Category: Uncategorized

Pandya et al., available on arXiv

Abstract: We characterize mass, momentum, energy and metal outflow rates of multi-phase galactic winds in a suite of FIRE-2 cosmological “zoom-in” simulations from the Feedback in Realistic Environments (FIRE) project. We analyze simulations of low-mass dwarfs, intermediate-mass dwarfs, Milky Way-mass halos, and high-redshift massive halos. Consistent with previous work, we find that dwarfs eject about 100 times more gas from their interstellar medium (ISM) than they form in stars, while this mass “loading factor” drops below one in massive galaxies. Most of the mass is carried by the hot phase (>10^5 K) in massive halos and the warm phase (10^3−10^5 K) in dwarfs; cold outflows (<10^3 K) are negligible except in high-redshift dwarfs. Energy, momentum and metal loading factors from the ISM are of order unity in dwarfs and significantly lower in more massive halos. Hot outflows have 2-5x higher specific energy than needed to escape from the gravitational potential of dwarf halos; indeed, in dwarfs, the mass, momentum, and metal outflow rates increase with radius whereas energy is roughly conserved, indicating swept up halo gas. Instantaneous mass loading factors tend to be larger during more powerful starbursts and when the inner halo is not virialized, but we see effectively no trend with the dense ISM gas fraction. We discuss how our results can guide future controlled numerical experiments that aim to elucidate the key parameters governing galactic winds and the resulting preventative feedback.

Yu et al., available on arXiv

Abstract: We investigate thin and thick stellar disc formation in Milky-Way-mass galaxies using twelve FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars formed during the late-time steady phase have more circular orbits and thin-disc-like morphology at z=0, whilst stars born during the bursty phase have more radial orbits and thick-disc structure. The median age of thick-disc stars at z=0 correlates strongly with this transition time. We also find that galaxies with an earlier transition from bursty to steady star formation have a higher thin-disc fractions at z=0. Three of our systems have minor mergers with LMC-size satellites during the thin-disc phase. These mergers trigger short starbursts but do not destroy the thin disc nor alter broad trends between the star formation transition time and thin/thick disc properties. If our simulations are representative of the Universe, then stellar archaeological studies of the Milky Way (or M31) provide a window into past star-formation modes in the Galaxy. Current age estimates of the Galactic thick disc would suggest that the Milky Way transitioned from bursty to steady phase ~6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognisable thin disc.

Santistevan et al., available on arXiv

Abstract: In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower-mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disk. However, recent observations of stars in the MW show that metal-poor ([Fe/H] < -2) stars are preferentially on prograde orbits with respect to the disk. Using the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor stars. Almost all (11 of 12) of our simulations have metal-poor stars preferentially on prograde orbits today and throughout most of their history: we thus predict that this is a generic feature of MW/M31-mass galaxies. The typical prograde-to-retrograde ratio is ~ 2:1, which depends weakly on stellar metallicity at [Fe/H] < -1. These trends predicted by our simulations agree well with MW observations. Prograde metal-poor stars originate largely from a single LMC/SMC-mass gas-rich galaxy merger, typically 7-12.5 Gyr ago, which deposited both existing metal-poor stars and significant gas on an orbital vector that sparked the formation of and/or shaped the orientation of a long-lived stellar disk, giving rise to a prograde bias for all low-metallicity stars. We also find sub-dominant contributions from in-situ stars formed in the host galaxy before this merger, and in some cases, additional massive mergers. We find few clear correlations between any properties of our MW/M31-mass galaxies at z=0 and the degree of this prograde bias as a result of diverse merger scenarios.

Parsotan et al., available on arXiv

Abstract: The galaxy size-stellar mass and central surface density-stellar mass relationships are observational constraints on galaxy formation models. However, inferring the physical size of a galaxy from observed stellar emission is non-trivial due to various observational effects. Consequently, forward-modeling light-based sizes from simulations is desirable. In this work, we use the SKIRT dust radiative transfer code to generate synthetic observations of massive galaxies (M∗∼10^11 Msun at z=2, hosted by haloes of mass Mhalo∼10^12.5 Msun) from high-resolution cosmological zoom-in simulations that form part of the Feedback In Realistic Environments (FIRE) project. The simulations used in this paper include explicit stellar feedback but no active galactic nucleus (AGN) feedback. From each mock observation, we infer the effective radius (Re), as well as the stellar mass surface density within this radius and within 1kpc (Σe and Σ1, respectively). We first investigate how well the intrinsic half-mass radius and stellar mass surface density can be inferred from observables. The predicted sizes and surface densities are within a factor of two of the intrinsic values. We then compare our predictions to the observed size-mass relationship and the Σ1−M⋆ and Σe−M⋆ relationships. At z≳2, the simulated massive galaxies are in general agreement with observational scaling relations. At z≲2, they evolve to become too compact but still star-forming, in the stellar mass and redshift regime where many of them should be quenched. Our results suggest that some additional source of feedback, such as AGN driven outflows, is necessary in order to decrease the central densities of the simulated massive galaxies to bring them into agreement with observations at z≲2.

Bellardini et al., available on arXiv

Abstract: We use FIRE-2 simulations to examine 3-D variations of gas-phase elemental abundances of [O/H], [Fe/H], and [N/H] in 11 Milky Way (MW) and M31-mass galaxies across their formation histories at z≤1.5 (tlookback≤9.4 Gyr), motivated by characterizing the initial conditions of stars for chemical tagging. Gas within 1 kpc of the disk midplane is vertically homogeneous to ≲0.008 dex at all z≤1.5. We find negative radial gradients (metallicity decreases with galactocentric radius) at all times, which steepen over time from ≈−0.01 dex kpc−1 at z=1 (tlookback=7.8 Gyr) to ≈−0.03 dex kpc−1 at z=0, and which broadly agree with observations of the MW, M31, and nearby MW/M31-mass galaxies. Azimuthal variations at fixed radius are typically 0.14 dex at z=1, reducing to 0.05 dex at z=0. Thus, over time radial gradients become steeper while azimuthal variations become weaker (more homogeneous). As a result, azimuthal variations were larger than radial variations at z≳0.8 (tlookback≳6.9 Gyr). Furthermore, elemental abundances are measurably homogeneous (to ≲0.05 dex) across a radial range of ΔR≈3.5 kpc at z≳1 and ΔR≈1.7 kpc at z=0. We also measure full distributions of elemental abundances, finding typically negatively skewed normal distributions at z≳1 that evolve to typically Gaussian distributions by z=0. Our results on gas abundances inform the initial conditions for stars, including the spatial and temporal scales for applying chemical tagging to understand stellar birth in the MW.

Flores Velazquez et al., available on arXiv

Abstract: Understanding the rate at which stars form is central to studies of galaxy formation. Observationally, the star formation rates (SFRs) of galaxies are measured using the luminosity in different frequency bands, often under the assumption of a time-steady SFR in the recent past. We use star formation histories (SFHs) extracted from cosmological simulations of star-forming galaxies from the FIRE project to analyze the time-scales to which the Hα and far-ultraviolet (FUV) continuum SFR indicators are sensitive. In these simulations, the SFRs are highly time variable for all galaxies at high redshift, and continue to be bursty to z=0 in dwarf galaxies. When FIRE SFHs are partitioned into their bursty and time-steady phases, the best-fitting FUV time-scale fluctuates from its ~10 Myr value when the SFR is time-steady to >~100 Myr immediately following particularly extreme bursts of star formation during the bursty phase. On the other hand, the best-fitting averaging time-scale for Hα is generally insensitive to the SFR variability in the FIRE simulations and remains ~5 Myr at all times. These time-scales are shorter than the 100 Myr and 10 Myr time-scales sometimes assumed in the literature for FUV and H-alpha, respectively, because while the FUV emission persists for stellar populations older than 100 Myr, the time-dependent luminosities are strongly dominated by younger stars. Our results confirm that the ratio of SFRs inferred using Hα vs. FUV can be used to probe the burstiness of star formation in galaxies.

Li et al., available on arXiv

Abstract: Observations of UV metal absorption lines have provided insight into the structure and composition of the circumgalactic medium (CGM) around galaxies. We compare these observations with the low-redshift (z<=0.3) CGM around dwarf galaxies in high-resolution cosmological zoom-in runs in the FIRE-2 simulation suite. We select simulated galaxies that match the halo mass, stellar mass, and redshift of the observed samples. We produce absorption measurements using Trident for UV transitions of C IV, O VI, Mg II and Si III. The FIRE equivalent width (EW) distributions and covering fractions for the C IV ion are broadly consistent with observations inside 0.5Rvir, but are under-predicted for O VI, Mg II, and Si III. The absorption strengths of the ions in the CGM are moderately correlated with the masses and star formation activity of the galaxies. The correlation strengths increase with the ionization potential of the ions. The structure and composition of the gas from the simulations exhibit three zones around dwarf galaxies characterized by distinct ion column densities: the disky ISM, the inner CGM (the wind-dominated regime), and the outer CGM (the IGM accretion-dominated regime). We find that the outer CGM in the simulations is nearly but not quite supported by thermal pressure, so it is not in hydrostatic equilibrium (HSE), resulting in halo-scale bulk inflow and outflow motions. The net gas inflow rates are comparable to the SFR of the galaxy, but the bulk inflow and outflow rates are greater by an order of magnitude, with velocities comparable to the virial velocity of the halo. These roughly virial velocities (∼100 km/s) produce large EWs in the simulations. This supports a picture for dwarf galaxies in which the dynamics of the CGM at large scales are coupled to the small-scale star formation activity near the centre of their halos.

Ji et al., available on arXiv

Abstract: We study the impact of cosmic rays (CRs) on the structure of virial shocks, using a large suite of high-resolution cosmological FIRE-2 simulations accounting for CR injection by supernovae. In massive (Mhalo>=10^11 Msun), low-redshift (z<~1−2) halos, which are expected to form "hot halos" with slowly-cooling gas in quasi-hydrostatic equilibrium (with a stable virial shock), our simulations without CRs do exhibit clear virial shocks. The cooler phase condensing out from inflows becomes pressure-confined to over-dense clumps, embedded in low-density, volume-filling hot gas whose cooling time is much longer than inflow time. The gas thus transitions sharply from cool free-falling inflow, to hot and thermal-pressure supported at approximately the virial radius (~Rvir), and the shock is quasi-spherical. With CRs, we previously argued that halos in this particular mass and redshift range build up CR-pressure-dominated gaseous halos. Here, we show that when CR pressure dominates over thermal pressure, there is no significant virial shock. Instead, inflowing gas is gradually decelerated by the CR pressure gradient and the gas is relatively subsonic out to and even beyond Rvir. Rapid cooling also maintains sub-virial temperatures in the inflowing gas within ~Rvir.

Samuel et al., available on arXiv

Abstract: We examine the prevalence, longevity, and causes of planes of satellite dwarf galaxies, as observed in the Local Group. We use 14 Milky Way/Andromeda-(MW/M31) mass host galaxies from the FIRE-2 simulations. We select the 14 most massive satellites by stellar mass within 300 kpc of each host and correct for incompleteness from the foreground galactic disk when comparing to the MW. We find that MW-like planes as spatially thin and/or kinematically coherent as observed are uncommon, but they do exist in our simulations. Spatially thin planes occur in 1-2 per cent of snapshots during z=0-0.2, and kinematically coherent planes occur in 5 per cent of snapshots. These planes are generally short-lived, surviving for < 500 Myr. However, if we select hosts with an LMC-like satellite near first pericenter, the fraction of snapshots with MW-like planes increases dramatically to 7-16 per cent, with lifetimes of 0.7-3 Gyr, likely because of group accretion of satellites. We find that M31's satellite distribution is much more common: M31's satellites lie within about 1 sigma of the simulation median for every plane metric we consider. We find no significant difference in average satellite planarity for isolated hosts versus hosts in LG-like pairs. Baryonic and dark matter-only simulations exhibit similar levels of planarity, even though baryonic subhalos are less centrally concentrated within their host halos. We conclude that planes of satellites are not a strong challenge to LCDM cosmology.

Liang et al., available on arXiv

Abstract: The relation between infrared excess (IRX) and UV spectral slope (beta_UV) is an empirical probe of dust properties of galaxies. The shape, scatter, and redshift evolution of this relation are not well understood, however, leading to uncertainties in estimating the dust content and star formation rates (SFRs) of galaxies at high redshift. In this study, we explore the nature and properties of the IRX-beta_UV relation with a sample of z=2-6 galaxies (M*~10^9−10^12 Msun) extracted from high-resolution cosmological simulations (MassiveFIRE) of the Feedback in Realistic Environments (FIRE) project. The galaxies in our sample show an IRX-beta_UV relation that is in good agreement with the observed relation in nearby galaxies. IRX is tightly coupled to the UV optical depth, and is mainly determined by the dust-to-star geometry instead of total dust mass, while beta-UV is set both by stellar properties, UV optical depth, and the dust extinction law. Overall, much of the scatter in the IRX-beta_UV relation of our sample is found to be driven by variations of the intrinsic UV spectral slope. We further assess how the IRX-beta_UV relation depends on viewing direction, dust-to-metal ratio, birth-cloud structures, and the dust extinction law and we present a simple model that encapsulates most of the found dependencies. Consequently, we argue that the reported `deficit’ of the infrared/sub-millimetre bright objects at z>5 does not necessarily imply a non-standard dust extinction law at those epochs.