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In a Perspective article recently published by Nature Astronomy, Claude-André Faucher-Giguère summarizes recent progress in simulating galaxy formation from the largest to the smallest scales. This article situates the FIRE simulations in the broader context of this field.

New animations of gas flows in the circum-galactic medium and of supermassive black hole growth in the FIRE simulations from Anglés-Alcázar et al. (2017a) and Anglés-Alcázar et al. (2017b) have been posted in visualizations.

Liang et al., available on arXiv

Abstract: Recent long wavelength observations on the thermal dust continuum suggest that the Rayleigh-Jeans (RJ) tail can be used as a time-efficient quantitative probe of the dust and ISM mass in high-z galaxies. We use high-resolution cosmological simulations from the Feedback in Realistic Environment (FIRE) project to analyze the dust emission of M_*>10^10 Msun galaxies at z=2-4. Our simulations (MassiveFIRE) explicitly include various forms of stellar feedback, and they produce the stellar masses and star formation rates of high-z galaxies in agreement with observations. Using radiative transfer modelling, we show that sub-millimeter (sub-mm) luminosity and molecular ISM mass are tightly correlated and that the overall normalization is in quantitative agreement with observations. Notably, sub-mm luminosity traces molecular ISM mass even during starburst episodes as dust mass and mass-weighted temperature evolve only moderately between z=4 and z=2, including during starbursts. Our finding supports the empirical approach of using broadband sub-mm flux as a proxy for molecular gas content in high-z galaxies. We thus expect single-band sub-mm observations with ALMA to dramatically increase the sample size of high-z galaxies with reliable ISM masses in the near future.

El-Badry et al., available on arXiv

Abstract: 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_form >~ 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 in situ but are accreted during hierarchical assembly. These ex situ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed in situ in the solar neighborhood, and in some simulations, even in the galactic center. Secondly, old stars formed in situ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at z >~ 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 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 < [Fe/H] < -2) than the most extreme metal-poor stars.

Bozek et al., available on arXiv

Abstract: We study the impact of a warm dark matter (WDM) cosmology on dwarf galaxy formation through a suite of cosmological hydrodynamical zoom-in simulations of Mhalo ~ 10^10 Msun dark matter halos as part of the Feedback in Realistic Environments (FIRE) project. A main focus of this paper is to evaluate the combined effects of dark matter physics and stellar feedback on the well-known small-scale issues found in cold dark matter (CDM) models. We find that the z=0 stellar mass of a galaxy is strongly correlated with the central density of its host dark matter halo at the time of formation, z_f, in both CDM and WDM models. WDM halos follow the same Mstar(z=0)-Vmax(z_f) relation as in CDM, but they form later, are less centrally dense, and therefore contain galaxies that are less massive than their CDM counterparts. As a result, the impact of baryonic effects on the central gravitational potential is typically diminished relative to CDM. However, the combination of delayed formation in WDM and energy input from stellar feedback results in dark matter profiles with lower overall densities. The WDM galaxies studied here have a wider diversity of star formation histories (SFHs) than the same systems simulated in CDM, and the two lowest Mstar WDM galaxies form all of their stars at late times. The discovery of young ultra-faint dwarf galaxies with no ancient star formation — which do not exist in our CDM simulations — would therefore provide evidence in support of WDM.

Fitts et al., available on arXiv

Abstract: We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 high-resolution cosmological zoom-in simulations, all with masses of Mh ~ 10^10 Msun (and Mstar ~ 10^5-10^7 Msun) at z=0, from the Feedback in Realistic Environments (FIRE) project. The stellar populations of these dwarf galaxies at z=0 are formed essentially entirely “in situ”: over 90% of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have >70% of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to z~3, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo 5 galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at z=0B. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for next-generation facilities. The small contribution of accreted stars make it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star color-magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual star formation histories of merged dwarfs.

El-badry et al., available on arXiv

Abstract: The shape of a galaxy’s spatially unresolved, globally integrated 21-cm emission line depends on its internal gas kinematics: galaxies with rotation-supported gas disks produce double-horned profiles with steep wings, while galaxies with dispersion-supported gas produce Gaussian-like profiles with sloped wings. Using mock observations of simulated galaxies from the FIRE project, we show that one can therefore constrain a galaxy’s gas kinematics from its unresolved 21-cm line profile. In particular, we find that the kurtosis of the 21-cm line increases with decreasing V/sigma, and that this trend is robust across a wide range of masses, signal-to-noise ratios, and inclinations. We then quantify the shapes of 21-cm line profiles from a morphologically unbiased sample of ~2000 low-redshift, HI-detected galaxies with Mstar = 10^(7-11) Msun and compare to the simulated galaxies. At Mstar >~ 10^10 Msun, both the observed and simulated galaxies produce double-peaked lines with low kurtosis and steep wings, consistent with rotation-supported disks. Both the observed and simulated line profiles become more Gaussian-like (higher kurtosis and less-steep wings) at lower masses, indicating increased dispersion support. However, the simulated galaxies transition from rotation to dispersion support more strongly: at Mstar = 10^(8-10) Msun, most of the simulations produce more Gaussian-like profiles than typical observed galaxies with similar mass, indicating that gas in the low-mass simulated galaxies is, on average, overly dispersion-supported. Most of the lower-mass simulated galaxies also have somewhat lower gas fractions than the median of the observed population. The simulations nevertheless reproduce the observed line-width baryonic Tully-Fisher relation, which is insensitive to rotation vs. dispersion support.

Lamberts et al., available on arXiv

Abstract: Binary black holes are the primary endpoint of massive stellar evolution. Their properties provide a unique opportunity to constrain binary evolution, which is still poorly understood. In this paper, we predict the inventory of binary black holes and their merger products in/around the Milky Way, and detail their main properties. We present the first combination of a high-resolution cosmological simulation of a Milky Way-mass galaxy with a binary population synthesis model. The hydrodynamic simulation, taken from the FIRE project, provides a cosmologically realistic star formation history for the galaxy and its stellar halo and satellites. We apply a metallicity-dependent evolutionary model to the star particles to produce individual binary black holes. We find that a million binary black holes have merged in the model Milky Way, and 3 million binaries are still present, with an average mass of 28 Msun per binary. Because the black hole progenitors are biased towards low metallicity stars, half reside in the stellar halo and satellites and 40 per cent of the binaries were formed outside the main galaxy. This trend increases with the masses of the black holes. The numbers and mass distribution of the merged systems is compatible with the LIGO/Virgo detections. Observations of these black holes will be challenging, both with electromagnetic methods and LISA. We find that a cosmologically realistic star formation history, with self-consistent metal enrichment and Galactic accretion history, are key ingredients for determining binary black hole rates that can be compared with observations to constrain massive binary evolution.

Sanderson et al., available on arXiv

Abstract: We use cosmological hydrodynamical simulations of Milky-Way-mass galaxies from the FIRE project to evaluate various strategies for estimating the mass of a galaxy’s accreted stellar halo from deep, integrated-light images. We find good agreement with observations if we mimic observational methods to measure the mass of a stellar “halo” component, selecting stars via projected radius relative to the disk scale length or by their surface brightness. However, these observational methods systematically underestimate the true stellar halo mass, defined in the simulation as the mass of accreted stars formed outside of the host galaxy, by up to a factor of ten. Furthermore, these observational selection strategies introduce spurious dependencies on the stellar mass and size of galaxies that can obscure the trends predicted by cosmological simulations. This problem persists whether galaxies are viewed edge-on or face-on. We show that metallicity or color information may provide a solution. Absent additional data, we caution that estimates of stellar halo masses from images alone should be taken as lower limits.

Garrison-Kimmel et al., available on arXiv

Movies of the evolution of Milky Way-mass FIRE-2 galaxies are available here

Abstract: We use hydrodynamic cosmological zoom-in simulations from the FIRE project to explore the morphologies and kinematics of fifteen Milky Way (MW)-mass galaxies. Our sample ranges from compact, bulge-dominated systems with 90% of their stellar mass within 2.5 kpc to well-ordered disks that reach >~15 kpc. The gas in our galaxies always forms a thin, rotation-supported disk at z=0, with sizes primarily determined by the gas mass. For stars, we quantify kinematics and morphology both via the fraction of stars on disk-like orbits and with the radial extent of the stellar disk. In this mass range, stellar morphology and kinematics are poorly correlated with the properties of the halo available from dark matter-only simulations (halo merger history, spin, or formation time). They more strongly correlate with the gaseous histories of the galaxies: those that maintain a high gas mass in the disk after z~1 develop well-ordered stellar disks. The best predictor of morphology we identify is the spin of the gas in the halo at the time the galaxy formed 1/2 of its stars (i.e. the gas that builds the galaxy). High-z mergers, before a hot halo emerges, produce some of the most massive bulges in the sample (from compact disks in gas-rich mergers), while later-forming bulges typically originate from internal processes, as satellites are stripped of gas before the galaxies merge. Moreover, most stars in z=0 MW-mass galaxies (even z=0 bulge stars) form in a disk: >~60-90% of stars begin their lives rotationally supported.