Turbulent Structure of the Interstellar Medium

2006

We report on a study of interstellar turbulence driven by both correlated and isolated supernova explosions. We use three-dimensional hydrodynamic models of a vertically stratified interstellar medium run with the adaptive mesh refinement code Flash at a maximum resolution of 2 pc, with a grid size of 0.5 × 0.5 × 10 kpc. Cold dense clouds form even in the absence of self-gravity due to the collective action of thermal instability and supersonic turbulence. Studying these clouds, we show that it can be misleading to predict physical properties such as the star formation rate or the stellar initial mass function using numerical simulations that do not include self-gravity of the gas. Even if all the gas in turbulently Jeans unstable regions in our simulation is assumed to collapse and form stars in local freefall times, the resulting total collapse rate is significantly lower than the value consistent with the input supernova rate. The amount of mass available for collapse depends on scale, suggesting a simple translation from the density PDF to the stellar IMF may be questionable. Even though the supernova-driven turbulence does produce compressed clouds, it also opposes global collapse. The net effect of supernova-driven turbulence is to inhibit star formation globally by decreasing the amount of mass unstable to gravitational collapse.

The Astrophysical Journal, 2012

In order to investigate the origin of the interstellar turbulence, detailed observations in the CO J = 1-0 and 3-2 lines have been carried out in an interacting region of a molecular cloud with an H II region. As a result, several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion are detected, whose systemic velocities have a relatively large scatter of a few km s −1 . It is suggested that the cloud is composed of small-scale dense and cold structures and their overlapping effect makes it appear to be a turbulent entity as a whole. This picture strongly supports the two-phase model of turbulent medium driven by thermal instability proposed previously. On the surface of the present cloud, the turbulence is likely to be driven by thermal instability following ionization shock compression and UV irradiation. Those small scale structures with line width of ∼ 0.6 km s −1 have a relatively high CO line ratio of J =3-2 to 1-0, 1 R 3−2/1−0 2. The large velocity gradient analysis implies that the 0.6 km s −1 width component cloudlets have an average density of 10 3−4 cm −3 , which is relatively high at cloud edges, but their masses are only 0.05 M .

Astronomy and Astrophysics, 2010

Context. It is well established that the atomic interstellar hydrogen is filling the galaxies and constitutes the building blocks of molecular clouds. Aims. To understand the formation and the evolution of molecular clouds, it is necessary to investigate the dynamics of turbulent and thermally bistable as well as barotropic flows. Methods. We perform high resolution 3-dimensional hydrodynamical simulations of 2-phase, isothermal and polytropic flows. Results. We compare the density probability distribution function (PDF) and Mach number density relation in the various simulations and conclude that 2-phase flows behave rather differently than polytropic flows. We also extract the clumps and study their statistical properties such as the mass spectrum, mass-size relation and internal velocity dispersion. In each case, it is found that the behavior is well represented by a simple power law. While the various exponents inferred are very similar for the 2-phase, isothermal and polytropic flows, we nevertheless find significant differences, as for example the internal velocity dispersion, which is smaller for 2-phase flows. Conclusions. The structure statistics are very similar to what has been inferred from observations, in particular the mass spectrum, the mass-size relation and the velocity dispersion-size relation are all power laws whose indices well agree with the observed values. Our results suggest that in spite of various statistics being similar for 2-phase and polytropic flows, they nevertheless present significant differences, stressing the necessity to consider the proper thermal structure of the interstellar atomic hydrogen for computing its dynamics as well as the formation of molecular clouds.

Annual Review of Astronomy and Astrophysics, 2004

▪ Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of high...

The Astrophysical Journal, 2011

We have used the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer and the Expanded Very Large Array to obtain a high-resolution map of the CO(6-5) and CO(1-0) emission in the lensed, star-forming galaxy SMM J2135−0102 at z = 2.32. The kinematics of the gas are well described by a model of a rotationally supported disk with an inclination-corrected rotation speed, v rot = 320 ± 25 km s −1 , a ratio of rotational-to-dispersion support of v/σ = 3.5 ± 0.2, and a dynamical mass of (6.0 ± 0.5) × 10 10 M within a radius of 2.5 kpc. The disk has a Toomre parameter, Q = 0.50 ± 0.15, suggesting that the gas will rapidly fragment into massive clumps on scales of L J ∼ 400 pc. We identify star-forming regions on these scales and show that they are ∼10× denser than those in quiescent environments in local galaxies, and significantly offset from the local molecular cloud scaling relations (Larson's relations). The large offset compared to local molecular cloud line-width-size scaling relations implies that supersonic turbulence should remain dominant on scales ∼100× smaller than in the kinematically quiescent interstellar medium (ISM) of the Milky Way, while the molecular gas in SMM J2135 is expected to be ∼50× denser than that in the Milky Way on all scales. This is most likely due to the high external hydrostatic pressure we measure for the ISM, P tot /k B ∼ (2 ± 1) × 10 7 K cm −3 . In such highly turbulent ISM, the subsonic regions of gravitational collapse (and star formation) will be characterized by much higher critical densities, n crit > = 10 8 cm −3 , a factor 1000× more than the quiescent ISM of the Milky Way.

The Astrophysical Journal, 2006

To study how supernova feedback structures the turbulent interstellar medium, we construct 3D models of ver-tically stratified gas stirred by discrete supernova explosions, including vertical gravitational fields and parameterized heating and cooling. The models ...

arXiv: Astrophysics of Galaxies, 2021

Supersonic gas turbulence is a ubiquitous property of the interstellar medium. The level of turbulence, quantified by the gas velocity dispersion (g), is observed to increase with the star formation rate (SFR) rate of a galaxy, but it is yet not established whether this trend is driven by stellar feedback or gravitational instabilities. In this work we carry out hydrodynamical simulations of entire disc galaxies, with different gas fractions, to understand the origins of the SFRg relation. We show that disc galaxies reach the same levels of turbulence regardless of the presence of stellar feedback processes, and argue that this is an outcome of the way disc galaxies regulate their gravitational stability. The simulations match the SFRg relation up to SFRs of the order of tens of M yr −1 and g ∼ 50 km s −1 in neutral hydrogen and molecular gas, but fail to reach the very large values (> 100 km s −1) reported in the literature for rapidly star forming galaxies. We demonstrate that such high values of g can be explained by 1) insufficient beam smearing corrections in observations, and 2) stellar feedback being coupled to the ionised gas phase traced by recombination lines. Given that the observed SFRg relation is composed of highly heterogeneous data, with g at high SFRs almost exclusively being derived from H observations of high redshift galaxies with complex morphologies, we caution against analytical models that attempt explain the SFRg relation without accounting for these effects.

The Astrophysical Journal, 2012

Stellar feedback drives the circulation of matter from the disk to the halo of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a vertical column of the interstellar medium with initial conditions typical of the solar circle in which supernovae drive turbulence and determine the vertical stratification of the medium. The simulations were run using a stable, positivity-preserving scheme for ideal MHD implemented in the FLASH code. We find that the majority (≈ 90%) of the mass is contained in thermally-stable temperature regimes of cold molecular and atomic gas at T < 200 K or warm atomic and ionized gas at 5000 K < T < 10 4.2 K, with strong peaks in probability distribution functions of temperature in both the cold and warm regimes. The 200 − 10 4.2 K gas fills 50−60% of the volume near the plane, with hotter gas associated with supernova remnants (30−40%) and cold clouds (< 10%) embedded within. At |z| ∼ 1 − 2 kpc, transition-temperature (10 5 K) gas accounts for most of the mass and volume, while hot gas dominates at |z| > 3 kpc. The magnetic field in our models has no significant impact on the scale heights of gas in each temperature regime; the magnetic tension force is approximately equal to and opposite the magnetic pressure, so the addition of the field does not significantly affect the vertical support of the gas. The addition of a magnetic field does reduce the fraction of gas in the cold (< 200 K) regime with a corresponding increase in the fraction of warm (∼ 10 4 K) gas. However, our models lack rotational shear and thus have no largescale dynamo, which reduces the role of the field in the models compared to reality. The supernovae drive oscillations in the vertical distribution of halo gas, with the period of the oscillations ranging from ≈ 30 Myr in the T < 200 K gas to ∼ 100 Myr in the 10 6 K gas, in line with predictions by Walters & Cox.

The Astrophysical Journal, 1999

We present the results of a velocity correlation study of the high latitude cloud MBM16 using a fully sampled 12 CO map, supplemented by new 13 CO data. We find a correlation length of 0.4 pc. This is similar in size to the formaldehyde clumps described in our previous study.

The Astrophysical Journal, 2009

Formation of interstellar clouds as a consequence of thermal instability is studied using twodimensional two-fluid magnetohydrodynamic simulations. We consider the situation of converging, supersonic flows of warm neutral medium in the interstellar medium that generate a shocked slab of thermally unstable gas in which clouds form. We found, as speculated in paper I, that in the shocked slab magnetic pressure dominates thermal pressure and the thermal instability grows in the isochorically cooling, thermally unstable slab that leads formation of HI clouds whose number density is typically n 100 cm -3 , even if the angle between magnetic field and converging flows is small. We also found that even if there is a large dispersion of magnetic field, evolution of the shocked slab is essentially determined by the angle between the mean magnetic field and converging flows. Thus, the direct formation of molecular clouds by piling up warm neutral medium does not seem a typical molecular cloud formation process, unless the direction of supersonic converging flows is biased to the orientation of mean magnetic field by some mechanism. However, when the angle is small, the HI shell generated as a result of converging flows is massive and possibly evolves into molecular clouds, provided gas in the massive HI shell is piled up again along the magnetic field line. We expect that another subsequent shock wave can pile up again the gas of the massive shell and produce a larger cloud. We thus emphasize the importance of multiple episodes of converging flows, as a typical formation process of molecular clouds.