Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions

Leroy, Adam K.; Usero, A.; Schruba, Andreas; Bigiel, Frank; Kruijssen, J. M. Diederik; Kepley, Amanda A.; Blanc, Guillermo A.; Bolatto, Alberto D.; Cormier, D.; Gallagher, Molly; Hughes, Annie; Jiménez-Donaire, María J.; Rosolowsky, Erik; Schinnerer, E.

doi:10.3847/1538-4357/835/2/217

Cited by 79 publications

(161 citation statements)

References 78 publications

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“…Observations of higher critical density tracers (e.g., HCN) provide information on the gas density distribution. While cloud-scale mapping of dense gas tracers remains a challenge even with ALMA, coarser (kpcscale) observations provide valuable insight too (e.g., Usero et al 2015;Bigiel et al 2016;Gallagher et al 2018), and can be further refined by modeling of the un-resolved ISM structure (Leroy et al 2017b). All these steps are goals of the PHANGS 2 collaboration.…”

Section: Discussionmentioning

confidence: 99%

How Galactic Environment Affects the Dynamical State of Molecular Clouds and Their Star Formation Efficiency

Schruba

Kruijssen

Leroy

2019

ApJ

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125

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We investigate how the dynamical state of molecular clouds relates to host galaxy environment, and how this impacts the star formation efficiency in the Milky Way and seven nearby galaxies. We compile measurements of molecular cloud and host galaxy properties and determine mass-weighted mean cloud properties for entire galaxies and distinct subregions within. We find molecular clouds to be in ambient pressure-balanced virial equilibrium, where clouds in gas-rich, molecular-dominated, high-pressure regions are close to self-virialization, whereas clouds in gas-poor, atomic-dominated, low-pressure environments achieve a balance between their internal kinetic pressure and external pressure from the ambient medium. The star formation efficiency per free-fall time of molecular clouds is low ∼0.1%−1% and shows systematic variations of 2 dex as a function of the virial parameter and host galactic environment. The trend observed for clouds in low-pressure environments-as the solar neighborhood-is well matched by state-of-the-art turbulence-regulated models of star formation. However, these models substantially overpredict the low observed star formation efficiencies of clouds in high-pressure environments, which suggests the importance of additional physical parameters not yet considered by these models.

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Section: Discussionmentioning

confidence: 99%

How Galactic Environment Affects the Dynamical State of Molecular Clouds and Their Star Formation Efficiency

Schruba

Kruijssen

Leroy

2019

ApJ

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125

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“…Schuster et al 2007;Leroy et al 2009;Bolatto et al 2013;Sandstrom et al 2013). The effective critical density for exciting CO(2-1) is higher than for CO(1-0) (∼ 10 3 cm −3 and ∼ 10 2 cm −3 , respectively; Leroy et al 2017a), implying that this tracer is less affected by optical depth. In addition, the mapping of CO(2-1) at a given resolution with ALMA is more efficient than for CO(1-0), which makes it a commonly observed transition for extragalactic studies of molecular gas and the tracer of choice in the PHANGS-ALMA survey (A. K. Leroy et al in prep.).…”

Section: Molecular Gas Tracermentioning

confidence: 92%

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

Chevance

Kruijssen

Hygate

et al. 2019

Monthly Notices of the Royal Astronomical Society

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250

300

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It remains a major challenge to derive a theory of cloud-scale ( 100 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-Hα 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 Myr, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities Σ H 2 8 M pc −2 , the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at Σ H 2 8 M 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 Hα (75−90 per cent of the cloud lifetime), GMCs disperse within just 1−5 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 HII regions are the fundamental units undergoing these lifecycles, with mean separations of 100−300 pc in star-forming discs. Future work should characterise the multi-scale physics and mass flows driving these lifecycles.

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“…On the other hand, the high-J lines might still trace the "density contrast" properly thus they are good tracers of the molecular gas undergoing star formation. Leroy et al (2017) show that the emissivity of the highdensity tracers (J = 1 − 0) depends strongly on the density distribution of the gas, and the corresponding line ratios can reflect the change in the gas-density structure. Therefore the difference between Fig.…”

Section: The Relationships Between Star-formation Efficiency Dense-gmentioning

confidence: 95%

The MALATANG survey: dense gas and star formation from high-transition HCN and HCO+ maps of NGC 253

Jiang

Greve

Gao

et al. 2020

Monthly Notices of the Royal Astronomical Society

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To study the high-transition dense-gas tracers and their relationships to the star formation of the inner ∼ 2 kpc circumnuclear region of NGC 253, we present HCN J = 4 − 3 and HCO + J = 4 − 3 maps obtained with the James Clerk Maxwell Telescope (JCMT). With the spatially resolved data, we compute the concentration indices r 90 /r 50 for the different tracers. HCN and HCO + 4-3 emission features tend to be centrally concentrated, which is in contrast to the shallower distribution of CO 1-0 and the stellar component. The dense-gas fraction ( f dense , traced by the velocity-integratedintensity ratios of HCN/CO and HCO + /CO) and the ratio R 31 (CO 3-2/1-0) decline towards larger galactocentric distances, but increase with higher SFR surface density. The radial variation and the large scatter of f dense and R 31 imply distinct physical conditions in different regions of the galactic disc. The relationships of f dense versus Σ stellar , and SFE dense versus Σ stellar are explored. SFE dense increases with higher Σ stellar in this galaxy, which is inconsistent with previous work that used HCN 1-0 data. This implies that existing stellar components might have different effects on the high-J HCN and HCO + than their low-J emission. We also find that SFE dense seems to be decreasing with higher f dense which is consistent with previous works, and it suggests that the ability of the dense gas to form stars diminishes when the average density of the gas increases. This is expected in a scenario where only the regions with high-density contrast collapse and form stars.

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Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions

Cited by 79 publications

References 78 publications

How Galactic Environment Affects the Dynamical State of Molecular Clouds and Their Star Formation Efficiency

How Galactic Environment Affects the Dynamical State of Molecular Clouds and Their Star Formation Efficiency

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

The MALATANG survey: dense gas and star formation from high-transition HCN and HCO+ maps of NGC 253

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