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Stellar clusters are critical constituents within galaxies: They are the result of highest-density star formation, and through their spatially and temporally correlated feedback, they regulate their host galaxy evolution. We present a novel numerical method to model star clusters as individual units of star formation using sink particles. In our method, star clusters grow via gas accretion and via merging with less massive clusters. We describe the implementation in the radiation hydrodynamics code GIZMO and run a large grid of marginally bound, turbulent clouds of 107 M ⊙ to explore the effect of modeling ingredients on the evolution of the clouds and the star clusters. We find both gas accretion and mergers to be critical processes to form star clusters of masses up to ∼105–106 M ⊙, while ionizing radiation is the main feedback mechanism regulating the growth of star clusters. The majority of our star clusters assemble their mass in 0.3–2.6 Myr, and the most massive ones take ∼10 Myr. By removing high-density gas by accretion, our sink-based cluster formation prescription allows the newly formed star clusters to inject their stellar feedback in less dense environments. This makes feedback more efficient at ionizing and disrupting the cloud than if we were to use a standard star formation approach, indicating that our numerical method is the missing critical step to model the interplay between star clusters and their host galaxies. [ABSTRACT FROM AUTHOR]

We use a neural network model and Atacama Large Millimeter/submillimeter Array (ALMA) observations of HCN and HNC to constrain the physical conditions, most notably the cosmic-ray ionization rate (CRIR, ζ), in the Central Molecular Zone (CMZ) of the starburst galaxy NGC 253. Using output from the chemical code UCLCHEM, we train a neural network model to emulate UCLCHEM and derive HCN and HNC molecular abundances from a given set of physical conditions. We combine the neural network with radiative transfer modeling to generate modeled integrated intensities, which we compare to measurements of HCN and HNC from the ALMA Large Program ALCHEMI. Using a Bayesian nested sampling framework, we constrain the CRIR, molecular gas volume and column densities, kinetic temperature, and beam-filling factor across NGC 253's CMZ. The neural network model successfully recovers UCLCHEM molecular abundances with ∼3% error and, when used with our Bayesian inference algorithm, increases the parameter-inference speed tenfold. We create images of these physical parameters across NGC 253's CMZ at 50 pc resolution and find that the CRIR, in addition to the other gas parameters, is spatially variable with ζ ∼ a few ×10−14 s−1 at r ≳ 100 pc from the nucleus, increasing to ζ > 10−13 s−1 at its center. These inferred CRIRs are consistent within 1 dex with theoretical predictions based on nonthermal emission. Additionally, the high CRIRs estimated in NGC 253's CMZ can be explained by the large number of cosmic-ray-producing sources as well as a potential suppression of cosmic-ray diffusion near their injection sites. [ABSTRACT FROM AUTHOR]

We use a sample of 73 simulated satellite and central dwarf galaxies spanning a stellar mass range of 105.3–109.1 M ⊙ to investigate the origin of their stellar age gradients. We find that dwarf galaxies often form their stars "inside-out," i.e., the stars form at successively larger radii over time. However, the oldest stars get reshuffled beyond the star-forming radius by fluctuations in the gravitational potential well caused by stellar feedback (the same mechanisms that cause dwarfs to form dark matter cores). The result is that many dwarfs appear to have an "outside-in" age gradient at z = 0, with younger stellar populations more centrally concentrated. However, for the reshuffled galaxies with the most extended star formation, young stars can form out to the large radii to which the old stars have been reshuffled, erasing the age gradient. We find that major mergers do not play a significant role in setting the age gradients of dwarfs. We find similar age gradient trends in satellites and field dwarfs, suggesting that environment plays only a minor role, if any. Finally, we find that the age gradient trends are imprinted on the galaxies at later times, suggesting that the stellar reshuffling dominates after the galaxies have formed 50% of their stellar mass. The later reshuffling is at odds with results from the fire-2 simulations. Hence, age gradients offer a test of current star formation and feedback models that can be probed via observations of resolved stellar populations. [ABSTRACT FROM AUTHOR]

The scaling of galaxy properties with halo mass suggests that feedback loops regulate star formation, but there is no consensus yet about how those feedback loops work. To help clarify discussions of galaxy-scale feedback, Paper I presented a very simple model for supernova feedback that it called the minimalist regulator model. This follow-up paper interprets that model and discusses its implications. The model itself is an accounting system that tracks all of the mass and energy associated with a halo's circumgalactic baryons—the central galaxy's atmosphere. Algebraic solutions for the equilibrium states of that model reveal that star formation in low-mass halos self-regulates primarily by expanding the atmospheres of those halos, ultimately resulting in stellar masses that are insensitive to the mass-loading properties of galactic winds. What matters most is the proportion of supernova energy that couples with circumgalactic gas. However, supernova feedback alone fails to expand galactic atmospheres in higher-mass halos. According to the minimalist regulator model, an atmospheric contraction crisis ensues, which may be what triggers strong black hole feedback. The model also predicts that circumgalactic medium properties emerging from cosmological simulations should depend largely on the specific energy of the outflows they produce, and we interpret the qualitative properties of several numerical simulations in light of that prediction. [ABSTRACT FROM AUTHOR]

This paper presents a new framework for understanding the relationship between a galaxy and its circumgalactic medium (CGM). It focuses on how imbalances between heating and cooling cause either expansion or contraction of the CGM. It does this by tracking all of the mass and energy associated with a halo's baryons, including their gravitational potential energy, even if feedback has pushed some of those baryons beyond the halo's virial radius. We show how a star-forming galaxy's equilibrium state can be algebraically derived within the context of this framework, and we analyze how the equilibrium star formation rate depends on supernova feedback. We consider the consequences of varying the mass loading parameter η M ≡ M ̇ wind / M ̇ * relating a galaxy's gas mass outflow rate ( M ̇ wind ) to its star formation rate ( M ̇ * ) and obtain results that challenge common assumptions. In particular, we find that equilibrium star formation rates in low-mass galaxies are generally insensitive to mass loading, and when mass loading does matter, increasing it actually results in more star formation because more supernova energy is needed to resist atmospheric contraction. [ABSTRACT FROM AUTHOR]

We use resolved spectroscopy from MaNGA to investigate the significance of both local and global properties of galaxies to the cessation of star formation at kpc scales. Quenched regions are identified from a sample of isolated disk galaxies by a single-parameter criterion D n (4000) - log EW(H α) > 1.6 − log 2 = 1.3 , and are divided into gas-rich quenched regions (GRQRs) and gas-poor quenched regions according to the surface density of cold gas (Σgas). Both types of quenched regions tend to be hosted by non-AGN galaxies with relatively high mass (M * ≳ 1010 M ⊙) and red colors (NUV − r ≳ 3), as well as low star formation rate and high central density at fixed mass. They span wide ranges in other properties including structural parameters that are similar to the parent sample, indicating that the conditions responsible for quenching in gas-rich regions are largely independent of the global properties of galaxies. We train random forest classifiers and regressors for predicting quenching in our sample with 15 local/global properties. Σ* is the most important property for quenching, especially for GRQRs. These results strongly indicate the important roles of low-mass hot evolved stars, which are numerous and long-lived in quenched regions and can provide substantial radiation pressure to support the surrounding gas against gravitational collapse. The different feature importance for quenching, as found previously by A. F. L. Bluck et al., is partly due to the different definitions of quenched regions, particularly the different requirements on EW(H α). [ABSTRACT FROM AUTHOR]

Bubbles driven by energy and mass injection from small scales are ubiquitous in astrophysical fluid systems and essential to feedback across multiple scales. In particular, O stars in young clusters produce high-velocity winds that create hot bubbles in the surrounding gas. We demonstrate that the dynamical evolution of these bubbles is critically dependent upon the geometry of their interfaces with their surroundings and the nature of heat transport across these interfaces. These factors together determine the amount of energy that can be lost from the interior through cooling at the interface, which in turn determines the ability of the bubble to do work on its surroundings. We further demonstrate that the scales relevant to physical dissipation across this interface are extremely difficult to resolve in global numerical simulations of bubbles for parameter values of interest. This means the dissipation driving evolution of these bubbles in numerical simulations is often of a numerical nature. We describe the physical and numerical principles that determine the level of dissipation in these simulations; we use this, along with a fractal model for the geometry of the interfaces, to explain differences in convergence behavior between hydrodynamical and magnetohydrodynamical simulations presented here. We additionally derive an expression for momentum as a function of bubble radius expected when the relevant dissipative scales are resolved and show that it still results in efficiently cooled solutions, as postulated in previous work. [ABSTRACT FROM AUTHOR]

Molecular outflows are expected to play a key role in galaxy evolution at high redshift. To study the impact of outflows on star formation at the epoch of reionization, we performed sensitive Atacama Large Millimeter/submillimeter Array observations of OH 119 μ m toward J2054-0005, a luminous quasar at z = 6.04. The OH line is detected and exhibits a P-Cygni profile that can be fitted with a broad blueshifted absorption component, providing unambiguous evidence of an outflow, and an emission component at near-systemic velocity. The mean and terminal outflow velocities are estimated to be v out ≈ 670 and 1500 km s−1, respectively, making the molecular outflow in this quasar one of the fastest at the epoch of reionization. The OH line is marginally spatially resolved for the first time in a quasar at z > 6, revealing that the outflow extends over the central 2 kpc region. The mass outflow rate is comparable to the star formation rate ( M ̇ out / SFR ∼ 2 ), indicating rapid (∼107 yr) quenching of star formation. The mass outflow rate in a sample of star-forming galaxies and quasars at 4 < z < 6.4 exhibits a positive correlation with the total infrared luminosity, although the scatter is large. Owing to the high outflow velocity, a large fraction (up to ∼50%) of the outflowing molecular gas may be able to escape from the host galaxy into the intergalactic medium. [ABSTRACT FROM AUTHOR]

We present the first results from a high-resolution simulation with a focus on galactic wind driving for an isolated galaxy with a halo mass of ∼1011 M ⊙ (similar to the Large Magellanic Cloud) and a total gas mass of ∼6 × 108 M ⊙, resulting in ∼108 gas cells at ∼4 M ⊙ mass resolution. We adopt a resolved stellar feedback model with nonequilibrium cooling and heating, including photoelectric heating and photoionizing radiation, as well as supernovae, coupled to the second-order meshless finite-mass method for hydrodynamics. These features make this the largest resolved interstellar medium (ISM) galaxy model run to date. We find mean star formation rates around 0.05 M ⊙ yr−1 and evaluate typical time-averaged loading factors for mass (η M ∼ 1.0, in good agreement with recent observations) and energy (η E ∼ 0.01). The bulk of the mass of the wind is transported by the warm (T < 5 × 105 K) phase, while there is a similar amount of energy transported in the warm and the hot phases (T > 5 × 105 K). We find an average opening angle of 30° for the wind, decreasing with higher altitude above the midplane. The wind mass loading is decreasing (flat) for the warm (hot) phase as a function of the star formation surface rate density ΣSFR, while the energy loading shows inverted trends with ΣSFR, decreasing for the warm wind and increasing for the hot wind, although with very shallow slopes. These scalings are in good agreement with previous simulations of resolved wind driving in the multiphase ISM. [ABSTRACT FROM AUTHOR]

We present the results of 300 pc resolution Atacama Large Millimeter/submillimeter Array imaging of the [O iii ] 88 μm line and dust continuum emission from a z = 8.312 Lyman-break galaxy MACS0416_Y1. The velocity-integrated [O iii ] emission has three peaks that are likely associated with three young stellar clumps of MACS0416_Y1, while the channel map shows a complicated velocity structure with little indication of a global velocity gradient unlike what was found in [C ii ] 158 μm at a larger scale, suggesting random bulk motion of ionized gas clouds inside the galaxy. In contrast, dust emission appears as two individual clumps apparently separating or bridging the [O iii ]/stellar clumps. The cross-correlation coefficient between dust and ultraviolet-related emission (i.e., [O iii ] and ultraviolet continuum) is unity on a galactic scale, while it drops at <1 kpc, suggesting well-mixed geometry of multiphase interstellar media on subkiloparsec scales. If the cutoff scale characterizes different stages of star formation, the cutoff scale can be explained by gravitational instability of turbulent gas. We also report on a kiloparsec-scale off-center cavity embedded in the dust continuum image. This could be a superbubble producing galactic-scale outflows, since the energy injection from the 4 Myr starburst suggested by a spectral energy distribution analysis is large enough to push the surrounding media creating a kiloparsec-scale cavity. [ABSTRACT FROM AUTHOR]

Precipitation is potentially a mechanism through which the circumgalactic medium (CGM) can regulate a galaxy's star formation. Here, we present idealized simulations of isolated Milky Way–like galaxies intended to examine the ability of galaxies to self-regulate their star formation, in particular via precipitation. We also examine the impact of rotation in the CGM. Using six simulations, we explore variations in the initial CGM t cool/ t ff ratio and rotation profile. Those variations affect the amount of gas accretion and star formation within the galactic disk. To encourage this accretion and better study its dependence on CGM structure, we gradually increase the efficiency of stellar feedback during the first half of our simulations. Yet despite this gradual increase, the resulting outflows quickly evacuate large, hot cavities within the CGM and even beyond r 200. Some of the CGM gas avoids interacting with the cavities and is able to feed the disk along its midplane, but the cooling of feedback-heated gas far from the midplane is too slow to supply the disk with additional gas. Our simulations illustrate the importance of physical mechanisms in the outer CGM and IGM for star formation regulation in Milky Way–scale halos. [ABSTRACT FROM AUTHOR]

We present results on the properties of extreme gas outflows in massive (M * ∼ 1011 M ⊙), compact, starburst (star formation rate, SFR∼ 200 M ⊙ yr−1) galaxies at z = 0.4–0.7 with very high star formation surface densities (ΣSFR ∼ 2000 M ⊙ yr−1 kpc−2). Using optical Keck/HIRES spectroscopy of 14 HizEA starburst galaxies, we identify outflows with maximum velocities of 820–2860 km s−1. High-resolution spectroscopy allows us to measure precise column densities and covering fractions as a function of outflow velocity and characterize the kinematics and structure of the cool gas outflow phase (T ∼ 104 K). We find substantial variation in the absorption profiles, which likely reflects the complex morphology of inhomogeneously distributed, clumpy gas and the intricacy of the turbulent mixing layers between the cold and hot outflow phases. There is not a straightforward correlation between the bursts in the galaxies' star formation histories and their wind absorption line profiles, as might naively be expected for starburst-driven winds. The lack of strong Mg ii absorption at the systemic velocity is likely an orientation effect, where the observations are down the axis of a blowout. We infer high mass outflow rates of ∼50–2200 M ⊙ yr−1, assuming a fiducial outflow size of 5 kpc, and mass loading factors of η ∼ 5 for most of the sample. While these values have high uncertainties, they suggest that starburst galaxies are capable of ejecting very large amounts of cool gas that will substantially impact their future evolution. [ABSTRACT FROM AUTHOR]

We investigate the connection between the regulation of star formation and the cycling of baryons both within and in and out of galaxies. We use idealized numerical simulations of Milky Way–mass galaxies, in which we vary the galaxy morphology and stellar feedback strength. By following individual gas parcels through the disk, spiral arms, and massive star-forming clumps, we quantify how gas moves through the different phases of the interstellar medium (ISM) and forms stars. We show that the residence time of gas in the dense ISM phase (τ SF), the nature of spiral arms, and the clump properties depend on both the galaxy morphology and stellar feedback. We quantify signatures of the baryon cycle within galaxies using the temporal and spatial power spectrum density (PSD) of the star formation rate (SFR). Stronger stellar feedback leads to more bursty star formation while the correlation timescale of the SFH is longer, because stronger feedback dissolves the dense ISM phase, leading to a more homogeneous ISM and a decrease in τ SF. The bulge strength has a similar effect: the deep gravitational potential in a bulge-dominant galaxy imposes a strong shear force that breaks apart gas clumps in the ISM; this subsequently inhibits the fragmentation of gas and therefore the star formation in the disk, leading to a decrease in the spatial power on scales of ∼1 kpc. We conclude that measurements of the temporal and spatial PSD of the SFR can provide constraints on the baryon cycle and the star formation process. [ABSTRACT FROM AUTHOR]

Massive, young stars are the main source of energy that maintains multiphase structure and turbulence in the interstellar medium (ISM), and without this "feedback" the star formation rate (SFR) would be much higher than is observed. Rapid energy loss in the ISM and efficient energy recovery by stellar feedback lead to coregulation of SFRs and the ISM state. Realistic approaches to this problem should solve for the dynamical evolution of the ISM, including star formation and the input of feedback energy self-consistently and accurately. Here, we present the TIGRESS-NCR numerical framework, in which UV radiation, supernovae, cooling and heating processes, and gravitational collapse are modeled explicitly. We use an adaptive ray-tracing method for UV radiation transfer from star clusters represented by sink particles, accounting for attenuation by dust and gas. We solve photon-driven chemical equations to determine the abundances of hydrogen (time dependent) and carbon/oxygen-bearing species (steady state), which then set cooling and heating rates self-consistently. Applying these methods, we present high-resolution magnetohydrodynamics simulations of differentially rotating local galactic disks representing typical conditions of nearby star-forming galaxies. We analyze ISM properties and phase distributions and show good agreement with existing multiwavelength galactic observations. We measure midplane pressure components (turbulent, thermal, and magnetic) and the weight, demonstrating that vertical dynamical equilibrium holds. We quantify the ratios of pressure components to the SFR surface density, which we call the feedback yields. The TIGRESS-NCR framework will allow for a wide range of parameter exploration, including in low-metallicity systems. [ABSTRACT FROM AUTHOR]