arXiv subjects

Eve C. Ostriker

Publications and source records attributed to Eve C. Ostriker.

At least 145 records · Page 8Linked to original sources

arXiv2010

Galactic Spiral Shocks with Thermal Instability in Vertically Stratified Galactic Disks

Galactic spiral shocks are dominant morphological features and believed to be responsible for substructure formation within spiral arms in disk galaxies. They can also contribute a substantial amount of kinetic energy to the interstellar gas by tapping the (differential) rotational motion. We use numerical hydrodynamic simulations to investigate dynamics and structure of spiral shocks with thermal instability in vertically stratified galactic disks, focusing on environmental conditions (of heating and the galactic potential) similar to the Solar neighborhood. We initially consider an isothermal disk in vertical hydrostatic equilibrium and let it evolve subject to interstellar cooling and heating as well as a stellar spiral potential. Due to thermal instability, a disk with surface density $Σ_0 \geq 6.7\Surf$ rapidly turns to a thin dense slab near the midplane sandwiched between layers of rarefied gas. The imposed spiral potential leads to a vertically curved shock that exhibits strong flapping motions in the plane perpendicular to the arm. The overall flow structure at saturation is comprised of arm, postshock expansion zone, and interarm regions that occupy typically 10\%, 20\%, and 70\% of the arm-to-arm distance, in which the gas resides for 15\%, 30\%, and 55\% of the arm-to-arm crossing time, respectively. The flows are characterized by transitions from rarefied to dense phases at the shock and from dense to rarefied phases in the postshock expansion zone, although gas with too-large postshock-density does not undergo this return phase transition, instead forming dense condensations. If self-gravity is omitted, the shock flapping drives random motions in the gas, but only up to $\sim 2-3 \kms$ in the in-plane direction and less than $2\kms$ in the vertical direction. Time-averaged shock profiles show... (abridged).

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arXiv2009

Protostar Formation in Supersonic Flows: Growth and Collapse of Spherical Cores

We present a unified model for molecular core formation and evolution, based on numerical simulations of converging, supersonic flows. Our model applies to star formation in GMCs dominated by large-scale turbulence, and contains four main stages: core building, core collapse, envelope infall, and late accretion. During the building stage, cores form out of dense, post-shock gas, and become increasingly centrally stratified as the mass grows over time. When the shock radius defining the core boundary exceeds $R\approx 4 a (4πG ρ_{mean})^{-1/2}$, where $a$ is the isothermal sound speed, a wave of collapse propagates from the edge to the center. During the building and collapse stages, density profiles can be fit by Bonnor-Ebert profiles with temperature 1.2 - 2.9 times the true value. As found previously for initially static equilibria, outside-in collapse leads to a Larson-Penston density profile $ρ\approx 8.86 a^2/(4 πG r^2)$. The third stage, consisting of an inside-out wave of gravitational rarefaction leading to $ρ\propto r^{-3/2}$, $v\propto r^{-1/2}$, is also similar to that for initially-static spheres, as originally described by Shu. We find that the collapse and infall stages have comparable duration, $\sim t_{ff}$, consistent with estimates for observed prestellar and protostellar (Class 0/I) cores. Core building takes longer, but does not produce high-contrast objects until shortly before collapse. The time to reach core collapse, and the core mass at collapse, decrease with increasing inflow Mach number. For all cases the accretion rate is $\gg a^3/G$ early on but sharply drops off; the final system mass depends on the duration of late-stage accretion, set by large-scale conditions in a cloud.

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arXiv2008

Cloud and Star Formation in Disk Galaxy Models with Feedback

We include feedback in global hydrodynamic simulations in order to study the star formation properties, and gas structure and dynamics, in models of galactic disks. We extend previous models by implementing feedback in gravitationally bound clouds: momentum is injected at a rate proportional to the star formation rate. This mechanical energy disperses cloud gas back into the surrounding ISM, truncating star formation in a given cloud, and raising the overall level of ambient turbulence. Propagating star formation can however occur as expanding shells collide, enhancing the density and triggering new cloud and star formation. By controlling the momentum injection per massive star and the specific star formation rate in dense gas, we find that the negative effects of high turbulence outweigh the positive ones, and in net feedback reduces the fraction of dense gas and thus the overall star formation rate. The properties of the large clouds that form are not, however, very sensitive to feedback, with cutoff masses of a few million solar masses, similar to observations. We find a relationship between the star formation rate surface density and the gas surface density with a power law index ~2 for our models with the largest dynamic range, consistent with theoretical expectations for our model of disk flaring. We point out that the value of the "Kennicutt-Schmidt" index depends on the thickness of the disk. With our simple feedback prescription (a single combined star formation event per cloud), we find that global spiral patterns are not sustained; less correlated feedback and smaller scale turbulence appear to be necessary for spiral patterns to persist.

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arXiv2008

Galactic Spiral Shocks with Thermal Instability

Using one-dimensional hydrodynamic simulations including interstellar heating, cooling, and thermal conduction, we investigate nonlinear evolution of gas flow across galactic spiral arms. We model the gas as a non-self-gravitating, unmagnetized fluid, and follow its interaction with a stellar spiral potential in a local frame comoving with the stellar pattern. Initially uniform gas rapidly separates into warm and cold phases as a result of thermal instability (TI), and also forms a quasi-steady shock that prompts phase transitions. After saturation, the flow follows a recurring cycle: warm and cold phases in the interarm region are shocked and immediately cool to become a denser cold medium in the arm; post-shock expansion reduces the mean density to the unstable regime in the transition zone and TI subsequently mediates evolution back into warm and cold interarm phases. For our standard model with n_0 = 2 cm^-3, the gas resides in the dense arm, thermally-unstable transition zone, and interarm region for 14%, 22%, 64% of the arm-to-arm crossing time. These regions occupy 1%, 16%, and 83% of the arm-to-arm distance, respectively. Gas at intermediate temperatures represents ~25-30% of the total mass, similar to the fractions estimated from HI observations. Despite transient features and multiphase structure, the time-averaged shock profiles can be matched to that of a diffusive isothermal medium with temperature 1,000 K and "particle" mean free path of l_0 = 100 pc. Finally, we quantify numerical conductivity associated with translational motion of phase-separated gas on the grid, and show that convergence of numerical results requires the numerical conductivity to be comparable to or smaller than the physical conductivity. (Abridged)

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arXiv2007

Theory of Star Formation

We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation -- turbulence, magnetic fields, and self-gravity -- are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

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arXiv2007

Kinematics of Spiral Arm Streaming in M51

We use CO and H alpha velocity fields to study the gas kinematics in the spiral arms and interarms of M51 (NGC 5194), and fit the 2D velocity field to estimate the radial and tangential velocity components as a function of spiral phase (arm distance). We find large radial and tangential streaming velocities, which are qualitatively consistent with the predictions of density wave theory and support the existence of shocks. The streaming motions are complex, varying significantly across the galaxy as well as along and between arms. Aberrations in the velocity field indicate that the disk is not coplanar, perhaps as far in as 20\arcsec\ (800 pc) from the center. Velocity profile fits from CO and H alpha are typically similar, suggesting that most of the H alpha emission originates from regions of recent star formation. We also explore vortensity and mass conservation conditions. Vortensity conservation, which does not require a steady state, is empirically verified. The velocity and density profiles show large and varying mass fluxes, which are inconsistent with a steady flow for a single dominant global spiral mode. We thus conclude that the spiral arms cannot be in a quasi-steady state in any rotating frame, and/or that out of plane motions may be significant.

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arXiv2007

Models of Vertically Stratified Two-Phase ISM Disks with MRI-Driven Turbulence

We have performed time-dependent numerical simulations of the interstellar medium (ISM) which account for galactic shear and magnetic fields, vertical gravity, and a radiative cooling function for atomic gas. This allows us to study the magnetorotational instability (MRI) in cloudy, vertically-stratified disks. As in previous unstratified models, we find that thermal instability interacts with MRI-driven turbulence and galactic shear to produce a network of cold, dense, filamentary clouds embedded in a warm diffuse ambient medium. This structure strongly resembles the morphology of HI gas observed in the 21 cm line. There is significant thermally-unstable gas, but the density and temperature distributions retain the twin peaks of the classical two-phase ISM. We analyze the vertical distributions of density and various pressure terms, and address what supports the ISM vertically. Turbulent velocities of the cold gas increase as the cold mass fraction decreases, but are generally low ~1-3 km/s near the midplane; they increase to > 5 km/s at high z. Finally, we argue that in the outer parts of galactic disks, MRI is likely able to prevent the development of self-gravitating instabilities and hence suppress star formation, even if cold gas is present.

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arXiv2007

Gravitational Runaway and Turbulence Driving in Star-Gas Galactic Disks

Galactic disks consist of both stars and gas. The gas is more dynamically responsive than the stars, and strongly nonlinear structures and velocities can develop in the ISM even while stellar surface density perturbations remain fractionally small. We use 2D numerical simulations to explore formation of bound clouds and turbulence generation in the gas of two-component galactic disks. We represent the stars with collisionless particles and follow their orbits using a PM method, and treat the gas as an isothermal, unmagnetized fluid. The two components interact through a combined gravity. Using stellar parameters typical of mid-disk conditions, we find that models with gaseous Toomre parameter Q_g < Q_c ~ 1.4 experience gravitational runaway and eventually form bound condensations. This Q_c value is nearly the same as previously found for razor-thin, gas-only models, indicating that the destabilizing effect of live stars is offsets the reduced self-gravity of thick disks. This result is also consistent with empirical studies showing that star formation is suppressed when Q_g > 1-2. The bound gaseous clouds that form have mass 6x10^7 Msun each; these represent superclouds that would subsequently fragment into GMCs. Self-gravity and sheared rotation also interact to drive turbulence in the gas when Q_g > Q_c. This turbulence is anisotropic, with more power in sheared than compressive motions. The gaseous velocity dispersion is ~ 0.6 times the thermal speed when Q_g ~ Q_c. This suggests that gravity is important in driving ISM turbulence in many spiral galaxies, since the low efficiency of star formation naturally leads to a state of marginal instability.

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arXiv2006

Interstellar Turbulence Driving by Galactic Spiral Shocks

Spiral shocks are potentially a major source of turbulence in the interstellar medium. To address this problem quantitatively, we use numerical simulations to investigate gas flow across spiral arms in vertically stratified, self-gravitating, magnetized models of galactic disks. Our models are isothermal, quasi-axisymmetric, and local in the quasi-radial direction while global in the vertical direction. We find that a stellar spiral potential perturbation promptly induces a spiral shock in the gas flow. For vertically stratified gas disks, the shock front in the radial-vertical plane is in general curved, and never achieves a steady state. This behavior is in sharp contrast to spiral shocks in two-dimensional (thin) disks, which are generally stationary. The non-steady motions in our models include large-amplitude quasi-radial flapping of the shock front. This flapping feeds random gas motions on the scale of the vertical disk thickness, which then cascades to smaller scales. The induced gas velocity dispersion in quasi-steady state exceeds the sonic value for a range of shock strengths, suggesting that spiral shocks are indeed an important generator of turbulence in disk galaxies.

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arXiv2006

An HST Archival Survey of Feathers in Spiral Galaxies

We present a survey of spiral arm extinction substructure referred to as feathers in 223 spiral galaxies using HST WFPC2 images. The sample includes all galaxies in the RC3 catalog with cz < 5000 km/s, B_T < 15, i < 60 degrees, and types Sa--Sd with well-exposed broadband WFPC2 images. The detection frequency of delineated, periodic feathers in this sample is 20% (45 of 223). This work is consistent with Lynds (1970), who concluded that feathers are common in prototypical Sc galaxies; we find that feathers are equally common in Sb galaxies. Sb--Sc galaxies without clear evidence for feathers either had poorer quality images, or flocculent or complex structure. We did not find clearly defined feathers in any Scd--Sd galaxy. The probability of detecting feathers was highest (83%) for spirals with well-defined primary dust lanes (PDLs; the lanes which line the inner edge of an arm); well-defined PDLs were only noted in Sab--Sc galaxies. Consistent with earlier work, we find that neighboring feathers tend to have similar shapes and pitch angles. OB associations are often found lining feathers, and many feathers transition to the stellar substructures known as spurs (Elmegreen 1980). We find that feathers are coincident with interarm filaments strikingly revealed in Spitzer 8 micron images. Comparison with CO 1-0 maps of NGC 0628 and NGC 5194 from BIMA SONG shows that feathers originate at the PDL coincident with gas surface density peaks. Contrary to the appearance at 8 microns, the CO maps show that gas surface density in feathers decreases rapidly with distance from the PDL. Also, we find that the spacing between feathers decreases with increasing gas surface density, consistent with formation via a gravitational instability.

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arXiv2006

Global Modeling of Spur Formation in Spiral Galaxies

We investigate the formation of substructure in spiral galaxies using global MHD simulations, including gas self-gravity. Our models extend previous local models by Kim and Ostriker (2002) by including the full effects of curvilinear coordinates, a realistic log-spiral perturbation, self-gravitational contribution from 5 radial wavelengths of the spiral shock, and variation of density and epicyclic frequency with radius. We show that with realistic Toomre Q values, self-gravity and galactic differential rotation produce filamentary gaseous structures with kpc-scale separations, regardless of the strength -- or even presence -- of a stellar spiral potential. However, the growth of sheared features distinctly associated with the spiral arms, described as spurs or feathers in optical and IR observations of many spiral galaxies, requires a sufficiently strong spiral potential in self gravitating models. Unlike independently-growing ''background'' filaments, the orientation of arm spurs depends on galactic location. Inside corotation, spurs emanate outward, on the convex side of the arm; outside corotation, spurs grow inward, on the concave side of the arm. Based on spacing, orientation, and the relation to arm clumps, it is possible to distinguish ''true spurs'' that originate as instabilities in the spiral arms from independently growing ''background'' filaments. Our models also suggest that magnetic fields are important in preserving grand design spiral structure when gas in the arms fragments via self-gravity into GMCs.

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arXiv2006

Formation of Spiral-Arm Spurs and Bound Clouds in Vertically Stratified Galactic Gas Disks

(Abridged) We investigate the growth of spiral-arm substructure in vertically stratified, self-gravitating, galactic gas disks, using local numerical MHD simulations. Our new models extend our previous 2D studies (Kim & Ostriker 2002), which showed that a magnetized spiral shock in a thin disk can undergo magneto-Jeans instability (MJI), resulting in interarm spur structures and massive fragments. Similar spur features have recently been seen in high-resolution observations of several galaxies. Here, we consider two sets of numerical models: 2D models that use a thick-disk gravitational kernel, and 3D runs with explicit vertical stratification. When disks are sufficiently magnetized and self-gravitating, the result in both sorts of models is the growth of spiral-arm substructure similar to that in our previous razor-thin models. Reduced self-gravity in thick disks increases the spur spacing to ~10 times the Jeans length at the arm peak. Bound clouds that form from spur fragmentation have masses ~(1-3)x10^7 Msun each, a factor ~3-8 times larger than in razor-thin models with the same gas surface density and stellar spiral arm strength. We find that unmagnetized or weakly magnetized 2D models are unstable to the wiggle instability (WI) previously identified by Wada & Koda (2004), and proposed as a potential spur- and clump-forming mechanism. However,our fully 3D models do not show this effect. Non-steady motions and strong vertical shear prevent coherent vortical structures from forming, evidently suppressing the WI that appears in 2D (isothermal) runs. We also find no clear traces of Parker instability in the nonlinear spiral arm substructures that emerge (in self-gravitating models), although conceivably Parker modes may help seed the MJI at early stages since azimuthal wavelengths are similar.

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arXiv2005

The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks

We study the stability of proto-planetary disks with vertical velocity gradients in their equilibrium rotation rates; such gradients are expected to develop when dust settles into the midplane. Using a linear stability analysis of a simple three-layer model, we show that the onset of instability occurs at a larger value of the Richardson number, and therefore for a thicker layer, when the effects of Coriolis forces are included. This analysis also shows that even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones. These conclusions are corroborated by a large number of nonlinear numerical simulations with two different parameterized prescriptions for the initial (continuous) dust distributions. Based on these numerical experiments, the Richardson number required for marginal stability is more than an order of magnitude larger than the traditional 1/4 value. The dominant modes that grow have horizontal wavelengths of several initial dust scale heights, and in nonlinear stages mix solids fairly homogeneously over a comparable vertical range. We conclude that gravitational instability may be more difficult to achieve than previously thought, and that the vertical distribution of matter within the dust layer is likely globally, rather than locally, determined.

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arXiv2005

Saturated-State Turbulence and Structure from Thermal and Magnetorotational Instability in the ISM: Three-Dimensional Numerical Simulations

This paper reports on 3D numerical simulations of dynamics and thermodynamics in the diffuse ISM. Our models are local, account for sheared galactic rotation, magnetic fields, and realistic cooling, and resolve scales ~1-200 pc. This combination permits the study of quasi-steady turbulence in a cloudy medium representing the warm/cold atomic ISM. Turbulence is driven by the magnetorotational instability (MRI); our models are the first to study the saturated state of MRI under strongly inhomogeneous conditions, with cloud/intercloud contrasts of about 100. For volume-averaged densities n=0.25 - 4 cm^3, the saturated-state velocity dispersion ranges from 8-1 km/s, scaling as n^-0.77. The MRI is therefore likely quite important in driving turbulence in low-density ISM regions, both at high latitudes in the inner Galaxy,and throughout the far outer Galaxy. The MRI may be key to suppressing star formation at large radii in spiral galaxies, where without MRI-driven turbulence, a thin, cold layer could form and become gravitationally instable. Our presentation also includes analysis of thermal structure (temperature and density PDFs are broadened and include extreme departures from equilibrium, but retain the classical two-phase bimodal character); results on the distribution of clump masses (the mass spectrum peaks at about 100 solar masses); comparisons of saturated-state MRI scalings with single-phase simulation results (we find B^2 is independent of n); and examples of synthetic HI line profile maps (showing that physical clumps are not easily distinguished in velocity components, and vice versa).

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arXiv2003

Thermal and Magnetorotational Instability in the ISM: Two-Dimensional Numerical Simulations

The structure and dynamics of diffuse gas in the Milky Way and other disk galaxies may be strongly influenced by thermal and magnetorotational instabilities (TI and MRI) on scales of about 1-100 pc. We initiate a study of these processes, using two-dimensional numerical hydrodynamic and magnetohydrodynamic (MHD) simulations with conditions appropriate for the atomic interstellar medium (ISM). We demonstrate, consistent with previous work, that nonlinear development of ``pure TI'' produces a network of filaments that condense into cold clouds at their intersections, yielding a distinct two-phase warm/cold medium within about 20 Myr. TI-driven turbulent motions of the clouds saturate at subsonic amplitudes for uniform initial P/k=2000 K cm^-3. MRI has previously been studied in near-uniform media; our simulations include both TI+MRI models, which begin from uniform-density conditions, and cloud+MRI models, which begin with a two-phase cloudy medium. Both the TI+MRI and cloud+MRI models show that MRI develops within a few galactic orbital times, just as for a uniform medium. The mean separation between clouds can affect which MRI mode dominates the evolution. Provided intercloud separations do not exceed half the MRI wavelength, we find the MRI growth rates are similar to those for the corresponding uniform medium. This opens the possibility, if low cloud volume filling factors increase MRI dissipation times compared to those in a uniform medium, that MRI-driven motions in the ISM could reach amplitudes comparable to observed HI turbulent linewidths.

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arXiv2003

Magnetorotationally-Driven Galactic Turbulence and the Formation of Giant Molecular Clouds

Using local 3D MHD simulations, we investigate ways in which galactic turbulence associated with the magnetorotational instability (MRI) may influence the formation and properties of GMCs. Our disk models are vertically stratified and subject to uniform shear. Initial magnetic fields are weak and purely vertical. For simplicity, we adopt an isothermal equation of state. We find that MRI-driven turbulence develops rapidly, with the saturated-state Shakura & Sunyaev parameter alpha~(0.15-0.3) dominated by Maxwell stresses. Many of the dimensionless characteristics of the turbulence (e.g. the ratio of the Maxwell to Reynolds stresses) are similar to results from previous MRI studies of accretion disks, hence insensitive to the degree of vertical disk compression, shear rate, and the presence of self-gravity. The density-weighted velocity dispersions in non- or weakly self-gravitating disks are sigma_x ~ sigma_y ~ (0.4-0.6)c_s and sigma_z~(0.2-0.3)c_s, suggesting that MRI can contribute significantly to the observed level of galactic turbulence. The saturated-state magnetic field strength B ~ 2 μG is similar to typical galactic values. When self-gravity is strong enough, MRI-driven high-amplitude density perturbations are swing-amplified to form Jeans-mass (~10^7 Msun) bound clouds. Compared to previous unmagnetized or strongly-magnetized disk models, the threshold for nonlinear instability in the present models occurs for surface densities at least 50% lower, corresponding to the Toomre parameter Q_th~1.6. We present evidence that self-gravitating clouds like GMCs formed under conditions similar to our models can lose much of their original spin angular momenta by magnetic braking, preferentially via fields threading near-perpendicularly to their spin axes.

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arXiv2003

Spectral Properties of Compressible Magnetohydrodynamic Turbulence from Numerical Simulations

We analyze the spectral properties of driven, supersonic compressible magnetohydrodynamic (MHD) turbulence obtained via high-resolution numerical experiments, for application to understanding the dynamics of giant molecular clouds. Via angle-averaged power spectra, we characterize the transfer of energy from the intermediate, driving scales down to smaller dissipative scales, and also present evidence for inverse cascades that achieve modal-equipartition levels on larger spatial scales. Investigating compressive versus shear modes separately, we evaluate their relative total power, and find that as the magnetic field strength decreases, (1) the shear fraction of the total kinetic power decreases, and (2) slopes of power-law fits over the inertial range steepen. To relate to previous work on incompressible MHD turbulence, we present qualitative and quantitative measures of the scale-dependent spectral anisotropy arising from the shear-Alfvén cascade, and show how these vary with changing mean magnetic field strength. Finally, we propose a method for using anisotropy in velocity centroid maps as a diagnostic of the mean magnetic field strength in observed cloud cores.

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arXiv2002

Three-Dimensional Simulations of Parker, Magneto-Jeans, and Swing Instabilities in Shearing Galactic Gas Disks

We use 3D MHD simulations to investigate nonlinear development of the Parker, magneto-Jeans (MJI), and swing mechanisms in galactic disks. The model disks are local, isothermal, and begin from a vertically-stratified equilibrium. We first construct axisymmetric equilibria and examine their stability. Finite disk thickness reduces the critical Toomre Q parameter below unity; we find Q_c \~ 0.75, 0.72, and 0.57 for β=\infty, 10, and 1 cases, respectively. We then pursue fully 3D models. In non-self-gravitating cases, the peak density enhancement from the `pure' Parker instability is less a factor of two. The dominant growing modes have radial wavelengths comparable to the disk scale height H, much shorter than the azimuthal wavelength (~10-20 H). Shearing disks, being more favorable to midplane-symmetric modes, have somewhat different late-time magnetic field profiles from nonshearing disks, but otherwise saturated states are similar. Late-time velocity fluctuations at 10% of the sound speed persist, but no characteristic structural signatures of Parker modes remain in the new equilibria. In self-gravitating cases, the development of density structure is qualitatively similar to our previous results from thin-disk simulations. The Parker instability, although it may help seed structure or tip the balance under marginal conditions, appears to play a secondary role. In shearing disks with Q less than a threshold level ~ 1, swing amplification can produce bound clouds of a few times the local Jeans mass. The most powerful cloud-condensing mechanism appears to be the MJI. Our simulations show that condensations of a local Jeans mass (~3\times 10^7 M_sun) grow very rapidly, supporting the idea that MJI is at least partly responsible for the formation of bound cloud complexes in spiral galaxies.

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