Turbulent Interstellar Medium Research Articles

Analysis of Scattering from Archival Pulsar Data using a CLEAN-based Method

In this work, we adopted a CLEAN-based method to determine the scatter time, τ, from archived pulsar profiles under both the thin screen and uniform medium scattering models and to calculate the scatter time frequency scale index α, where . The value of α is −4.4, if a Kolmogorov spectrum of the interstellar medium turbulence is assumed. We deconvolved 1342 profiles from 347 pulsars over a broad range of frequencies and dispersion measures. In our survey, in the majority of cases the scattering effect was not significant compared to pulse profile widths. For a subset of 21 pulsars scattering at the lowest frequencies was large enough to be measured. Because reliable scatter time measurements were determined only for the lowest frequency, we were limited to using upper limits on scatter times at higher frequencies for the purpose of our scatter time frequency slope estimation. We scaled the deconvolved scatter time to 1 GHz assuming and considered our results in the context of other observations which yielded a broad relation between scatter time and dispersion measure.

Prospects of Measuring the Angular Power Spectrum of the Diffuse Galactic Synchrotron Emission with SKA1 Low

The diffuse Galactic synchrotron emission (DGSE) is the most important diffuse foreground component for future cosmological 21-cm observations. The DGSE is also an important probe of the cosmic ray electron and magnetic field distributions in the turbulent interstellar medium (ISM) of our Galaxy. In this paper we briefly review the Tapered Gridded Estimator (TGE) which can be used to quantify the angular power spectrum of the sky signal directly from the visibilities measured in radio-interferometric observations. The salient features of the TGE are (1.) it deals with the gridded data which makes it computationally very fast (2.) it avoids a positive noise bias which normally arises from the system noise inherent to the visibility data, and (3.) it allows us to taper the sky response and thereby suppresses the contribution from unsubtracted point sources in the outer parts and the sidelobes of the antenna beam pattern. We also summarize earlier work where the TGE was used to measure the C_l of the DGSE using 150 MHz GMRT data. Earlier measurements of the angular power spectrum are restricted to smaller angular multipole l ~ 10^3 for the DGSE, the signal at the larger l values is dominated by the residual point sources after source subtraction. The higher sensitivity of the upcoming SKA1 Low will allow the point sources to be subtracted to a fainter level than possible with existing telescopes. We predict that it will be possible to measure the angular power spectrum of the DGSE to larger values of l with SKA1 Low. Our results show that it should be possible to achieve l_{max} ~ 10^4 and ~ 10^5 with 2 minutes and 10 hours of observations respectively.

How stellar feedback simultaneously regulates star formation and drives outflows

We present an analytic model for how momentum deposition from stellar feedback simultaneously regulates star formation and drives outflows in a turbulent interstellar medium (ISM). Because the ISM is turbulent, a given patch of ISM exhibits sub-patches with a range of surface densities. The high-density patches are 'pushed' by feedback, thereby driving turbulence and self-regulating local star formation. Sufficiently low-density patches, however, are accelerated to above the escape velocity before the region can self-adjust and are thus vented as outflows. In the turbulent-pressure-supported regime, when the gas fraction is $\gtrsim 0.3$, the ratio of the turbulent velocity dispersion to the circular velocity is sufficiently high that at any given time, of order half of the ISM has surface density less than the critical value and thus can be blown out on a dynamical time. The resulting outflows have a mass-loading factor ($\eta \equiv M_{\rm out}/M_{\star}$) that is inversely proportional to the gas fraction times the circular velocity. At low gas fractions, the star formation rate needed for local self-regulation, and corresponding turbulent Mach number, decline rapidly; the ISM is 'smoother', and it is actually more difficult to drive winds with large mass-loading factors. Crucially, our model predicts that stellar-feedback-driven outflows should be suppressed at $z \lesssim 1$ in $M_{\star} \gtrsim 10^{10} M_{\odot}$ galaxies. This mechanism allows massive galaxies to exhibit violent outflows at high redshifts and then 'shut down' those outflows at late times, thereby enabling the formation of a smooth, extended thin stellar disk. We provide simple fitting functions for $\eta$ that should be useful for sub-resolution and semi-analytic models. [abridged]

Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM

Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-ray absorption in excess of the Milky Way. The column densities of metals inferred from UV and optical spectra differ from those derived from soft X-ray spectra, at times by an order of magnitude, with the latter being higher. The origin of the soft X-ray absorption excess observed in GRB X-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous investigations on the effect of hot material both internal and external to the GRB host galaxy on our X-ray afterglow observations. Nevertheless, all models proposed so far have either only been able to account for a subset of our observations (i.e. at z > 2), or they have required fairly extreme conditions to be present within the absorbing material. In this paper, we investigate the absorption of the GRB afterglow by a collisionally ionised and turbulent interstellar medium (ISM). We find that a dense (3 per cubic centimeters) collisionally ionised ISM could produce UV/optical and soft X-ray absorbing column densities that differ by a factor of 10, however the UV/optical and soft X-ray absorbing column densities for such sightlines and are 2-3 orders of magnitude lower in comparison to the GRB afterglow spectra. For those GRBs with a larger soft X-ray excess of up to an order of magnitude, the contribution in absorption from a turbulent ISM as considered here would ease the required conditions of additional absorbing components, such as the GRB circumburst medium and intergalactic medium.

Probing interstellar turbulence in cirrus with deep optical imaging: no sign of energy dissipation at 0.01 pc scale

Diffuse Galactic light has been observed in the optical since the 1930s. We propose that, when observed in the optical with deep imaging surveys, it can be used as a tracer of the turbulent cascade in the diffuse interstellar medium (ISM), down to scales of about 1 arcsec. Here we present a power spectrum analysis of the dust column density of a diffuse cirrus at high Galactic latitude (l ~ 198 deg, b ~ 32 deg) as derived from the combination of a MegaCam g-band image, obtained as part of the MATLAS large programme at the CFHT, with Planck radiance and Wise 12 micron data. The combination of these three datasets have allowed us to compute the density power spectrum of the HI over scales of more than three orders of magnitude. We found that the density field is well described by a single power law over scales ranging from 0.01 to 50 pc. The exponent of the power spectrum, gamma=-2.9 +- 0.1, is compatible with what is expected for thermally bi-stable and turbulent HI. We did not find any steepening of the power spectrum at small scales indicating that the typical scale at which turbulent energy is dissipated in this medium is smaller than 0.01pc. The ambipolar diffusion scenario that is usually proposed as the main dissipative agent, is consistent with our data only if the density of the cloud observed is higher than the typical values assumed for the cold neutral medium gas. We discuss the new avenue offered by deep optical imaging surveys for the study of the low density ISM structure and turbulence.

Relativistic jet feedback in high-redshift galaxies – I. Dynamics

We present the results of three dimensional relativistic hydrodynamic simulations of interaction of AGN jets with a dense turbulent two-phase interstellar medium, which would be typical of high redshift galaxies. We describe the effect of the jet on the evolution of the density of the turbulent ISM. The jet driven energy bubble affects the gas to distances up to several kiloparsecs from the injection region. The shocks resulting from such interactions create a multi-phase ISM and radial outflows. One of the striking result of this work is that low power jets (P_jet < 10^{43} erg/s) although less efficient in accelerating clouds, are trapped in the ISM for a longer time and hence affect the ISM over a larger volume. Jets of higher power drill through with relative ease. Although the relativistic jets launch strong outflows, there is little net mass ejection to very large distances, supporting a galactic fountain scenario for local feedback.

SCINTILLATION ARCS IN LOW-FREQUENCY OBSERVATIONS OF THE TIMING-ARRAY MILLISECOND PULSAR PSR J0437–4715

ABSTRACT Low-frequency observations of pulsars provide a powerful means for probing the microstructure in the turbulent interstellar medium (ISM). Here we report on high-resolution dynamic spectral analysis of our observations of the timing-array millisecond pulsar PSR J0437–4715 with the Murchison Widefield Array (MWA), enabled by our recently commissioned tied-array beam processing pipeline for voltage data recorded from the high time resolution mode of the MWA. A secondary spectral analysis reveals faint parabolic arcs akin to those seen in high-frequency observations of pulsars with the Green Bank and Arecibo telescopes. Data from Parkes observations at a higher frequency of 732 MHz reveal a similar parabolic feature with a curvature that scales approximately as the square of the observing wavelength (λ 2) to the MWA's frequency of 192 MHz. Our analysis suggests that scattering toward PSR J0437–4715 predominantly arises from a compact region about 115 pc from the Earth, which matches well with the expected location of the edge of the Local Bubble that envelopes the local Solar neighborhood. As well as demonstrating new and improved pulsar science capabilities of the MWA, our analysis underscores the potential of low-frequency pulsar observations for gaining valuable insights into the local ISM and for characterizing the ISM toward timing-array pulsars.

THE SPATIAL DISTRIBUTION OF THE YOUNG STELLAR CLUSTERS IN THE STAR-FORMING GALAXY NGC 628

We present a study of the spatial distribution of the stellar cluster populations in the star forming galaxy NGC 628. Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young (<100 Myr) stellar clusters within the galaxy, identified from a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent $\alpha$. We recover a weighted mean index of $\alpha$ ~ -0.8 for all spatial scales below the break at 3".3 (158 pc at a distance of 9.9 Mpc) and an index of $\alpha$ ~ -0.18 above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.

Dynamo saturation in direct simulations of the multi‐phase turbulent interstellar medium

AbstractThe ordered magnetic field observed via polarised synchrotron emission in nearby disc galaxies can be explained by a mean‐field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical‐flux initial conditions are potentially able to influence this dynamo via the occurrence of the magnetorotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean‐field dynamo. This is motivated by the observation that different saturation behaviour was previously obtained for different supernova rates. We perform direct numerical simulations (DNS) of three‐dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing‐periodic boundary conditions horizontally. Unlike in our previous work, we also impose a vertical seed magnetic field. We run the simulations until the growth of the magnetic energy becomes negligible. We furthermore perform simulations of equivalent 1D dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. We compare the saturation of the magnetic field in the DNS with the algebraic quenching of a mean‐field dynamo. The final magnetic field strength found in the direct simulation is in excellent agreement with a quenched α) dynamo. For supernova rates representative of the Milky Way, field losses via a Galactic wind are likely responsible for saturation. We conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. We propose that a mean field approach with algebraic quenching may serve as a simple sub‐grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cosmic ray confinement and transport models for probing their putative sources

Recent efforts in cosmic ray (CR) confinement and transport theory are discussed. Three problems are addressed as being crucial for understanding the present day observations and their possible telltale signs of the CR origin. The first problem concerns CR behavior right after their release from a source, such as a supernova remnant. At this phase, the CRs are confined near the source by self-emitted Alfven waves. The second is the problem of diffusive propagation of CRs through the turbulent interstellar medium. This is a seemingly straightforward and long-resolved problem, but it remains controversial and reveals paradoxes. A resolution based on the Chapman-Enskog asymptotic CR transport analysis, that also includes magnetic focusing, is suggested. The third problem is about a puzzling sharp (∼10°) anisotropies in the CR arrival directions that might bear on important clues of their transport between the source and observer. The overarching goal is to improve our understanding of all aspects of the CR's source escape and ensuing propagation through the galaxy to the level at which their sources can be identified observationally.

The resolution bias: low-resolution feedback simulations are better at destroying galaxies

Feedback from super-massive black holes (SMBHs) is thought to play a key role in regulating the growth of host galaxies. Cosmological and galaxy formation simulations using smoothed particle hydrodynamics (SPH), which usually use a fixed mass for SPH particles, often employ the same sub-grid Active galactic nuclei (AGN) feedback prescription across a range of resolutions. It is thus important to ask how the impact of the simulated AGN feedback on a galaxy changes when only the numerical resolution (the SPH particle mass) changes. We present a suite of simulations modelling the interaction of an AGN outflow with the ambient turbulent and clumpy interstellar medium (ISM) in the inner part of the host galaxy at a range of mass resolutions. We find that, with other things being equal, degrading the resolution leads to feedback becoming more efficient at clearing out all gas in its path. For the simulations presented here, the difference in the mass of the gas ejected by AGN feedback varies by more than a factor of ten between our highest and lowest resolution simulations. This happens because feedback-resistant high density clumps are washed out at low effective resolutions. We also find that changes in numerical resolution lead to undesirable artifacts in how the AGN feedback affects the AGN immediate environment.

Properties of interstellar filaments as derived from Herschel, Planck, and molecular line observations

AbstractRecent Herschel and Planck observations of submillimeter dust emission revealed the omnipresence of filamentary structures in the interstellar medium (ISM). The ubiquity of filaments in quiescent clouds as well as in star-forming regions indicates that the formation of filamentary structures is a natural product of the physics at play in the magnetized turbulent cold ISM. An analysis of more than 270 filaments observed with Herschel in 8 regions of the Gould Belt, shows that interstellar filaments are characterized by a narrow distribution of central width sharply peaked at ~0.1 pc, while they span a wide column density range. Molecular line observations of a sample of these filaments show evidence of an increase in the velocity dispersion of dense filaments with column density, suggesting an evolution in mass per unit length due to accretion of surrounding material onto these star-forming filaments. The analyses of Planck dust polarization observations show that both the mean magnetic field and its fluctuations along the filaments are different from those of their surrounding clouds. This points to a coupling between the matter and the $\vec{B}$-field in the filament formation process. These observational results, derived from dust and gas tracers in total and polarized intensity, set strong constraints on our understanding of the formation and evolution of filaments in the ISM. They provide important clues on the initial conditions of the star formation process along interstellar filaments.

The density variance–Mach number relation in isothermal and non-isothermal adiabatic turbulence

The density variance { Mach number relation of the turbulent interstellar medium is relevant for theoretical models of the star formation rate, eciency, and the initial mass function of stars. Here we use high-resolution hydrodynamical simulations with grid resolutions of up to 1024 3 cells to model compressible turbulence in a regime similar to the observed interstellar medium. We use Fyris Alpha, a shock-capturing code employing a high-order Godunov scheme to track large density variations induced by shocks. We investigate the robustness of the standard relation between the logarithmic density variance ( 2 ) and the sonic Mach number (M) of isothermal interstellar turbulence, in the non-isothermal regime. Specically, we test ideal gases with diatomic molecular ( = 7=5) and monatomic ( = 5=3) adiabatic indices. A periodic cube of gas is stirred with purely solenoidal forcing at low wavenumbers, leading to a fullydeveloped turbulent medium. We nd that as the gas heats in adiabatic compressions, it evolves along the relationship in the density variance { Mach number plane, but deviates signicantly from the standard expression for isothermal gases. Our main

THERMAL INSTABILITY AND MULTI-PHASE INTERSTELLAR MEDIUM IN THE FIRST GALAXIES

We examine the linear stability and nonlinear growth of the thermal instability in isobarically contracting gas with various metallicities and FUV field strengths. When the H2 cooling is suppressed by FUV fields (G_0>10^-3) or the metallicity is high enough (Z/Zs>10^-3), the interstellar medium is thermally unstable in the temperature range 100-7000 K owing to the cooling by CII and OI fine-structure lines. In this case, a bi-phasic medium with a bimodal density probability distribution is formed as a consequence of the thermal instability. The characteristic scales of the thermal instability become smaller with increasing metallicity. Comparisons of the nonlinear simulations with different resolution indicates that the maximum scale of the thermal instability should be resolved with more than 60 cells to follow runaway cooling driven by the thermal instability. Under sufficiently weak FUV fields and with low metallcity, the density range of the thermal instability shrinks owing to dominance of H2 cooling. As the FUV intensity is reduced, bi-phasic structure becomes less remarkable and disappears eventually. Our basic results suggest that, in early galaxies, i) the thermal instability has little effect for the medium with Z/Zs<10^-4, ii) fragmentation by the thermal instability could determine mass spectrum of star clusters for 10^-4<Z/Zs<0.04, and iii) thermally bistable turbulent interstellar medium like our galaxy becomes ubiquitous for Z/Zs>0.04, although the threshold metallicity depends on the conditions such as thermal pressure, FUV strength and redshift.

Supernova feedback in an inhomogeneous interstellar medium

Supernova (SN) feedback is one of the key processes shaping the interstellar medium (ISM) of galaxies. SNe contribute to (and in some cases may dominate) driving turbulence in the ISM and accelerating galactic winds. Modern cosmological simulations have sufficient resolution to capture the main structures in the ISM of galaxies, but are typically still not capable of explicitly resolving all of the small-scale stellar feedback processes, including the expansion of supernova remnants (SNRs). We perform a series of controlled three-dimensional hydrodynamic (adaptive mesh refinement) simulations of single SNRs expanding in an inhomogeneous density field with statistics motivated by those of the turbulent ISM. We use these to quantify the momentum and thermal energy injection from SNe as a function of spatial scale and the density, metallicity, and structure of the ambient medium. We develop a series of analytic formulae that we fit to the simulations. These formulae can be used as a basis for a more predictive sub-resolution model for SN feedback for galaxy formation simulations. We then use simulations of multiple, stochastically driven SNe that resolve the key phases of SNRs to test the sub-resolution model, and show that it accurately captures the turbulent kinetic energy and thermal energy in the ISM. By contrast, proposed SN feedback models in the literature based on `delayed cooling' significantly overpredict the late-time thermal energy and momentum in SNRs.

PROBING THE PHYSICAL CONDITIONS OF ATOMIC GAS AT HIGH REDSHIFT

A new method is used to measure the physical conditions of the gas in damped Lyman-alpha systems (DLAs). Using high resolution absorption spectra of a sample of 80 DLAs, we are able to measure the ratio of the upper to lower fine-structure levels of the ground state of C II and Si II. These ratios are determined solely by the physical conditions of the gas. We explore the allowed physical parameter space using a Monte Carlo Markov Chain method to constrain simultaneously the temperature, neutral hydrogen density, and electron density of each DLA. The results indicate that at least 5 % of all DLAs have the bulk of their gas in a dense, cold phase with typical densities of ~100 cm-3 and temperatures below 500 K. We further find that the typical pressure of DLAs in our sample is log(P/k) = 3.4 [K cm-3], which is comparable to the pressure of the local interstellar medium (ISM), and that the components containing the bulk of the neutral gas can be quite small with absorption sizes as small as a few parsec. We show that the majority of the systems are consistent with having densities significantly higher than expected from a purely canonical WNM, indicating that significant quantities of dense gas (i.e. n_H > 0.1 cm-3) are required to match observations. Finally, we identify 8 systems with positive detections of Si II*. These systems have pressures (P/k) in excess of 20000 K cm-3, which suggest that these systems tag a highly turbulent ISM in young, star-forming galaxies.

Models of diffuse Hα in the interstellar medium: the relative contributions from in situ ionization and dust scattering

Using three dimensional Monte Carlo radiation transfer models of photoionisation and dust scattering, we explore different components of the widespread diffuse H$\alpha$ emission observed in the interstellar medium of the Milky Way and other galaxies. We investigate the relative contributions of H$\alpha$ from recombination emission in ionised gas and H$\alpha$ that originates in H{\sc ii} regions near the Galactic midplane and scatters off high altitude dust in the diffuse interstellar medium. For the radiation transfer simulations we consider two geometries for the interstellar medium: a three dimensional fractal geometry that reproduces the average density structure inferred for hydrogen in the Milky Way, and a density structure from a magneto hydrodynamic simulation of a supernovae driven turbulent interstellar medium. Although some sight lines that are close to H{\sc ii} regions can be dominated by scattered light, overall we find that less than $\sim 20\%$ of the total H$\alpha$ intensity in our simulations can be attributed to dust scattering. Our findings on the relative contribution of scattered H$\alpha$ are consistent with previous observational and theoretical analyses. We also investigate the relative contributions of dust scattering and in situ ionisation of high density dust clouds in the diffuse gas. Dust scattering in these partially ionised clouds contribute $\sim 40\%$ to the total intensity of H$\alpha$.

3D morphology of a random field from its 2D cross-section

Abstract The two aspect ratios of randomly oriented triaxial ellipsoids (representing isosurfaces of an isotropic 3D random field) can be determined from a single 2D cross-section of their sample using the probability density function (PDF) of the filamentarityF of individual structures seen in cross-section (F = 0 for a circle and F = 1 for a line). The PDF of F has a robust form with a sharp maximum and truncation at larger F, and the most probable and maximum values of F are uniquely and simply related to the two aspect ratios of the triaxial ellipsoids. The parameters of triaxial ellipsoids of randomly distributed sizes can still be recovered from the PDF of F. This method is applicable to many shape-recognition problems, here illustrated by the neutral hydrogen density in the turbulent interstellar medium of the Milky Way. The gas distribution is shown to be filamentary with axis ratios of about 1:2:20.

The dynamics of collapsing cores and star formation

Low-mass stars are generally understood to form by the gravitational collapse of the dense molecular clouds known as starless cores. Continuum observations have not been able to distinguish among the several different hypotheses that describe the collapse because the predicted density distributions are the almost the same, as they are for all spherical self-gravitating clouds. However, the predicted contraction velocities are different enough that the models can be discriminated by comparing the velocities at large and small radii. This can be done by observing at least two different molecular line transitions that are excited at different densities. For example, the spectral lines of the H2O (110 - 101) and C18O (1-0) have critical densities for collisional de-excitation that differ by 5 orders of magnitude. We compare observations of these lines from the contracting starless core L1544 against the spectra predicted for several different hypothetical models of contraction including the Larson-Penston flow, the inside-out collapse of the singular isothermal sphere, the quasi-equilibrium contraction of an unstable Bonnor-Ebert sphere, and the non-equilibrium collapse of an over-dense Bonnor-Ebert sphere. Only the model of the unstable quasi-equilibrium Bonnor-Ebert sphere is able to produce the observed shapes of both spectral lines. This model allows us to interpret other observations of molecular lines in L1544 to find that the inward velocities seen in observations of CS(2-1) and N2H+ are located within the starless core itself, in particular in the region where the density profile follows an inverse square law. If these conclusions were to hold in the analysis of other starless cores, this would imply that the formation of hydrostatic clouds within the turbulent interstellar medium is not only possible but not exceptional and may be an evolutionary phase in low-mass star formation.

Simulating the H2 content of high-redshift galaxies

We introduce a sub-grid model for the non-equilibrium abundance of molecular hydrogen in cosmological simulations of galaxy formation. We improve upon previous work by accounting for the unresolved structure of molecular clouds in a phenomenological way which combines both observational and numerical results on the properties of the turbulent interstellar medium. We apply the model to a cosmological simulation of the formation of a Milky-Way-sized galaxy at z=2, and compare the results to those obtained using other popular prescriptions that compute the equilibrium abundance of H2. In these runs we introduce an explicit link between star formation and the local H2 abundance, and perform an additional simulation in which star formation is linked directly to the density of cold gas. In better agreement with observations, we find that the simulated galaxy produces less stars and harbors a larger gas reservoir when star formation is regulated by molecular hydrogen. In this case, the galaxy is composed of a younger stellar population as early star formation is inhibited in small, metal poor dark-matter haloes which cannot efficiently produce H2. The number of luminous satellites orbiting within the virial radius of the galaxy at z=2 is reduced by 10-30 per cent in models with H2-regulated star formation.