Spiral-arm instability – II. Magnetic destabilization

Inoue, Shigeki; Yoshida, Naoki

doi:10.1093/mnras/stz584

Cited by 19 publications

(8 citation statements)

References 101 publications

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“…29,122,124 Another explanation is that the presence of a magnetic field may actively promote the destabilisation of the gas by opposing the Coriolis force, transferring angular momentum away from growing condensations. 29,122,125 Whatever the source of the reduced shear, so long as it is efficiently reduced (cf. Ref.…”

Section: Origins Of the Periodic Density Fluctuationsmentioning

confidence: 99%

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

et al. 2020

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The density structure of the interstellar medium (ISM) determines where stars form and release energy, momentum, and heavy elements, driving galaxy evolution. [1][2][3][4] Density variations are seeded and amplified by gas motion, but the exact nature of this motion is unknown across spatial scale and galactic environment. 5 Although dense star-forming gas likely emerges from a combination of instabilities, 6, 7 convergent flows, 8 and turbulence, 9 establishing the precise origin is challenging because it requires quantifying gas motion over many orders of magnitude in spatial scale. Here we measure [10][11][12] the motion of molecular gas in the Milky Way and in nearby galaxy NGC 4321, assembling observations that span an unprecedented spatial dynamic range (10 −1 −10 3 pc). We detect ubiquitous velocity fluctuations across all spatial scales and galactic environments. Statistical analysis of these fluctuations indicates how star-forming gas is assembled. We discover oscillatory gas flows with wavelengths ranging from 0.3−400 pc. These flows are coupled to regularly-spaced density enhancements that likely form via gravitational instabilities. 13,14 We also identify stochastic and scale-free velocity and density fluctuations, consistent with the structure generated in turbulent flows. 9 Our results demonstrate that ISM structure cannot be considered in isolation. Instead, its formation and evolution is controlled by nested, interdependent flows of matter covering many orders of magnitude in spatial scale.We use observations that trace molecular gas in a variety of galactic environments and span a wide range of spatial scales. We measure the position-position-velocity (p-p-v) structure of the molecular ISM from 0.1 pc scales, relevant for individual star-forming cores, up to the scales of individual giant molecular clouds (GMCs), now accessible in nearby galaxies using facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA). On large (from 100 pc to >1000 pc) scales, we analyse observations of nearby galaxy NGC 4321 from the Physics at High Angular resolution in Nearby GalaxieS (PHANGS-ALMA) survey. At intermediate (from 1 pc to 100 pc) scales, we target both the Galactic disc and the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way with data from the Galactic Ring Survey 15 and the Mopra CMZ survey, 16 respectively. On small scales (from 0.1 pc to around 10 pc) we include observations of two dense molecular clouds, G035.39-00.33 17 in the Galactic disc and G0.253+0.016 11 in the CMZ.We extract the kinematics of the gas using spectral decomposition, modelling each spectrum as a set of individual Gaussian emission features. [10][11][12] Spectral decomposition is advantageous because it yields a description of all prominent emission features observed in spectroscopic data. We visualise the results using the peak intensities and velocity centroids of the modelled emission features (Figure 1). This method facilitates the detection of small fluctuations in velocity, which can o...

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Section: Origins Of the Periodic Density Fluctuationsmentioning

confidence: 99%

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

et al. 2020

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“…Including thermal (magnetic) pressure can significantly increase min(S tot ) if its sound (Alfvén) velocity is comparable to the kinetic velocity dispersion. Meanwhile, effects of magnetic fields are complicated; toroidal fields can destabilise spiral arms by cancelling Coriolis force (Elmegreen 1987(Elmegreen , 1994Inoue & Yoshida 2019a). However, molecular gas is generally cold therefore does not have a such high sound velocity.…”

Section: Other Uncertainties and Limitationsmentioning

confidence: 99%

Instability analysis for spiral arms of local galaxies: M51, NGC3627 and NGC628

Inoue,

Takagi,

Miyazaki

et al. 2021

Preprint

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We investigate dynamical states of grand-design spiral arms in three local galaxies: M51, NGC3627 and NGC628. Based on linear perturbation analysis considering multiple components in the galaxies, we compute instability parameters of the spiral arms using their observational data and argue whether the arms will fragment by their self-gravity. Our analysis utilises observations of carbon monoxide (CO), 21-centimetre line emission and multi-band photometric images for molecular gas, atomic gas and stellar components in the arms, respectively. We find that the grand-design arms of these galaxies indicate marginally stable states, and hence they are not on the way to fragment. We consider this to be consistent with the commonness of spiral galaxies and the relative rarity of fragmented discs at low redshifts. In the analysis, molecular gas is the dominant component to determine the (in)stability of the arms, whereas atomic gas and stars are far less important. Therefore, the results of our analysis are sensitive to an assumed CO-to-H 2 conversion factor. If we assume a typical scatter of the measurements and admit nearly twice as large a conversion factor as our fiducial value, our analysis results in predicting the instability for the spiral arms. More sophisticated determination of the conversion factor is required for more accurate analysis for the (in)stability of spiral arms.

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“…In this Letter, we perform simulations of CRGs in Section 2 and apply our linear perturbation analysis presented in Section 3 to the simulation results. Our analysis has been successfully applied to the (in)stability of spiral arms (Inoue & Yoshida 2018, 2019, 2020, and it is also well-suited for the dynamical analysis of ring structures. We demonstrate that our analysis can characterise the dynamical (in)stability of the rings in our simulations.…”

Section: Introductionmentioning

confidence: 99%

Fragmentation of ring galaxies and transformation to clumpy galaxies

Inoue,

Yoshida,

Hernquist

2021

Preprint

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We study the fragmentation of collisional ring galaxies (CRGs) using our linear perturbation analysis that computes the physical conditions of gravitational instability. We adopt the analysis to our CRG simulations, and show that our analysis accurately characterises the stability and the onset of fragmentation that is determined by the balance of self-gravity against pressure and Coriolis forces. In addition, since the orthodox 'density-wave' model is inapplicable to such self-gravitating rings, we devise a simple model that describes the rings propagating as material waves. We find that the toy model can predict the behaviour of the rings in our simulations. We also apply our instability analysis to a CRG discovered at a high redshift z = 2.19. We find that a quite high velocity dispersion is required for the stability of the ring, and therefore the CRG can be unstable against ring fragmentation. CRGs are rarely observed at high redshifts, and it may be because CRGs are usually too faint. Since the fragmentation can induce active star formation and make the ring bright enough to observe, the instability may explain the rarity. Fragmenting CRGs would evolve into clumpy galaxies with low surface densities in their inter-clump regions.

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Spiral-arm instability – II. Magnetic destabilization

Cited by 19 publications

References 101 publications

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

Instability analysis for spiral arms of local galaxies: M51, NGC3627 and NGC628

Fragmentation of ring galaxies and transformation to clumpy galaxies

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