arXiv subjects

Emily C. Cunningham

Publications and source records attributed to Emily C. Cunningham.

At least 19 recordsLinked to original sources

arXiv2026

Extragalactic Stellar Streams in Time-Dependent Cosmological Halos

Upcoming and ongoing surveys will detect thousands of stellar streams around galaxies other than the Milky Way. Studies from the Milky Way have shown that time-dependent evolution of the Galactic halo plays a key role in shaping stellar streams, but remains unexplored for extragalactic stellar streams. We use the FIRE-2 m12m cosmological zoom-in simulation, mock observed as an extragalactic system including three stellar streams, to examine how halo time-dependence affects progenitor and host halo inference from extragalactic systems. We show that two of the three m12m streams are well reproduced in a static halo if we only allow for tidal stripping near pericenter. We apply the extragalactic stream fitting code X-Stream to each mock observed stream, and obtain constraints on the host dark matter halo and stream properties. Using on-sky morphology alone and then fixing the progenitor radial velocities, we compare recovered orbits and halo parameters to the FIRE-2 m12m ground truth. For the longest stream with a looped segment, we find unbiased strong constraints on progenitor and halo properties. For the shortest stream, we find limits on orbital parameters, but no constraints on progenitor and halo mass unless we include fainter, more extended debris. For the most massive stream, which was not well produced in a static halo, the recovered orbital parameters are biased, reflecting unmodeled time-dependence. We conclude that imaging of stream debris from extragalactic dwarf galaxies can, in some cases, be used to infer present-day dark matter halo properties, even in a cosmological environment.

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arXiv2026

The Importance of Galaxy-Wide Star Formation in Driving Winds at z~1

In this work, we study winds for a representative sample of 86 star-forming galaxies (SFGs) at z~1 with $M_\star = 10^{9.0}-10^{11.5} M_\odot$, by measuring the Mg II line profiles in deep Keck spectra. A total of 50 (58\%) are found to have winds. Unlike local starburst galaxies, the wind detection rate does not exhibit a threshold in star-formation rate (SFR) density $\Sigma_\mathrm{SFR}$ at 0.1 Msun/yr/kpc$^2$, but shows a gradual decline around this value. We find correlations between wind velocity $v_\mathrm{wind}$ and SFR, $\Sigma_\mathrm{SFR}$, and stellar mass, as per previous studies. Intriguingly, the z~1 SFGs appear to follow the same $v_\mathrm{wind}$-SFR relation as local starbursts. A combined fit gives: log $v_\mathrm{wind}$ = 0.16 log SFR + 2.4 (3-sigma significance). This unified relation spans over 4 dex in SFR and agrees with Illustris-TNG. No unified relation is found between $v_\mathrm{wind}$ and stellar mass, sSFR, or $\Sigma_\mathrm{SFR}$. This suggests winds might be most closely associated with SFR. We examine whether winds in z~1 SFGs are driven by their most compact star-forming regions. To do so, we consider whether the relation between $v_\mathrm{wind}$ and the $\Sigma_\mathrm{SFR}$ measured from only these regions is stronger than that for the galaxy-wide $\Sigma_\mathrm{SFR}$. We do not find a stronger correlation, suggesting that winds are most related to $\Sigma_\mathrm{SFR}$ of the entire galaxy. Collectively, these findings suggest a picture in which galaxy-wide star formation plays an important role in driving winds at z~1. Wind bubbles from all star-forming regions could combine momentum and help lift their entrained gas out of the galaxy.

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arXiv2025

Kinematics of Distant Milky Way Halo RR Lyrae Stars out to 160 kpc

We present a kinematical study of the outer halo (r_GC approximately 60 to 160 kpc) of the Milky Way based on spectroscopy of 55 RR Lyrae stars obtained with the ESI instrument on the Keck II telescope. Our spectroscopic targets were selected from three photometric surveys: NGVS, DES, and Pan-STARRS1. We derive center-of-mass radial velocities with uncertainties of 6 to 35 km s^-1. The halo velocity dispersion measured from our sample is 70 plus/minus 7 km s^-1. The velocity field shows a possible dipole-like structure, with redshifted northern and blueshifted southern hemispheres. Fitting a Milky Way - Large Magellanic Cloud dipole perturbation model yields a weak or marginal dipole signal with amplitude -30 (+16, -20) km s^-1 and apex direction (l, b) = (-38.2 (+42.4, -31.5), -41.3 (+27.9, -23.8)) deg, along with a bulk compression velocity of -16 plus/minus 11 km s^-1. Although limited by sky coverage and sample size, our results are consistent with the presence of LMC-induced disequilibrium in the distant halo beyond 100 kpc. In addition to the 55 RR Lyrae stars, our spectroscopy reveals that 10 additional photometrically selected RR Lyrae candidates are actually quasar or blazar contaminants, highlighting the need for caution regarding such contaminants in sparsely sampled photometric surveys. Our study demonstrates that single-epoch spectroscopy of RR Lyrae stars is a viable method for probing the kinematics of the outer halo, and future surveys such as Rubin LSST and DESI-II have the potential to significantly advance this effort.

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arXiv2025

Shaping the Milky Way. II. The dark matter halo's response to the LMC's passage in a cosmological context

The distribution of dark matter in the Milky Way (MW) is expected to exhibit a large-scale dynamical response to the recent infall of the LMC. This event produces a dynamical friction wake and shifts the MW's halo density center. The structure of this response encodes information about the LMC- MW mass ratio, the LMC's orbit, the MW halo's pre-infall structure and could provide constraints on dark matter physics. To extract this information, a method to separate these effects and recover the initial shape of the MW's halo is required. Here, we use basis function expansions to analyze the halo response in eighteen simulations of MW-LMC-like interactions from the MWest cosmological, dark-matter-only zoom-in simulations. The results show that mergers similar to the LMC consistently generate a significant dipole and a secondary quadrupole response in the halo. The dipole arises from the host density center displacement and halo distortions, and its amplitude scales as the square of the MW-LMC mass ratio, peaking 0.2-0.7 Gyr after the LMC's pericenter. The quadrupole's strength depends primarily on the original axis ratios of the host halo, though contributions from the dynamical friction wake cause it to peak less than 0.3 Gyr before pericenter. Future measurements of both the dipole and quadrupole imprints of the LMC's passage in the density of the MW's stellar halo should be able to disentangle these effects and provide insight into the initial structure of the MW's halo, the MW's response, and the mass of the LMC.

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arXiv2025

Dancing Streams In Merging Halos: Stellar Streams in a MW--LMC-like merger

Stellar streams -- formed from tidally stripped globular clusters or dwarf galaxies -- are sensitive tracers of a galaxy's accretion history and gravitational potential. While numerous streams are known in the Milky Way (MW), the formation and evolution of stellar streams have been primarily studied in isolated settings. The impact of subsequent galaxy interactions on stellar streams remains largely unexplored. Understanding merger-induced effects is however, crucial given the accretion of the Large Magellanic Cloud (LMC) onto the MW, and the fact that for example M31 and Cen A have experienced recent mergers. We analyze the detailed evolution of 1024 stellar streams during a complete MW--LMC-like merger, systematically varying initial stream properties and considering various orbits for the infalling perturber. We find that an MW--LMC mass-ratio merger significantly alters stellar stream properties, including energy, angular momentum, orbit, and morphology. Some streams exhibit dramatic morphological changes or develop complex substructures, while others see substantial shifts in energy and/or angular momentum, or re-orient their orbital plane. Interestingly, strong morphological alterations do not necessarily correlate with large changes in energy or orbit. A few streams split apart with parts moving to different orbits, appearing disconnected in position and kinematics despite their common origin. Strong effects correlate with close encounters between stream particles and the infalling perturber at various times during the merger. Our findings highlight the considerable impact of significant accretion events on the properties of stellar streams, and the challenge to recover the initial orbits of streams from their appearance at the present-day. Visualizations of the detailed evolution of all 1024 stellar streams are available at https://dancingstreamsinmerginghalos.github.io.

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arXiv2024

Strong Chemical Tagging in FIRE: Intra and Inter-Cluster Chemical Homogeneity in Open Clusters in Milky Way-like Galaxy Simulations

Open star clusters are the essential building blocks of the Galactic disk; "strong chemical tagging" - the premise that all star clusters can be reconstructed given chemistry information alone - is a driving force behind many current and upcoming large Galactic spectroscopic surveys. In this work, we characterize abundance patterns for 9 elements (C, N, O, Ne, Mg, Si, S, Ca, and Fe) in open clusters (OCs) in three galaxies (m12i, m12f, and m12m) from the Latte suite of FIRE-2 simulations to investigate if strong chemical tagging is possible in these simulations. We select young massive (>=10^(4.6) Msun) OCs formed in the last ~100 Myr and calculate the intra- and inter-cluster abundance scatter for these clusters. We compare these results with analogous calculations drawn from observations of OCs in the Milky Way. We find the intra-cluster scatter of the observations and simulations to be comparable. While the abundance scatter within each cluster is minimal (<0.020 dex), the mean abundance patterns of different clusters are not unique. We also calculate the chemical difference in intra- and inter-cluster star pairs and find it, in general, to be so small that it is difficult to distinguish between stars drawn from the same OC or from different OCs. Despite tracing three distinct nucleosynthetic families (core-collapse supernovae, white dwarf supernovae, and stellar winds), we conclude that these elemental abundances do not provide enough discriminating information to use strong chemical tagging for reliable OC membership.

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arXiv2024

Efficient and accurate force replay in cosmological-baryonic simulations

We construct time-evolving gravitational potential models for a Milky Way-mass galaxy from the FIRE-2 suite of cosmological-baryonic simulations using basis function expansions. These models capture the angular variation with spherical harmonics for the halo and azimuthal harmonics for the disk, and the radial or meridional plane variation with splines. We fit low-order expansions (4 angular/harmonic terms) to the galaxy's potential for each snapshot, spaced roughly 25 Myr apart, over the last 4 Gyr of its evolution, then extract the forces at discrete times and interpolate them between adjacent snapshots for forward orbit integration. Our method reconstructs the forces felt by simulation particles with high fidelity, with 95% of both stars and dark matter, outside of self-gravitating subhalos, exhibiting errors $\leq$4% in both the disk and the halo. Imposing symmetry on the model systematically increases these errors, particularly for disk particles, which show greater sensitivity to imposed symmetries. The majority of orbits recovered using the models exhibit positional errors $\leq$10% for 2-3 orbital periods, with higher errors for orbits that spend more time near the galactic center. Approximate integrals of motion are retrieved with high accuracy even with a larger potential sampling interval of 200 Myr. After 4 Gyr of integration, 43% and 70% of orbits have total energy and angular momentum errors within 10%, respectively. Consequently, there is higher reliability in orbital shape parameters such as pericenters and apocenters, with errors $\sim$10% even after multiple orbital periods. These techniques have diverse applications, including studying satellite disruption in cosmological contexts.

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arXiv2023

On the co-rotation of Milky Way satellites: LMC-mass satellites induce apparent motions in outer halo tracers

Understanding the physical mechanism behind the formation of a co-rotating thin plane of satellite galaxies, like the one observed around the Milky Way (MW), has been challenging. The perturbations induced by a massive satellite galaxy, like the Large Magellanic Cloud (LMC) provide valuable insight into this problem. The LMC induces an apparent co-rotating motion in the outer halo by displacing the inner regions of the halo with respect to the outer halo. Using the Latte suite of FIRE-2 cosmological simulations of MW-mass galaxies, we confirm that the apparent motion of the outer halo induced by the infall of a massive satellite changes the observed distribution of orbital poles of outer-halo tracers, including satellites. We quantify the changes in the distribution of orbital poles using the two-point angular correlation function and find that all satellites induce changes. However, the most massive satellites with pericentric passages between 30-100kpc induce the largest changes. The best LMC-like satellite analog shows the largest change in orbital pole distribution. The dispersion of orbital poles decreases by 20° during the first two pericentric passages. Even when excluding the satellites brought in with the LMC-like satellite, there is clustering of orbital poles. These results suggest that in the MW, the recent pericentric passage of the LMC should have changed the observed distribution of orbital poles of all other satellites. Therefore, studies of kinematically-coherent planes of satellites that seek to place the MW in a cosmological context should account for the existence of a massive satellite like the LMC.

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arXiv2023

Bar formation and destruction in the FIRE-2 simulations

The physical mechanisms responsible for bar formation and destruction in galaxies remain a subject of debate. While we have gained valuable insight into how bars form and evolve from isolated idealized simulations, in the cosmological domain, galactic bars evolve in complex environments with mergers, gas accretion events, in presence of turbulent Inter Stellar Medium (ISM) with multiple star formation episodes, in addition to coupling to their host galaxies' dark matter halos. We investigate bar formation in 13 Milky Way-mass galaxies from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulations. 8 of the 13 simulated galaxies form bars at some point during their history: three from tidal interactions and five from internal evolution of the disk. The bars in FIRE-2 are generally shorter than the corotation radius (mean bar radius $\sim 1.53$ kpc), have a wide range of pattern speeds (36--97 km s$^{-1}$kpc$^{-1}$), and live for a wide range of dynamical times (2--160 bar rotations). We find that bar formation in FIRE-2 galaxies is influenced by satellite interactions and the stellar-to-dark matter mass ratio in the inner galaxy, but neither is a sufficient condition for bar formation. Bar formation is more likely to occur, and the bars formed are stronger and longer-lived, if the disks are kinematically cold; galaxies with high central gas fractions and/or vigorous star formation, on the other hand, tend to form weaker bars. In the case of the FIRE-2 galaxies these properties combine to produce ellipsoidal bars with strengths $A_2/A_0 \sim$ 0.1--0.2.

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arXiv2023

LMC-driven anisotropic boosts in stream--subhalo interactions

Dark Matter (DM) subhalos are predicted to perturb stellar streams; stream morphologies and dynamics can constrain the mass distribution of subhalos. Using FIRE-2 simulations of Milky Way-mass galaxies, we show that presence of a Large Magellanic Cloud (LMC)--analog significantly changes stream-subhalo encounter rates. Three key factors drive these changes. First, the LMC--analog brings in many subhalos, increasing encounter rates for streams near the massive satellite by up to 20--40%. Second, the LMC--analog displaces the host from its center-of-mass (inducing reflex motion), causing a north-south asymmetry in the density and radial velocity distribution of subhalos. This asymmetry results in encounter rates varying by 50--70% across the sky at the same distance. Finally, the LMC--mass satellite induces a density wake in the host's DM halo, further boosting the encounter rates near the LMC--analog. We also explore the influence of stream orbital properties, finding a 50% increase in encounters for streams moving retrograde to the LMC--analog's orbit in the opposite hemisphere. The dependence of encounter rates on stream location and orbit has important implications for where to search for new streams with spurs and gaps in the Milky Way.

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arXiv2023

Structure, Kinematics, and Observability of the Large Magellanic Cloud's Dynamical Friction Wake in Cold vs. Fuzzy Dark Matter

The Large Magellanic Cloud (LMC) will induce a dynamical friction (DF) wake on infall to the Milky Way (MW). The MW's stellar halo will respond to the gravity of the LMC and the dark matter (DM) wake, forming a stellar counterpart to the DM wake. This provides a novel opportunity to constrain the properties of the DM particle. We present a suite of high-resolution, windtunnel-style simulations of the LMC's DF wake that compare the structure, kinematics, and stellar tracer response of the DM wake in cold DM (CDM), with and without self-gravity, vs. fuzzy DM (FDM) with $m_a = 10^{-23}$ eV. We conclude that the self-gravity of the DM wake cannot be ignored. Its inclusion raises the wake's density by $\sim 10\%$, and holds the wake together over larger distances ($\sim$ 50 kpc) than if self-gravity is ignored. The DM wake's mass is comparable to the LMC's infall mass, meaning the DM wake is a significant perturber to the dynamics of MW halo tracers. An FDM wake is more granular in structure and is $\sim 20\%$ dynamically colder than a CDM wake, but with comparable density. The granularity of an FDM wake increases the stars' kinematic response at the percent level compared to CDM, providing a possible avenue of distinguishing a CDM vs. FDM wake. This underscores the need for kinematic measurements of stars in the stellar halo at distances of 70-100 kpc.

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arXiv2023

The proto-galaxy of Milky Way-mass haloes in the FIRE simulations

Observational studies are finding stars believed to be relics of the earliest stages of hierarchical mass assembly of the Milky Way (i.e., proto-Galaxy). In this work, we contextualize these findings by studying the masses, ages, spatial distributions, morphology, kinematics, and chemical compositions of proto-galaxy populations from the 13 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. Our findings indicate that proto-Milky Way populations: i) can have a stellar mass range between $1\times10^{8}<\mathrm{M}_{\star}<2\times10^{10}[\mathrm{M}_{\odot}]$, a virial mass range between $3\times10^{10}<\mathrm{M}_{\star}<6\times10^{11}[\mathrm{M}_{\odot}]$, and be as young as $8 \lesssim \mathrm{Age} \lesssim 12.8$ [Gyr] ($1\lesssim z \lesssim 6$); ii) are predominantly centrally concentrated, with $\sim50\%$ of the stars contained within $5-10$ kpc; iii) on average show weak but systematic net rotation in the plane of the host's disc at $z=0$ (i.e., $0.25\lesssim\langle\kappa/\kappa_{\mathrm{disc}}\rangle\lesssim0.8$); iv) present [$\alpha$/Fe]-[Fe/H] compositions that overlap with the metal-poor tail of the host's old disc; v) tend to assemble slightly earlier in Local Group-like environments than in systems in isolation. Interestingly, we find that ~60% of the proto-Milky Way galaxies are comprised by 1 dominant system ($1/5\lesssim$M$_{\star}$/M$_{\star,\mathrm{proto-Milky Way}}$$\lesssim4/5$) and 4-5 lower mass systems (M$_{\star}$/M$_{\star,\mathrm{proto-Milky Way}}$$\lesssim1/10$); the other ~40% are comprised by 2 dominant systems and 3-4 lower mass systems. These massive/dominant proto-Milky Way fragments can be distinguished from the lower mass ones in chemical-kinematic samples, but appear (qualitatively) indistinguishable from one another. Our results could help observational studies disentangle if the Milky Way formed from one or two dominant systems.

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arXiv2023

Chemical Cartography of the Sagittarius Stream with Gaia

The stellar stream connected to the Sagittarius (Sgr) dwarf galaxy is the most massive tidal stream that has been mapped in the Galaxy, and is the dominant contributor to the outer stellar halo of the Milky Way. We present metallicity maps of the Sgr stream, using 34,240 red giant branch stars with inferred metallicities from Gaia BP/RP spectra. This sample is larger than previous samples of Sgr stream members with chemical abundances by an order of magnitude. We measure metallicity gradients with respect to Sgr stream coordinates $(Λ, B)$, and highlight the gradient in metallicity with respect to stream latitude coordinate $B$, which has not been observed before. We find $\nabla \mathrm{[M/H]} = -2.48 \pm 0.08 \times 10^{-2}$ dex/deg above the stream track ($B>B_0$ where $B_0=1.5$ deg is the latitude of the Sgr remnant) and $\nabla \mathrm{[M/H]} =- 2.02 \pm 0.08 \times 10^{-2}$ dex/deg below the stream track ($B<B_0$). By painting metallicity gradients onto a tailored N-body simulation of the Sgr stream, we find that the observed metallicities in the stream are consistent with an initial radial metallicity gradient in the Sgr dwarf galaxy of $\sim -0.1$ to $-0.2$ dex/kpc, well within the range of observed metallicity gradients in Local Group dwarf galaxies. Our results provide novel observational constraints for the internal structure of the dwarf galaxy progenitor of the Sgr stream. Leveraging new large datasets in conjunction with tailored simulations, we can connect the present day properties of disrupted dwarfs in the Milky Way to their initial conditions.

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arXiv2023

RomAndromeda: The Roman Survey of the Andromeda Halo

As our nearest large neighbor, the Andromeda Galaxy provides a unique laboratory for investigating galaxy formation and the distribution and substructure properties of dark matter in a Milky Way-like galaxy. Here, we propose an initial 2-epoch ($Δt\approx 5$yr), 2-band Roman survey of the entire halo of Andromeda, covering 500 square degrees, which will detect nearly every red giant star in the halo (10$σ$ detection in F146, F062 of 26.5, 26.1AB mag respectively) and yield proper motions to $\sim$25 microarcsec/year (i.e., $\sim$90 km/s) for all stars brighter than F146 $\approx 23.6$ AB mag (i.e., reaching the red clump stars in the Andromeda halo). This survey will yield (through averaging) high-fidelity proper motions for all satellites and compact substructures in the Andromeda halo and will enable statistical searches for clusters in chemo-dynamical space. Adding a third epoch during the extended mission will improve these proper motions by $\sim t^{-1.5}$, to $\approx 11$ km/s, but this requires obtaining the first epoch in Year 1 of Roman operations. In combination with ongoing and imminent spectroscopic campaigns with ground-based telescopes, this Roman survey has the potential to yield full 3-d space motions of $>$100,000 stars in the Andromeda halo, including (by combining individual measurements) robust space motions of its entire globular cluster and most of its dwarf galaxy satellite populations. It will also identify high-velocity stars in Andromeda, providing unique information on the processes that create this population. These data offer a unique opportunity to study the immigration history, halo formation, and underlying dark matter scaffolding of a galaxy other than our own.

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arXiv2023

HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra

The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions (HALO7D) survey measures the kinematics and chemical properties of stars in the Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured proper motions of MW halo main sequence turn-off (MSTO) stars in the four CANDELS fields. HALO7D consists of deep pencil beams, making it complementary to other contemporary wide-field surveys. We present the [Fe/H] and [$α$/Fe] abundances for 113 HALO7D stars in the Galactocentric radial range of $\sim 10-40$ kpc. Using the full 7D chemodynamical data (3D positions, 3D velocities, and abundances) of HALO7D, we measure the velocity anisotropy, $β$, of the halo velocity ellipsoid for each field and for different metallicity-binned subsamples. We find that two of the four fields have stars on very radial orbits, while the remaining two have stars on more isotropic orbits. Separating the stars into high, mid, and low [Fe/H] bins at $-2.2$ dex and $-1.1$ dex for each field separately, we find differences in the anisotropies between the fields and between the bins; some fields appear dominated by radial orbits in all bins while other fields show variation between the [Fe/H] bins. These chemodynamical differences are evidence that the HALO7D fields have different fractional contributions from the progenitors that built up the MW stellar halo. Our results highlight the additional information that is available on smaller spatial scales when compared to results from a spherical average of the stellar halo.

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arXiv2022

Implications of the Milky Way travel velocity for dynamical mass estimates of the Local Group

The total mass of the Local Group (LG) is a fundamental quantity that enables interpreting the orbits of its constituent galaxies and placing the LG in a cosmological context. One of the few methods that allows inferring the total mass directly is the "Timing Argument," which models the relative orbit of the Milky Way (MW) and M31 in equilibrium. The MW itself is not in equilibrium, a byproduct of its merger history including the recent pericentric passage of the LMC, and recent work has found that the MW disk is moving with a lower bound "travel velocity" of $\sim 32~{\rm km}~{\rm s}^{-1}$ with respect to the outer stellar halo. Previous Timing Argument measurements attempt to account for this non-equilibrium state, but have been restricted to theoretical predictions for the impact of the LMC specifically. In this paper, we quantify the impact of a travel velocity on recovered LG mass estimates using several different compilations of recent kinematic measurements of M31. We find that incorporating the measured value of the travel velocity lowers the inferred LG mass by 10--12\% compared to a static MW halo. Measurements of the travel velocity with more distant tracers could yield even larger values, which would further decrease the inferred LG mass. Therefore, the newly measured travel velocity directly implies a lower LG mass than from a model with a static MW halo and must be considered in future dynamical studies of the Local Volume.

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arXiv2022

The observable properties of galaxy accretion events in Milky Way-like galaxies in the FIRE-2 cosmological simulations

In the $Λ$-Cold Dark Matter model of the Universe, galaxies form in part through accreting satellite systems. Previous work have built an understanding of the signatures of these processes contained within galactic stellar halos. This work revisits that picture using seven Milky Way-like galaxies in the \textit{Latte} suite of FIRE-2 cosmological simulations. The resolution of these simulations allows a comparison of contributions from satellites above M$_{*}$$\gtrsim$10$\times$$^{7}$M$_{\odot}$, enabling the analysis of observable properties for disrupted satellites in a fully self-consistent and cosmological context. Our results show that, the time of accretion and the stellar mass of an accreted satellite are fundamental parameters that in partnership dictate the resulting spatial distribution, orbital energy, and [$α$/Fe]-[Fe/H] compositions of the stellar debris of such mergers $at$ $present$ $day$. These parameters also govern the resulting dynamical state of an accreted galaxy at $z=0$, leading to the expectation that the inner regions of the stellar halo (R$_{\mathrm{GC}}$ $\lesssim$30 kpc) should contain fully phase-mixed debris from both lower and higher mass satellites. In addition, we find that a significant fraction of the lower mass satellites accreted at early times deposit debris in the outer halo (R$_{\mathrm{GC}}$ $>$50 kpc) that are $not$ fully phased-mixed, indicating that they could be identified in kinematic surveys. Our results suggest that, as future surveys become increasingly able to map the outer halo of our Galaxy, they may reveal the remnants of long-dead dwarf galaxies whose counterparts are too faint to be seen $in$ $situ$ in higher redshift surveys.

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arXiv2022

Constraining the Tilt of the Milky Way's Dark Matter Halo with the Sagittarius Stream

Recent studies have suggested that the Milky Way (MW)'s Dark Matter (DM) halo may be significantly tilted with respect to its central stellar disk, a feature that might be linked to its formation history. In this work, we demonstrate a method of constraining the orientation of the minor axis of the DM halo using the angle and frequency variables. This method is complementary to other traditional techniques, such as orbit fitting. We first test the method using a simulated tidal stream evolving in a realistic environment inside an MW-mass host from the FIRE cosmological simulation, showing that the theoretical description of a stream in the action-angle-frequency formalism still holds for a realistic dwarf galaxy stream in a cosmological potential. Utilizing the slopes of the line in angle and frequency space, we show that the correct rotation frame yields a minimal slope difference, allowing us to put a constraint on the minor axis location. Finally, we apply this method to the Sagittarius stream's leading arm. We report that the MW's DM halo is oblate with the flattening parameter in the potential $q\sim0.7-0.9$ and the minor axis pointing toward $(\ell,b) = (42^{o},48^{o})$. Our constraint on the minor axis location is weak and disagrees with the estimates from other works; we argue that the inconsistency can be attributed in part to the observational uncertainties and in part to the influence of the Large Magellanic Cloud.

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