Category: Uncategorized

Vargya et al., available on arXiv

Abstract: Self-interacting dark matter (SIDM) models offer one way to reconcile inconsistencies between observations and predictions from collisionless cold dark matter (CDM) models on dwarf-galaxy scales. In order to incorporate the effects of both baryonic and SIDM interactions, we study a suite of cosmological-baryonic simulations of Milky-Way (MW)-mass galaxies from the Feedback in Realistic Environments (FIRE-2) project where we vary the SIDM self-interaction cross-section sigma/m. We compare the shape of the main dark matter (DM) halo at redshift z=0 predicted by SIDM simulations (at sigma/m=0.1, 1, and 10 cm^2 g^−1) with CDM simulations using the same initial conditions. In the presence of baryonic feedback effects, we find that SIDM models do not produce the large differences in the inner structure of MW-mass galaxies predicted by SIDM-only models. However, we do find that the radius where the shape of the total mass distribution begins to differ from that of the stellar mass distribution is dependent on sigma/m. This transition could potentially be used to set limits on the SIDM cross-section in the MW.

Orr et al., available on arXiv

Abstract: We examine the azimuthal variations in gas-phase metallicity profiles in simulated Milky Way mass disk galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite, which includes a sub-grid turbulent metal mixing model. We produce spatially resolved maps of the disks at z~0 with pixel sizes ranging from 250 to 750 pc, analogous to modern integral field unit (IFU) galaxy surveys, mapping the gas-phase metallicities in both the cold & dense gas and the ionized gas correlated with HII regions. We report that the spiral arms alternate in a pattern of metal rich and metal poor relative to the median metallicity on the order of <~0.1 dex, appearing generally in this sample of flocculent spirals. The pattern persists even in a simulation with different strengths of metal mixing, indicating that the pattern emerges from physics above the sub-grid scale. Local enrichment does not appear to be the dominant source of the azimuthal metallicity variations at z~0: there is no correlation with local star formation on these spatial scales. Rather, the arms are moving inwards and outwards relative to each other, carrying their local metallicity gradients with them radially before mixing into the larger-scale interstellar medium. We propose that the arms act as freeways channeling relatively metal poor gas radially inwards, and relatively enriched gas radially outwards.

Porter et al., available on arXiv

Abstract: We present an analysis of spatially resolved gas-phase metallicity relations in five dwarf galaxies (Mhalo~10^11 Msun, M*~10^8.8−10^9.6 Msun) from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, which include an explicit model for sub-grid turbulent mixing of metals in gas, near z~0, over a period of 1.4 Gyrs, and compare our findings with observations. While these dwarf galaxies represent a diverse sample, we find that all simulated galaxies match the observed mass-metallicity (MZR) and mass-metallicity gradient (MZGR) relations. We note that in all five galaxies, the metallicities are effectively identical between phases of the interstellar medium (ISM), with 95% being within +/-0.1 dex between various ISM phases, including the cold and dense gas (T<500 K and nH>1 cm^−3), ionized gas (near the Halpha T~10^4 K ridge-line), and nebular regions (ionized gas where the 10 Myr-averaged star formation rate is non-zero). We find that most of the scatter in relative metallicity between cold and dense gas and ionized gas/nebular regions can be attributed to either local starburst events or metal-poor inflows. We also note the presence of a major merger in one of our galaxies, m11e, with a substantial impact on the metallicity distribution in the spatially resolved map, showing two strong metallicity peaks and triggering a starburst in the main galaxy.

Shipp et al., available on arXiv

Abstract: We present the first detailed study comparing the populations of stellar streams in cosmological simulations to observed Milky Way dwarf galaxy streams. In particular, we compare streams identified around Milky Way analogs in the FIRE-2 simulations to stellar streams observed by the Southern Stellar Stream Spectroscopic Survey (S5). For an accurate comparison between the stream populations, we produce mock Dark Energy Survey (DES) observations of the FIRE streams and estimate the detectability of their tidal tails and progenitors. The number and stellar mass distributions of detectable stellar streams is consistent between observations and simulations. However, there are discrepancies in the distributions of pericenters and apocenters, with the detectable FIRE streams, on average, forming at larger pericenters (out to > 110 kpc) and surviving only at larger apocenters (> 40 kpc) than those observed in the Milky Way. We find that the population of high-stellar mass dwarf galaxy streams in the Milky Way is incomplete. Interestingly, a large fraction of the FIRE streams would only be detected as satellites in DES-like observations, since their tidal tails are too low-surface brightness to be detectable. We thus predict a population of yet-undetected tidal tails around Milky Way satellites, as well as a population of fully undetected low surface brightness stellar streams, and estimate their detectability with the Rubin Observatory. Finally, we discuss the causes and implications of the discrepancies between the stream populations in FIRE and the Milky Way, and explore future avenues for tests of satellite disruption in cosmological simulations.

Shen et al., available on arXiv

Abstract: We analyze the first set of cosmological baryonic zoom-in simulations of galaxies in dissipative self-interacting dark matter (dSIDM). The simulations utilize the FIRE-2 galaxy formation physics with the inclusion of dissipative dark matter self-interactions modelled as a constant fractional energy dissipation (fdiss=0.5). In this paper, we examine the properties of dwarf galaxies with M∗∼10^5-10^9 Msun in both isolation and within Milky Way-mass hosts. For isolated dwarfs, we find more compact galaxy sizes and promotion of stellar/neutral gas disk formation in dSIDM with (sigma/m)<=1 cm^2 g^−1 but they are still consistent with observed galaxy sizes and masses. In addition, as a result of the steeper central density profiles developed in dSIDM, the sub-kpc circular velocities of isolated dwarfs in models with (sigma/m)>=0.1 cm^2 g^−1 are enhanced by about a factor of two, which are still consistent with the measured stellar velocity dispersions of Local Group dwarfs but in tension with the HI rotation curves of more massive field dwarfs. Meanwhile, for satellites of the simulated Milky Way-mass hosts, the median circular velocity profiles are marginally affected by dSIDM physics, but dSIDM may help address the missing compact dwarf satellites in CDM. The number of satellites is slightly enhanced in dSIDM, but the differences are small compared with the large host-to-host variations. In conclusion, the dSIDM models with constant cross-section (sigma/m)>~0.1 cm^2 g^−1 (assuming fdiss=0.5) are effectively ruled out in bright dwarfs (Mhalo~10^11 Msun) by circular velocity constraints. However, models with lower effective cross-sections (at this halo mass/velocity scale) are still viable and can give rise to non-trivial observable signatures.

Shen et al., available on arXiv

Abstract: We present the first set of cosmological baryonic zoom-in simulations of galaxies including dissipative self-interacting dark matter (dSIDM). These simulations utilize the Feedback In Realistic Environments (FIRE-2) galaxy formation physics, but allow the dark matter to have dissipative self-interactions analogous to Standard Model forces, parameterized by the self-interaction cross-section per unit mass, (sigma/m), and the dimensionless degree of dissipation, 0~0.1 cm^2 g^−1 (and fdiss=0.5 as fiducial). The power-law slopes asymptote to α~-1.5 in low-mass dwarfs independent of cross-section, which arises from a dark matter “cooling flow”. Through comparisons with dark matter only simulations, we find the profile in this regime is insensitive to the inclusion of baryons. However, when (sigma/m)eff<<0.1cm^2 g^−1, baryonic effects can produce cored density profiles comparable to non-dissipative cold dark matter (CDM) runs but at smaller radii. Simulated galaxies with (sigma/m)>~10cm^2 g^−1 develop significant coherent rotation of dark matter, accompanied by halo deformation, but this is unlike the well-defined thin “dark disks” often attributed to baryon-like dSIDM. The density profiles in this high cross-section model exhibit lower normalizations given the onset of halo deformation. For our surveyed dSIDM parameters, halo masses and galaxy stellar masses do not show appreciable difference from CDM, but dark matter kinematics and halo concentrations/shapes can differ.

Wellons et al., available on arXiv

Abstract: Feedback from accreting supermassive black holes (SMBHs) is thought to be a primary driver of quenching in massive galaxies, but the best way to implement SMBH physics into galaxy formation simulations remains ambiguous. As part of the Feedback in Realistic Environments (FIRE) project, we explore the effects of different modeling choices for SMBH accretion and feedback in a suite of ~500 cosmological zoom-in simulations across a wide range of halo mass (10^10-10^13 Msun). Within the suite, we vary the numerical schemes for BH accretion and feedback, the accretion efficiency, and the strength of mechanical, radiative, and cosmic ray feedback independently. We then compare the outcomes to observed galaxy scaling relations. We find several models that satisfy the observational constraints, and for which the energetics in different feedback channels are physically plausible. Interestingly, cosmic rays accelerated by SMBHs play an important role in many successful models. However, it is non-trivial to reproduce scaling relations across halo mass, and many model variations produce qualitatively incorrect results regardless of parameter choices. The growth of stellar and BH mass are closely related: for example, over-massive BHs tend to over-quench galaxies. BH mass is most strongly affected by the choice of accretion efficiency in high-mass halos, but by feedback efficiency in low-mass halos. The amount of star formation suppression by SMBH feedback in low-mass halos is determined primarily by the time-integrated feedback energy. For massive galaxies, the “responsiveness” of a model (i.e. how quickly and powerfully the BH responds to gas available for accretion) is an additional important factor for quenching.

Grudic et al., available on arXiv

Abstract: The properties of young star clusters formed within a galaxy are thought to vary in different interstellar medium (ISM) conditions, but the details of this mapping from galactic to cluster scales are poorly understood due to the large dynamic range involved in galaxy and star cluster formation. We introduce a new method for modeling cluster formation in galaxy simulations: mapping giant molecular clouds (GMCs) formed self-consistently in a FIRE-2 MHD galaxy simulation onto a cluster population according to a GMC-scale cluster formation model calibrated to higher-resolution simulations, obtaining detailed properties of the galaxy’s star clusters in mass, metallicity, space, and time. We find ~10% of all stars formed in the galaxy originate in gravitationally-bound clusters overall, and this fraction increases in regions with elevated Sigma_gas and Sigma_SFR, because such regions host denser GMCs with higher star formation efficiency. These quantities vary systematically over the history of the galaxy, driving variations in cluster formation. The mass function of bound clusters varies — no single Schechter-like or power-law distribution applies at all times. In the most extreme episodes, clusters as massive as 7*10^6 Msun form in massive, dense clouds with high star formation efficiency. The initial mass-radius relation of young star clusters is consistent with an environmentally-dependent 3D density that increases with Sigma_gas and Sigma_SFR. The model does not reproduce the age and metallicity statistics of old (>11 Gyr) globular clusters found in the Milky Way, possibly because it forms stars more slowly at z>3.

Rodriguez et al., available on arXiv

Abstract: The current generation of galaxy simulations can resolve individual giant molecular clouds, the progenitors of dense star clusters. But the evolutionary fate of these young massive clusters (YMCs), and whether they can become the old globular clusters (GCs) observed in many galaxies, is determined by a complex interplay of internal dynamical processes and external galactic effects. We present the first star-by-star N-body models of massive N~10^5-10^7) star clusters formed in a FIRE-2 MHD simulation of a Milky Way-mass galaxy, with all of the relevant initial conditions and galactic tidal effects extracted directly from the cosmological simulation. We randomly select 895 (~30%) of the YMCs with >6*10^4 Msun from Grudic et al. 2022 and integrate them to the present day using the Cluster Monte Carlo Code, CMC. This procedure predicts a MW-like system with 148 GCs, most of which were formed during the early, bursty mode of star formation in the galaxy. Our GCs are younger, less massive, and more core collapsed than clusters in the Milky Way or M31. This is a direct result of the assembly history and age-metallicity relationship of the GCs’ host galaxy: younger clusters are preferentially born in stronger galactic tidal fields and initially retain fewer stellar-mass black holes, causing them to lose mass faster and reach core collapse sooner than their older counterparts. Our results suggest that the masses and core/half-light radii of GCs are shaped not only by internal dynamical processes, but by the specific evolutionary history of their host galaxies as well. These results emphasize that $N$-body studies with realistic stellar physics are crucial to understanding the evolution and present-day properties of galactic GC systems.

Sameie et al., available on arXiv

Abstract: We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies (Mvir~10^10 Msun at z=0) as part of the “Feedback in Realistic Environment” (FIRE) project. Compact (r1/2<30 pc), relatively massive (0.5*10^5<~Mstar/Msun<~5*10^5), self-bound stellar clusters form at 11>~z>~5 in progenitors with Mvir~10^9 Msun. Cluster formation is triggered when at least 10^7 Msun of dense, turbulent gas reaches Sigma_gas~10^4 Msun pc^−2 as a result of the compressive effects of supernova feedback or from cloud-cloud collisions. The clusters can survive for 2−3Gyr; absent numerical effects, they would likely 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 halos 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 the James Webb Space Telescope. By contrast, the star clusters’ host galaxies will remain undetectable.