Tracing the Formation of Molecular Clouds in a Low-metallicity Galaxy: An H i Narrow Self-absorption Survey of the Large Magellanic Cloud

Liu, Boyang; Li, Di; Qian, Lei; Wong, Tony; Goldsmith, Paul F.

doi:10.3847/1538-4357/ab54cd

Cited by 7 publications

(5 citation statements)

References 72 publications

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“…This is significantly shorter than the derived molecular cloud lifetime (see Section 3.2 ) and suggests that the cloud-wide conversion from atomic to molecular gas is relatively rapid. This is in agreement with the small cold H I /CO ratio measured towards molecular clouds using H I narrow self-absorption in the LMC (Liu et al 2019 ), showing that molecular clouds are constituted of more than 99 per cent of molecular gas. Similar results are also observed for molecular clouds in the Milky Way (Li & Goldsmith 2003 ;Kr čo & Goldsmith 2010 ).…”

Section: The Formation Of Molecular Cloudssupporting

confidence: 90%

Towards a multitracer timeline of star formation in the LMC – II. The formation and destruction of molecular clouds

Ward

Kruijssen

Chevance

et al. 2022

Monthly Notices of the Royal Astronomical Society

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The time-scales associated with various stages of the star formation process represent major unknowns in our understanding of galactic evolution, as well as of star and planet formation. This is the second paper in a series aiming to establish a multi-tracer time-line of star formation in the Large Magellanic Cloud (LMC), focusing on the lifecycle of molecular clouds. We use a statistical method to determine a molecular cloud lifetime in the LMC of $t_{\text{CO}}=11.8^{+2.7}_{-2.2}$ Myr. This short time-scale is similar to the cloud dynamical time, and suggests that molecular clouds in the LMC are largely decoupled from the effects of galactic dynamics and have lifetimes set by internal processes. This provides a clear contrast to atomic clouds in the LMC, of which the lifetimes are correlated with galactic dynamical time-scales. We additionally derive the time-scale for which molecular clouds and H ii regions co-exist as $t_{\text{fb}}=1.2^{+0.3}_{-0.2}$ Myr, implying an average feedback front expansion velocity of 12 km s−1, consistent with expansion velocities of H ii regions in the LMC observed directly using optical spectroscopy. Taken together, these results imply that the molecular cloud lifecycle in the LMC proceeds rapidly and is regulated by internal dynamics and stellar feedback. We conclude by discussing our measurements in the context of previous work in the literature, which reported considerably longer lifetimes for molecular clouds in the LMC, and find that these previous findings resulted from a subjective choice in timeline calibration that is avoided by our statistical methodology.

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Section: The Formation Of Molecular Cloudssupporting

confidence: 90%

Towards a multitracer timeline of star formation in the LMC – II. The formation and destruction of molecular clouds

Ward

Kruijssen

Chevance

et al. 2022

Monthly Notices of the Royal Astronomical Society

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show abstract

“…This is significantly shorter than the derived molecular cloud lifetime (see Section 3.2) and suggests that the cloudwide conversion from atomic to molecular gas is relatively rapid. This is in agreement with the small cold H /CO ratio measured towards molecular clouds using H narrow self-absorption in the LMC (Liu et al 2019), showing that molecular clouds are constituted of more than 99 per cent of molecular gas. Similar results are also observed for molecular clouds in the Milky Way (Li & Goldsmith 2003;Krčo & Goldsmith 2010).…”

Section: H𝛼supporting

confidence: 90%

Towards a multi-tracer timeline of star formation in the LMC -- II. The formation and destruction of molecular clouds

Ward,

Kruijssen,

Chevance

et al. 2022

Preprint

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The time-scales associated with various stages of the star formation process represent major unknowns in our understanding of galactic evolution, as well as of star and planet formation. This is the second paper in a series aiming to establish a multi-tracer time-line of star formation in the Large Magellanic Cloud (LMC), focusing on the lifecycle of molecular clouds. We use a statistical method to determine a molecular cloud lifetime in the LMC of 𝑡 CO = 11.8 +2.7 −2.2 Myr. This short time-scale is similar to the cloud dynamical time, and suggests that molecular clouds in the LMC are largely decoupled from the effects of galactic dynamics and have lifetimes set by internal processes. This provides a clear contrast to atomic clouds in the LMC, of which the lifetimes are correlated with galactic dynamical time-scales. We additionally derive the time-scale for which molecular clouds and H regions co-exist as 𝑡 fb = 1.2 +0.3 −0.2 Myr, implying an average feedback front expansion velocity of 12 km s −1 , consistent with expansion velocities of H regions in the LMC observed directly using optical spectroscopy. Taken together, these results imply that the molecular cloud lifecycle in the LMC proceeds rapidly and is regulated by internal dynamics and stellar feedback. We conclude by discussing our measurements in the context of previous work in the literature, which reported considerably longer lifetimes for molecular clouds in the LMC, and find that these previous findings resulted from a subjective choice in timeline calibration that is avoided by our statistical methodology.

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“…In agreement with this, Koch et al (2019) find the velocity at peak H and CO brightness temperature to be typically consistent within the 2.6 km s −1 CO velocity channel size, rather than additional scatter or offsets in this relation if the CO peaked at a strong self-absorption feature. In qualitative agreement with these results, Liu et al (2019) use CO components to derive H self-absorption in the LMC and find relatively small corrections on 15 pc scales.…”

Section: Limitations Of the H Modelssupporting

confidence: 72%

“…As we note in §6.4, using constraints from other spectral lines can assist in resolving between this degeneracy by restricting where we search for signatures of opaque H . Additionally, these comparisons could lead to HISA detections (e.g., Liu et al 2019;Wang et al 2020b).…”

Section: Future Directions For Identifying Opaque H In Nearby Galaxiesmentioning

confidence: 99%

A lack of constraints on the cold opaque HI mass: HI spectra in M31 and M33 prefer multi-component models over a single cold opaque component

Koch,

Rosolowsky,

Leroy

et al. 2021

Preprint

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Previous work has argued that atomic gas mass estimates of galaxies from 21-cm H emission are systematically low due to a cold opaque atomic gas component. If true, this opaque component necessitates a ∼ 35% correction factor relative to the mass from assuming optically-thin H emission. These mass corrections are based on fitting H spectra with a single opaque component model that produces a distinct "top-hat" shaped line profile. Here, we investigate this issue using deep, high spectral resolution H VLA observations of M31 and M33 to test if these top-hat profiles are instead superpositions of multiple H components along the line-of-sight. We fit both models and find that > 80% of the spectra strongly prefer a multi-component Gaussian model while < 2% prefer the single opacity-corrected component model. This strong preference for multiple components argues against previous findings of lines-of-sight dominated by only cold H . Our findings are enabled by the improved spectral resolution (0.42 km s −1 ), whereas coarser spectral resolution blends multiple components together. We also show that the inferred opaque atomic ISM mass strongly depends on the goodness-of-fit definition and is highly uncertain when the inferred spin temperature has a large uncertainty. Finally, we find that the relation of the H surface density with the dust surface density and extinction has significantly more scatter when the inferred H opacity correction is applied. These variations are difficult to explain without additionally requiring large variations in the dust properties. Based on these findings, we suggest that the opaque H mass is best constrained by H absorption studies.

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Tracing the Formation of Molecular Clouds in a Low-metallicity Galaxy: An H i Narrow Self-absorption Survey of the Large Magellanic Cloud

Cited by 7 publications

References 72 publications

Towards a multitracer timeline of star formation in the LMC – II. The formation and destruction of molecular clouds

Towards a multitracer timeline of star formation in the LMC – II. The formation and destruction of molecular clouds

Towards a multi-tracer timeline of star formation in the LMC -- II. The formation and destruction of molecular clouds

A lack of constraints on the cold opaque HI mass: HI spectra in M31 and M33 prefer multi-component models over a single cold opaque component

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