The Self-gravitating Gas Fraction and the Critical Density for Star Formation (original) (raw)
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The PdBI Arcsecond Whirlpool Survey (PAWS): The Role of Spiral Arms in Cloud and Star Formation
The Astrophysical Journal
The process that leads to the formation of the bright star forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment in the nearby grand-design spiral galaxy M 51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the (ionized, atomic, molecular, dusty) interstellar medium (ISM) with star formation tracers (HII regions, young < 10 Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs, (5) tentative evidence for star formation
An Updated View of Giant Molecular Clouds, Gas Flows and Star Formation in M51 with PAWS
Proceedings of the International Astronomical Union, 2012
We present an overview of the latest results from the PdBI Arcsecond Whirlpool Survey (PAWS, PI: E. Schinnerer), which has mapped CO(1-0) emission in the nearby granddesign spiral galaxy M51 at 40pc resolution. Our data are sensitive to GMCs above 10 5 M , allowing the construction of the largest GMC catalog to date-containing over 1500 objectsusing the CPROPS algorithm (Rosolowsky & Leroy 2006). In the inner disk of M51, the properties of the CO emission show significant variation that can be linked to the dynamical environment in which the molecular gas is located. We find that dynamically distinct regions host clouds with different properties and exhibit different GMC mass spectra, as well as distinct patterns of star formation. To understand how this sensitivity to environment emerges, we consider the role of pressure on GMC stabilization (including shear and star formation feedback-driven turbulence). We suggest that, in the presence of significant external pressure, streaming motions driven by the spiral arm can act to reduce the surface pressure on clouds. The resulting stabilization impacts the global pattern of star formation and can account for the observed nonmonotonic radial dependence of the gas depletion time. Our findings have implications for the observed scatter in the standard GMC relations and extragalactic star formation laws.
2014
We present an analytical model of the relation between the surface density of gas and star formation rate in galaxies and clouds, as a function of the presence of supersonic turbulence and the associated structure of the interstellar medium. The model predicts a power-law relation of index 3/2, flattened under the effects of stellar feedback at high densities or in very turbulent media, and a break at low surface densities when ISM turbulence becomes too weak to induce strong compression. This model explains the diversity of star formation laws and thresholds observed in nearby spirals and their resolved regions, the Small Magellanic Cloud, high-redshift disks and starbursting mergers, as well as Galactic molecular clouds. While other models have proposed interstellar dust content and molecule formation to be key ingredients to the observed variations of the star formation efficiency, we demonstrate instead that these variations can be explained by interstellar medium turbulence and structure in various types of galaxies.
SHORT GMC LIFETIMES: AN OBSERVATIONAL ESTIMATE WITH THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS)
The Astrophysical Journal, 2015
We describe and execute a novel approach to observationally estimate the lifetimes of giant molecular clouds (GMCs). We focus on the cloud population in the zone between the two main spiral arms in M51, i.e. the inter-arm region, where cloud destruction via shear and star formation feedback dominates over cloud formation processes. By monitoring the change in GMC number densities and ensemble properties from one side of the inter-arm to the other, we estimate the cloud lifetime as a fraction of the inter-arm travel time. We find that cloud lifetimes in M51's inter-arm are finite and short, i.e. 20 to 30 Myr. Such short lifetimes suggest that cloud evolution is influenced by environment, in which processes are sufficient to disrupt GMCs after a few free-fall times. Over most of the region under investigation shear appears to regulate cloud lifetimes. As the shear timescale increases with galactocentric radius, we expect cloud destruction to switch primarily to star formation feedback at larger radii. We identify a transition from sheardominated to star formation feedback-dominated cloud disruption through a change in the behavior of the inter-arm GMC number density. The signature suggests that shear is more efficient at completely dispersing clouds, whereas star formation feedback tends to transform the cloud population, e.g. by fragmenting high mass clouds into lower mass pieces. Compared to the characteristic timescale for molecular hydrogen in M51, our short cloud lifetime measurements suggest that gas can remain molecular while clouds disperse and reassemble. We propose that galaxy dynamics regulates the cycling of molecular material from diffuse to bound-and ultimately star-forming-objects, and hence contributes to long observed molecular gas depletion times in normal disk galaxies. We also speculate that, in more extreme environments such as elliptical galaxies and concentrated galaxy centers, star formation can be suppressed when the shear timescale becomes so short that some clouds never have the opportunity to collapse and form stars.
ORION2: A magnetohydrodynamics code for star formation
Journal of Open Source Software, 2021
The formation of stars and stellar clusters remains a grand challenge problem in astrophysics that has important implications for the evolution of the interstellar medium as well as shaping the evolution of galaxies. The computational challenges are formidable and involve a coupling of highly non-linear physical processes such as hydrodynamics, self-gravity, magnetic fields, radiation transfer, supersonic turbulence, ionization, protostellar outflows, stellar winds and chemistry that have both disparate timescales as well as operate over many decades of physical length scale. These processes can regulate the feedback from nascent protostars onto the surrounding turbulent gas clouds that are the embryos of new star formation, and as a result, the feedback itself can influence the gaseous reservoir feeding newly formed protostars which in turn influence the star formation process.
IC 3418: STAR FORMATION IN A TURBULENT WAKE
The Astrophysical Journal, 2010
Galaxy Evolution Explorer observations of IC 3418, a low surface brightness galaxy in the Virgo Cluster, revealed a striking 17 kpc UV tail of bright knots and diffuse emission. Hα imaging confirms that star formation is ongoing in the tail. IC 3418 was likely recently ram pressure stripped on its first pass through Virgo. We suggest that star formation is occurring in molecular clouds that formed in IC 3418's turbulent stripped wake. Tides and ram pressure stripping (RPS) of molecular clouds are both disfavored as tail formation mechanisms. The tail is similar to the few other observed starforming tails, all of which likely formed during RPS. The tails' morphologies reflect the forces present during their formation and can be used to test for dynamical coupling between molecular and diffuse gas, thereby probing the origin of the star forming molecular gas.
Conference summary: triggered star formation in a turbulent ISM
Proceedings of the International Astronomical Union, 2006
While the overall star formation rate in a galaxy appears to depend primarily on the gas mass and density, with the timescale for conversion of gas into stars given by the dynamical time, turbulence and explosions are still important for the process of star formation because they control the birth correlations in space and time. Most star formation appears triggered by some specific process, whether it is a galactic spiral shock, the expansion of a superbubble, the compression of a bright-rimmed globule, or some seemingly random compressive event in a supersonically turbulent flow. These processes give space and time sequences for star birth that are well observed. Many examples were given at this conference. Shocks are the link between large-scale but weak galactic processes and small-scale but strong final collapses. The rate limiting step is on the largest scale, where the dynamical time is slowest. Both gravitational instabilities and pressurized triggering seem to work on the s...
Monthly Notices of the Royal Astronomical Society, 2024
We present a numerical study of the evolution of power-law tails (PLTs) in the (column-)density distributions (N-PDF, ρ-PDF) in contracting star-forming clumps in primordial gas, without and with some initial rotational and/or turbulent support. In all considered runs multiple PLTs emerge shortly after the formation of the first protostar. The first PL T (PL T 1) in the ρ-PDF is a stable feature with slope q = −1.3 which corresponds – under the condition of preserved spherical symmetry – to the outer envelope of the protostellar object with density profile ρ ∝ l−2 in the classical Larson–Penston collapse model, where l is the radius. The second PL T (PL T 2) in the ρ-PDF is stable in the pure-infall runs but fluctuates significantly in the runs with initial support against gravity as dozens of protostars form and their mutual tidal forces change the density structure. Its mean slope, <q_2> = −2, corresponds to a density profile of ρ ∝ l−3/2 which describes a core in free fall in the classical Larson–Penston collapse model or an attractor solution at scales with dominating protostellar gravity. PLT 1 and PLT 2 in the N-PDFs are generally consistent with the observational data of Galactic low-mass star-forming regions from Herschel data. In the runs with initial support against gravity a third PL T (PL T 3) in the ρ-PDFs appears simultaneously with or after the emergence of PLT 2. It is very shallow, with mean slope of <q_3> −1, and is associated with the formation of thin protostellar accretion discs.
The Astrophysical Journal, 2014
Using data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated the largest extragalactic giant molecular cloud (GMC) catalog to date, containing 1507 individual objects. GMCs in the inner M51 disk account for only 54% of the total 12 CO(1-0) luminosity of the survey, but on average they exhibit physical properties similar to Galactic GMCs. We do not find a strong correlation between the GMC size and velocity dispersion, and a simple virial analysis suggests that ∼30% of GMCs in M51 are unbound. We have analyzed the GMC properties within seven dynamically motivated galactic environments, finding that GMCs in the spiral arms and in the central region are brighter and have higher velocity dispersions than inter-arm clouds. Globally, the GMC mass distribution does not follow a simple power-law shape. Instead, we find that the shape of the mass distribution varies with galactic environment: the distribution is steeper in inter-arm region than in the spiral arms, and exhibits a sharp truncation at high masses for the nuclear bar region. We propose that the observed environmental variations in the GMC properties and mass distributions are a consequence of the combined action of large-scale dynamical processes and feedback from high-mass star formation. We describe some challenges of using existing GMC identification techniques for decomposing the 12 CO(1-0) emission in molecule-rich environments, such as M51's inner disk.