Turbulent Velocity Dispersion Research Articles

Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

The interstellar medium in the Milky Way's Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near $z=0$ to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of $0.01 that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion of $ on scales of $20 in the CMZ ring. The MRI alone sustains a velocity dispersion of $ Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density as $B 102 \, (n/10^3 \ G $. Finally, we discuss the implications of our results within the observational context in the CMZ.

Interstellar Polarization Survey. IV. Characterizing the Magnetic Field Strength and Turbulent Dispersion Using Optical Starlight Polarization in the Diffuse Interstellar Medium

Angular dispersion functions are typically used to estimate the fluctuations in polarization angle around the mean magnetic field orientation in dense regions, such as molecular clouds. The technique provides accurate turbulent-to-regular magnetic field ratios, 〈Bt2〉1/2/Bpos , which are often underestimated by the classic Davis–Chandrasekhar–Fermi method. We assess the technique's suitability to characterize the turbulent and regular plane-of-sky magnetic field in low-density structures of the nearby interstellar medium (ISM), particularly when the turbulence outer scale, δ, is smaller than the smallest scale observed, ℓ min. We use optical polarization maps of three intermediate-latitude fields (∣b∣ ≳ 7.°5) with dimensions of 0.°3 × 0.°3, sourced from the Interstellar Polarization Survey–General ISM catalog. We decomposed the H i emission detected by the Galactic All-Sky Survey within our fields to estimate the multiphase ISM properties associated with the structure coupled to the magnetic field. We produced maps of the plane-of-sky magnetic field strength (B pos), mass density (ρ), and turbulent velocity dispersion (σ v,turb). In the regions with well-defined structures at d < 400 pc, the average B – ranges from ∼3 μG to ∼9 μ G, depending on the method and physical properties. In the region where structures extend up to 1000 pc, B pos varies from ∼1 to ∼3 μ G. The results agree with previous estimations in the local, diffuse ISM. Finally, optical starlight polarization and thermal dust polarization at 353 GHz consistently reveal a highly regular plane-of-sky magnetic field orientation unfazed by diffuse dust structures observed at 12 μm.

A thermal–kinetic subgrid model for supernova feedback in simulations of galaxy formation

ABSTRACT We present a subgrid model for supernova feedback designed for cosmological simulations of galaxy formation that may include a cold interstellar medium (ISM). The model uses thermal and kinetic channels of energy injection, which are built upon the stochastic kinetic and thermal models for stellar feedback used in the owls and eagle simulations, respectively. In the thermal channel, the energy is distributed statistically isotropically and injected stochastically in large amounts per event, which minimizes spurious radiative energy losses. In the kinetic channel, we inject the energy in small portions by kicking gas particles in pairs in opposite directions. The implementation of kinetic feedback is designed to conserve energy, linear and angular momentum, and is statistically isotropic. To test the model, we run simulations of isolated Milky Way-mass and dwarf galaxies, in which the gas is allowed to cool down to 10 K. Using the thermal and kinetic channels together, we obtain smooth star formation histories and powerful galactic winds with realistic mass loading factors. Furthermore, the model produces spatially resolved star formation rates (SFRs) and velocity dispersions that are in agreement with observations. We vary the numerical resolution by several orders of magnitude and find excellent convergence of the global SFRs and wind mass loading. We show that large thermal energy injections generate a hot phase of the ISM and modulate the star formation by ejecting gas from the disc, while the low-energy kicks increase the turbulent velocity dispersion in the neutral ISM, which in turn helps suppress star formation.

ThePlanckclusters in the LOFAR sky

Context.The footprint of the recent second data release of the LOFAR Two-metre Sky Survey (LoTSS-DR2) covers 309PlanckSunyaev-Zeldovich (SZ) selected galaxy clusters, 83 of which host a radio halo and 26 host a radio relic(s). It provides an excellent opportunity to statistically study the properties of extended cluster radio sources, especially their connection with merging activities.Aims.We quantify cluster dynamic states to investigate their relation with the occurrence of extended radio sources. We also search for connections between intracluster medium (ICM) turbulence and nonthermal characteristics of radio halos in the LoTSS-DR2.Methods.We analyzedXMM-NewtonandChandraarchival X-ray data of allPlanckSZ clusters in the footprint of LoTSS-DR2. We computed concentration parameters and centroid shifts that indicate the dynamic states of the clusters. We also performed a power spectral analysis of the X-ray surface brightness fluctuations to investigate large-scale density perturbations and estimate the turbulent velocity dispersion. Furthermore, we searched for the relation between radio halo power and the turbulent dissipation flux channeled to particle acceleration.Results.The concentration parameters measured by the two telescopes agree well, but the centroid shift has a larger scatter. The surface brightness power spectral analysis results in a large scatter of the surface brightness and density fluctuation amplitudes. We therefore only found a marginal anticorrelation between density fluctuations and cluster relaxation state, and we did not find a correlation between density fluctuations and radio halo power. Nevertheless, the injected power for particle acceleration calculated from turbulent dissipation is correlated with the radio halo power, where the best-fit unity slope supports the turbulent (re)acceleration scenario. Two different acceleration models, transit-time damping and adiabatic stochastic acceleration, cannot be distinguished due to the large scatter of the estimated turbulent Mach number. We introduced a new quantity [kBT ⋅ YX]rRH, which is proportional to the turbulent acceleration power assuming a constant Mach number. This quantity is strongly correlated with radio halo power, where the slope is also unity.

Deciphering the radio–star formation correlation on kpc scales

Given the multiple energy-loss mechanisms of cosmic-ray (CR) electrons in galaxies, the tightness of the infrared (IR)–radio continuum correlation is surprising. As the radio continuum emission at GHz frequencies is optically thin, this offers the opportunity to obtain unbiased star formation rates (SFRs) from radio-continuum flux-density measurements. The calorimeter theory can naturally explain the tightness of the far-infrared (FIR)–radio correlation but makes predictions that do not agree with observations. Noncalorimeter models often have to involve a conspiracy to maintain the tightness of the FIR–radio correlation. We extended a published analytical model of galactic disks by including a simplified prescription for the synchrotron emissivity. The galactic gas disks of local spiral galaxies, low-z starburst galaxies, high-z main sequence star-forming galaxies, and high-z starburst galaxies are treated as turbulent clumpy accretion disks. The magnetic field strength is determined by the equipartition between the turbulent kinetic and the magnetic energy densities. Our fiducial model, which includes neither galactic winds nor CR electron secondaries, reproduces the observed radio continuum spectral energy distributions of most (∼70%) of the galaxies. Except for the local spiral galaxies, fast galactic winds can potentially make the conflicting models agree with observations. The observed IR–radio correlations are reproduced by the model within 2σ of the joint uncertainty of model and data for all datasets. The model agrees with the observed SFR–radio correlations within ∼4σ. Energy equipartition between the CR particles and the magnetic field only approximately holds in our models of main sequence star-forming galaxies. If a CR electron calorimeter is assumed, the slope of the IR–radio correlation flattens significantly. Inverse Compton losses are not dominant in the starburst galaxies because in these galaxies not only the gas density but also the turbulent velocity dispersion is higher than in normal star-forming galaxies. Equipartition between the turbulent kinetic and magnetic field energy densities then leads to very high magnetic field strengths and very short synchrotron timescales. The exponents of our model SFR–radio correlations at 150 MHz and 1.4 GHz are very close to one.

On the scale height of the molecular gas disc in Milky Way-like galaxies

ABSTRACT We study the relationship between the scale height of the molecular gas disc and the turbulent velocity dispersion of the molecular interstellar medium within a simulation of a Milky Way-like galaxy in the moving-mesh code arepo. We find that the vertical distribution of molecular gas can be described by a Gaussian function with a uniform scale height of ∼50 pc. We investigate whether this scale height is consistent with a state of hydrostatic balance between gravity and turbulent pressure. We find that the hydrostatic prediction using the total turbulent velocity dispersion (as one would measure from kiloparsec-scale observations) gives an overestimate of the true molecular disc scale height. The hydrostatic prediction using the velocity dispersion between the centroids of discrete giant molecular clouds (cloud–cloud velocity dispersion) leads to more accurate estimates. The velocity dispersion internal to molecular clouds is elevated by the locally enhanced gravitational field. Our results suggest that observations of molecular gas need to reach the scale of individual molecular clouds in order to accurately determine the molecular disc scale height.

Turbulence generation by shock interaction with a highly nonuniform medium.

An initially planar shock wave propagating into a medium of nonuniform density will be perturbed, leading to the generation of postshock velocity perturbations. Using numerical simulations we study this phenomenon in the case of highly nonuniform density (order-unity normalized variance, σ_{ρ}/ρ[over ¯]∼1) and strong shocks (shock Mach numbers M[over ¯]_{s}≳10). This leads to a highly disrupted shock and a turbulent postshock flow. We simulate this interaction for a range of shock drives and initial density configurations meant to mimic those which might be presently achieved in experiments. Theoretical considerations lead to scaling relations, which are found to reasonably predict the postshock turbulence properties. The turbulent velocity dispersion and turbulent Mach number are found to depend on the preshock density dispersion and shock speed in a manner consistent with the linear Richtymer-Meshkov instability prediction. We also show a dependence of the turbulence generation on the scale of density perturbations. The postshock pressure and density, which can be substantially reduced relative to the unperturbed case, are found to be reasonably predicted by a simplified analysis that treats the extended shock transition region as a single normal shock.

Bursting Bubbles: Feedback from Clustered Supernovae and the Trade-off Between Turbulence and Outflows

We present an analytic model for clustered supernovae (SNe) feedback in galaxy disks, incorporating the dynamical evolution of superbubbles formed from spatially overlapping SNe remnants. We propose two realistic outcomes for the evolution of superbubbles in galactic disks: (1) the expansion velocity of the shock front falls below the turbulent velocity dispersion of the interstellar medium in the galaxy disk, whereupon the superbubble stalls and fragments, depositing its momentum entirely within the galaxy disk; or (2) the superbubble grows in size to reach the gas scale height, breaking out of the galaxy disk and driving galactic outflows/fountains. In either case, we find that superbubble breakup/breakout almost always occurs before the last Type II SN (≲40 Myr) in the recently formed star cluster, assuming a standard high-end initial mass function slope, and scalings between stellar lifetimes and masses. The threshold between these two cases implies a break in the effective strength of feedback in driving turbulence within galaxies, and a resulting change in the scalings of, for example, star formation rates with gas surface density (the Kennicutt–Schmidt relation) and the star formation efficiency in galaxy disks.

Chaotic and Clumpy Galaxy Formation in an Extremely Massive Reionization-era Halo

Abstract The SPT 0311–58 system at z = 6.900 is an extremely massive structure within the reionization epoch and offers a chance to understand the formation of galaxies at an extreme peak in the primordial density field. We present 70 mas Atacama Large Millimeter/submillimeter Array observations of the dust continuum and [C ii] 158 μm emission in the central pair of galaxies and reach physical resolutions of ∼100–350 pc, among the most detailed views of any reionization-era system to date. The observations resolve the source into at least a dozen kiloparsec-size clumps. The global kinematics and high turbulent velocity dispersion within the galaxies present a striking contrast to recent claims of dynamically cold thin-disk kinematics in some dusty galaxies just 800 Myr later at z ∼ 4. We speculate that both gravitational interactions and fragmentation from massive parent disks have likely played a role in the overall dynamics and formation of clumps in the system. Each clump individually is comparable in mass to other 6 &lt; z &lt; 8 galaxies identified in rest-UV/optical deep field surveys, but with star formation rates elevated by a factor of ~3-5. Internally, the clumps themselves bear close resemblance to greatly scaled-up versions of virialized cloud-scale structures identified in low-redshift galaxies. Our observations are qualitatively similar to the chaotic and clumpy assembly within massive halos seen in simulations of high-redshift galaxies.

A new method for measuring the 3D turbulent velocity dispersion of molecular clouds

ABSTRACT The structure and star formation activity of a molecular cloud are fundamentally linked to its internal turbulence. However, accurately measuring the turbulent velocity dispersion is challenging due to projection effects and observational limitations, such as telescope resolution, particularly for clouds that include non-turbulent motions, such as large-scale rotation. Here, we develop a new method to recover the 3D turbulent velocity dispersion (σv,3D) from position–position–velocity (PPV) data. We simulate a rotating, turbulent, collapsing molecular cloud, and compare its intrinsic σv,3D with three different measures of the velocity dispersion accessible in PPV space: (1) the spatial mean of the 2nd-moment map, σi, (2) the standard deviation of the gradient/rotation-corrected 1st-moment map, σ(c − grad), and (3) a combination of (1) and (2), called the ‘gradient-corrected parent velocity dispersion’, $\sigma _{\mathrm{(p}-\mathrm{grad)}}=(\sigma _{\mathrm{i}}^2+\sigma _{(\mathrm{c}-\mathrm{grad)}}^2)^{1/2}$. We show that the gradient correction is crucial in order to recover purely turbulent motions of the cloud, independent of the orientation of the cloud with respect to the line of sight. We find that with a suitable correction factor and appropriate filters applied to the moment maps, all three statistics can be used to recover σv,3D, with method 3 being the most robust and reliable. We determine the correction factor as a function of the telescope beam size for different levels of cloud rotation, and find that for a beam full width at half-maximum f and cloud radius R, the 3D turbulent velocity dispersion can best be recovered from the gradient-corrected parent velocity dispersion via $\sigma _{v,\mathrm{3D}}= \left[(-0.29\pm 0.26)\, f/R + 1.93 \pm 0.15\right] \sigma _{\mathrm{(p}-\mathrm{grad)}}$ for f/R &amp;lt; 1, independent of the level of cloud rotation or LOS orientation.

Turbulence in the intragroup and circumgalactic medium

Context. In massive objects, such as galaxy clusters, the turbulent velocity dispersion, σturb, is tightly correlated to both the object mass, M, and the thermal energy. Aims. Here, we investigate whether these scaling laws extend to lower-mass objects in dark-matter filaments. Methods. We perform a cosmological zoom-in simulation of a filament using an adaptive filtering technique for the resolved velocity component and a subgrid-scale model to account for the unresolved component. We then compute the mean turbulent and thermal energies for all halos in the zoom-in region and compare different definitions of halo averages. Averaging constrained by density and temperature thresholds is favored over averages solely based on virial spheres. Results. We find no clear trend for the turbulent velocity dispersion versus halo mass, but significant correlation and a scaling law with exponent α ∼ 0.5 between the turbulent velocity dispersion and thermal energy that agrees with a nearly constant turbulent Mach number, similar to more massive objects. Conclusions. We conclude that the self-similar energetics proposed for galaxy clusters extends down to the circumgalactic medium of individual galaxies.

The dense warm ionized medium in the inner Galaxy

Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for a widely distributed highly ionized warm interstellar gas with a density intermediate between the warm ionized medium (WIM) and compact H II regions suggests that there is a major gap in our understanding of the interstellar gas. Aims. Our goal is to investigate the properties of the dense WIM in the Milky Way using spectrally resolved SOFIA GREAT [N II] 205 μm fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrally unresolved Herschel PACS [N II] 122μm data, and spectrally resolved 12CO. Methods. We observed eight lines of sight (LOS) in the 20° &lt; l &lt; 30° region in the Galactic plane. We analyzed spectrally resolved lines of [N II] at 205 μm and RRL observations, along with the spectrally unresolved Herschel PACS 122 μm emission, using excitation and radiative transfer models to determine the physical parameters of the dense WIM. We derived the kinetic temperature, as well as the thermal and turbulent velocity dispersions from the [N II] and RRL linewidths. Results. The regions with [N II] 205 μm emission are characterized by electron densities, n(e) ~ 10−35 cm−3, temperatures range from 3400 to 8500 K, and nitrogen column densities N(N+) ~ 7 × 1016 to 3 × 1017 cm−2. The ionized hydrogen column densities range from 6 × 1020 to 1.7 × 1021 cm−2 and the fractional nitrogen ion abundance x(N+) ~ 1.1 × 10−4 to 3.0 × 10−4, implying an enhanced nitrogen abundance at a distance ~4.3 kpc from the Galactic Center. The [N II] 205 μm emission lines coincide with CO emission, although often with an offset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions, which themselves are associated with molecular gas. Conclusions. These dense ionized regions are found to contribute ≳50% of the observed [C II] intensity along these LOS. The kinetic temperatures we derive are too low to explain the presence of N+ resulting from electron collisional ionization and/or proton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation from nearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium.

Momentum feedback from marginally resolved H ii regions in isolated disc galaxies

ABSTRACT We present a novel, physically motivated sub-grid model for H ii region feedback within the moving mesh code arepo, accounting for both the radiation pressure-driven and thermal expansion of the ionized gas surrounding young stellar clusters. We apply this framework to isolated disc galaxy simulations with mass resolutions between 103 and 105 M⊙ per gas cell. Each simulation accounts for the self-gravity of the gas, the momentum and thermal energy from supernovae, the injection of mass by stellar winds, and the non-equilibrium chemistry of hydrogen, carbon, and oxygen. We reduce the resolution dependence of our model by grouping those H ii regions with overlapping ionization front radii. The Strömgren radii of the grouped H ii regions are at best marginally resolved, so that the injection of purely thermal energy within these radii has no effect on the interstellar medium. By contrast, the injection of momentum increases the fraction of cold and molecular gas by more than 50 per cent at mass resolutions of 103 M⊙, and decreases its turbulent velocity dispersion by ∼10 km s−1. The mass-loading of galactic outflows is decreased by an order of magnitude. The characteristic lifetime of the least-massive molecular clouds ($M/{\rm M}_\odot \lesssim 5.6 \times 10^4$) is reduced from ∼18 to $\lesssim 10$ Myr, indicating that H ii region feedback is effective in destroying these clouds. Conversely, the lifetimes of intermediate-mass clouds ($5.6 \times 10^4 \lesssim M/{\rm M}_\odot \lesssim 5 \times 10^5$) are elongated by ∼7 Myr, likely due to a reduction in supernova clustering. The derived cloud lifetimes span the range from 10 to 40 Myr, in agreement with observations. All results are independent of whether the momentum is injected from a ‘spherical’ or a ‘blister-type’ H ii region.

Low star formation efficiency due to turbulent adiabatic compression in the Taffy bridge

The Taffy system (UGC 12914/15) consists of two massive spiral galaxies that had a head-on collision about 20 Myr ago. It represents an ideal laboratory for studying the reaction of the interstellar medium (ISM) to a high-speed (∼1000 km s−1) gas-gas collision. New sensitive, high-resolution (2.7″ or ∼800 pc) CO(1−0) observations of the Taffy system with the IRAM Plateau de Bure Interferometer (PdBI) are presented. The total CO luminosity of the Taffy system detected with the PdBI is LCO, tot = 4.8 × 109 K km s−1 pc2, 60% of the CO luminosity found with the IRAM 30 m telescope. About 25% of the total interferometric CO luminosity stems from the bridge region. Assuming a Galactic N(H2)/ICO conversion factor for the galactic disks and a third of this value for the bridge gas, about 10% of the molecular gas mass is located in the bridge region. The giant H II region close to UGC 12915 is located at the northern edge of the high-surface-brightness giant molecular cloud association (GMA), which has the highest velocity dispersion among the bridge GMAs. The bridge GMAs are clearly not virialized because of their high velocity dispersion. Three dynamical models are presented and while no single model reproduces all of the observed features, they are all present in at least one of the models. Most of the bridge gas detected in CO does not form stars. We suggest that turbulent adiabatic compression is responsible for the exceptionally high velocity dispersion of the molecular ISM and the suppression of star formation in the Taffy bridge. In this scenario the turbulent velocity dispersion of the largest eddies and turbulent substructures or clouds increase such that giant molecular clouds are no longer in global virial equilibrium. The increase in the virial parameter leads to a decrease in the star formation efficiency. The suppression of star formation caused by turbulent adiabatic compression was implemented in the dynamical simulations and decreased the star formation rate in the bridge region by ∼90%. Most of the low-surface-density, CO-emitting gas will disperse without forming stars but some of the high-density gas will probably collapse and form dense star clusters, such as the luminous H II region close to UGC 12915. We suggest that globular clusters and super star clusters formed and still form through the gravitational collapse of gas previously compressed by turbulent adiabatic compression during galaxy interactions.

Turbulent Gas in Lensed Planck-selected Starbursts at z ∼ 1–3.5

Abstract Dusty star-forming galaxies at high redshift (1 &lt; z &lt; 3) represent the most intense star-forming regions in the universe. Key aspects to these processes are the gas heating and cooling mechanisms, and although it is well known that these galaxies are gas-rich, little is known about the gas excitation conditions. Only a few detailed radiative transfer studies have been carried out owing to a lack of multiple line detections per galaxy. Here we examine these processes in a sample of 24 strongly lensed star-forming galaxies identified by the Planck satellite (LPs) at z ∼ 1.1–3.5. We analyze 162 CO rotational transitions (ranging from J up = 1 to 12) and 37 atomic carbon fine-structure lines ([C i]) in order to characterize the physical conditions of the gas in the sample of LPs. We simultaneously fit the CO and [C i] lines and the dust continuum emission, using two different non-LTE, radiative transfer models. The first model represents a two-component gas density, while the second assumes a turbulence-driven lognormal gas density distribution. These LPs are among the most gas-rich, IR-luminous galaxies ever observed (μ L L IR ( 8 − 1000 μ m ) ∼ 10 13 − 14.6 L ⊙; 〈 μ L M ISM 〉 = (2.7 ± 1.2) × 1012 M ⊙, with μ L ∼ 10–30 the average lens magnification factor). Our results suggest that the turbulent interstellar medium present in the LPs can be well characterized by a high turbulent velocity dispersion ( 〈 ΔV turb 〉 ∼ 100 km s−1) and ratios of gas kinetic temperature to dust temperature 〈 T kin/T d 〉 ∼ 2.5, sustained on scales larger than a few kiloparsecs. We speculate that the average surface density of the molecular gas mass and IR luminosity, Σ M ISM ∼ 103–4 M ⊙ pc−2 and Σ L IR ∼ 1011–12 L ⊙ kpc−2, arise from both stellar mechanical feedback and a steady momentum injection from the accretion of intergalactic gas.

Nonadiabatic Turbulence Driving during Gravitational Collapse

Abstract We investigate the generation of turbulence during the prestellar gravitational contraction of a turbulent spherical core. We define the ratio g of the one-dimensional turbulent velocity dispersion to the gravitational velocity to then analytically estimate g under the assumptions of (a) equipartition or virial equilibrium between the gravitational ( ) and turbulent kinetic ( ) energies and (b) stationarity of transfer from gravitational to turbulent energy (implying cst). In the equipartition and virial cases, we find and , respectively; in the stationary case we find , where η is an efficiency factor, is the energy injection scale of the turbulence, and R is the core’s radius. Next, we perform AMR simulations of the prestellar collapse of an isothermal, transonic turbulent core at two different resolutions, and a nonturbulent control simulation. We find that the turbulent simulations collapse at the same rate as the nonturbulent one, so that the turbulence generation does not significantly slow down the collapse. We also find that (a) the simulations approach near balance between the rates of energy injection from the collapse and of turbulence dissipation; (b) , close to the “virial” value (turbulence is 30% ∼ 40% of nonthermal linewidth); (c) the injection scale is , and (d) the “turbulent pressure” scales as , an apparently nearly adiabatic scaling. We propose that this scaling and the nearly virial values of the turbulent velocity dispersion may be reconciled with the nondelayed collapse rate if the turbulence is dissipated as soon as it is generated.

Illuminating a tadpole’s metamorphosis III: quantifying past and present environmental impact

ABSTRACT We combine Multi-Unit Spectroscopic Explorer and Atacama Large Millimeter/sub-millimeter Array observations with theoretical models to evaluate how a tadpole-shaped globule located in the Carina Nebula has been influenced by its environment. This globule is now relatively small (radius ∼2500 au), hosts a protostellar jet+outflow (HH 900), and, with a blueshifted velocity of ∼10 km s−1, is travelling faster than it should be if its kinematics were set by the turbulent velocity dispersion of the precursor cloud. Its outer layers are currently still subject to heating, but comparing the internal and external pressures implies that the globule is in a post-collapse phase. Intriguingly the outflow is bent, implying that the Young Stellar Object (YSO) responsible for launching it is comoving with the globule, which requires that the star formed after the globule was up to speed since otherwise it would have been left behind. We conclude that the most likely scenario is one in which the cloud was much larger before being subject to radiatively driven implosion, which accelerated the globule to the high observed speeds under the photoevaporative rocket effect and triggered the formation of the star responsible for the outflow. The globule may now be in a quasi-steady state following collapse. Finally, the HH 900 YSO is likely ≳1 M⊙ and may be the only star forming in the globule. It may be that this process of triggered star formation has prevented the globule from fragmenting to form multiple stars (e.g. due to heating) and has produced a single higher mass star.

Forming early-type galaxies without AGN feedback: a combination of merger-driven outflows and inefficient star formation

ABSTRACT Regulating the available gas mass inside galaxies proceeds through a delicate balance between inflows and outflows, but also through the internal depletion of gas due to star formation. At the same time, stellar feedback is the internal engine that powers the strong outflows. Since star formation and stellar feedback are both small-scale phenomena, we need a realistic and predictive subgrid model for both. We describe the implementation of supernova momentum feedback and star formation based on the turbulence of the gas in the ramses code. For star formation, we adopt the so-called multifreefall model. The resulting star formation efficiencies can be significantly smaller or bigger than the traditionally chosen value of $1\, {\rm per\, cent}$. We apply these new numerical models to a prototype cosmological simulation of a massive halo that features a major merger which results in the formation of an early-type galaxy without using AGN feedback. We find that the feedback model provides the first-order mechanism for regulating the stellar and baryonic content in our simulated galaxy. At high redshift, the merger event pushes gas to large densities and large turbulent velocity dispersions, such that efficiencies come close to $10\, {\rm per\, cent}$, resulting in large star formation rate (SFR). We find small molecular gas depletion time during the starburst, in perfect agreement with observations. Furthermore, at late times, the galaxy becomes quiescent with efficiencies significantly smaller than $1\, {\rm per\, cent}$, resulting in small SFR and long molecular gas depletion time.

Solar Center–limb Variation of the Strengths of Spectral Lines: Classification and Interpretation of Observed Trends

The equivalent widths (W) of 565 spectral lines in the wavelength range of 4690--6870A were evaluated at 31 consecutive points from the solar disk center (mu=cos(theta)=1) to near the limb (mu=0.25) by applying the synthetic spectrum-fitting technique, in order to clarify the nature of their center--limb variations, especially the observed slope differing from line to line and its interpretation in terms of line properties. We found that the distribution of the gradient beta (= -dlog W/dlog mu) well correlates with that of dlog W/dlog T index, which means that the center-to-limb variation of W is determined mainly by the T-sensitivity of individual lines because the line-forming region shifts towards upper layers of lower T as we go toward the limb. Further, the key to understanding the behavior of dlog W/dlog T (depending on the temperature sensitivity of number population) is whether the considered species is in minor population stage or major population stage, by which the distribution of beta is explained in terms of differences in excitation potential and line strengths. All the center--limb data of equivalent widths (as well as line-of-sight turbulent velocity dispersions, elemental abundances, and mean line-formation depths derived as by-products) along with the solar spectra used for our analysis are made available as on-line materials.

Cosmic-ray propagation in the bi-stable interstellar medium

Context. Cosmic rays propagate through the galactic scales down to the smaller scales at which stars form. Cosmic rays are close to energy equipartition with the other components of the interstellar medium and can provide a support against gravity if pressure gradients develop. Aims. We study the propagation of cosmic rays within the turbulent and magnetised bi-stable interstellar gas. The conditions necessary for cosmic-ray trapping and cosmic-ray pressure gradient development are investigated. Methods. We derived an analytical value of the critical diffusion coefficient for cosmic-ray trapping within a turbulent medium, which follows the observed scaling relations. We then presented a numerical study using 3D simulations of the evolution of a mixture of interstellar gas and cosmic rays, in which turbulence is driven at varying scales by stochastic forcing within a box of 40 pc. We explored a large parameter space in which the cosmic-ray diffusion coefficient, the magnetisation, the driving scale, and the amplitude of the turbulence forcing, as well as the initial cosmic-ray energy density, vary. Results. We identify a clear transition in the interstellar dynamics for cosmic-ray diffusion coefficients below a critical value deduced from observed scaling relations. This critical diffusion depends on the characteristic length scale L of Dcrit ≃ 3.1 × 1023 cm2 s−1(L/1 pc)q+1, where the exponent q relates the turbulent velocity dispersion σ to the length scale as σ ~ Lq. Hence, in our simulations this transition occurs around Dcrit ≃ 1024–1025 cm2 s−1. The transition is recovered in all cases of our parameter study and is in very good agreement with our simple analytical estimate. In the trapped cosmic-ray regime, the induced cosmic-ray pressure gradients can modify the gas flow and provide a support against the thermal instability development. We discuss possible mechanisms that can significantly reduce the cosmic-ray diffusion coefficients within the interstellar medium. Conclusions. Cosmic-ray pressure gradients can develop and modify the evolution of thermally bi-stable gas for diffusion coefficients D0 ≤ 1025 cm2 s−1 or in regions where the cosmic-ray pressure exceeds the thermal one by more than a factor of ten. This study provides the basis for further works including more realistic cosmic-ray diffusion coefficients, as well as local cosmic-ray sources.