The Structure and Kinematics of the Magellanic Stellar Periphery

The Large and Small Magellanic Clouds (LMC/SMC) are of fundamental importance in near-field cosmology. As the closest pair of interacting dwarf galaxies, they constitute the prototype system for studying the influence of tidal interactions on galaxy evolution. However, the orbital and interaction history of the Clouds -- critical to understanding these influences -- remains relatively unconstrained. This thesis aims to understand the effects of past interactions between the Magellanic Clouds by performing the first dedicated kinematic study of the Magellanic stellar outskirts. Stars in these regions are strongly susceptible to external perturbations, and the resulting structural and kinematic signatures are persistent: evidenced by a wealth of substructure observed across the Magellanic periphery. To kinematically study these structures, I have developed and led the Magellanic Edges Survey (MagES), a spectroscopic study using the 2dF+AAOmega instrument on the Anglo-Australian Telescope targeting ~8700 red clump and red giant branch stars across the periphery of the Clouds. In combination with astrometric measurements and high-precision photometry from the Gaia satellite, the survey provides 3D kinematics and abundance information critical for understanding the effects of dynamical perturbations on the Magellanic system. As a first demonstration of the efficacy of MagES data, the kinematics of two fields in the northern LMC disk are investigated. These are found to exhibit relatively undisturbed disk-like kinematics, enabling calculation of the most distant direct mass estimate for the LMC. Focus is then directed to a large arm-like substructure to the north of the LMC. This structure is determined to be made from disturbed LMC disk material, with its discrepant kinematics relative to the disk indicative that it was strongly perturbed during a recent interaction with the Milky Way. Comparison with dynamical models reveals the feature has not been closely influenced by close passages or disk crossings of the SMC around the LMC within the past Gyr, but earlier SMC passages may have contributed to its formation. These are the first kinematic constraints on the dynamical history of the Clouds prior to their most recent pericentric passage ~150 Myr ago. Finally, the kinematic and structural properties of the outer LMC at galactocentric radii beyond 10 degrees are explored. The northeastern LMC disk is remarkably undisturbed, with geometry and kinematics near-identical to those at smaller radii. In contrast, the western and southern LMC disk are highly disturbed, with deviations exceeding 25 km s^{-1} from equilibrium disk kinematics, and significantly elevated velocity dispersions. Red clump stars in these regions are also significantly brighter than expected for an undisturbed disk, indicating substantial warping. It is further demonstrated that several substructures to the south of the LMC, including two claw-like features and a long arm-like structure extending around the southeastern disk, are comprised of predominantly LMC material -- as opposed to SMC debris -- but display strongly disturbed kinematics. Comparisons with dynamical models reveal the western LMC disk is likely significantly affected by an SMC crossing of the LMC disk plane ~400 Myr ago. However, southern substructures appear considerably more complex than observed in any models, plausibly requiring multiple previous interactions with the SMC to fully explain their observed dynamical properties. In summary, the MagES data presented in this thesis provide a set of unprecedented empirical constraints on the interaction history of the Clouds that will be critical for guiding future numerical models aiming to accurately describe the complex evolution of the Magellanic system

The Magellanic Edges Survey III. Kinematics of the disturbed LMC outskirts

We explore the structural and kinematic properties of the outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. Even at large galactocentric radii ($8^\circ<R<11^\circ$), we find the north-eastern LMC disk is relatively unperturbed: its kinematics are consistent with a disk of inclination ~$36.5^\circ$ and line-of-nodes position angle ~$145^\circ$ east of north. In contrast, fields at similar radii in the southern and western disk are significantly perturbed from equilibrium, with non-zero radial and vertical velocities, and distances significantly in front of the disk plane implied by our north-eastern fields. We compare our observations to simple dynamical models of the Magellanic/Milky Way system which describe the LMC as a collection of tracer particles within a rigid potential, and the Small Magellanic Cloud (SMC) as a rigid Hernquist potential. A possible SMC crossing of the LMC disk plane ~400 Myr ago, in combination with the LMC's infall to the Milky Way potential, can qualitatively explain many of the perturbations in the outer disk. Additionally, we find the claw-like and arm-like structures south of the LMC have similar metallicities to the outer LMC disk ([Fe/H]~-1), and are likely comprised of perturbed LMC disk material. The claw-like substructure is particularly disturbed, with out-of-plane velocities >60 km s$^{-1}$ and apparent counter-rotation relative to the LMC's disk motion. More detailed N-body models are necessary to elucidate the origin of these southern features, potentially requiring repeated interactions with the SMC prior to ~1 Gyr ago.Comment: Accepted by MNRA

Resolved SPLASH Chemodynamics in Andromeda's PHAT Stellar Halo and Disk: On the Nature of the Inner Halo Along the Major Axis

Stellar kinematics and metallicity are key to exploring formation scenarios for galactic disks and halos. In this work, we characterized the relationship between kinematics and photometric metallicity along the line-of-sight to M31's disk. We combined optical HST/ACS photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey with Keck/DEIMOS spectra from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. The resulting sample of 3536 individual red giant branch stars spans 4-19 projected kpc, making it a useful probe of both the disk and inner halo. We separated these stars into disk and halo populations by modeling the line-of-sight velocity distributions as a function of position across the disk region, where 70.9% stars have a high likelihood of belonging to the disk and 17.1% to the halo. Although stellar halos are typically thought to be metal-poor, the kinematically identified halo contains a significant population of stars (29.4%) with disk-like metallicity ([Fe/H]$_{\rm phot} \sim -0.10$). This metal-rich halo population lags the gaseous disk to a similar extent as the rest of the halo, indicating that it does not correspond to a canonical thick disk. Its properties are inconsistent with those of tidal debris originating from the Giant Stellar Stream merger event. Moreover, the halo is chemically distinct from the phase-mixed component previously identified along the minor axis (i.e., away from the disk), implying contributions from different formation channels. These metal-rich halo stars provide direct chemodynamical evidence in favor of the previously suggested "kicked-up'' disk population in M31's inner stellar halo.Comment: Submitted to AJ. Conclusions on page 20. 18 figures, 2 tables, 4 appendice

The Magellanic Edges Survey I: Description and first results

We present an overview of, and first science results from, the Magellanic Edges Survey (MagES), an ongoing spectroscopic survey mapping the kinematics of red clump and red giant branch stars in the highly substructured periphery of the Magellanic Clouds. In conjunction with Gaia astrometry, MagES yields a sample of ∼7000 stars with individual 3D velocities that probes larger galactocentric radii than most previous studies. We outline our target selection, observation strategy, data reduction, and analysis procedures, and present results for two fields in the northern outskirts (>10◦ on-sky from the centre) of the Large Magellanic Cloud (LMC). One field, located in the vicinity of an arm-like overdensity, displays apparent signatures of perturbation away from an equilibrium disc model. This includes a large radial velocity dispersion in the LMC disc plane, and an asymmetric line-of-sight velocity distribution indicative of motions vertically out of the disc plane for some stars. The second field reveals 3D kinematics consistent with an equilibrium disc, and yields Vcirc = 87.7 +- 8.0 km s-1 at a radial distance of ∼10.5 kpc from the LMC centre. This leads to an enclosed mass estimate for the LMC at this radius of (1.8 +- 0.3) x 1010 M.Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Based on data acquired at the Anglo-Australian Observatory. SK is partially supported by National Science Foundation (NSF) grants AST-1813881 and AST1909584 and Heising-Simons foundation grant 2018-1030. DMN acknowledges support from the National Aeronautics and Space Administration (NASA) under award number 80NSSC19K0589. AK gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation) – Project-ID 138713538 – SFB 881 (‘The Milky Way System’), subprojects A03, A05, and A11

S5S^5: Probing the Milky Way and Magellanic Clouds potentials with the 6-D map of the Orphan-Chenab stream

We present a 6-D map of the Orphan-Chenab (OC) stream by combining the data from 5 years of Southern Stellar Stream Spectroscopic Survey $S^5$ observations with Gaia EDR3 data. We reconstruct the proper motion, radial velocity, distance and on-sky track of stream stars with spline models and extract the stellar density along the stream. The stream has a total luminosity of $M_V=-8.2$ and an average metallicity of $[Fe/H]=-1.9$, similar to classical MW satellites like Draco. The stream shows drastic changes in its physical width varying from 200 pc to 1 kpc, a constant line of sight velocity dispersion of 5 km/s, but an increase in the velocity dispersion along the stream near pericenter to $\sim$ 10 km/s. Despite the large apparent variation in the stellar number density along the stream, the flow rate of stars along the stream is remarkably constant. We model the 6-D stream track by a Lagrange-point stripping method with a flexible MW potential in the presence of a moving extended LMC potential. This allows us to constrain the mass profile of the MW within the distance range 15.6 < r < 55.5 kpc, with the best measured enclosed mass of $(2.85\pm 0.1)\times10^{11}\,M_\odot$ within 32.4 kpc. With the OC stream's closest approach distance to the LMC of $\sim 21$ kpc, our stream measurements are highly sensitive to the LMC mass profile with the most precise measurement of the LMC's enclosed mass being at 32.8 kpc with $M=(7.02\pm 0.9)\times10^{10}\, {M}_\odot$. We confidently detect that the LMC DM halo extends to at least 53 kpc. The fitting of the OC stream allows us to constrain the past LMC trajectory and the degree of dynamical friction it experienced. We demonstrate that the stars on the OC stream show large energy and angular momentum spreads caused by the LMC perturbation and revealing the limitations of orbital invariants for substructure identification in the MW halo.Comment: submitted to MNRAS; comments welcome; data released with the paper is available on Zenodo https://zenodo.org/record/722265

Signature of a massive rotating metal-poor star imprinted in the Phoenix stellar stream*

The Phoenix stellar stream has a low intrinsic dispersion in velocity and metallicity that implies the progenitor was probably a low mass globular cluster. In this work we use Magellan/MIKE high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity ($\sigma(\mathrm{[Fe~II/H]}) = 0.04^{+0.11}_{-0.03}$) and a large peak-to-peak range in [Na/Fe] and [Al/Fe] abundance ratios, consistent with the light element abundance patterns seen in the most metal-poor globular clusters. However, unlike any other globular cluster, we also find an intrinsic spread in [Sr II/Fe] spanning $\sim$1 dex, while [Ba II/Fe] shows nearly no intrinsic spread ($\sigma(\mathrm{[Ba~II/H]}) = {0.03}^{+0.10}_{-0.02}$). This abundance signature is best interpreted as slow neutron capture element production from a massive fast-rotating metal-poor star ($15-20 \mathrm{M}_\odot$, $v_\mathrm{ini}/v_\mathrm{crit} = 0.4$, $[\mathrm{Fe/H}] = -3.8$). The low inferred cluster mass suggests the system would have been unable to retain supernovae ejecta, implying that any massive fast-rotating metal-poor star that enriched the interstellar medium must have formed and evolved before the globular cluster formed. Neutron capture element production from asymptotic giant branch stars or magneto-rotational instabilities in core-collapse supernovae provide poor fits to the observations. We also report one Phoenix stream star to be a lithium-rich giant ($A(\mathrm{Li}) = 3.1 \pm 0.1$). At $[\mathrm{Fe/H}] = -2.93$ it is among the most metal-poor lithium-rich giants known.Comment: Accepted to ApJ 2021-07-0

Broken into Pieces::ATLAS and Aliqa Uma as One Single Stream

We present the first spectroscopic measurements of the ATLAS and Aliqa Uma streams from the Southern Stellar Stream Spectroscopic Survey ($S^5$), in combination with the photometric data from the Dark Energy Survey and astrometric data from $Gaia$. From the coherence of spectroscopic members in radial velocity and proper motion, we find out that these two systems are extremely likely to be one stream with discontinuity in morphology and density on the sky (the "kink" feature). We refer to this entire stream as the ATLAS-Aliqa Uma stream, or the AAU stream. We perform a comprehensive exploration of the effect of baryonic substructures and find that only an encounter with the Sagittarius dwarf $\sim 0.5$ Gyr ago can create a feature similar to the observed "kink". In addition, we also identify two gaps in the ATLAS component associated with the broadening in the stream width (the "broadening" feature). These gaps have likely been created by small mass perturbers, such as dark matter halos, as the AAU stream is the most distant cold stream known with severe variations in both the stream surface density and the stream track on the sky. With the stream track, stream distance and kinematic information, we determine the orbit of the AAU stream and find that it has been affected by the Large Magellanic Cloud, resulting in a misalignment between the proper motion and stream track. Together with the Orphan-Chenab Stream, AAU is the second stream pair that has been found to be a single stream separated into two segments by external perturbation.Comment: 33 pages, 22 figures (including 1 movie), 3 tables. Accepted for publication in Ap

The southern stellar stream spectroscopic survey (S (5)): Overview, target selection, data reduction, validation, and early science

We introduce the southern stellar stream spectroscopy survey (S5), an on-going program to map the kinematics and chemistry of stellar streams in the southern hemisphere. The initial focus of S5 has been spectroscopic observations of recently identified streams within the footprint of the dark energy survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9 m Anglo-Australian Telescope’s 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S5 has mapped nine DES streams and three streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S5 program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction, and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S5, including future public data releases, can be found at http://s5collab.github.io

The Southern Stellar Stream Spectroscopic Survey (S5): Chemical Abundances of Seven Stellar Streams

We present high-resolution Magellan/MIKE spectroscopy of 42 red giant stars in seven stellar streams confirmed by the Southern Stellar Stream Spectroscopic Survey (S5): ATLAS, Aliqa Uma, Chenab, Elqui, Indus, Jhelum, and Phoenix. Abundances of 30 elements have been derived from over 10,000 individual line measurements or upper limits using photometric stellar parameters and a standard LTE analysis. This is currently the most extensive set of element abundances for stars in stellar streams. Three streams (ATLAS, Aliqa Uma, and Phoenix) are disrupted metal-poor globular clusters, although only weak evidence is seen for the light-element anticorrelations commonly observed in globular clusters. Four streams (Chenab, Elqui, Indus, and Jhelum) are disrupted dwarf galaxies, and their stars display abundance signatures that suggest progenitors with stellar masses ranging from 106 to 107 M⊙. Extensive description is provided for the analysis methods, including the derivation of a new method for including the effect of stellar parameter correlations on each star's abundance and uncertainty.A.R.C. is supported in part by the Australian Research Council through a Discovery Early Career Researcher Award (DE190100656). Parts of this research were supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) through project No. CE170100013. A.B.P. acknowledges support from NSF grant AST-1813881. S.K. is partially supported by NSF grants AST-1813881 and AST-1909584 and HeisingSimons foundation grant 2018-1030. S.L.M. and J.D.S. acknowledge support from the Australian Research Council through Discovery Project grant DP180101791