arXiv subjects

Philip F. Hopkins

Publications and source records attributed to Philip F. Hopkins.

At least 235 records · Page 13Linked to original sources

arXiv2019

Observing Galaxy Evolution in the Context of Large-Scale Structure

Galaxies form and evolve in the context of their local and large-scale environments. Their baryonic content that we observe with imaging and spectroscopy is intimately connected to the properties of their dark matter halos, and to their location in the "cosmic web" of large-scale structure. Very large spectroscopic surveys of the local universe (e.g., SDSS and GAMA) measure galaxy positions (location within large-scale structure), statistical clustering (a direct constraint on dark matter halo masses), and spectral features (measuring physical conditions of the gas and stars within the galaxies, as well as internal velocities). Deep surveys with the James Webb Space Telescope (JWST) will revolutionize spectroscopic measurements of redshifts and spectral properties for galaxies out to the epoch of reionization, but with numerical statistics and over cosmic volumes that are too small to map large-scale structure and to constrain halo properties via clustering. Here, we consider advances in understanding galaxy evolution that would be enabled by very large spectroscopic surveys at high redshifts: very large numbers of galaxies (outstanding statistics) over large co-moving volumes (large-scale structure on all scales) over broad redshift ranges (evolution over most of cosmic history). The required observational facility can be established as part of the probe portfolio by NASA within the next decade.

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arXiv2019

Local simulations of MRI turbulence with meshless methods

The magneto-rotational instability (MRI) is one of the most important processes in sufficiently ionized astrophysical disks. Grid-based simulations, especially those using the local shearing box approximation, provide a powerful tool to study the ensuing nonlinear turbulence. On the other hand, while meshless methods have been widely used in both cosmology, galactic dynamics, and planet formation they have not been fully deployed on the MRI problem. We present local unstratified and vertically stratified MRI simulations with two meshless MHD schemes: a recent implementation of SPH MHD (Price2012), and a MFM MHD scheme with a constrained gradient divergence cleaning scheme, as implemented in the GIZMO code \citep{Hopkins2017}. Concerning variants of the SPH hydro force formulation we consider both the "vanilla" SPH and the PSPH variant included in GIZMO. We find, as expected, that the numerical noise inherent in these schemes affects turbulence significantly. A high order kernel, free of the pairing instability, is necessary. Both schemes can adequately simulate MRI turbulence in unstratified shearing boxes with net vertical flux. The turbulence, however, dies out in zero-net-flux unstratified boxes, probably due to excessive and numerical dissipation. In zero-net-flux vertically stratified simulations, MFM can reproduce the MRI dynamo and its characteristic butterfly diagram for several tens of orbits before ultimately decaying. In contrast, extremely strong toroidal fields, as opposed to sustained turbulence, develop in equivalent simulations using SPH MHD. This unphysical state in SPH MHD is likely caused by a combination of excessive artificial viscosity, numerical resistivity, and the relatively large residual errors in the divergence of the magnetic field remaining even after cleaning procedures are applied.

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arXiv2019

A Predicted Correlation Between Age Gradient and Star Formation History in FIRE Dwarf Galaxies

We explore the radial variation of star formation histories in dwarf galaxies simulated with Feedback In Realistic Environments (FIRE) physics. The sample contains 9 low-mass field dwarfs with M_ star = 10^5 - 10^7 M_sun from previous FIRE results, and a new suite of 17 higher mass field dwarfs with M_star = 10^7 - 10^9 M_sun introduced here. We find that age gradients are common in our dwarfs, with older stars dominant at large radii. The strength of the gradient correlates with overall galaxy age such that earlier star formation produces a more pronounced gradient. The relation between formation time and strength of the gradient is driven by both mergers and star-formation feedback. Mergers can both steepen and flatten the age gradient depending on the timing of the merger and star formation history of the merging galaxy. In galaxies without significant mergers, early feedback pushes stars to the outskirts at early times. Interestingly, among galaxies without mergers, those with large dark matter cores have flatter age gradients because these galaxies have more late-time feedback. If real galaxies have age gradients as we predict, stellar population studies that rely on sampling a limited fraction of a galaxy can give a biased view of its global star formation history. We show that central fields can be biased young by a few Gyrs while outer fields are biased old. Fields positioned near the 2D half-light radius will provide the least biased measure of a dwarf galaxy's global star formation history.

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arXiv2018

Dynamic Localised Turbulent Diffusion and its Impact on the Galactic Ecosystem

Modelling the turbulent diffusion of thermal energy, momentum, and metals is required in all galaxy evolution simulations due to the ubiquity of turbulence in galactic environments. The most commonly employed diffusion model, the Smagorinsky model, is known to be over-diffusive due to its strong dependence on the fluid velocity shear. We present a method for dynamically calculating a more accurate, locally appropriate, turbulent diffusivity: the dynamic localised Smagorinsky model. We investigate a set of standard astrophysically-relevant hydrodynamical tests, and demonstrate that the dynamic model curbs over-diffusion in non-turbulent shear flows and improves the density contrast in our driven turbulence experiments. In galactic discs, we find that the dynamic model maintains the stability of the disc by preventing excessive angular momentum transport, and increases the metal-mixing timescale in the interstellar medium. In both our isolated Milky Way-like galaxies and cosmological simulations, we find that the interstellar and circumgalactic media are particularly sensitive to the treatment of turbulent diffusion. We also examined the global gas enrichment fractions in our cosmological simulations, to gauge the potential effect on the formation sites and population statistics of Population III stars and supermassive black holes, since they are theorised to be sensitive to the metallicity of the gas out of which they form. The dynamic model is, however, not for galaxy evolution studies only. It can be applied to all astrophysical hydrodynamics simulations, including those modelling stellar interiors, planetary formation, and star formation.

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arXiv2018

Cosmic Rays or Turbulence can Suppress Cooling Flows (Where Thermal Heating or Momentum Injection Fail)

The quenching `maintenance' and `cooling flow' problems are important from the Milky Way through massive cluster elliptical galaxies. Previous work has shown that some source of energy beyond that from stars and pure magnetohydrodynamic processes is required, perhaps from AGN, but even the qualitative form of this energetic input remains uncertain. Different scenarios include thermal `heating,' direct wind or momentum injection, cosmic ray heating or pressure support, or turbulent `stirring' of the intra-cluster medium (ICM). We investigate these in $10^{12}-10^{14}\,{\rm M}_{\odot}$ halos using high-resolution non-cosmological simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model, including simplified toy energy-injection models, where we arbitrarily vary the strength, injection scale, and physical form of the energy. We explore which scenarios can quench without violating observational constraints on energetics or ICM gas. We show that turbulent stirring in the central $\sim100\,$kpc, or cosmic-ray injection, can both maintain a stable low-SFR halo for $>$Gyr timescales with modest energy input, by providing a non-thermal pressure which stably lowers the core density and cooling rates. In both cases, associated thermal-heating processes are negligible. Turbulent stirring preserves cool-core features while mixing condensed core gas into the hotter halo and is by far the most energy efficient model. Pure thermal heating or nuclear isotropic momentum injection require vastly larger energy, are less efficient in lower-mass halos, easily over-heat cores, and require fine-tuning to avoid driving unphysical temperature gradients or gas expulsion from the halo center.

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arXiv2018

Be it therefore resolved: Cosmological Simulations of Dwarf Galaxies with Extreme Resolution

We study a suite of extremely high-resolution cosmological FIRE simulations of dwarf galaxies ($M_{\rm halo} \lesssim 10^{10}$$M_{\odot}$), run to $z=0$ with $30 M_{\odot}$ resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with $M_{\rm halo} \gtrsim 10^{8.6} M_{\odot}$ is populated by a resolved {\em stellar} galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; $M_{\ast}<10^{5}\,M_{\odot}$) have their star formation truncated early ($z\gtrsim2$), likely by reionization, while classical dwarfs ($M_{\ast}>10^{5} M_{\odot}$) continue forming stars to $z<0.5$. The systems have bursty star formation (SF) histories, forming most of their stars in periods of elevated SF strongly clustered in both space and time. This allows our dwarf with $M_{\ast}/M_{\rm halo} > 10^{-4}$ to form a dark matter core $>200$pc, while lower-mass UFDs exhibit cusps down to $\lesssim100$pc, as expected from energetic arguments. Our dwarfs with $M_{\ast}>10^{4}\,M_{\odot}$ have half-mass radii ($R_{\rm 1/2}$) in agreement with Local Group (LG) dwarfs; dynamical mass vs. $R_{1/2}$ and the degree of rotational support also resemble observations. The lowest-mass UFDs are below surface brightness limits of current surveys but are potentially visible in next-generation surveys (e.g. LSST). The stellar metallicities are lower than in LG dwarfs; this may reflect pre-enrichment of the LG by the massive hosts or Pop-III stars. Consistency with lower resolution studies implies that our simulations are numerically robust (for a given physical model).

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arXiv2018

The Impact of Enhanced Halo Resolution on the Simulated Circumgalactic Medium

Traditional cosmological hydrodynamics simulations fail to spatially resolve the circumgalatic medium (CGM), the reservoir of tenuous gas surrounding a galaxy and extending to its virial radius. We introduce the technique of Enhanced Halo Resolution (EHR), enabling more realistic physical modeling of the simulated CGM by consistently forcing gas refinement to smaller scales throughout the virial halo of a simulated galaxy. We investigate the effects of EHR in the Tempest simulations, a suite of Enzo-based cosmological zoom simulations following the evolution of an L* galaxy, resolving spatial scales of 500 comoving pc out to 100 comoving kpc in galactocentric radius. Among its many effects, EHR (1) changes the thermal balance of the CGM, increasing its cool gas content and decreasing its warm/hot gas content; (2) preserves cool gas structures for longer periods; and (3) enables these cool clouds to exist at progressively smaller size scales. Observationally, this results in a boost in "low ions" like H I and a drop in "high ions" like O VI throughout the CGM. These effects of EHR do not converge in the Tempest simulations, but extrapolating these trends suggests that the CGM in reality is a mist consisting of ubiquitous, small, long-lived, cool clouds suspended in a hot medium at the virial temperature of the halo. Additionally, we explore the physical mechanisms to explain why EHR produces the above effects, proposing that it works both by (1) better sampling the distribution of CGM phases enabling runaway cooling in the denser, cooler tail of the phase distribution; and (2) preventing cool gas clouds from artificially mixing with the ambient hot halo and evaporating. Evidence is found for both EHR mechanisms occurring in the Tempest simulations.

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arXiv2018

Ubiquitous Instabilities of Dust Moving in Magnetized Gas

Squire & Hopkins (2017) showed that coupled dust-gas mixtures are generically subject to 'resonant drag instabilities' (RDIs), which drive violently-growing fluctuations in both. But the role of magnetic fields and charged dust has not yet been studied. We therefore explore the RDI in gas which obeys ideal MHD and is coupled to dust via both Lorentz forces and drag, with an external acceleration (e.g., gravity, radiation) driving dust drift through gas. We show this is always unstable, at all wavelengths and non-zero values of dust-to-gas ratio, drift velocity, dust charge, 'stopping time' or drag coefficient (for any drag law), or field strength; moreover growth rates depend only weakly (sub-linearly) on these parameters. Dust charge and magnetic fields do not suppress instabilities, but give rise to a large number of new instability 'families,' each with distinct behavior. The 'MHD-wave' (magnetosonic or Alfven) RDIs exhibit maximal growth along 'resonant' angles where the modes have a phase velocity matching the corresponding MHD wave, and growth rates increase without limit with wavenumber. The 'gyro' RDIs are driven by resonances between drift and Larmor frequencies, giving growth rates sharply peaked at specific wavelengths. Other instabilities include 'acoustic' and 'pressure-free' modes (previously studied), and a family akin to cosmic ray instabilities which appear when Lorentz forces are strong and dust streams super-Alfvenically along field lines. We discuss astrophysical applications in the warm ISM, CGM/IGM, HII regions, SNe ejecta/remnants, Solar corona, cool-star winds, GMCs, and AGN.

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arXiv2018

The Resonant Drag Instability (RDI): Acoustic Modes

Recently, Squire & Hopkins (2017) showed any coupled dust-gas mixture is subject to a class of linear 'resonant drag instabilities' (RDI). These can drive large dust-to-gas ratio fluctuations even at arbitrarily small dust-to-gas mass ratios. Here, we identify and study both resonant and new non-resonant instabilities, in the simple case where the gas satisfies neutral hydrodynamics and supports acoustic waves ($ω^{2}=c_{s}^{2}\,k^{2}$). The gas and dust are coupled via an arbitrary drag law and subject to external accelerations (e.g. gravity, radiation pressure). If there is any dust drift velocity, the system is unstable. The instabilities exist for {\em all} dust-to-gas ratios $μ$ and their growth rates depend only weakly on $μ$ around resonance, as $\simμ^{1/3}$ or $\sim μ^{1/2}$ (depending on wavenumber). The behavior changes depending on whether the drift velocity is larger or smaller than the sound speed $c_{s}$. In the supersonic regime a 'resonant' instability appears with growth rate increasing without limit with wavenumber, even for vanishingly small $μ$ and values of the coupling strength ('stopping time'). In the subsonic regime non-resonant instabilities always exist, but their growth rates no longer increase indefinitely towards small wavelengths. The dimensional scalings and qualitative behavior of the instability do not depend sensitively on the drag law or equation-of-state of the gas. The instabilities directly drive exponentially growing dust-to-gas-ratio fluctuations, which can be large even when the modes are otherwise weak. We discuss physical implications for cool-star winds, AGN-driven winds and torii, and starburst winds: the instabilities alter the character of these outflows and could drive clumping and/or turbulence in the dust and gas.

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arXiv2018

How To Model Supernovae in Simulations of Star and Galaxy Formation

We study the implementation of mechanical feedback from supernovae (SNe) and stellar mass loss in galaxy simulations, within the Feedback In Realistic Environments (FIRE) project. We present the FIRE-2 algorithm for coupling mechanical feedback, which can be applied to any hydrodynamics method (e.g. fixed-grid, moving-mesh, and mesh-less methods), and black hole as well as stellar feedback. This algorithm ensures manifest conservation of mass, energy, and momentum, and avoids imprinting 'preferred directions' on the ejecta. We show that it is critical to incorporate both momentum and thermal energy of mechanical ejecta in a self-consistent manner, accounting for SNe cooling radii when they are not resolved. Using idealized simulations of single SN explosions, we show that the FIRE-2 algorithm, independent of resolution, reproduces converged solutions in both energy and momentum. In contrast, common 'fully-thermal' (energy-dump) or 'fully-kinetic' (particle-kicking) schemes in the literature depend strongly on resolution: when applied at mass resolution >100 solar masses, they diverge by orders-of-magnitude from the converged solution. In galaxy-formation simulations, this divergence leads to orders-of-magnitude differences in galaxy properties, unless those models are adjusted in a resolution-dependent way. We show that all models that individually time-resolve SNe converge to the FIRE-2 solution at sufficiently high resolution. However, in both idealized single-SN simulations and cosmological galaxy-formation simulations, the FIRE-2 algorithm converges much faster than other sub-grid models without re-tuning parameters.

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arXiv2018

Under the Firelight: Stellar Tracers of the Local Dark Matter Velocity Distribution in the Milky Way

The Gaia era opens new possibilities for discovering the remnants of disrupted satellite galaxies in the Solar neighborhood. If the population of local accreted stars is correlated with the dark matter sourced by the same mergers, one can then map the dark matter distribution directly. Using two cosmological zoom-in hydrodynamic simulations of Milky Way-mass galaxies from the Latte suite of Fire-2 simulations, we find a strong correlation between the velocity distribution of stars and dark matter at the solar circle that were accreted from luminous satellites. This correspondence holds for dark matter that is either relaxed or in kinematic substructure called debris flow, and is consistent between two simulated hosts with different merger histories. The correspondence is more problematic for streams because of possible spatial offsets between the dark matter and stars. We demonstrate how to reconstruct the dark matter velocity distribution from the observed properties of the accreted stellar population by properly accounting for the ratio of stars to dark matter contributed by individual mergers. After demonstrating this method using the Fire-2 simulations, we apply it to the Milky Way and use it to recover the dark matter velocity distribution associated with the recently discovered stellar debris field in the Solar neighborhood. Based on results from Gaia, we estimate that $42 ^{+26}_{-22}\%$ of the local dark matter that is accreted from luminous mergers is in debris flow.

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arXiv2018

Nonlinear Evolution of the Resonant Drag Instability in Magnetized Gas

We investigate, for the first time, the nonlinear evolution of the magnetized "resonant drag instabilities" (RDIs). We explore magnetohydrodynamic (MHD) simulations of gas mixed with (uniform) dust grains subject to Lorentz and drag forces, using the GIZMO code. The magnetized RDIs exhibit fundamentally different behaviour than the purely acoustic RDIs. The dust organizes into coherent structures and the system exhibits strong dust-gas separation. In the linear and early nonlinear regime, the growth rates agree with linear theory and the dust self-organizes into two-dimensional planes or "sheets." Eventually the gas develops fully nonlinear, saturated Alfvénic and compressible fast-mode turbulence, which fills the under-dense regions with a small amount of dust, and drives a dynamo which saturates at equipartition of kinetic and magnetic energy. The dust density fluctuations exhibit significant non-Gaussianity, and the power spectrum is strongly weighted towards the largest (box-scale) modes. The saturation level can be understood via quasi-linear theory, as the forcing and energy input via the instabilities becomes comparable to saturated tension forces and dissipation in turbulence. The magnetized simulation presented here is just one case; it is likely that the magnetic RDIs can take many forms in different parts of parameter space.

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arXiv2018

The physics of Lyman-alpha escape from high-redshift galaxies

Lyman-alpha (Lyα) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. We present a detailed Lyα radiative transfer study of a cosmological zoom-in simulation from the Feedback In Realistic Environments (FIRE) project. We focus on the time, spatial, and angular properties of the Lyα emission over a redshift range of z = 5-7, after escaping the galaxy and being transmitted through the intergalactic medium (IGM). Over this epoch, our target galaxy has an average stellar mass of $M_{\rm star} \approx 5 \times 10^8 {\rm M}_\odot$. We find that many of the interesting features of the Lyα line can be understood in terms of the galaxy's star formation history. The time variability, spatial morphology, and anisotropy of Lyα properties are consistent with current observations. For example, the rest frame equivalent width has a ${\rm EW}_{{\rm Ly}α,0} > 20 {\rm Å}$ duty cycle of 62% with a non-negligible number of sightlines with $> 100 {\rm Å}$, associated with outflowing regions of a starburst with greater coincident UV continuum absorption, as these conditions generate redder, narrower (or single peaked) line profiles. The lowest equivalent widths correspond to cosmological filaments, which have little impact on UV continuum photons but efficiently trap Lyα and produce bluer, broader lines with less transmission through the IGM. We also show that in dense self-shielding, low-metallicity filaments and satellites Lyα radiation pressure can be dynamically important. Finally, despite a significant reduction in surface brightness with increasing redshift, Lyα detections and spectroscopy of high-$z$ galaxies with the upcoming James Webb Space Telescope is feasible.

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arXiv2018

On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds

Measurements of the star formation efficiency (SFE) of giant molecular clouds (GMCs) in the Milky Way generally show a large scatter, which could be intrinsic or observational. We use magnetohydrodynamic simulations of GMCs (including feedback) to forward-model the relationship between the true GMC SFE and observational proxies. We show that individual GMCs trace broad ranges of observed SFE throughout collapse, star formation, and disruption. Low measured SFEs (<<1%) are "real" but correspond to early stages, the true "per-freefall" SFE where most stars actually form can be much larger. Very high (>>10%) values are often artificially enhanced by rapid gas dispersal. Simulations including stellar feedback reproduce observed GMC-scale SFEs, but simulations without feedback produce 20x larger SFEs. Radiative feedback dominates among mechanisms simulated. An anticorrelation of SFE with cloud mass is shown to be an observational artifact. We also explore individual dense "clumps" within GMCs and show that (with feedback) their bulk properties agree well with observations. Predicted SFEs within the dense clumps are ~2x larger than observed, possibly indicating physics other than feedback from massive (main sequence) stars is needed to regulate their collapse.

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arXiv2018

The Elephant in the Room: The Importance of Where and When Massive Stars Form in Molecular Clouds

Most simulations of galaxies and massive giant molecular clouds (GMCs) cannot explicitly resolve the formation (or predict the main-sequence masses) of individual stars. So they must use some prescription for the amount of feedback from an assumed population of massive stars (e.g. sampling the initial mass function [IMF]). We perform a methods study of simulations of a star-forming GMC with stellar feedback from UV radiation, varying only the prescription for determining the luminosity of each stellar mass element formed (according to different IMF sampling schemes). We show that different prescriptions can lead to widely varying (factor of ~3) star formation efficiencies (on GMC scales) even though the average mass-to-light ratios agree. Discreteness of sources is important: radiative feedback from fewer, more-luminous sources has a greater effect for a given total luminosity. These differences can dominate over other, more widely-recognized differences between similar literature GMC-scale studies (e.g. numerical methods, cloud initial conditions, presence of magnetic fields). Moreover the differences in these methods are not purely numerical: some make different implicit assumptions about where and how massive stars form, and this remains deeply uncertain in star formation theory.

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arXiv2018

The failure of stellar feedback, magnetic fields, conduction, and morphological quenching in maintaining red galaxies

The quenching "maintenance'" and related "cooling flow" problems are important in galaxies from Milky Way mass through clusters. We investigate this in halos with masses $\sim 10^{12}-10^{14}\,{\rm M}_{\odot}$, using non-cosmological high-resolution hydrodynamic simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We specifically focus on physics present without AGN, and show that various proposed "non-AGN" solution mechanisms in the literature, including Type Ia supernovae, shocked AGB winds, other forms of stellar feedback (e.g. cosmic rays), magnetic fields, Spitzer-Braginskii conduction, or "morphological quenching" do not halt or substantially reduce cooling flows nor maintain "quenched" galaxies in this mass range. We show that stellar feedback (including cosmic rays from SNe) alters the balance of cold/warm gas and the rate at which the cooled gas within the galaxy turns into stars, but not the net baryonic inflow. If anything, outflowing metals and dense gas promote additional cooling. Conduction is important only in the most massive halos, as expected, but even at $\sim 10^{14}\,{\rm M}_{\odot}$ reduces inflow only by a factor $\sim 2$ (owing to saturation effects and anisotropic suppression). Changing the morphology of the galaxies only slightly alters their Toomre-$Q$ parameter, and has no effect on cooling (as expected), so has essentially no effect on cooling flows or maintaining quenching. This all supports the idea that additional physics, e.g., AGN feedback, must be important in massive galaxies.

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arXiv2018

From the Top Down and Back Up Again: Star Cluster Structure from Hierarchical Star Formation

Young massive star clusters spanning $\sim 10^4 - 10^8 M_\odot$ in mass have been observed to have similar surface brightness profiles. Recent hydrodynamical simulations of star cluster formation have also produced star clusters with this structure. We argue analytically that this type of mass distribution arises naturally in the relaxation from a hierarchically-clustered distribution of stars into a monolithic star cluster through hierarchical merging. We show that arbitrary initial profiles will tend to converge to a universal profile under hierarchical merging, owing to phase-space mixing obeying certain conservation constraints. We perform $N$-body simulations of a pairwise merger of model star clusters and find that mergers readily produce the shallow surface brightness profiles observed in young massive clusters. Finally, we simulate the relaxation of a hierarchically-clustered mass distribution constructed from an idealized fragmentation model. Assuming only power-law spatial and kinematic scaling relations, these numerical experiments are able to reproduce the surface density profiles of observed young massive star clusters. Thus we provide physical motivation for the structure of young massive clusters within the paradigm of hierarchical star formation. This has important implications for the structure of nascent globular clusters.

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arXiv2018

Where are the most ancient stars in the Milky Way?

The oldest stars in the Milky Way (MW) bear imprints of the Galaxy's early assembly history. We use FIRE cosmological zoom-in simulations of three MW-mass disk galaxies to study the spatial distribution, chemistry, and kinematics of the oldest surviving stars ($z_{\rm form} \gtrsim 5$) in MW-like galaxies. We predict the oldest stars to be less centrally concentrated at $z=0$ than stars formed at later times as a result of two processes. First, the majority of the oldest stars are not formed $\textit{in situ}$ but are accreted during hierarchical assembly. These $\textit{ex situ}$ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed $\textit{in situ}$ in the solar neighborhood, and in some simulations, even in the galactic center. Secondly, old stars formed $\textit{in situ}$ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at $z\gtrsim 2$, similar to the process that creates dark matter cores and expands stellar orbits in bursty dwarf galaxies. The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines $\textit{away}$ from the bulge and inner halo than for inward-looking sight lines. Although the task of identifying specific stars as ancient remains challenging, we anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before $z=5$. We predict most of these targets to have higher metallicity ($-3 < \rm [Fe/H] < -2$) than the most extreme metal-poor stars.

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