scholarly journals Global dynamics of the interstellar medium in magnetized disc galaxies

2019 ◽  
Vol 489 (4) ◽  
pp. 5004-5021 ◽  
Author(s):  
Bastian Körtgen ◽  
Robi Banerjee ◽  
Ralph E Pudritz ◽  
Wolfram Schmidt
Keyword(s):  
Star Formation ◽  
Magnetic Fields ◽  
Interstellar Medium ◽  
Growth Time ◽  
Global Dynamics ◽  
Fragmentation Pattern ◽  
Giant Molecular Clouds ◽  
Stellar Feedback ◽  
Magnetohydrodynamic Simulations ◽  
Disc Galaxies

ABSTRACT Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impact on gas dynamics and star formation in galaxies remains controversial. We use a suite of global magnetohydrodynamic simulations of isolated disc galaxies to study the influence of magnetic fields on the diffuse and dense gas in the discs. We find that the magnetic field acts in multiple ways. Stronger magnetized discs fragment earlier due to the shorter growth time of the Parker instability. Due to the Parker instability in the magnetized discs, we also find cold ($T \lt 50\, \mathrm{K}$) and dense ($n\sim 10^3 {--}10^4\, \mathrm{cm}^{-3}$) gas several hundred pc above/below the mid-plane without any form of stellar feedback. In addition, magnetic fields change the fragmentation pattern. While in the hydrodynamic case, the disc breaks up into ring-like structures, magnetized discs show the formation of filamentary entities that extent both in the azimuthal and radial direction. These kpc scale filaments become magnetically (super-)critical very quickly and allow for the rapid formation of massive giant molecular clouds. Our simulations suggest that major differences in the behaviour of star formation – due to a varying magnetization – in galaxies could arise.

scholarly journals On the nature of variations in the measured star formation efficiency of molecular clouds

2019 ◽  
Vol 488 (2) ◽  
pp. 1501-1518 ◽  
Author(s):  
Michael Y Grudić ◽  
Philip F Hopkins ◽  
Eve J Lee ◽  
Norman Murray ◽  
Claude-André Faucher-Giguère ◽  
...  
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Main Sequence ◽  
Giant Molecular Clouds ◽  
Main Sequence Stars ◽  
Stellar Feedback ◽  
Magnetohydrodynamic Simulations ◽  
Formation Efficiency ◽  
The Relationship ◽  
Very High

Abstract Measurements of the star formation efficiency (SFE) of giant molecular clouds (GMCs) in the Milky Way generally show a large scatter, which could be intrinsic or observational. We use magnetohydrodynamic simulations of GMCs (including feedback) to forward-model the relationship between the true GMC SFE and observational proxies. We show that individual GMCs trace broad ranges of observed SFE throughout collapse, star formation, and disruption. Low measured SFEs (${\ll} 1\hbox{ per cent}$) are ‘real’ but correspond to early stages; the true ‘per-freefall’ SFE where most stars actually form can be much larger. Very high (${\gg} 10\hbox{ per cent}$) values are often artificially enhanced by rapid gas dispersal. Simulations including stellar feedback reproduce observed GMC-scale SFEs, but simulations without feedback produce 20× larger SFEs. Radiative feedback dominates among mechanisms simulated. An anticorrelation of SFE with cloud mass is shown to be an observational artefact. We also explore individual dense ‘clumps’ within GMCs and show that (with feedback) their bulk properties agree well with observations. Predicted SFEs within the dense clumps are ∼2× larger than observed, possibly indicating physics other than feedback from massive (main-sequence) stars is needed to regulate their collapse.

scholarly journals Effects of initial density profiles on massive star cluster formation in giant molecular clouds

2021 ◽  
Author(s):  
Yingtian Chen ◽  
Hui Li ◽  
Mark Vogelsberger
Keyword(s):  
Angular Momentum ◽  
Star Formation ◽  
Molecular Clouds ◽  
Globular Clusters ◽  
Cluster Formation ◽  
Initial Density ◽  
Giant Molecular Clouds ◽  
Density Profiles ◽  
Stellar Feedback ◽  
Gas Accretion

Abstract We perform a suite of hydrodynamic simulations to investigate how initial density profiles of giant molecular clouds (GMCs) affect their subsequent evolution. We find that the star formation duration and integrated star formation efficiency of the whole clouds are not sensitive to the choice of different profiles but are mainly controlled by the interplay between gravitational collapse and stellar feedback. Despite this similarity, GMCs with different profiles show dramatically different modes of star formation. For shallower profiles, GMCs first fragment into many self-gravitation cores and form sub-clusters that distributed throughout the entire clouds. These sub-clusters are later assembled ‘hierarchically’ to central clusters. In contrast, for steeper profiles, a massive cluster is quickly formed at the center of the cloud and then gradually grows its mass via gas accretion. Consequently, central clusters that emerged from clouds with shallower profiles are less massive and show less rotation than those with the steeper profiles. This is because 1) a significant fraction of mass and angular momentum in shallower profiles is stored in the orbital motion of the sub-clusters that are not able to merge into the central clusters 2) frequent hierarchical mergers in the shallower profiles lead to further losses of mass and angular momentum via violent relaxation and tidal disruption. Encouragingly, the degree of cluster rotations in steeper profiles is consistent with recent observations of young and intermediate-age clusters. We speculate that rotating globular clusters are likely formed via an ‘accretion’ mode from centrally-concentrated clouds in the early Universe.

scholarly journals Disruption of giant molecular clouds and formation of bound star clusters under the influence of momentum stellar feedback

2019 ◽  
Vol 487 (1) ◽  
pp. 364-380 ◽  
Author(s):  
Hui Li ◽  
Mark Vogelsberger ◽  
Federico Marinacci ◽  
Oleg Y Gnedin
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Star Clusters ◽  
Giant Molecular Clouds ◽  
Density Profiles ◽  
Stellar Feedback ◽  
Wide Range ◽  
Gas Removal ◽  
Formation Efficiency ◽  
Stellar Density

Abstract Energetic feedback from star clusters plays a pivotal role in shaping the dynamical evolution of giant molecular clouds (GMCs). To study the effects of stellar feedback on the star formation efficiency of the clouds and the dynamical response of embedded star clusters, we perform a suite of isolated GMC simulations with star formation and momentum feedback subgrid models using the moving-mesh hydrodynamics code Arepo. The properties of our simulated GMCs span a wide range of initial mass, radius, and velocity configurations. We find that the ratio of the final stellar mass to the total cloud mass, ϵint, scales strongly with the initial cloud surface density and momentum feedback strength. This correlation is explained by an analytic model that considers force balancing between gravity and momentum feedback. For all simulated GMCs, the stellar density profiles are systematically steeper than that of the gas at the epochs of the peaks of star formation, suggesting a centrally concentrated stellar distribution. We also find that star clusters are always in a sub-virial state with a virial parameter ∼0.6 prior to gas expulsion. Both the sub-virial dynamical state and steeper stellar density profiles prevent clusters from dispersal during the gas removal phase of their evolution. The final cluster bound fraction is a continuously increasing function of ϵint. GMCs with star formation efficiency smaller than 0.5 are still able to form clusters with large bound fractions.

scholarly journals The lifecycle of molecular clouds in nearby star-forming disc galaxies

2019 ◽  
Vol 493 (2) ◽  
pp. 2872-2909 ◽  
Author(s):  
Mélanie Chevance ◽  
J M Diederik Kruijssen ◽  
Alexander P S Hygate ◽  
Andreas Schruba ◽  
Steven N Longmore ◽  
...  
Keyword(s):  
Star Formation ◽  
Flux Ratio ◽  
Molecular Gas ◽  
H Ii Regions ◽  
Dynamical Processes ◽  
Nearby Star ◽  
Star Forming ◽  
Spatially Resolved ◽  
Stellar Feedback ◽  
Disc Galaxies

ABSTRACT It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically $10\!-\!30\,{\rm Myr}$, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $\Sigma _{\rm H_2}\ge 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $\Sigma _{\rm H_2}\le 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just $1\!-\!5\,{\rm Myr}$ once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H ii regions are the fundamental units undergoing these lifecycles, with mean separations of $100\!-\!300\,{{\rm pc}}$ in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.

scholarly journals Massive core/star formation triggered by cloud–cloud collision: Effect of magnetic field

2020 ◽  
Author(s):  
Nirmit Sakre ◽  
Asao Habe ◽  
Alex R Pettitt ◽  
Takashi Okamoto
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Strong Magnetic Field ◽  
Molecular Clouds ◽  
Weak Magnetic Field ◽  
Dense Cores ◽  
Cloud Models ◽  
Low Mass ◽  
Magnetohydrodynamic Simulations

Abstract We study the effect of magnetic field on massive dense core formation in colliding unequal molecular clouds by performing magnetohydrodynamic simulations with sub-parsec resolution (0.015 pc) that can resolve the molecular cores. Initial clouds with the typical gas density of the molecular clouds are immersed in various uniform magnetic fields. The turbulent magnetic fields in the clouds consistent with the observation by Crutcher et al. (2010, ApJ, 725, 466) are generated by the internal turbulent gas motion before the collision, if the uniform magnetic field strength is 4.0 μG. The collision speed of 10 km s−1 is adopted, which is much larger than the sound speeds and the Alfvén speeds of the clouds. We identify gas clumps with gas densities greater than 5 × 10−20 g cm−3 as the dense cores and trace them throughout the simulations to investigate their mass evolution and gravitational boundness. We show that a greater number of massive, gravitationally bound cores are formed in the strong magnetic field (4.0 μG) models than the weak magnetic field (0.1 μG) models. This is partly because the strong magnetic field suppresses the spatial shifts of the shocked layer that should be caused by the nonlinear thin shell instability. The spatial shifts promote the formation of low-mass dense cores in the weak magnetic field models. The strong magnetic fields also support low-mass dense cores against gravitational collapse. We show that the numbers of massive, gravitationally bound cores formed in the strong magnetic field models are much larger than in the isolated, non-colliding cloud models, which are simulated for comparison. We discuss the implications of our numerical results on massive star formation.

scholarly journals Star formation in galaxy mergers with realistic models of stellar feedback and the interstellar medium

2013 ◽  
Vol 430 (3) ◽  
pp. 1901-1927 ◽  
Author(s):  
Philip F. Hopkins ◽  
Thomas J. Cox ◽  
Lars Hernquist ◽  
Desika Narayanan ◽  
Christopher C. Hayward ◽  
...  
Keyword(s):  
Star Formation ◽  
Interstellar Medium ◽  
Galaxy Mergers ◽  
Stellar Feedback

scholarly journals The Role of Magnetic Fields in Star Formation

2013 ◽  
Vol 9 (S302) ◽  
pp. 10-20 ◽  
Author(s):  
Ralph E. Pudritz ◽  
Mikhail Klassen ◽  
Helen Kirk ◽  
Daniel Seifried ◽  
Robi Banerjee
Keyword(s):  
Angular Momentum ◽  
Star Formation ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Star Clusters ◽  
Young Stars ◽  
Giant Molecular Clouds ◽  
Jets And Outflows ◽  
Radiation Fields

AbstractStars are born in turbulent, magnetized filamentary molecular clouds, typically as members of star clusters. Several remarkable technical advances enable observations of magnetic structure and field strengths across many physical scales, from galactic scales on which giant molecular clouds (GMCs) are assembled, down to the surfaces of magnetized accreting young stars. These are shedding new light on the role of magnetic fields in star formation. Magnetic fields affect the gravitational fragmentation and formation of filamentary molecular clouds, the formation and fragmentation of magnetized disks, and finally to the shedding of excess angular momentum in jets and outflows from both the disks and young stars. Magnetic fields play a particularly important role in angular momentum transport on all of these scales. Numerical simulations have provided an important tool for tracking the complex process of the collapse and evolution of protostellar gas since several competing physical processes are at play - turbulence, gravity, MHD, and radiation fields. This paper focuses on the role of magnetic fields in three crucial regimes of star formation: the formation of star clusters emphasizing fragmentation, disk formation and the origin of early jets and outflows, to processes that control the spin evolution of young stars.

Formation of structure in star-forming clouds

1990 ◽  
Vol 68 (9) ◽  
pp. 808-823
Author(s):  
Ralph E. Pudritz
Keyword(s):  
Star Formation ◽  
Magnetic Fields ◽  
Interstellar Medium ◽  
Physical Properties ◽  
Molecular Clouds ◽  
Fundamental Theory ◽  
Wave Pressure ◽  
Star Forming ◽  
Effective Wave ◽  
Hydromagnetic Waves

Star formation occurs in massive, dense, molecular clouds in the interstellar medium. These clouds have a rich substructure consisting of dense clumps and extended filaments. Since stars only form within these dense clumps, any fundamental theory of star formation must predict their physical properties. This review focusses on the physics of molecular clouds and discusses in this context a particular mechanism for the formation of structure that is well supported by the observations. Strong hydromagnetic waves are likely to be excited in molecular clouds since it is observed that cloud magnetic fields have energy densities close to gravity. These waves support the cloud against global gravitational collapse by providing an effective wave "pressure". This review also shows that waves may control the formation of structure in molecular clouds.

scholarly journals Galactic star formation in parsec-scale resolution simulations

2010 ◽  
Vol 6 (S270) ◽  
pp. 487-490
Author(s):  
Leila C. Powell ◽  
Frederic Bournaud ◽  
Damien Chapon ◽  
Julien Devriendt ◽  
Adrianne Slyz ◽  
...  
Keyword(s):  
Star Formation ◽  
High Resolution ◽  
Interstellar Medium ◽  
Molecular Clouds ◽  
Giant Molecular Clouds ◽  
Gas Distribution ◽  
Computational Problem ◽  
Cold Gas

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.

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