Formation Of Giant Molecular Clouds Research Articles

The Role of Pressure in Giant Molecular Cloud Formation

We examine the hypothesis that hydrostatic pressure alone determines the ratio of atomic to molecular gas averaged over a particular radius in disk galaxies. The hypothesis implies that the transition radius, the location where the ratio is unity, should always occur at the same value of stellar surface density in all galaxies. We examine data for 28 galaxies and find that the stellar surface density at the transition radius is indeed constant to within 40% at a value of 120 M☉ pc-2. If the hypothesis can be confirmed at all radii within a large range of galaxy types and metallicities, combining it with the observed relation between the star formation rate and H2 surface density may enable us to derive a physically motivated star formation prescription with wide applicability.

Is the Parker Instability a Viable Mechanism for the Formation of Giant Molecular Clouds?

The stratified interstellar medium has been known to be subject to the Parker instability, and the instability has been considered as a plausible mechanism for the formation of giant molecular clouds (GMCs). By presenting our recent efforts to understand the instability through linear analyses and numerical simulations, we raise a negative point of view that the Parker instability alone could not form GMCs.

Magnetorotationally Driven Galactic Turbulence and the Formation of Giant Molecular Clouds

Giant molecular clouds (GMCs), where most stars form, may originate from self-gravitating instabilities in the interstellar medium. Using local three-dimensional magnetohydrodynamic simulations, we investigate ways in which galactic turbulence associated with the magnetorotational instability (MRI) may influence the formation and properties of these massive, self-gravitating clouds. Our disk models are vertically stratified with both gaseous and stellar gravity and subject to uniform shear corresponding to a flat rotation curve. Initial magnetic fields are assumed to be weak and purely vertical. For simplicity, we adopt an isothermal equation of state with sound speed cs = 7 km s-1. We find that MRI-driven turbulence develops rapidly, with the saturated-state Shakura & Sunyaev parameter α ~ 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—although self-gravity enhances fluctuation amplitudes slightly. The density-weighted velocity dispersions in non- or weakly self-gravitating disks are σx ~ σy ~ (0.4-0.6)cs and σz ~ (0.2-0.3)cs, suggesting that MRI can contribute significantly to the observed level of galactic turbulence. The saturated-state magnetic field strength ~ 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 (~107 M☉) 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 Qth ~ 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 nearly perpendicularly to their spin axes. Finally, we discuss the present results within the larger theoretical and observational context, outlining directions for future study.

Giant Molecular Clouds in M33. I. BIMA All‐Disk Survey

We present the first interferometric CO(J=1->0) map of the entire H-alpha disk of M33. The 13 diameter synthesized beam corresponds to a linear resolution of 50 pc, sufficient to distinguish individual giant molecular clouds (GMCs). From these data we generated a catalog of 148 GMCs with an expectation that no more than 15 of the sources are spurious. The catalog is complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of 2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that our survey detects ~50% of the CO flux, hence that the total molecular mass of M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our catalog are confined largely to the central region (R < 4 kpc). They show a remarkable spatial and kinematic correlation with overdense HI filaments; the geometry suggests that the formation of GMCs follows that of the filaments. The GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper than that found in the Milky Way and in the LMC. Combined with the total mass, this steep function implies that the GMCs in M33 form with a characteristic mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions, implying that the GMCs have a short quiescent period. Our results suggest the rapid assembly of molecular clouds from atomic gas, with prompt onset of massive star formation.

The effect of particle number in N-body simulations of the formation of giant molecular clouds

In this paper, we study the effect of particle number in the N-body simulation of the formation of giant molecular clouds. The result shows that in the case of formation by aggregation of “unit” clouds, if we take the mean density of the unit clouds to be a constant (or their effective radius to be proportional to the cubic roots of their mass), then the disruption rate of the GMCs will be independent of the initial number of unit clouds.

The effect of rigid rotation in the formation of giant molecular clouds by aggregation

The formation of giant molecular clouds (GMCs) by aggregation in a rigidly rotating disk galaxy is studied. The result shows that the GMCs so formed consist mainly of individual clouds located close to one another from inelastic collision and self-gravitation under a velocity dispersion. The clouds so formed are not very massive. If there is differential rotation, then there is a greater chance of more massive clouds being formed.

Formation of giant molecular clouds by aggregation in a galactic disc with rigid rotation

We have explored a model for the formation of giant molecular clouds through accretion of smaller clouds; the star disc is assumed to be rigidly rotating in this model. The main results are: (1) The aggregates consist mainly of the neighbouring clouds whose collision was induced by the velocity dispersion. In this model aggregates take a shorter time to form compared to the model which includes differential rotation, although the aggregates tend to be less massive. (2) By the same token, the mass of these aggregates does not increase further. The inclusion of differential rotation would have allowed for the possibility of distinct aggregates colliding amongst themselves to form larger aggregates.

Large-Scale Gravitational Instability in the Formation of GMCS

Two models of molecular cloud in disk galaxies are proposed to investigate the formation of giant molecular clouds (GMCs) under the gravitational instability and random collision using PP(Particle–Particle) simulation. Having analysed simulation outputs of the two models and comparing them with observation, we are able to draw some general conclusions, the most significant ones of which are: 1) Similar to results obtained previously, the gravitational instability can make small clouds form large clouds faster than random collision. 2) The differential rotation in gravitational instability model plays a positive role in agglomeration of molecular clouds.

Three-dimensional Evolution of the Parker Instability under a Uniform Gravity

Using an isothermal MHD code, we have performed three-dimensional, high-resolution simulations of the Parker instability. The initial equilibrium system is composed of exponentially-decreasing isothermal gas and magnetic field (along the azimuthal direction) under a uniform gravity. The evolution of the instability can be divided into three phases: linear, nonlinear, and relaxed. During the linear phase, the perturbations grow exponentially with a preferred scale along the azimuthal direction but with smallest possible scale along the radial direction, as predicted from linear analyses. During the nonlinear phase, the growth of the instability is saturated and flow motion becomes chaotic. Magnetic reconnection occurs, which allows gas to cross field lines. This, in turn, results in the redistribution of gas and magnetic field. The system approaches a new equilibrium in the relaxed phase, which is different from the one seen in two-dimensional works. The structures formed during the evolution are sheet-like or filamentary, whose shortest dimension is radial. Their maximum density enhancement factor relative to the initial value is less than 2. Since the radial dimension is too small and the density enhancement is too low, it is difficult to regard the Parker instability alone as a viable mechanism for the formation of giant molecular clouds.

The effect of spiral structure on the formation of giant molecular clouds: Zhang Tong-jie & Song Guo-zuan Shanghai Observatory, Chinese Academy of Sciences, Shanghai 200030

The effect of spiral structure on the formation of giant molecular clouds: Zhang Tong-jie & Song Guo-zuan Shanghai Observatory, Chinese Academy of Sciences, Shanghai 200030

Giant molecular cloud formation through the Parker instability in a skewed magnetic field

view Abstract Citations (16) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Giant Molecular Cloud Formation through the Parker Instability in a Skewed Magnetic Field Hanawa, Tomoyuki ; Matsumoto, Ryoji ; Shibata, Kazunari Abstract The effect of the magnetic skew on the Parker instability is investigated by means of the linear stability analysis for a gravitationally stratified gas layer permeated by a horizontal magnetic field. When the magnetic field is skewed (i.e., the field line direction is a function of the height), the wavelength of the most unstable mode is $ \lambda \; \sim \; 10 H $ where $ H $ is the pressure scale height. The growth rate of the short wavelength modes is greatly reduced when the gradient in the magnetic field direction exceeds 0.5 radian per scale height. Our results indicate that the Parker instability in a skewed magnetic field preferentially forms large scale structures like giant molecular clouds. Publication: The Astrophysical Journal Pub Date: July 1992 DOI: 10.1086/186454 arXiv: arXiv:astro-ph/9205002 Bibcode: 1992ApJ...393L..71H Keywords: Interstellar Gas; Interstellar Magnetic Fields; Magnetic Field Configurations; Magnetohydrodynamic Stability; Molecular Clouds; Cosmic Plasma; Gravitational Effects; Astrophysics; INSTABILITIES; ISM: MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS: MHD; Astrophysics E-Print: 12 pages full text sources arXiv | ADS |

Formation of giant molecular clouds and helical magnetic fields by the Parker instability

USING the Nagoya telescope1, Uchida et al.2 found an unusual helical filamentary structure, spinning about its long axis, in the L1641 cloud in the Orion cloud complex. Noting that this structure is consistent with a helically twisted magnetic field inferred from optical polarization observations3,4, they argued that the helical filament is a manifestation of torsional magnetohydrodynamic (Alfven) waves draining angular momentum from a nearby massive cloud, thus promoting collapse and star formation. Here we present an alternative interpretation. We suggest that the Orion molecular cloud complex formed through the Parker instability5 (the buoyancy of a magnetic field entrained in matter), and that the helical filament is the result of spinning gas falling along the magnetic field and twisting it. The twisted magnetic field, unlike a purely planar field, suppresses the Parker instability on small scales, allowing the generation of finite clouds rather than general turbulence.

Formation of Giant Molecular Clouds Associated with Spiral Structure

AbstractMolecular hydrogen in spiral galaxies is distributed in clumps, i.e., molecular clouds, which have mass between 103M⊙ and 106M⊙ and a mass spectrum of n(m) ∝ m−1.6. Molecular clouds with masses greater than 105M⊙, are called giant molecular clouds (GMCs). It is generally accepted that GMCs are formed by the coalescence of molecular clouds through their collision. This process is studied by both numerical simulation and numerical integration. The observation with high resolution identified a great number of CO emission cores in galaxies. Based on this result, the aggregation or clustering formation of GMCs is numerically simulated. In the process of either coalescence or clustering, spiral perturbation plays an important role.

The role of orbital dynamics and cloud-cloud collisions in the formation of giant molecular clouds in global spiral structures

The role of orbit crowding and cloud-cloud collisions in the formation of GMCs and their organization in global spiral structure is investigated. Both N-body simulations of the cloud system and a detailed analysis of individual particle orbits are used to develop a conceptual understanding of how individual clouds participate in the collective density response. Detailed comparisons are made between a representative cloud-particle simulation in which the cloud particles collide inelastically with one another and give birth to and subsequently interact with young star associations and stripped down simulations in which the cloud particles are allowed to follow ballistic orbits in the absence of cloud-cloud collisions or any star formation processes. Orbit crowding is then related to the behavior of individual particle trajectories in the galactic potential field. The conceptual picture of how GMCs are formed in the clumpy ISMs of spiral galaxies is formulated, and the results are compared in detail with those published by other authors.

Formation of Giant Molecular Clouds by Coagulation of Small Molecular Clouds in a Spiral Gravitational Potential

The formation process of giant molecular clouds (GMCs) is investigated from the standpoint of the coagulation theory of molecular clouds. Small clouds collide with each other and grow to become massive ones. Ultimately they form GMCs with a finite lifetime. The occurrence of star formation in a GMC destroys it and consequently small clouds are formed again. We study the time evolution of the clouds which move through a spiral gravitational potential by an N-body simulation. Then the ensemble of clouds responds to the spiral potential and forms a spiral structure similar to that produced by hydrodynamical galactic shock. It is shown that GMCs are formed in the spiral arm region by collisions between clouds. The distribution of GMCs indicates their short lifetime, of the order of a few times 107 years.

Evolution of Galaxies with Periodic Star Formation

Interchange processes in the interstellar medium, which is composed of diffuse gas, small clouds and giant molecular clouds are examined. It is stressed that the formation of giant molecular clouds through gravitational instability of a cloud ensemble leads to periodic star formation. The evolution of galaxies with such periodic star formation episodes is studied, especially paying attention to the effect of gas infall.

The Evolution of Giant Molecular Clouds

We discuss the origin, lifetime, destruction, spatial distribution and relation to star formation of giant molecular clouds. A coagulation model including the effects of spiral density wave shocks is described. We explore implications for CO observations of external galaxies. The collective effects of OB star winds and supernova remnants in disrupting clouds are considered.