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    The radial acceleration relation (RAR) connects the total gravitational acceleration of a galaxy at a given radius, atot(r), with that accounted for by baryons at the same radius, abar(r). The shape and tightness of the RAR for rotationally-supported galaxies have characteristics in line with MOdified Newtonian Dynamics (MOND) and can also arise within the cosmological constant + cold dark matter (ΛCDM) paradigm. We use zoom simulations of 20 galaxies with stellar masses of M⋆ ≃ 107–11 M⊙ to study the RAR in the FIRE-2 simulations. We highlight the existence of simulated galaxies with non-monotonic RAR tracks that ‘hook’ down from the average relation. These hooks are challenging to explain in Modified Inertia theories of MOND, but naturally arise in all of our ΛCDM-simulated galaxies that are dark-matter dominated at small radii and have feedback-induced cores in their dark matter haloes. We show, analytically and numerically, that downward hooks are expected in such cored haloes because they have non-monotonic acceleration profiles. We also extend the relation to accelerations below those traced by disc galaxy rotation curves. In this regime, our simulations exhibit ‘bends’ off of the MOND-inspired extrapolation of the RAR, which, at large radii, approach atot ≈ abar/fb, where fb is the cosmic baryon fraction. Future efforts to search for these hooks and bends in real galaxies will provide interesting tests for MOND and ΛCDM.

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    We introduce an analytic surface density profile for dark matter haloes that accurately reproduces the structure of simulated haloes of mass Mvir = 107–1011 M⊙, making it useful for modelling line-of-sight (LOS) perturbers in strong gravitational lensing models. The two-parameter function has an analytic deflection potential and is more accurate than the projected Navarro, Frenk, and White profile commonly adopted at this mass scale for perturbers, especially at the small radii of most relevant for lensing perturbations. Using a characteristic radius, R−1, where the log slope of surface density is equal to −1, and an associated surface density, Σ−1, we can represent the expected lensing signal from LOS haloes statistically, for an ensemble of halo orientations, using a distribution of projected concentration parameters, $\mathcal {C}_{\rm vir} := r_{\rm vir}/ R_{-1}$. Though an individual halo can have a projected concentration that varies with orientation with respect to the observer, the range of projected concentrations correlates with the usual three-dimensional halo concentration in a way that enables ease of use.

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  3. Abstract

    We study how supersonic streaming velocities of baryons relative to dark matter—a large-scale effect imprinted at recombination and coherent over ∼3 Mpc scales—affect the formation of dwarf galaxies atz≳ 5. We perform cosmological hydrodynamic simulations, including and excluding streaming velocities, in regions centered on halos withMvir(z= 0) ≈ 1010M; the simulations are part of the Feedback In Realistic Environments (FIRE) project and run with FIRE-3 physics. Our simulations comprise many thousands of systems with halo masses betweenMvir= 2 × 105Mand 2 × 109Min the redshift rangez= 20–5. A few hundred of these galaxies form stars and have stellar masses ranging from 100 to 107M. While star formation is globally delayed by approximately 50 Myr in the streaming relative to nonstreaming simulations and the number of luminous galaxies is correspondingly suppressed at high redshift in the streaming runs, these effects decay with time. Byz= 5, the properties of the simulated galaxies are nearly identical in the streaming versus nonstreaming runs, indicating that any effects of streaming velocities on the properties of galaxies at the mass scale of classical dwarfs and larger do not persist toz= 0.

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  4. ABSTRACT We introduce a suite of cosmological volume simulations to study the evolution of galaxies as part of the Feedback in Realistic Environments project. FIREbox, the principal simulation of the present suite, provides a representative sample of galaxies (∼1000 galaxies with $M_{\rm star}\gt 10^8\, M_\odot$ at z  = 0) at a resolution ($\Delta {}x\sim {}20\, {\rm pc}$ , $m_{\rm b}\sim {}6\times {}10^4\, M_\odot$ ) comparable to state-of-the-art galaxy zoom-in simulations. FIREbox captures the multiphase nature of the interstellar medium in a fully cosmological setting (L = 22.1 Mpc) thanks to its exceptionally high dynamic range (≳106) and the inclusion of multichannel stellar feedback. Here, we focus on validating the simulation predictions by comparing to observational data. We find that star formation rates, gas masses, and metallicities of simulated galaxies with $M_{\rm star}\lt 10^{10.5-11}\, M_\odot$ broadly agree with observations. These galaxy scaling relations extend to low masses ($M_{\rm star}\sim {}10^7\, M_\odot$ ) and follow a (broken) power-law relationship. Also reproduced are the evolution of the cosmic HI density and the HI column density distribution at z ∼ 0–5. At low z , FIREbox predicts a peak in the stellar-mass–halo-mass relation but also a higher abundance of massive galaxies and a higher cosmic star formation rate density than observed, showing that stellar feedback alone is insufficient to reproduce the properties of massive galaxies at late times. Given its high resolution and sample size, FIREbox offers a baseline prediction of galaxy formation theory in a ΛCDM Universe while also highlighting modelling challenges to be addressed in next-generation galaxy simulations. 
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    We explore the properties of Milky Way (MW) subhaloes in self-interacting dark matter models for moderate cross-sections of 1–5 cm2 g−1 using high-resolution zoom-in N-body simulations. We include the gravitational potential of a baryonic disc and bulge matched to the MW, which is critical for getting accurate predictions. The predicted number and distribution of subhaloes within the host halo are similar for 1 and 5 cm2 g−1 models, and they agree with observations of MW satellite galaxies only if subhaloes with peak circular velocity over all time >7 km s−1 are able to form galaxies. We do not find distinctive signatures in the pericentre distribution of the subhaloes that could help distinguish the models. Using an analytical model to extend the simulation results, we are able to show that subhaloes in models with cross-sections between 1 and 5 cm2 g−1 are not dense enough to match the densest ultrafaint and classical dwarf spheroidal galaxies in the MW. This motivates exploring velocity-dependent cross-sections with values larger than 5 cm2 g−1 at the velocities relevant for the satellites such that core collapse would occur in some of the ultrafaint and classical dwarf spheroidals.

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    We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies $(M_{\rm vir} \approx 10^{10}\, {\rm M}_\odot$ at z = 0) as part of the ‘Feedback in Realistic Environment’ (FIRE) project. Compact (r1/2 < 30 pc), relatively massive (0.5 × 105 ≲ M⋆/M⊙ ≲ 5 × 105), self-bound stellar clusters form at 11 ≳ z ≳ 5 in progenitors with $M_{\rm vir} \approx 10^9\, \mathrm{M}_{\odot }$. Cluster formation is triggered when at least $10^7\, \mathrm{M}_{\odot }$ of dense, turbulent gas reaches $\Sigma _{\rm gas} \approx 10^4\, {\rm M}_\odot \, {\rm pc}^{-2}$ as a result of the compressive effects of supernova feedback or from cloud–cloud collisions. The clusters can survive for $2-3\, {\rm Gyr}$; absent numerical effects, they could possibly survive substantially longer, perhaps to z = 0. The longest lived clusters are those that form at significant distance – several hundreds of pc – from their host galaxy. We therefore predict that globular clusters forming in progenitors of present-day dwarf galaxies will be offset from any pre-existing stars within their host dark matter haloes as opposed to deeply embedded within a well-defined galaxy. Properties of the nascent clusters are consistent with observations of some of the faintest and most compact high-redshift sources in Hubble Space Telescope lensing fields and are at the edge of what will be detectable as point sources in deep imaging of non-lensed fields with JWST. By contrast, the star clusters’ host galaxies will remain undetectable.

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  7. Abstract

    We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD;MV> − 7.0,4.9<log10(M*(z=0)/M)<5.5) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass byz= 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, Andxiii, which formed only 10% of its stellar mass byz= 5, and 75% in a rapid burst atz∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This “young” UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs (M*(z= 5) ≲ 5 × 104M) are likely quenched by reionization, whereas more-massive M31 UFDs (M*(z= 5) ≳ 105M) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs.

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    Free, publicly-accessible full text available October 1, 2024
  8. Abstract We describe a public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation (available at ) from the Feedback In Realistic Environments (FIRE) project. FIRE-2 simulations achieve parsec-scale resolution to explicitly model the multiphase interstellar medium while implementing direct models for stellar evolution and feedback, including stellar winds, core-collapse and Type Ia supernovae, radiation pressure, photoionization, and photoelectric heating. We release complete snapshots from three suites of simulations. The first comprises 20 simulations that zoom in on 14 Milky Way (MW)–mass galaxies, five SMC/LMC-mass galaxies, and four lower-mass galaxies including one ultrafaint; we release 39 snapshots across z = 0–10. The second comprises four massive galaxies, with 19 snapshots across z = 1–10. Finally, a high-redshift suite comprises 22 simulations, with 11 snapshots across z = 5–10. Each simulation also includes dozens of resolved lower-mass (satellite) galaxies in its zoom-in region. Snapshots include all stored properties for all dark matter, gas, and star particles, including 11 elemental abundances for stars and gas, and formation times (ages) of star particles. We also release accompanying (sub)halo catalogs, which include galaxy properties and member star particles. For the simulations to z = 0, including all MW-mass galaxies, we release the formation coordinates and an “ex situ” flag for all star particles, pointers to track particles across snapshots, catalogs of stellar streams, and multipole basis expansions for the halo mass distributions. We describe publicly available python packages for reading and analyzing these simulations. 
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    Self-interacting dark matter (SIDM) models have received great attention over the past decade as solutions to the small-scale puzzles of astrophysics. Though there are different implementations of dark matter (DM) self-interactions in N-body codes of structure formation, there has not been a systematic study to compare the predictions of these different implementations. We investigate the implementation of dark matter self-interactions in two simulation codes:gizmo and arepo. We begin with identical initial conditions for an isolated 1010 M⊙ dark matter halo and investigate the evolution of the density and velocity dispersion profiles in gizmo and arepo for SIDM cross-section over mass of 1, 5, and 50 $\rm cm^2\, g^{-1}$. Our tests are restricted to the core expansion phase, where the core density decreases and core radius increases with time. We find better than 30 per cent agreement between the codes for the density profile in this phase of evolution, with the agreement improving at higher resolution. We find that varying code-specific SIDM parameters changes the central halo density by less than 10 per cent outside of the convergence radius. We argue that SIDM core formation is robust across the two different schemes and conclude that these codes can reliably differentiate between cross-sections of 1, 5, and 50 $\rm cm^2\, g^{-1}$, but finer distinctions would require further investigation.

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    Characterizing the predicted environments of dwarf galaxies like the Large Magellanic Cloud (LMC) is becoming increasingly important as next-generation surveys push sensitivity limits into this low-mass regime at cosmological distances. We study the environmental effects of LMC-mass haloes (M200m ∼ 1011 M⊙) on their populations of satellites (M⋆ ≥ 104 M⊙) using a suite of zoom-in simulations from the Feedback In Realistic Environments (FIRE) project. Our simulations predict significant hot coronas with T ∼ 105 K and Mgas ∼ 109.5 M⊙. We identify signatures of environmental quenching in dwarf satellite galaxies, particularly for satellites with intermediate mass (M⋆ = 106–107 M⊙). The gas content of such objects indicates ram pressure as the likely quenching mechanism, sometimes aided by star formation feedback. Satellites of LMC-mass hosts replicate the stellar mass dependence of the quiescent fraction found in satellites of Milky Way-mass hosts (i.e. that the quiescent fraction increases as stellar mass decreases). Satellites of LMC-mass hosts have a wider variety of quenching times when compared to the strongly bimodal distribution of quenching times of nearby centrals. Finally, we identify significant tidal stellar structures around four of our six LMC analogues, suggesting that stellar streams may be common. These tidal features originated from satellites on close orbits, extend to ∼80 kpc from the central galaxy, and contain ∼106–107 M⊙ of stars.

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