high-resolution spectroscopy presented here provides detailed chemical abundances and more precise velocities, facilitating a revised dynamical analysis for signs of tidal disruption and a collective analysis of the detailed chemistry to evaluate astrophysical scenarios for the origin of the spatially extended feature. We discuss these signatures here and assess the evidence for several formation scenarios for the extended feature of Tucana II, highlighting how such studies of the outskirts of the UFD population as a whole can inform which scenarios may be preferred.

Leo T is the lowest mass galaxy known to contain neutral gas and to show signs of recent star formation, which makes it a valuable laboratory for studying the nature of gas and star formation at the limits of where galaxies are found to have rejuvenating episodes of star formation.

Here we discuss a novel study of Leo T that uses data from the MUSE integral field spectrograph and photometric data from HST. The high sensitivity of MUSE allowed us to increase the number of Leo T stars observed spectroscopically from 19 to 75. We studied the age and metallicity of these stars and identified two populations, all consistent with similar metallicity of [Fe/H] ∼ − 1.5 dex, suggesting that a large fraction of metals were ejected. Within the young population, we discovered three emission line Be stars, supporting the conclusion that rapidly rotating massive stars are common in metal-poor environments. We find differences in the dynamics of young and old stars, with the young population having a velocity dispersion consistent with the kinematics of the cold component of the neutral gas. This finding directly links the recent star formation in Leo T with the cold component of the neutral gas.

To investigate whether an m = 1 perturbation in the halo potential could be a mechanism creating observed kinematical asymmetry in the rotation curve of the dwarf irregular galaxy WLM, we fit a theoretical rotational velocity associated with such a perturbation in the halo potential model to the observed data. We show that a lopsided halo potential model can explain the asymmetry in the kinematic data reasonably well. To investigate how well such a small perturbation in the halo potential represents the observed asymmetries in the rotation curve and in the velocity field of WLM, we measure the value of the perturbation parameter Ɛper from the kinematic data on the rotation curve of both sides in the outer part. We obtain the value of the perturbation parameter denoting the perturbed halo potential of about 0.2.

Almost 30 years have passed since not only the universal density distribution of dark matter (DM) halos in cosmological N-body simulations, but also the scalings between properties of DM halos represented by concentration-mass (c-M) relations are proposed. We derive the c-M relation for sub galactic halos (subhalos) of Milky Way (MW)-sized host halo using the result of the ultra-high resolution cosmological N-body simulation, Phi-4096, with a particle mass of 5.13 × 103 h−1M⊙. This c-M relation is confirmed to be consistent with a c-M relation from near the free streaming scale to the galactic scale proposed in a literature. One of our main findings is that the c-M relation can reproduce observational properties of DM halos from dwarf galaxies to clusters of galaxies. In addition, we provide a testable prediction of the density distributions of MW subhalos for future observations.

In this work we present preliminary results from a systematic search for new Local Group dwarf galaxies in the deep, wide-field Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). The first major result from this work is the discovery of a candidate ultra faint dwarf galaxy, Boötes V. This satellite of the Milky Way has a physical half-light radius of , an absolute V-band magnitude of −4.5 ± 0.4 mag, and resides at a heliocentric distance of approximately 100 kpc. We also announce the discovery of a faint, compact star cluster, UNIONS 1. The end goal of this work will be to put firm constraints on the detectability and completeness of Local Group dwarf galaxies given the broad sky coverage and excellent photometric depth of UNIONS.

The Galactic dwarf spheroidal galaxies (dSphs) provide valuable insight into dark matter (DM) properties and its role in galaxy formation. Their close proximity enables the measurement of line-of-sight velocities for resolved stars, which allows us to study DM halo structure. However, uncertainties in DM mass profile determination persist due to the degeneracy between DM mass density and velocity dispersion tensor anisotropy. Overcoming this requires large kinematic samples and identification of foreground contamination. With 1.25 deg2 and 2394 fibers, PFS plus pre-imaging with Hyper Suprime Cam will make significant progress in this undertaking.

Gaia EDR3 has provided proper motions of Milky Way (MW) dwarf galaxies with an unprecedented accuracy, which allows us to investigate their orbital properties. We found that the total energy and angular momentum of MW dwarf galaxies are much larger than that of MW K-giant stars, Sagittarius stream stars and globular clusters. It suggests that many MW dwarf galaxies have had a recent infall into the MW halo. We confirmed that MW dwarf galaxies lie near their pericenters, which suggests that they do not behave like satellite systems derived from Lambda-Cold-Dark-Matter cosmological simulations. These new results require revisiting the origin of MW dwarf galaxies, e.g., if they came recently, they were likely to have experienced gas removal due to the ram pressure induced by MW’s hot gas, and to be affected by MW tides. We will discuss the consequences of these processes on their mass estimation.

The controversy “dark matter vs. modified gravity” constitutes a major topic of discussion. It was proposed that dynamical friction could be used to discriminate between the two alternatives. Analytic calculations indicate that, with modified gravity, globular clusters (GCs) of low-mass galaxies experience much stronger dynamical friction than in the equivalent system with Newtonian gravity and dark matter. As a result, in modified gravity the old GCs of low mass galaxies should have already settled in the centers of the galaxies. This is not observed. Here we report on our efforts to verify the analytic results by self-consistent simulations with the MOND-type (modified Newtonian dynamics) gravity. The core stalling mechanism, that was not considered in the analytic calculations, prevents GCs to settle in centers of ultra-diffuse galaxies. For isolated dwarf galaxies, which are gas-rich objects, supernova explosions prevent the GCs from settling.

Prolate rotation is characterized by a significant stellar rotation around a galaxy’s major axis, which contrasts with the more common oblate rotation. Prolate rotation is thought to be due to major mergers and thus studies of prolate-rotating systems can help us better understand the hierarchical process of galaxy evolution. Dynamical studies of such galaxies are important to find their gravitational potential profile, total mass, and dark matter fraction. Recently, it has been shown in a cosmological simulation that it is possible to form a prolate-rotating dwarf galaxy following a dwarf-dwarf merger event. The simulation also shows that the unusual prolate rotation can be time enduring. In this particular example, the galaxy continued to rotate around its major axis for at least 7.4 Gyr (from the merger event until the end of the simulation). In this project, we use mock observations of the hydro-dynamically simulated prolate-rotating dwarf galaxy to fit various stages of its evolution with Jeans dynamical models. The Jeans models successfully fit the early oblate state before the major merger event, and also the late prolate stages of the simulated galaxy, recovering its mass distribution, velocity dispersion, and rotation profile. We also ran a prolate-rotating N-body simulation with similar properties to the cosmologically simulated galaxy, which gradually loses its angular momentum on a short time scale ∼ 100 Myr. More tests are needed to understand why prolate rotation is time enduring in the cosmological simulation, but not in a simple N-body simulation.

Recent panoramic maps of the Magellanic system have revealed a wealth of low-surface-brightness stellar substructures surrounding both the Large and Small Magellanic Clouds (LMC/SMC); clear evidence of tidal interactions between the two Clouds, as well as with the Milky Way. The Magellanic Edges Survey (MagES), a spectroscopic survey that targets red clump and red giant branch stars across the outskirts of both Clouds, aims to characterise these features and shed light on their formation. We summarise recent results from MagES, which suggest multiple previous LMC-SMC interactions are required to fully explain the observed dynamical properties of the Clouds.

The formation of the Magellanic Bridge during an encounter between the Magellanic Clouds ∼ 200 Myr ago would be imprinted in the chemical evolution and kinematics of its stellar population, with sites of active star formation. Since it contains hundreds of stellar clusters and associations, we combined deep photometry from VISCACHA and SMASH surveys to explore this topic, by deriving structural parameters, age, metallicity, distance and mass for 33 Bridge clusters with robust statistical tools. We identified a group of 13 clusters probably stripped from the Small Magellanic Cloud (0.5 − 4.7 Gyr, [Fe/H] < −0.6 dex) and another 15 probably formed in-situ (< 200 Myr, [Fe/H] ∼ −0.4 dex). Two metallicity dips were detected in the age-metallicity relation, coeval to the Stream and Bridge formation epochs. Cluster masses range from 500 to ∼ 104 M⊙, and a new estimate of 3 − 5 × 105 M⊙ is obtained for the Bridge stellar mass.

Observational studies have identified several sub-structures in different regions of the Magellanic Clouds, the nearest pair of interacting dwarf satellites of the Milky Way. By studying the metallicity of the sources in these sub-structures, we aim to shed light on the possible origin of these sub-structures. Spectroscopic metallicities exist only for a few thousand sources, mostly giant stars located in specific regions of the galaxies. These metallicities come from different instruments at various spectral resolutions, and systematic uncertainties hamper comparisons and draw firm conclusions about their origin. The third data release of Gaia has provided us with ∼ 0.17 million XP spectra of the different stellar populations in the SMC alone as faint as ∼ 18 mags in the G band, which are spread across ∼ 10° from the SMC centre. We aim to determine the metallicities of these sources based on synthetic Strömgren photometry derived from XP spectra and produce a high-resolution metallicity map of the SMC. Our metallicity gradient estimate of the SMC turns out to be −0.062 ± 0.009 dex/deg. This is comparable with the previous estimates, which also validates our method of metallicity estimation. We aim to apply this method to other stellar populations and to the LMC to create a high-resolution metallicity map of the Magellanic Clouds.

We have introduced a quantitative and automated method to parameterize star clusters in the Magellanic Clouds (MCs) using the Gaia DR3 data. We used the existing cluster catalogs and extracted their Gaia DR3 data and nearby field regions. We automated the Field Star Decontamination (FSD) algorithm with multiple annular field regions for isolated clusters. We estimated the LMC and SMC clusters’ age, extinction, distance modulus, and metallicity using a Bayesian approach. We expect to parameterize many clusters in the outer LMC with the help of the wide coverage of the Gaia data. We aim to identify correlated cluster formation episodes between the MCs, thereby throwing light on their interaction history. Here we present the preliminary results of this study.

The Small Magellanic Cloud (SMC), as one of the nearest galaxies to us, provides a superb laboratory for studying resolved stars in exquisite detail. We present here the largest sample of SMC Red Giant Branch (RGB) stars (∼6000) observed with the AAOmega spectrograph fed by the Two Degree Field (2dF) multi-object system at the Anglo-Australian Telescope of the Siding Spring Observatory (Australia). Metallicities were recovered using a direct estimation of [Fe/H] from the equivalent widths of the Calcium triplet (CaT). We discuss the potential implications of the metallicity gradients and compare them with previous determinations of star formation history to assess the consequences of encounters between the SMC and the Large Magellanic Cloud.

Dark matter (DM) halo angular momentum is very challenging to determine from observations of galaxies. In this study, we present a new hybrid method of estimating the dimensionless halo angular momentum, halo spin of a gas-rich dwarf barred galaxy UGC5288 using N-Body/SPH simulations. We forward model the galaxy disk properties- stellar and gas mass, surface densities, disk scalelengths, bar length and bar ellipticity from observations. We use the HI rotation curve to constrain the DM halo density profile and further use the bar properties to determine the models that best represent the observed baryonic disk. We compare the halo spin profile from our models to the halo spin profiles of similar mass dwarf galaxy analogues of UGC5288 in the TNG50 simulations. The halo spin profile from our simulated models matches within ballpark values of the median spin profile of UGC5288 analogues in the TNG50 simulations, although there are some uncertainties due to the DM halo evolutionary history.

The Magellanic Stream is a lengthy, ribbon-like gas structure stretching 200 degrees across the sky and surrounding the Large and Small Magellanic Clouds. These two galaxies are the brightest dwarf galaxies orbiting the Milky Way (MW). The Stream is a major subject of study in galactic dynamics because it provides insights into the evolution of galaxies, including the MW and the Magellanic Clouds, its companion dwarf satellites, and the interstellar medium. Gas flows play a key role in galaxies’ growth, evolution, and sustainability, but many questions related to the Stream remain unanswered. Here, I will review the main advance in this subject of the last decade and posit new questions that need to be addressed.

In this contribution, we describe how we have used positions and velocities of 30 globular clusters in the Large Magellanic Cloud (LMC) to estimate its anisotropy and mass within 13 kpc, and then how we have used these estimates to extrapolate its virial mass. This is the first time that this family of mass estimation methods has been applied to the LMC. We also compare our estimate against other estimates of the LMC’s mass via different methods and discuss the broader context of our results.