Formation Of Giant Molecular Clouds Research Articles

Star cluster formation from giant molecular clouds in the Small Magellanic Cloud about 2 Gyr ago: their origin, structures, and kinematics

ABSTRACT Recent observations have found that the age distribution of star clusters (SCs) in the Small Magellanic Cloud (SMC) shows a sharp peak around 2 Gyr ago. However, it is theoretically unclear what physical processes are responsible for such sudden formation of SCs in the SMC. Here, we investigate whether massive SCs with initial masses more than $10^5\, \text{M}_\odot$ can be formed during tidal interaction of the SMC with the Large Magellanic Cloud (LMC) about 2 Gyr ago, based on our new simulations, which include molecular hydrogen formation on dust grains and SC formation within giant molecular clouds (GMCs). We find that the formation of GMCs with masses more than $10^5\, \text{M}_\odot$ can be dramatically enhanced due to the tidal force of the LMC–SMC interaction. We also find that gravitationally bound massive SCs can be formed within these GMCs, though their mean stellar densities ($10^4\, \text{M}_\odot \,\text{pc}^{-3}$) are systematically lower than those of the genuine globular clusters (GCs). All simulated SCs have diffuse extended stellar envelopes that were formed from multiple merging of sub clusters within their natal GMCs. Furthermore, we find that some of the simulated SCs can have considerable global internal rotation and sub-structures surrounding them. Based on these simulation results, we discuss the origin of the observed diverse properties of SCs in the SMC and the physical roles of galaxy interaction in the formation of massive SCs from GMCs.

Giant molecular cloud formation at the interface of colliding supershells in the large magellanic cloud

ABSTRACT We investigate the H i envelope of the young, massive GMCs in the star-forming regions N48 and N49, which are located within the high column density H i ridge between two kpc-scale supergiant shells, LMC 4 and LMC 5. New long-baseline H i 21 cm line observations with the Australia Telescope Compact Array (ATCA) were combined with archival shorter baseline data and single dish data from the Parkes telescope, for a final synthesized beam size of 24.75 arcsec by 20.48 arcsec, which corresponds to a spatial resolution of ∼6 pc in the LMC. It is newly revealed that the H i gas is highly filamentary and that the molecular clumps are distributed along filamentary H i features. In total 39 filamentary features are identified and their typical width is ∼21 (8–49) [pc]. We propose a scenario in which the GMCs were formed via gravitational instabilities in atomic gas which was initially accumulated by the two shells and then further compressed by their collision. This suggests that GMC formation involves the filamentary nature of the atomic medium.

G.A.S.

Context. Star formation in galaxies is inefficient, and understanding how star formation is regulated in galaxies is one of the most fundamental challenges of contemporary astrophysics. Radiative cooling, feedback from supernovae and active galactic nuclei (AGN), and large-scale dynamics and dissipation of turbulent energy act over various time and spatial scales and all regulate star formation in a complex gas cycle. Aims. This paper presents the physics implemented in a new semi-analytical model of galaxy formation and evolution called the Galaxy Assembler from dark-matter Simulation (G.A.S.). Methods. The fundamental underpinning of our new model is the development of a multiphase interstellar medium (ISM) in which energy produced by supernovae and AGN maintains an equilibrium between a diffuse, hot, and stable gas and a cooler, clumpy, and low-volume filling factor gas. The hot gas is susceptible to thermal and dynamical instabilities. We include a description of how turbulence leads to the formation of giant molecular clouds through an inertial turbulent energy cascade, assuming a constant kinetic energy transfer per unit volume. We explicitly modelled the evolution of the velocity dispersion at different scales of the cascade and accounted for thermal instabilities in the hot halo gas. Thermal instabilities effectively reduce the impact of radiative cooling and moderates accretion rates onto galaxies, and in particular, for those residing in massive haloes. Results. We show that rapid and multiple exchanges between diffuse and unstable gas phases strongly regulates star formation rates in galaxies because only a small fraction of the unstable gas is forming stars. We checked that the characteristic timescales describing the gas cycle, gas depletion timescale, and star-forming laws at different scales are in good agreement with observations. For high-mass haloes and galaxies, cooling is naturally regulated by the growth of thermal instabilities, so we do not need to implement strong AGN feedback in this model. Our results are also in good agreement with the observed stellar mass function from z ≃ 6.0 to z ≃ 0.5. Conclusion. Our model offers the flexibility to test the impact of various physical processes on the regulation of star formation on a representative population of galaxies across cosmic times. Thermal instabilities and the cascade of turbulent energy in the dense gas phase introduce a delay between gas accretion and star formation, which keeps galaxy growth inefficient in the early Universe. The main results presented in this paper, such as stellar mass functions, are available in the GALAKSIENN library.

On the observed diversity of star formation efficiencies in Giant Molecular Clouds

Observations find a median star formation efficiency per free-fall time in Milky Way Giant Molecular Clouds (GMCs) on the order of $\epsilon_{\rm ff}\sim 1\%$ with dispersions of $\sim0.5\,{\rm dex}$. The origin of this scatter in $\epsilon_{\rm ff}$ is still debated and difficult to reproduce with analytical models. We track the formation, evolution and destruction of GMCs in a hydrodynamical simulation of a Milky Way-like galaxy and by deriving cloud properties in an observationally motivated way, measure the distribution of star formation efficiencies which are in excellent agreement with observations. We find no significant link between $\epsilon_{\rm ff}$ and any measured global property of GMCs (e.g. gas mass, velocity dispersion). Instead, a wide range of efficiencies exist in the entire parameter space. From the cloud evolutionary tracks, we find that each cloud follow a \emph{unique} evolutionary path which gives rise to wide diversity in all properties. We argue that it is this diversity in cloud properties, above all else, that results in the dispersion of $\epsilon_{\rm ff}$.

Triggering the formation of the supergiant H ii region NGC 604 in M 33

Abstract Formation mechanism of a supergiant H ii region NGC 604 is discussed in terms of collision of H i clouds in M 33. An analysis of the archival H i data obtained with the Very Large Array (VLA) reveals complex velocity distributions around NGC 604. The H i clouds are composed of two velocity components separated by ∼20 km s−1 for an extent of ∼700 pc, beyond the size of the the H ii region. Although the H i clouds are not easily separated in velocity with some mixed component represented by merged line profiles, the atomic gas mass amounts to $6 \times 10^{6}\,M_{\odot }$ and $9 \times 10^{6}\,M_{\odot }$ for each component. These characteristics of H i gas and the distributions of dense molecular gas in the overlapping regions of the two velocity components suggest that the formation of giant molecular clouds and the following massive cluster formation have been induced by the collision of H i clouds with different velocities. Referring to the existence of a gas bridging feature connecting M 33 with M 31 reported by large-scale H i surveys, the disturbed atomic gas possibly represents the result of past tidal interaction between the two galaxies, which is analogous to the formation of the R 136 cluster in the LMC.

Multiarm spirals on the periphery of disc galaxies

Abstract Spiral patterns in some disc galaxies have two arms in the centre, and three or more arms on the periphery. The same result is also obtained in numerical simulations of stellar and gaseous discs.We argue that such patterns may occur due to fast cooling of the gas, resulting in formation of giant molecular clouds. The timescale of this process is 50 Myr, the factor of 10 shorter than of ordinary secular instability. The giant molecular clouds give rise to multiarm spirals through the mechanism of swing amplification.

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

Abstract Recent radio observations show that giant molecular cloud (GMC) mass functions noticeably vary across galactic disks. High-resolution magnetohydrodynamics simulations show that multiple episodes of compression are required for creating a molecular cloud in the magnetized interstellar medium. In this article, we formulate the evolution equation for the GMC mass function to reproduce the observed profiles, for which multiple compressions are driven by a network of expanding shells due to H ii regions and supernova remnants. We introduce the cloud–cloud collision (CCC) terms in the evolution equation in contrast to previous work (Inutsuka et al.). The computed time evolution suggests that the GMC mass function slope is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC effect is limited only in the massive end of the mass function. In addition, we identify a gas resurrection channel that allows the gas dispersed by massive stars to regenerate GMC populations or to accrete onto pre-existing GMCs. Our results show that almost all of the dispersed gas contributes to the mass growth of pre-existing GMCs in arm regions whereas less than 60% contributes in inter-arm regions. Our results also predict that GMC mass functions have a single power-law exponent in the mass range <105.5 (where represents the solar mass), which is well characterized by GMC self-growth and dispersal timescales. Measurement of the GMC mass function slope provides a powerful method to constrain those GMC timescales and the gas resurrecting factor in various environments across galactic disks.

Observing simulations: molecular clouds and their journey in the galaxy

In order to have a global picture of the cycle of matter in galaxies, we need to understand the interplay of large-scale galactic phenomena with the formation of giant molecular clouds (GMCs) and, ultimately, their subsequent star formation (SF). In this work, we study the population of GMCs within a smoothed particle hydrodynamics (SPH) simulation of a spiral galaxy, and investigate the link between the GMC properties and position with respect to spiral arms, both directly from the simulation (with the 3D densities of H 2 and CO) and from an observer's perspective (with CO emission in PPV space).

Simulations of supermassive black hole growth in high-redshift disc galaxies

Observations suggest that a large fraction of black hole growth occurs in normal star-forming disk galaxies. Here we describe simulations of black hole accretion in isolated disk galaxies with sufficient resolution (~5 pc) to track the formation of giant molecular clouds that feed the black hole. Black holes in z=2 gas-rich disks (fgas=50%) occasionally undergo ~10 Myr episodes of Eddington-limited accretion driven by stochastic collisions with massive, dense clouds. We predict that these gas-rich disks host weak AGNs 1/4 of the time, and moderate/strong AGNs 10% of the time. Averaged over 100 Myr timescales and the full distribution of accretion rates, the black holes grow at a few per cent of the Eddington limit -- sufficient to match observations and keep the galaxies on the MBH-Mbulge relation. This suggests that dense cloud accretion in isolated z=2 disks could dominate cosmic black hole growth. In z=0 disks with fgas=10%, Eddington-limited growth is extremely rare because typical gas clouds are smaller and more susceptible to disruption by AGN feedback. This results in an average black hole growth rate in high-fgas galaxies that is up to 1000 times higher than that in low-fgas galaxies. In all our simulations, accretion shows variability by factors of 10^4 on a variety of time scales, with variability at 1 Myr scales driven by the structure of the interstellar medium.

STAR FORMATION EFFICIENCIES AND LIFETIMES OF GIANT MOLECULAR CLOUDS IN THE MILKY WAY

We use a sample of the 13 most luminous WMAP Galactic free-free sources, responsible for 33% of the free- free emission of the Milky Way, to investigate star formation. The sample contains 40 star forming complexes; we combine this sample with giant molecular cloud (GMC) catalogs in the literature, to identify the host GMCs of 32 of the complexes. We estimate the star formation efficiency epsilon_GMC and star formation rate per free-fall time epsilon_ff. We find that epsilon_GMC ranges from 0.002 to 0.2, with an ionizing luminosity-weighted average epsilon_GMC = 0.08, compared to the Galactic average = 0.005. Turning to the star formation rate per free-fall time, we find values that range up to epsilon_ff = 1. Weighting by ionizing luminosity, we find an average of epsilon_ff = 0.16 - 0.24 depending on the estimate of the age of the system. Once again, this is much larger than the Galaxy-wide average value epsilon_ff = 0.008. We show that the lifetimes of giant molecular clouds at the mean mass found in our sample is 17 plus or minus 4 Myr, about two free-fall times. The GMCs hosting the most luminous clusters are being disrupted by those clusters. Accordingly, we interpret the range in epsilon_ff as the result of a time-variable star formation rate; the rate of star formation increases with the age of the host molecular cloud, until the stars disrupt the cloud. These results are inconsistent with the notion that the star formation rate in Milky Way GMCs is determined by the properties of supersonic turbulence

3D Magneto-hydrodynamic simulations of Parker instability with cosmic rays

This study investigates Parker instability in an interstellar medium (ISM) near the Galactic plane using three-dimensional magneto-hydrodynamic simulations. Parker instability arises from the presence of a magnetic field in a plasma, wherein the magnetic buoyant pressure expels the gas and cause the gas to move along the field lines. The process is thought to induce the formation of giant molecular clouds in the Galaxy. In this study, the effects of cosmic-ray (CR) diffusion are examined. The ISM at equilibrium is assumed to comprise a plasma fluid and a CR fluid at various temperatures, with a uniform magnetic field passing through it in the azimuthal direction of the Galactic disk. After a small perturbation, the unstable gas aggregates at the footpoint of the magnetic fields and forms dense blobs. The growth rate of the instability increases with the strength of the CR diffusion. The formation of dense clouds is enhanced by the effect of cosmic rays (CRs), whereas the shape of the clouds depends sensitively on the initial conditions of perturbation.

Galactic star formation in parsec-scale resolution simulations

AbstractThe interstellar medium (ISM) in galaxies is multiphase and cloudy, with stars forming in the very dense, cold gas found in Giant Molecular Clouds (GMCs). Simulating the evolution of an entire galaxy, however, is a computational problem which covers many orders of magnitude, so many simulations cannot reach densities high enough or temperatures low enough to resolve this multiphase nature. Therefore, the formation of GMCs is not captured and the resulting gas distribution is smooth, contrary to observations. We investigate how star formation (SF) proceeds in simulated galaxies when we obtain parsec-scale resolution and more successfully capture the multiphase ISM. Both major mergers and the accretion of cold gas via filaments are dominant contributors to a galaxy's total stellar budget and we examine SF at high resolution in both of these contexts.

GMC formation by agglomeration and self gravity

We investigate the formation of giant molecular clouds (GMCs) in spiral galaxies through both agglomeration of clouds in the spiral arms, and self gravity. The simulations presented include two-fluid models, which contain both cold and warm gas, although there is no heating or cooling between them. We find agglomeration is predominant when both the warm and cold components of the interstellar medium are effectively stable to gravitational instabilities. In this case, the spacing (and consequently mass) of clouds and spurs along the spiral arms is determined by the orbits of the gas particles and correlates with their epicyclic radii (or equivalently spiral shock strength). Notably GMCs formed primarily by agglomeration tend to be unbound associations of many smaller clouds, which disperse upon leaving the spiral arms. These GMCs are likely to be more massive in galaxies with stronger spiral shocks or higher surface densities. GMCs formed by agglomeration are also found to exhibit both prograde and retrograde rotation, a consequence of the clumpiness of the gas. At higher surface densities, self gravity becomes more important in arranging both the warm and cold gas into clouds and spurs, and determining the properties of the most massive GMCs. These massive GMCs can be distinguished by their higher angular momentum, exhibit prograde rotation and are more bound. For a 20 M ⊙ pc -2 disc, the spacing between the GMCs fits both the agglomeration and self gravity scenarios, as the maximum unstable wavelength of gravitational perturbations in the warm gas is similar to the spacing found when GMCs form solely by agglomeration.

On the Rapid Collapse and Evolution of Molecular Clouds

Stars generally form faster than the ambipolar diffusion time, suggesting that several processes short-circuit the delay and promote a rapid collapse. These processes are considered here, including turbulence compression in the outer parts of giant molecular cloud (GMC) cores and GMC envelopes, GMC core formation in an initially supercritical state, and compression-induced triggering in dispersing GMC envelopes. The classical issues related to star formation timescales are addressed: high molecular fractions, low efficiencies, long consumption times for CO and HCN, rapid GMC core disruption and the lack of a stable core, long absolute but short relative timescales with accelerated star formation, and the slow motions of protostars. We consider stimuli to collapse from changes in the density dependence of the ionization fraction, the cosmic ray ionization rate, and various dust properties at densities above ~105 cm-3. We favor the standard model of subcritical GMC envelops and suggest they would be long-lived if not for disruption by rapid star formation in GMC cores. The lifecycle of GMCs is illustrated by a spiral arm section in the Hubble Heritage image of M51, showing GMC formation, star formation, GMC disruption with lingering triggered star formation, and envelope dispersal. There is no delay between spiral arm dust lanes and star formation; the classical notion results from heavy extinction in the dust lane and triggered star formation during cloud dispersal. Differences in the IMF for the different modes of star formation are considered.

Giant molecular clouds and star formation in the Large Magellanic Cloud

AbstractIn order to elucidate star formation in the Large Magellanic Cloud, a complete survey of the molecular clouds was carried out by NANTEN. In this work, we compare 230 giant molecular clouds (GMCs), whose physical quantities are well determined, with young clusters and Hii regions. We find that about 76% of the GMCs are actively forming stars or clusters, while 24% show no signs of massive star or cluster formation. Effects of supergiant shells (SGSs) on the formation of GMCs and stars are also studied. The number and surface mass densities of the GMCs are higher by a factor of 1.5–2 at the edge of the SGSs than elsewhere. It is also found that young stellar clusters are more actively formed in the GMCs facing to the center of the SGSs. These results are consistent with the previous studies by Yamaguchi et al. and suggest the formation of GMCs and the cluster is triggered by dynamical effects of the SGSs.

Spiral arm triggering of star formation

AbstractWe present numerical simulations of the passage of clumpy gas through a galactic spiral shock, the subsequent formation of giant molecular clouds (GMCs) and the triggering of star formation. The spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. In addition to triggering the star formation process, the clumpy gas passing through the shock naturally generates the observed velocity dispersion size relation of molecular clouds. In this scenario, the internal motions of GMCs need not be turbulent in nature. The coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Globally unbound molecular clouds provides a simple explanation of the low efficiency of star formation. While dense regions in the shock become bound and collapse to form stars, the majority of the gas disperses as it leaves the spiral arm.

From Parker and thermal instabilities to the formation of giant molecular clouds

AbstractWe study the Parker instability (PI) under the influence of heating and cooling processes. We distinguish two models with different heating functions. On the base of linear stability analysis, we find that the properties of PI in such a system are significantly different with respect to the adiabatic and isothermal cases. We find that in case of thermally stable equilibrium, PI operating in presence of additional heating sources, postulated by Reynolds et al. (1999), and cooling described by Dalgarno & McCray (1972), leads to a decrease of temperature in magnetic valleys. Numerical simulations show that this behavior promotes a transition of gas to thermally unstable regime. In consequence the gas density grows up by an order of magnitude and the condensed gas clumps acquire compact shapes. Our results lead to the conclusion that PI can trigger the formation of giant molecular clouds complexes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the influence of cooling and heating processes on Parker instability

We study Parker instability (PI) operating in a non-adiabatic, gravitationally stratified, interstellar medium. We discuss models with two kinds of heating mechanisms. The first one results from photoionization models. The other, relying on supplemental sources, has been postulated by Reynolds, Haffner & Tufte. The cooling rate, corresponding to radiative losses, is an approximation to the one given by Dalgarno & McCray. An unperturbed state of the system represents a magnetohydrostatic and thermal equilibrium. We perform linear stability analysis by solving the boundary value problem. We find that the maximum growth rate of PI rises for increasing magnitudes of non-adiabatic effects. In the pure photoionization model, the maximum growth rate of the general non-adiabatic case coincides with the isothermal limit. Adding other sources of heat leads to a maximum growth rate that is larger than the one corresponding to the isothermal limit. We find that the influence of the supplemental heating on PI also leads to a decrease in temperature in magnetic valleys. Finally, we conclude that the initial gas cooling due to the action of PI may promote a subsequent onset of thermal instability in magnetic valleys and formation of giant molecular clouds.

Spiral shocks, triggering of star formation and the velocity dispersion in giant molecular clouds

We present numerical simulations of the passage of clumpy gas through a galactic spiral shock and the subsequent formation of giant molecular clouds (GMCs). The spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. The effect of the clumpiness of gas as it passes through the shock is to generate chaotic internal motions in the gas. The kinematics of these motions are found to agree with the observed velocity-dispersion/size relation found in star-forming regions. In contrast to the standard picture where continuously driven turbulence generates the density inhomogeneities in star-forming clouds, we find here that it is the clumpiness of the interstellar gas that produces the chaotic motions as it passes through the spiral shock and initiates the star formation process. The velocity dispersion can be understood as being due to the random mass loading of clumps as they converge in the spiral shock. In this model there is no need for any internal or external continuous driving mechanism for the 'turbulence'. The coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Indeed, while clearly some parts of the clouds are self-gravitating and able to form stars, most of the molecular material remains gravitationally unbound. This can provide a simple explanation for the low efficiency of star formation.

Detections of CO in Late-Type, Low Surface Brightness Spiral Galaxies

Using the IRAM 30 m telescope, we have obtained (CO)-C-12 J = 1-0 and 2-1 spectral line observations toward the nuclear regions of 15 edge-on, low surface brightness (LSB) spiral galaxies. Our sample comprises extreme late-type LSB spirals with disk-dominated morphologies and rotational velocities V-rot less than or similar to 120 km s(-1). We report detections of four galaxies in at least one transition (greater than or similar to 5 sigma); for the remainder of the sample we provide upper limits on the nuclear CO content. Adopting a standard Galactic I-CO-to-H-2 conversion factor implies molecular gas masses of (3.3-9.8) x 10(6) M-circle dot in the nuclear regions (inner 1.1-1.8 kpc) of the detected galaxies. Combining our new data with samples of late-type spirals from the literature, we find that CO-detected LSB spirals adhere to the same M-H2-far-infrared correlation as more luminous and higher surface brightness galaxies. The amount of CO in the central regions of late-type spirals appears to depend more strongly on mass than on central optical surface brightness, and CO detectability declines significantly for moderate to low surface brightness spirals with V-rot less than or similar to 90 km s(-1); no LSB spirals have so far been detected in CO below this threshold. Metallicity effects alone are unlikely to account for this trend, and we speculate that we are seeing the effects of a decrease in the mean fraction of a galaxy disk able to support giant molecular cloud formation with decreasing galaxy mass.