scholarly journals Towards a multitracer timeline of star formation in the LMC – I. Deriving the lifetimes of H i clouds

2020 ◽  
Vol 497 (2) ◽  
pp. 2286-2301 ◽  
Author(s):  
Jacob L Ward ◽  
Mélanie Chevance ◽  
J M Diederik Kruijssen ◽  
Alexander P S Hygate ◽  
Andreas Schruba ◽  
...  
Keyword(s):  
Star Formation ◽  
Time Scales ◽  
Time Scale ◽  
Molecular Clouds ◽  
Molecular Form ◽  
Cloud Formation ◽  
H Ii Regions ◽  
Star Forming ◽  
Atomic Gas ◽  
Gas Cloud

ABSTRACT The time-scales associated with the various stages of the star formation process remain poorly constrained. This includes the earliest phases of star formation, during which molecular clouds condense out of the atomic interstellar medium. We present the first in a series of papers with the ultimate goal of compiling the first multitracer timeline of star formation, through a comprehensive set of evolutionary phases from atomic gas clouds to unembedded young stellar populations. In this paper, we present an empirical determination of the lifetime of atomic clouds using the Uncertainty Principle for Star Formation formalism, based on the de-correlation of H α and H i emission as a function of spatial scale. We find an atomic gas cloud lifetime of 48$^{+13}_{-8}$ Myr. This time-scale is consistent with the predicted average atomic cloud lifetime in the LMC (based on galactic dynamics) that is dominated by the gravitational collapse of the mid-plane ISM. We also determine the overlap time-scale for which both H i and H α emissions are present to be very short (tover < 1.7 Myr), consistent with zero, indicating that there is a near-to-complete phase change of the gas to a molecular form in an intermediary stage between H i clouds and H ii regions. We utilize the time-scales derived in this work to place empirically determined limits on the time-scale of molecular cloud formation. By performing the same analysis with and without the 30 Doradus region included, we find that the most extreme star-forming environment in the LMC has little effect on the measured average atomic gas cloud lifetime. By measuring the lifetime of the atomic gas clouds, we place strong constraints on the physics that drives the formation of molecular clouds and establish a solid foundation for the development of a multitracer timeline of star formation in the LMC.

scholarly journals Stochastic modelling of star-formation histories II: star-formation variability from molecular clouds and gas inflow

2020 ◽  
Vol 497 (1) ◽  
pp. 698-725 ◽  
Author(s):  
Sandro Tacchella ◽  
John C Forbes ◽  
Neven Caplar
Keyword(s):  
Life Cycle ◽  
Star Formation ◽  
Time Scales ◽  
Time Scale ◽  
Molecular Clouds ◽  
Star Formation History ◽  
Small Scale ◽  
Inflow Rate ◽  
Wide Range ◽  
The Galaxy

ABSTRACT A key uncertainty in galaxy evolution is the physics regulating star formation, ranging from small-scale processes related to the life-cycle of molecular clouds within galaxies to large-scale processes such as gas accretion on to galaxies. We study the imprint of such processes on the time-variability of star formation with an analytical approach tracking the gas mass of galaxies (‘regulator model’). Specifically, we quantify the strength of the fluctuation in the star-formation rate (SFR) on different time-scales, i.e. the power spectral density (PSD) of the star-formation history, and connect it to gas inflow and the life-cycle of molecular clouds. We show that in the general case the PSD of the SFR has three breaks, corresponding to the correlation time of the inflow rate, the equilibrium time-scale of the gas reservoir of the galaxy, and the average lifetime of individual molecular clouds. On long and intermediate time-scales (relative to the dynamical time-scale of the galaxy), the PSD is typically set by the variability of the inflow rate and the interplay between outflows and gas depletion. On short time-scales, the PSD shows an additional component related to the life-cycle of molecular clouds, which can be described by a damped random walk with a power-law slope of β ≈ 2 at high frequencies with a break near the average cloud lifetime. We discuss star-formation ‘burstiness’ in a wide range of galaxy regimes, study the evolution of galaxies about the main sequence ridgeline, and explore the applicability of our method for understanding the star-formation process on cloud-scale from galaxy-integrated measurements.

scholarly journals Deterministic Self-Propagating Star Formation

1989 ◽  
Vol 120 ◽  
pp. 518-523
Author(s):  
Jan Palouš
Keyword(s):  
Star Formation ◽  
Simple Model ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Large Scale ◽  
Galactic Evolution ◽  
Cloud Formation ◽  
Rotating Disks ◽  
Atomic Gas ◽  
High Column

AbstractThe evolution of large scale expanding structures in differentially rotating disks is studied. High column densities in some places may eventually lead to molecular cloud formation and initiate also star-formation. After some time, multi-structured arms evolve, where regions of intensive star-formation are separated from each other by regions of atomic gas or molecular clouds. This is due to the deterministic nature and to the coherence of this process. A simple model of galactic evolution is introduced and the different behaviour of Sa, Sb, and Sc galaxies is shown.

scholarly journals Molecular line ratio diagnostics along the radial cut and dusty ultraviolet-bright clumps in a spiral galaxy NGC 0628

2020 ◽  
Vol 495 (3) ◽  
pp. 2682-2712
Author(s):  
Selçuk Topal
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Line Ratio ◽  
H Ii Regions ◽  
Giant Molecular Clouds ◽  
Star Forming ◽  
The Galaxy ◽  
Star Formation In Galaxies ◽  
Molecular Lines ◽  
Co Gas

ABSTRACT Molecular emission lines are essential tools to shed light on many questions regarding star formation in galaxies. Multiple molecular lines are particularly useful to probe different phases of star-forming molecular clouds. In this study, we investigate the physical properties of giant molecular clouds (GMCs) using multiple lines of CO, i.e. CO(1–0, 2–1, 3–2) and 13CO(1–0), obtained at selected 20 positions in the disc of NGC 0628. A total of 11 positions were selected over the radial cut, including the centre, and remaining 9 positions were selected across the southern and northern arms of the galaxy. A total of 13 out of 20 positions are brighter at $24\, \mu {\rm m}$ and ultraviolet (UV) emission and hosting significantly more H ii regions compared to the rest of the positions indicating opposite characteristics. Our line ratio analysis shows that the gas gets warmer and thinner as a function of radius from the galaxy centre up to 1.7 kpc, and then the ratios start to fluctuate. Our empirical and model results suggest that the UV-bright positions have colder and thinner CO gas with higher hydrogen and CO column densities. However, the UV-dim positions have relatively warmer CO gas with lower densities bathed in GMCs surrounded by less number of H ii regions. Analysis of multiwavelength infrared and UV data indicates that the UV-bright positions have higher star formation efficiency than that of the UV-dim positions.

scholarly journals ALMA CO observations of a giant molecular cloud in M 33: Evidence for high-mass star formation triggered by cloud–cloud collisions

2020 ◽  
Author(s):  
Hidetoshi Sano ◽  
Kisetsu Tsuge ◽  
Kazuki Tokuda ◽  
Kazuyuki Muraoka ◽  
Kengo Tachihara ◽  
...  
Keyword(s):  
Star Formation ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Line Emission ◽  
High Mass ◽  
H Ii Regions ◽  
Mass Star ◽  
Star Forming ◽  
Spatially Resolved ◽  
Giant Molecular Cloud

Abstract We report the first evidence for high-mass star formation triggered by collisions of molecular clouds in M 33. Using the Atacama Large Millimeter/submillimeter Array, we spatially resolved filamentary structures of giant molecular cloud 37 in M 33 using 12CO(J = 2–1), 13CO(J = 2–1), and C18O(J = 2–1) line emission at a spatial resolution of ∼2 pc. There are two individual molecular clouds with a systematic velocity difference of ∼6 km s−1. Three continuum sources representing up to ∼10 high-mass stars with spectral types of B0V–O7.5V are embedded within the densest parts of molecular clouds bright in the C18O(J = 2–1) line emission. The two molecular clouds show a complementary spatial distribution with a spatial displacement of ∼6.2 pc, and show a V-shaped structure in the position–velocity diagram. These observational features traced by CO and its isotopes are consistent with those in high-mass star-forming regions created by cloud–cloud collisions in the Galactic and Magellanic Cloud H ii regions. Our new finding in M 33 indicates that cloud–cloud collision is a promising process for triggering high-mass star formation in the Local Group.

scholarly journals The time-scales probed by star formation rate indicators for realistic, bursty star formation histories from the FIRE simulations

2020 ◽  
Author(s):  
José A Flores Velázquez ◽  
Alexander B Gurvich ◽  
Claude-André Faucher-Giguére ◽  
James S Bullock ◽  
Tjitske K Starkenburg ◽  
...  
Keyword(s):  
Star Formation ◽  
Time Scales ◽  
Galaxy Formation ◽  
Time Scale ◽  
High Redshift ◽  
Formation Rate ◽  
Star Forming ◽  
Best Fitting ◽  
Far Ultraviolet ◽  
Star Formation Histories

Abstract Understanding the rate at which stars form is central to studies of galaxy formation. Observationally, the star formation rates (SFRs) of galaxies are measured using the luminosity in different frequency bands, often under the assumption of a time-steady SFR in the recent past. We use star formation histories (SFHs) extracted from cosmological simulations of star-forming galaxies from the FIRE project to analyze the time-scales to which the Hα and far-ultraviolet (FUV) continuum SFR indicators are sensitive. In these simulations, the SFRs are highly time variable for all galaxies at high redshift, and continue to be bursty to z = 0 in dwarf galaxies. When FIRE SFHs are partitioned into their bursty and time-steady phases, the best-fitting FUV time-scale fluctuates from its ∼10 Myr value when the SFR is time-steady to ≳100 Myr immediately following particularly extreme bursts of star formation during the bursty phase. On the other hand, the best-fitting averaging time-scale for Hα is generally insensitive to the SFR variability in the FIRE simulations and remains ∼ 5 Myr at all times. These time-scales are shorter than the 100 Myr and 10 Myr time-scales sometimes assumed in the literature for FUV and Hα, respectively, because while the FUV emission persists for stellar populations older than 100 Myr, the time-dependent luminosities are strongly dominated by younger stars. Our results confirm that the ratio of SFRs inferred using Hα vs. FUV can be used to probe the burstiness of star formation in galaxies.

scholarly journals An uncertainty principle for star formation – IV. On the nature and filtering of diffuse emission

2019 ◽  
Vol 488 (2) ◽  
pp. 2800-2824 ◽  
Author(s):  
Alexander P S Hygate ◽  
J M Diederik Kruijssen ◽  
Mélanie Chevance ◽  
Andreas Schruba ◽  
Daniel T Haydon ◽  
...  
Keyword(s):  
Star Formation ◽  
Compact Star ◽  
Critical Length ◽  
Electromagnetic Spectrum ◽  
H Ii Regions ◽  
Diffuse Emission ◽  
Star Forming ◽  
Star Forming Regions ◽  
Compact Component ◽  
Astronomical Images

Abstract Diffuse emission is observed in galaxies in many tracers across the electromagnetic spectrum, including tracers of star formation, such as H α and ultraviolet (UV), and tracers of gas mass, such as carbon monoxide (CO) transition lines and the 21-cm line of atomic hydrogen (H i). Its treatment is key to extracting meaningful information from observations such as cloud-scale star formation rates. Finally, studying diffuse emission can reveal information about the physical processes taking place in the interstellar medium, such as chemical transitions and the nature of stellar feedback (through the photon escape fraction). We present a physically motivated method for decomposing astronomical images containing both diffuse emission and compact regions of interest, such as H ii regions or molecular clouds, into diffuse and compact component images through filtering in Fourier space. We have previously presented a statistical method for constraining the evolutionary timeline of star formation and mean separation length between compact star-forming regions with galaxy-scale observations. We demonstrate how these measurements are biased by the presence of diffuse emission in tracer maps and that by using the mean separation length as a critical length-scale to separate diffuse emission from compact emission, we are able to remove its biasing effect. Furthermore, this method provides, without the need for interferometry or ancillary spectral data, a measurement of the diffuse emission fraction in input tracer maps and decomposed diffuse and compact emission maps for further analysis.

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 Giant Molecular Clouds in the Galaxy: The Massachusetts-Stony Brook CO Galactic Plane Survey

1987 ◽  
Vol 115 ◽  
pp. 499-499 ◽  
Author(s):  
P. M. Solomon
Keyword(s):  
Molecular Clouds ◽  
Galactic Plane ◽  
Far Infrared ◽  
Formation Mechanisms ◽  
H Ii Regions ◽  
Giant Molecular Clouds ◽  
Star Forming ◽  
The Galaxy ◽  
Two Populations ◽  
Spiral Arm

The CO Galactic Plane Survey consists of 40,572 spectral line observations in the region between 1 = 8° to 90° and b = −1°.05 to +1°.05 spaced every 3 arc minutes, carried out with the FCRAO 14-m antenna. The velocity coverage from −100 to +200 km/s includes emission from all galactic radii. This high resolution survey was designed to observe and identify essentially all molecular clouds or cloud components larger than 10 parsecs in the inner galaxy. There are two populations of molecular clouds which separate according to temperature. The warm clouds are closely associated with H II regions, exhibit a non-axisymmetric galactic distribution and are a spiral arm population. The cold clouds are a disk population, are not confined to any patterns in longitude-velocity space and must be widespread in the galaxy both in and out of spiral arms. The correlation between far infrared luminosities from IRAS, and molecular masses from CO is utilized to determine a luminosity to mass ratio for the clouds. A face-on picture of the galaxy locating the warm population is presented, showing ring like or spiral arm features at R ∼ 5, 7.5 and 9 kpc. The cloud size and mass spectrum will be discussed and evidence presented showing the presence of clusters of giant molecular clouds with masses of 106 to 107 M⊙. The two populations of clouds probably have different star forming luminosity functions. The implication of the two populations for star formation mechanisms will be discussed.

scholarly journals The SFR Efficiency of HI Gas in the Outskirts of Star Forming Galaxies

2016 ◽  
Vol 11 (S321) ◽  
pp. 360-362
Author(s):  
Marc Rafelski
Keyword(s):  
Star Formation ◽  
Atomic Hydrogen ◽  
Rest Frame ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Hydrogen Gas ◽  
Star Forming ◽  
Uv Emission ◽  
Atomic Gas

AbstractIn order to understand the origin of the decreased star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in Damped Lyα Systems (DLAs) at z ~ 3, we measure the SFR efficiency of atomic gas at z ~ 1, z ~ 2, and z ~ 3 around star-forming galaxies. We create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies’ outskirts. We find that the SFR efficiency of Hi gas is ~ 3% of that predicted by the KS relation. We find no significant evolution in the SFR efficiency with redshift, although simulations and models predict a decreasing SFR efficiency with decreasing metallicity and thus with increasing redshift. We discuss possible explanations for this decreased efficiency without an evolution with redshift.

scholarly journals A Guide to Comparisons of Star Formation Simulations with Observations

2010 ◽  
Vol 6 (S270) ◽  
pp. 511-519 ◽  
Author(s):  
Alyssa A. Goodman
Keyword(s):  
Star Formation ◽  
Spectral Line ◽  
Molecular Clouds ◽  
Direct Relationship ◽  
Statistical Tests ◽  
Random Processes ◽  
Mass Function ◽  
Star Forming ◽  
Star Forming Regions ◽  
Taste Testing

AbstractWe review an approach to observation-theory comparisons we call “Taste-Testing.” In this approach, synthetic observations are made of numerical simulations, and then both real and synthetic observations are “tasted” (compared) using a variety of statistical tests. We first lay out arguments for bringing theory to observational space rather than observations to theory space. Next, we explain that generating synthetic observations is only a step along the way to the quantitative, statistical, taste tests that offer the most insight. We offer a set of examples focused on polarimetry, scattering and emission by dust, and spectral-line mapping in star-forming regions. We conclude with a discussion of the connection between statistical tests used to date and the physics we seek to understand. In particular, we suggest that the “lognormal” nature of molecular clouds can be created by the interaction of many random processes, as can the lognormal nature of the IMF, so that the fact that both the “Clump Mass Function” (CMF) and IMF appear lognormal does not necessarily imply a direct relationship between them.

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