Star-forming Interstellar Medium Research Articles

Toward Implementation of the Pressure-regulated, Feedback-modulated Model of Star Formation in Cosmological Simulations: Methods and Application to TNG

Traditional star formation subgrid models implemented in cosmological galaxy formation simulations, such as that of V. Springel & L. Hernquist (hereafter SH03), employ adjustable parameters to satisfy constraints measured in the local Universe. In recent years, however, theory and spatially resolved simulations of the turbulent, multiphase, star-forming interstellar medium (ISM) have begun to produce new first-principles models, which when fully developed can replace traditional subgrid prescriptions. This approach has advantages of being physically motivated and predictive rather than empirically tuned, and allowing for varying environmental conditions rather than being tied to local-Universe conditions. As a prototype of this new approach, by combining calibrations from the TIGRESS numerical framework with the pressure-regulated feedback-modulated (PRFM) theory, simple formulae can be obtained for both the gas depletion time and an effective equation of state. Considering galaxies in TNG50, we compare the “native” simulation outputs with postprocessed predictions from PRFM. At TNG50 resolution, the total midplane pressure is nearly equal to the total ISM weight, indicating that galaxies in TNG50 are close to satisfying vertical equilibrium. The measured gas scale height is also close to theoretical equilibrium predictions. The slopes of the effective equations of states are similar, but with effective velocity dispersion normalization from SH03 slightly larger than that from current TIGRESS simulations. Because of this and the decrease in PRFM feedback yield at high pressure, the PRFM model predicts shorter gas depletion times than the SH03 model at high densities and redshift. Our results represent a first step toward implementing new, numerically calibrated subgrid algorithms in cosmological galaxy formation simulations.

Regulating star formation in a magnetized disc galaxy

ABSTRACT We use high-resolution magnetohydrodynamic simulations of isolated disc galaxies to investigate the co-evolution of magnetic fields with a self-regulated, star-forming interstellar medium (ISM). The simulations are conducted using the ramses adaptive mesh refinement code on the standard agora initial condition, with gas cooling, star formation, and feedback. We run galaxies with a variety of initial magnetic field strengths. The fields evolve and achieve approximate saturation within 500 Myr, but at different levels. The galaxies reach a quasi-steady state, with slowly declining star formation due to both gas consumption and increase in the field strength at intermediate ISM densities. We connect this behaviour to differences in the gas properties and overall structure of the galaxies. Stronger magnetic fields limit supernova bubble sizes. Different cases support the ISM using varying combinations of magnetic pressure, turbulence, and thermal energy. Initially, $\gtrsim\!\! 1\ \mu \mathrm{ G}$ magnetic fields evolve modestly and dominate support at all radii. Conversely, initially weaker fields grow through feedback and turbulence but never dominate the support. This is reflected in the stability of the gas disc. This interplay determines the overall distribution of star formation in each case. We conclude that an initially weak field can grow to produce a realistic model of a local disc galaxy, but starting with typically assumed field strengths ($\gtrsim\!\! 1\ \mu \mathrm{ G}$) will not.

Metallicity Dependence of Pressure-regulated Feedback-modulated Star Formation in the TIGRESS-NCR Simulation Suite

We present a new suite of numerical simulations of the star-forming interstellar medium (ISM) in galactic disks using the TIGRESS-NCR framework. Distinctive aspects of our simulation suite are (1) sophisticated and comprehensive numerical treatments of essential physical processes including magnetohydrodynamics, self-gravity, and galactic differential rotation, as well as photochemistry, cooling, and heating coupled with direct ray-tracing UV radiation transfer and resolved supernova feedback and (2) wide parameter coverage including the variation in metallicity over Z′≡Z/Z⊙∼0.1-3 , gas surface density Σgas ∼ 5–150 M ⊙ pc−2, and stellar surface density Σstar ∼ 1–50 M ⊙ pc−2. The range of emergent star formation rate surface density is ΣSFR ∼ 10−4–0.5 M ⊙ kpc−2 yr−1, and ISM total midplane pressure is P tot/k B = 103–106 cm−3 K, with P tot equal to the ISM weight W . For given Σgas and Σstar, we find ΣSFR∝Z′0.3 . We provide an interpretation based on the pressure-regulated feedback-modulated (PRFM) star formation theory. The total midplane pressure consists of thermal, turbulent, and magnetic stresses. We characterize feedback modulation in terms of the yield ϒ, defined as the ratio of each stress to ΣSFR. The thermal feedback yield varies sensitively with both weight and metallicity as ϒth∝W−0.46Z′−0.53 , while the combined turbulent and magnetic feedback yield shows weaker dependence ϒturb+mag∝W−0.22Z′−0.18 . The reduction in ΣSFR at low metallicity is due mainly to enhanced thermal feedback yield, resulting from reduced attenuation of UV radiation. With the metallicity-dependent calibrations we provide, PRFM theory can be used for a new subgrid star formation prescription in cosmological simulations where the ISM is unresolved.

A high-redshift calibration of the [O I]-to-H I conversion factor in star-forming galaxies

The assembly and build-up of neutral atomic hydrogen (H I) in galaxies is one of the most fundamental processes in galaxy formation and evolution. Studying this process directly in the early universe is hindered by the weakness of the hyperfine 21-cm H Iline transition, impeding direct detections and measurements of the H Igas masses (MHI). Here we present a new method to inferMHIof high-redshift galaxies using neutral, atomic oxygen as a proxy. Specifically, we derive metallicity-dependent conversion factors relating the far-infrared [O I]-63 μm and [O I]-145 μm emission line luminosities andMHIin star-forming galaxies atz ≈ 2 − 6 using gamma-ray bursts (GRBs) as probes. We calibrate the [O I]-to-H Iconversion factor relying on a sample of local galaxies with direct measurements ofMHIand [O I]-63 μm and [O I]-145 μm line luminosities in addition to the SIGAMEhydrodynamical simulation framework at similar epochs (z ≈ 0). We find that the [O I]63 μm-to-H Iand [O I]145 μm-to-H Iconversion factors, here denotedβ[OI]−63 μmandβ[OI]−145 μm, respectively, universally appear to be anti-correlated with the gas-phase metallicity. The GRB measurements further predict a mean ratio ofL[OI]−63 μm/L[OI]−145 μm = 1.55 ± 0.12 and reveal generally less excited [C II] over [O I] compared to the local galaxy sample. Thez ≈ 0 galaxy sample also shows systematically higherβ[OI]−63 μmandβ[OI]−145 μmconversion factors than the GRB sample, indicating either suppressed [O I] emission in local galaxies likely due to their lower hydrogen densities or more extended, diffuse H Igas reservoirs traced by the H I21-cm. Finally, we apply these empirical calibrations to the few detections of [O I]-63 μm and [O I]-145 μm line transitions atz ≈ 2 from the literature and further discuss the applicability of these conversion factors to probe the H Igas content in the dense, star-forming interstellar medium (ISM) of galaxies well into the epoch of reionization.

Interplay of stellar and gas-phase metallicities: unveiling insights for stellar feedback modelling with Illustris, IllustrisTNG, and EAGLE

ABSTRACT The metal content of galaxies provides a window into their formation in the full context of the cosmic baryon cycle. In this study, we examine the relationship between stellar mass and stellar metallicity (MZ*R) in the hydrodynamic simulations Illustris, TNG, and EAGLE (Evolution and Assembly of GaLaxies and their Environment) to understand the global properties of stellar metallicities within the feedback paradigm employed by these simulations. Interestingly, we observe significant variations in the overall normalization and redshift evolution of the MZ*R across the three simulations. However, all simulations consistently demonstrate a tertiary dependence on the specific star formation rate (sSFR) of galaxies. This finding parallels the relationship seen in both simulations and observations between stellar mass, gas-phase metallicity, and some proxy of galaxy gas content (e.g. SFR, gas fraction, and atomic gas mass). Since we find this correlation exists in all three simulations, each employing a subgrid treatment of the dense, star-forming interstellar medium (ISM) to simulate smooth stellar feedback, we interpret this result as a fairly general feature of simulations of this kind. Furthermore, with a toy analytic model, we propose that the tertiary correlation in the stellar component is sensitive to the extent of the ‘burstiness’ of feedback within galaxies.

Constraining the halo-ISM connection through multi-transition carbon monoxide line-intensity mapping

Line-intensity mapping (LIM) surveys will characterise the cosmological large-scale structure of emissivity in a range of atomic and molecular spectral lines, but existing literature rarely considers whether these surveys can recover excitation properties of the tracer gas species, such as the carbon monoxide (CO) molecule. Combining basic empirical and physical assumptions with the off-the-shelf Radex radiative transfer code or a Gaussian process emulator of Radex outputs, we devise a basic dark matter halo model for CO emission by tying bulk CO properties to halo properties, exposing physical variables governing CO excitation as free parameters. The CO Mapping Array Project (COMAP) is working towards a multi-band survey programme to observe both CO(1–0) and CO(2–1) at z ∼ 7. We show that this programme, as well as a further `Triple Deluxe' extension to higher frequencies covering CO(3–2), is fundamentally capable of successfully recovering the connection between halo mass and CO abundances, and constraining the molecular gas kinetic temperature and density within the star-forming interstellar medium in ways that single-transition CO LIM cannot. Given a fiducial thermal pressure of ∼ 104 K cm-3 for molecular gas in halos of ∼ 1010 M ⊙, simulated multi-band COMAP surveys successfully recover the thermal pressure within 68% interval half-widths of 0.5–0.6 dex. Construction of multi-frequency LIM instrumentation to access multiple CO transitions is crucial in harnessing this capability, as part of a cosmic statistical probe of gas metallicity, dust chemistry, and other physical parameters in star-forming regions of the first galaxies and proto-galaxies out of reionisation.

Dwarf Galaxies Show Little ISM Evolution from z ∼ 1 to z ∼ 0: A Spectroscopic Study of Metallicity, Star Formation, and Electron Density

We present gas-phase metallicity measurements for 583 emission line galaxies at 0.3 < z < 0.85, including 388 dwarf galaxies with , and explore the dependence of the metallicity on the stellar mass and star formation (SF) properties of the galaxies. Metallicities are determined through the measurement of emission lines in very deep (∼7 hr exposure) Keck/DEIMOS spectra taken primarily from the HALO7D survey. We measure metallicity with three strong-line calibrations (O3Hβ, R23, and O3O2) for the overall sample, as well as with the faint [Ne iii]λ3869 and [O iii]λ4363 emission lines for 112 and 17 galaxies where robust detections were possible. We construct mass–metallicity relations (MZR) for each calibration method, finding MZRs consistent with other strong-line results at comparable redshift, as well as with z ∼ 0 galaxies. We quantify the intrinsic scatter in the MZR as a function of mass, finding it increases with lower stellar mass. We also measure a weak but significant correlation between increased MZR scatter and higher specific star formation rate (SFR). We find a weak influence of SFR in the fundamental metallicity relation as well, with an SFR coefficient of α = 0.21. Finally, we use the flux ratios of the [O ii]λ λ 3727,3729 doublet to calculate gas electron density in ∼1000 galaxies with as a function of redshift. We measure low electron densities (n e ∼ 25 cm−3) for z < 1 galaxies, again consistent with z ≈ 0 conditions, but measure higher densities (n e ∼ 100 cm−3) at z > 1. These results all suggest that there is little evolution in star-forming interstellar medium conditions from z ∼ 1 to z = 0, confirmed with a more complete sample of low-mass galaxies than has previously been available in this redshift range.

Photochemistry and Heating/Cooling of the Multiphase Interstellar Medium with UV Radiative Transfer for Magnetohydrodynamic Simulations

We present an efficient heating/cooling method coupled with chemistry and UV radiative transfer that can be applied to numerical simulations of the interstellar medium (ISM). We follow the time-dependent evolution of hydrogen species (H2, H, H+), assume carbon/oxygen species (C, C+, CO, O, and O+) are in formation–destruction balance given the nonsteady hydrogen abundances, and include essential heating/cooling processes needed to capture the thermodynamics of all ISM phases. UV radiation from discrete point sources and the diffuse background is followed through adaptive ray tracing and a six-ray approximation, respectively, allowing for H2 self-shielding; cosmic-ray heating and ionization are also included. To validate our methods and demonstrate their application for a range of density, metallicity, and radiation fields, we conduct a series of tests, including the equilibrium curves of thermal pressure versus density, the chemical and thermal structure in photodissociation regions, H i-to-H2 transitions, and the expansion of H ii regions and radiative supernova remnants. Careful treatment of photochemistry and cosmic-ray ionization is essential for many aspects of ISM physics, including identifying the thermal pressure at which cold and warm neutral phases coexist. We caution that many current heating and cooling treatments used in galaxy formation simulations do not reproduce the correct thermal pressure and ionization fraction in the neutral ISM. Our new model is implemented in the MHD code Athena and incorporated in the TIGRESS simulation framework, for use in studying the star-forming ISM in a wide range of environments.

PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions

ABSTRACT Determining the atomic and molecular content of the interstellar medium (ISM) is of fundamental importance to understand the star-formation process. Although there exist various three-dimensional hydro-chemical codes modelling the ISM, they are computationally expensive and inefficient for studies over a large parameter space. Building on our earlier approach, we present PDFchem, a novel algorithm that models the cold ISM at moderate and large scales using functions connecting the quantities of the local (AV, eff) and the observed (AV, obs) visual extinctions, and the local number density, nH, with probability density functions (PDF) of AV, obs on cloud scales typically tens-to-hundreds of pc as an input. For any given AV, obs-PDF, the algorithm instantly computes the average abundances of the most important species (H i, H2, C ii, C i, CO, OH, OH+, H2O+, CH, HCO+) and performs radiative transfer calculations to estimate the average emission of the most commonly observed lines ([C ii] 158$\mu$m, both [C i] fine-structure lines and the first five rotational transitions of 12CO). We examine two AV, obs-PDFs corresponding to a non-star-forming and a star-forming ISM region, under a variety of environmental parameters combinations. These cover far-ultraviolet intensities in the range of χ/χ0 = 10−1 − 103, cosmic ray ionization rates in the range of $\zeta _{\rm CR}=10^{-17}-10^{-13}\, {\rm s}^{-1}$ and metallicities in the range of $Z=0.1-2\, {\rm Z}_{\odot }$. PDFchem is fast, easy to use, reproduces the PDR quantities of the hydrodynamical models, and can be used directly with observed data of the cold ISM.

Pressure-regulated, Feedback-modulated Star Formation in Disk Galaxies

The star formation rate (SFR) in galactic disks depends on both the quantity of the available interstellar medium (ISM) gas and its physical state. Conversely, the ISM’s physical state depends on the SFR, because the “feedback” energy and momentum injected by recently formed massive stars is crucial to offsetting losses from turbulent dissipation and radiative cooling. The ISM’s physical state also responds to the gravitational field that confines it, with increased weight driving higher pressure. In a quasi-steady state, it is expected that the mean total pressure of different thermal phases will match each other, that the component pressures and total pressure will satisfy thermal and dynamical equilibrium requirements, and that the SFR will adjust as needed to provide the requisite stellar radiation and supernova feedback. The pressure-regulated, feedback-modulated (PRFM) theory of the star-forming ISM formalizes these ideas, leading to a prediction that the SFR per unit area, ΣSFR, will scale nearly linearly with ISM weight . In terms of the large-scale gas surface density Σgas, stellar plus dark matter density ρ sd, and effective ISM velocity dispersion σ eff, an observable weight estimator is , and this is predicted to match the total midplane pressure P tot. Using a suite of multiphase magnetohydrodynamic simulations run with the TIGRESS computational framework, we test the principles of the PRFM model and calibrate the total feedback yield ϒtot = P tot/ΣSFR ∼ 1000 km s−1, as well as its components. We compare the results from TIGRESS to theory, previous numerical simulations, and observations, finding excellent agreement.

The signature of large-scale turbulence driving on the structure of the interstellar medium

ABSTRACT The mechanisms that maintain turbulence in the interstellar medium (ISM) are still not identified. This work investigates how we can distinguish between two fundamental driving mechanisms: the accumulated effect of stellar feedback versus the energy injection from galactic scales. We perform a series of numerical simulations describing a stratified star-forming ISM subject to self-consistent stellar feedback. Large-scale external turbulent driving, of various intensities, is added to mimic galactic driving mechanisms. We analyse the resulting column density maps with a technique called Multi-scale non-Gaussian segmentation, which separates the coherent structures and the Gaussian background. This effectively discriminates between the various simulations and is a promising method to understand the ISM structure. In particular, the power spectrum of the coherent structures flattens above 60 pc when turbulence is driven only by stellar feedback. When large-scale driving is applied, the turn-over shifts to larger scales. A systematic comparison with the Large Magellanic Cloud (LMC) is then performed. Only 1 out of 25 regions has a coherent power spectrum that is consistent with the feedback-only simulation. A detailed study of the turn-over scale leads us to conclude that regular stellar feedback is not enough to explain the observed ISM structure on scales larger than 60 pc. Extreme feedback in the form of supergiant shells likely plays an important role but cannot explain all the regions of the LMC. If we assume ISM structure is generated by turbulence, another large-scale driving mechanism is needed to explain the entirety of the observations.

From giant clumps to clouds – II. The emergence of thick disc kinematics from the conditions of star formation in high redshift gas rich galaxies

ABSTRACT High-redshift disc galaxies are more gas rich, clumpier, and more turbulent than local Universe galaxies. This early era of galaxy formation imprints the distribution and kinematics of the stars that we observe today, but it is not yet well established how. In this work, we use simulations of isolated Milky Way-mass disc galaxies to study how kinematic properties of stars change when varying the gas fraction. This allows us to quantify the roles played by internal processes, e.g. gas turbulence and gravitational scattering off massive gas clumps, in establishing the observed stellar velocity dispersions and orbital eccentricities. We find that models with gas fractions &amp;gt;20 per cent feature a turbulent and clumpy interstellar medium (ISM), leading to zero-age stellar velocity dispersions $\sim 20\!-\!30\, {\rm km\, s}^{-1}$ and high-mean orbital eccentricities. Low eccentricities cannot arise from these physical conditions. For gas fractions below 20 per cent, the ISM becomes less turbulent, with stellar velocity dispersions $\lt 10\, {\rm km\, s}^{-1}$, and nearly circular orbits for young stars. The turbulence present in gas-rich high redshift galaxies hence acts as a ‘barrier’ against the formation of thin discs. We compare our findings to the Milky Way’s age–velocity dispersion relation and argue that velocity dispersions imprinted already at star formation by the ISM contribute significantly at all times. Finally, we show that observed orbital eccentricities in the Milky Way’s thick and thin discs can be explained entirely as imprints by the star-forming ISM, rather than by mergers or secular processes.

Observation and calibration strategies for large-scale multi-beam velocity-resolved mapping of the [CII] emission in the Orion molecular cloud

Context.The [CII] 158 μm far-infrared fine-structure line is one of the dominant cooling lines of the star-forming interstellar medium. Hence [CII] emission originates in and thus can be used to trace a range of ISM processes. Velocity-resolved large-scale mapping of [CII] in star-forming regions provides a unique perspective of the kinematics of these regions and their interactions with the exciting source of radiation.Aims.We explore the scientific applications of large-scale mapping of velocity-resolved [CII] observations. With the [CII] observations, we investigate the effect of stellar feedback on the ISM. We present the details of observation, calibration, and data reduction using a heterodyne array receiver mounted on an airborne observatory.Methods.A 1.15 square degree velocity-resolved map of the Orion molecular cloud centred on the bar region was observed using the German REceiver for Astronomy at Terahertz Frequencies (upGREAT) heterodyne receiver flying on board the Stratospheric Observatory for Infrared Astronomy. The data were acquired using the 14 pixels of the German REceiver for Astronomy at Terahertz Frequencies that were observed in an on-the-fly mapping mode. 2.4 million spectra were taken in total. These spectra were gridded into a three-dimensional cube with a spatial resolution of 14.1 arcseconds and a spectral resolution of 0.3 km s−1.Results.A square-degree [CII] map with a spectral resolution of 0.3 km s−1is presented. The scientific potential of this data is summarized with discussion of mechanical and radiative stellar feedback, filament tracing using [CII], [CII] opacity effects, [CII] and carbon recombination lines, and [CII] interaction with the large molecular cloud. The data quality and calibration is discussed in detail, and new techniques are presented to mitigate the effects of unavoidable instrument deficiencies (e.g. baseline stability) and thus to improve the data quality. A comparison with a smaller [CII] map taken with theHerschel/Heterodyne Instrument for the Far-Infrared spectrometer is presented.Conclusions.Large-scale [CII] mapping provides new insight into the kinematics of the ISM. The interaction between massive stars and the ISM is probed through [CII] observations. Spectrally resolving the [CII] emission is necessary to probe the microphysics induced by the feedback of massive stars. We show that certain heterodyne instrument data quality issues can be resolved using a spline-based technique, and better data correction routines allow for more efficient observing strategies.

SILCC VI – Multiphase ISM structure, stellar clustering, and outflows with supernovae, stellar winds, ionizing radiation, and cosmic rays

ABSTRACT We present simulations of the multiphase interstellar medium (ISM) at solar neighbourhood conditions including thermal and non-thermal ISM processes, star cluster formation, and feedback from massive stars: stellar winds, hydrogen ionizing radiation computed with the novel treeray radiative transfer method, supernovae (SN), and the injection of cosmic rays (CR). N-body dynamics is computed with a 4th-order Hermite integrator. We systematically investigate the impact of stellar feedback on the self-gravitating ISM with magnetic fields, CR advection and diffusion, and non-equilibrium chemical evolution. SN-only feedback results in strongly clustered star formation with very high star cluster masses, a bi-modal distribution of the ambient SN densities, and low volume-filling factors (VFF) of warm gas, typically inconsistent with local conditions. Early radiative feedback prevents an initial starburst, reduces star cluster masses and outflow rates. Furthermore, star formation rate surface densities of $\Sigma _{\dot{M}_\star } = 1.4-5.9 \times 10^{-3}$$\mathrm{M}_\odot \, \mathrm{yr}^{-1}\, \mathrm{kpc}^{-2}$, VFFwarm = 60–80 per cent as well as thermal, kinetic, magnetic, and cosmic ray energy densities of the model including all feedback mechanisms agree well with observational constraints. On the short, 100 Myr, time-scales investigated here, CRs only have a moderate impact on star formation and the multiphase gas structure and result in cooler outflows, if present. Our models indicate that at low gas surface densities SN-only feedback only captures some characteristics of the star-forming ISM and outflows/inflows relevant for regulating star formation. Instead, star formation is regulated on star cluster scales by radiation and winds from massive stars in clusters, whose peak masses agree with solar neighbourhood estimates.

N ii] Fine-structure Emission at 122 and 205 μm in a Galaxy at z = 2.6: A Globally Dense Star-forming Interstellar Medium

Abstract We present new observations with the Atacama Large Millimeter/submillimeter Array of the 122 and 205 μm fine-structure line emission of singly ionized nitrogen in a strongly lensed starburst galaxy at z = 2.6. The 122/205 μm [N ii] line ratio is sensitive to electron density, , in the ionized interstellar medium, and we use this to measure n e ≈ 300 cm−3, averaged across the galaxy. This is over an order of magnitude higher than the Milky Way average, comparable to localized Galactic star-forming regions. Combined with observations of the atomic carbon (C i) and carbon monoxide (CO J = 4–3) in the same system, we reveal the conditions in this intensely star-forming system. The majority of the molecular interstellar medium has been driven to high density, and the resultant conflagration of star formation produces a correspondingly dense ionized phase, presumably colocated with myriad H ii regions that litter the gas-rich disk.

A Framework for Multiphase Galactic Wind Launching Using TIGRESS

Abstract Galactic outflows have density, temperature, and velocity variations at least as large as those of the multiphase, turbulent interstellar medium (ISM) from which they originate. We have conducted a suite of parsec-resolution numerical simulations using the TIGRESS framework, in which outflows emerge as a consequence of interaction between supernovae (SNe) and the star-forming ISM. The outflowing gas is characterized by two distinct thermal phases, cool (T ≲ 104 K) and hot (T ≳ 106 K), with most mass carried by the cool phase and most energy and newly injected metals carried by the hot phase. Both components have a broad distribution of outflow velocity, and especially for cool gas this implies a varying fraction of escaping material depending on the halo potential. Informed by the TIGRESS results, we develop straightforward analytic formulae for the joint probability density functions (PDFs) of mass, momentum, energy, and metal loading as distributions in outflow velocity and sound speed. The model PDFs have only two parameters, star formation rate surface density and the metallicity of the ISM, and fully capture the behavior of the original TIGRESS simulation PDFs over . Employing PDFs from resolved simulations will enable implementations of subgrid models for galaxy formation with wind velocity and temperature (as well as total loading factors) that are based on theoretical predictions rather than empirical tuning. This is a critical step to incorporate advances from TIGRESS and other high-resolution simulations in future cosmological hydrodynamics and semi-analytic galaxy formation models. We release a Python package to prototype our model and to ease its implementation.

How do different spiral arm models impact the ISM and GMC population?

ABSTRACT The nature of galactic spiral arms in disc galaxies remains elusive. Regardless of the spiral model, arms are expected to play a role in sculpting the star-forming interstellar medium (ISM). As such, different arm models may result in differences in the structure of the ISM and molecular cloud properties. In this study, we present simulations of galactic discs subject to spiral arm perturbations of different natures. We find very little difference in how the cloud population or gas kinematics vary between the different grand design spirals, indicating that the ISM on cloud scales cares little about where spiral arms come from. We do, however, see a difference in the interarm/arm mass spectra, and minor differences in tails of the distributions of cloud properties (as well as radial variations in the stellar/gaseous velocity dispersions). These features can be attributed to differences in the radial dependence of the pattern speeds between the different spiral models, and could act as a metric of the nature of spiral structure in observational studies.

Diffuse Ionized Gas in Simulations of Multiphase, Star-forming Galactic Disks

Abstract It has been hypothesized that photons from young, massive star clusters are responsible for maintaining the ionization of diffuse warm ionized gas seen in both the Milky Way and other disk galaxies. For a theoretical investigation of the warm ionized medium (WIM), it is crucial to solve radiation-transfer equations where the interstellar medium (ISM) and clusters are modeled self-consistently. To this end, we employ a solar neighborhood model of Three-phase Interstellar Medium in Galaxies Resolving Evolution with Star Formation and Supernova Feedback (TIGRESS), a magnetohydrodynamic simulation of the multiphase, star-forming ISM, and post-process the simulation with an adaptive ray tracing method to transfer UV radiation from star clusters. We find that the WIM volume filling factor is highly variable, and sensitive to the rate of ionizing photon production and ISM structure. The mean WIM volume filling factor rises to ∼0.15 at . Approximately half of ionizing photons are absorbed by gas and half by dust; the cumulative ionizing photon escape fraction is 1.1%. Our time-averaged synthetic line profile matches Wisconsin Mapper observations on the redshifted (outflowing) side, but has insufficient intensity on the blueshifted side. Our simulation matches the Dickey–Lockman neutral density profile well, but only a small fraction of snapshots have high-altitude WIM density consistent with Reynolds Layer estimates. We compute a clumping correction factor that is remarkably constant with distance from the midplane and time; this can be used to improve estimates of ionized gas mass and mean electron density from observed surface brightness profiles in edge-on galaxies.

A Systematic Study of Galactic Outflows via Fluorescence Emission: Implications for Their Size and Structure

Abstract Galactic outflows play a major role in the evolution of galaxies, but the underlying physical processes are poorly understood. This is mainly because we have little information about the outflow structure, especially on large scales. In this paper, we probe the structure of galactic outflows in low-z starbursts using a combination of ultraviolet spectroscopy and imaging of the fluorescence emission lines (associated with transitions to excited fine-structure levels) and spectroscopy of the corresponding strongly blueshifted resonance absorption lines. We find that, in the majority of cases, the observed fluorescence emission lines are much weaker and narrower than the absorption lines, originating in the star-forming interstellar medium and/or the slowest-moving part of the inner outflow. In a minority of cases, the outflowing absorbing material does make a significant contribution to the fluorescence emission. These latter systems are characterized by both strong Lyα emission lines and weak low-ionization absorption lines (both known to be empirical signs of Lyman-continuum leakage). We argue that the observed weakness of emission from the outflow seen in the majority of cases is due to the missing emission arising on scales larger than those encompassed by the aperture of the Cosmic Origins Spectrograph on board the Hubble Space Telescope. This implies shallow radial density profiles in these outflows, and suggests that most of the observed absorbing material must be created/injected at radii much larger than that of the starburst. This has important implications both for our understanding of the physics of galactic outflows and for our estimation of their principal properties.

Kinematics and Dynamics of Multiphase Outflows in Simulations of the Star-forming Galactic Interstellar Medium

Abstract Galactic outflows produced by stellar feedback are known to be multiphase in nature. Observations and simulations indicate that the material within several kiloparsecs of galactic disk midplanes consists of warm clouds embedded within a hot wind. A theoretical understanding of the outflow phenomenon, including both winds and fountain flows, requires study of the interactions among thermal phases. We develop a method to quantify these interactions via measurements of mass, momentum, and energy flux exchanges using temporally and spatially averaged quantities and conservation laws. We apply this method to a star-forming interstellar medium simulation based on the TIGRESS framework, for solar neighborhood conditions. To evaluate the extent of interactions among the phases, we examine the validity of the “ballistic model,” which predicts the trajectories of the warm phase (5050 K &lt; T &lt; 2 × 104 K) treated as non-interacting clouds. This model is successful at intermediate vertical velocities ( ), but at higher velocities, we observe an excess in simulated warm outflow compared to the ballistic model. This discrepancy cannot be fully accounted for by cooling of high-velocity, intermediate-temperature (2 × 104 K &lt; T &lt; 5 × 105 K) gas. We examine the fluxes of mass, momentum, and energy and conclude that the warm phase gains mass via cooling of the intermediate phase and momentum from the hot (T &gt; 5 × 105 K) phase. The large energy flux from the hot outflow, transferred to the warm and intermediate phases, is quickly radiated away. A simple interaction model implies an effective warm cloud size in the fountain flow of a few 100 pc, showing that warm–hot flux exchange mainly involves a few large clouds rather than many small ones.