Galactic Shocks Research Articles

Modeling high-energy cosmic ray induced terrestrial muon flux: A lookup table

On geological timescales, the Earth is likely to be exposed to an increased flux of high-energy cosmic rays (HECRs) from astrophysical sources such as nearby supernovae, gamma-ray bursts or by galactic shocks. Typical cosmic ray energies may be much higher than the ≤ 1 GeV flux which normally dominates. These high-energy particles strike the Earth's atmosphere initiating an extensive air shower. As the air shower propagates deeper, it ionizes the atmosphere by producing charged secondary particles. Secondary particles such as muons and thermal neutrons produced as a result of nuclear interactions are able to reach the ground, enhancing the radiation dose. Muons contribute 85% to the radiation dose from cosmic rays. This enhanced dose could be potentially harmful to the biosphere. This mechanism has been discussed extensively in literature but has never been quantified. Here, we have developed a lookup table that can be used to quantify this effect by modeling terrestrial muon flux from any arbitrary cosmic ray spectra with 10 GeV to 1 PeV primaries. This will enable us to compute the radiation dose on terrestrial planetary surfaces from a number of astrophysical sources.

FRAGMENTATION IN THE FIRST GALAXIES

Motivated by recent simulations of galaxy formation in which protogalaxies acquire their baryonic content through cold accretion, we study the gravitational fragmentation of cold streams flowing into a typical first galaxy. We use a one-zone hydrodynamical model to examine the thermal evolution of the gas flowing into a 108 M☉ dark matter halo at redshift z = 10. The goal is to gain an understanding of the expected fragmentation mass scale and thus the characteristic mass of the first population of stars to form by shock fragmentation. Our model accurately describes the chemical and thermal evolution of the gas as we are specifically concerned with how the chemical abundances and initial conditions of the low-density, metal-enriched, cold accretion streams that pass an accretion shock alter the cooling properties and tendency to fragment in the post-shock gas. Cold accretion flows are not shock heated at the virial radius but instead flow along high-baryonic-density filaments of the cosmic web and penetrate deep into the host halo of the protogalaxy. In this physical regime, if molecular cooling is absent because of a strong Lyman–Werner background, we find there to be a sharp drop in the fragmentation mass at a metallicity of Z ∼ 10−4 Z☉. If, however, H2 and HD molecules are present, they dominate the cooling at T < 104 K, and metallicity then has no effect on the fragmentation properties of the cold stream. For a solar abundance pattern of metallicity, O is the most effective metal coolant throughout the evolution, while for a pair instability supernova (PISN) metallicity yield, Si+ is the most effective coolant. PISN abundance patterns also exhibit a slightly smaller critical metallicity. Dust grains are not included in our chemical model, but we argue that their inclusion would not significantly alter the results. We also find that this physical scenario allows for the formation of stellar clusters and large, 104 M☉ bound fragments, possibly the precursors to globular clusters and supermassive black holes. Finally, we conclude that the usual assumption of isobaricity for galactic shocks breaks down in gas of a sufficiently high metallicity, suggesting that metal cooling leads to thermal instabilities.

Atmospheric consequences of cosmic ray variability in the extragalactic shock model: 2. Revised ionization levels and their consequences

It has been suggested that galactic shock asymmetry induced by our galaxy's infall toward the Virgo Cluster may be a source of periodicity in cosmic ray exposure as the solar system oscillates perpendicular to the galactic plane, thereby, inducing an observed terrestrial periodicity in biodiversity. There are a number of plausible mechanisms by which cosmic rays might affect terrestrial biodiversity. Here we investigate one of these mechanisms, the consequent ionization and dissociation in the atmosphere, resulting in changes in atmospheric chemistry that culminate in the depletion of ozone and a resulting increase in the dangerous solar UVB flux on the ground. We use a heuristic model of the cosmic ray intensity enhancement originally suggested by Medvedev and Melott (2007) to compute steady state atmospheric effects. This paper is a reexamination of an issue we have studied before with a simplified approximation for the distribution of incidence angles. The new results are based on an improved ionization background computation averaged over a massive ensemble (about 7 × 105) shower simulations at various energies and incidence angles. We adopt a range with a minimal model and a fit to full exposure to the postulated extragalactic background. The atmospheric effects are greater than they were with our earlier, simplified ionization model. At the lower end of the intensity range, we find that the effects are too small to be of serious consequence. At the upper end of this range, ∼6% global average loss of ozone column density exceeds that currently experienced due to anthropogenic effects such as accumulated chlorofluorocarbons. We discuss some of the possible effects. The intensity of the atmospheric effects is less than those of a nearby supernova or galactic γ ray burst, but the duration of the effects would be about 106 times longer. Present UVB enhancement from current ozone depletion ∼3% is a documented stress on the biosphere, but a depletion of the magnitude found at the upper end of our range would approximately double the global average UVB flux. We conclude that for estimates at the upper end of the reasonable range of the cosmic ray variability over geologic time, the mechanism of atmospheric ozone depletion may provide a major biological stress, which could easily bring about major loss of biodiversity. It is possible that future high‐energy astrophysical observations will resolve the question of whether such depletion is likely.

Galactic scale star formation: Interplay between stellar spirals and the ISM

AbstractSpiral structures in the disk galaxies have been extensively studied by many theoretical papers, but conventional steady-state models are not consistent with what we observe in time-dependent, multi-dimensional numerical simulations and also in real galaxies. Here we review recent progress in numerical modeling of stellar and gas spirals in disk galaxies. The spiral arms excited in a stellar disk can last for 10 Gyrs without the ISM, but each spiral arm is short-lived and is recurrently formed. The stellar spirals are not waves propagating with a single pattern speed. The ISM is concentrated in local potential minima, which roughly follow the galactic rotation together with the stellar arms, therefore galactic dust lanes are not the classic ‘galactic shocks’.

Nonlinear spiral waves in a galactic disk

An analytical solution of the nonlinear dynamic equations for a galactic disk is found. This solution describes the spiral design of spiral galaxies. The logarithmic profile of the spiral design and relatively larger density fluctuations as compared to velocity-field fluctuations are explained. Breaking of the obtained solution takes place for a certain critical amplitude. This can be a mechanism for the formation of galactic shocks and narrow star formation regions.

Atmospheric consequences of cosmic‐ray variability in the extragalactic shock model

It has been suggested that galactic shock asymmetry induced by our galaxy's infall toward the Virgo Cluster may be a source of periodicity in cosmic‐ray exposure as the solar system oscillates normally to the galactic plane, thereby inducing an observed terrestrial periodicity in biodiversity. There are a number of plausible mechanisms by which cosmic rays might affect terrestrial biodiversity. Here we investigate one of these mechanisms, the consequent ionization and dissociation in the atmosphere, resulting in changes in atmospheric chemistry, which culminate in the depletion of ozone and a resulting increase in the dangerous solar UVB flux on the ground. We estimate the enhancement of cosmic‐ray intensity for a range of reasonable parameters of the galactic wind and galactic magnetic field, and use these to compute steady‐state atmospheric effects. At the lower end of this range, we find that the effects are far too small to be of serious consequence. At the upper end of this range, the level of ozone depletion approaches that currently experienced due to anthropogenic effects such as accumulated chlorofluorocarbons, i.e., ∼2.1% global average loss of ozone column density. We discuss some of the possible effects. While much smaller intensity than the atmospheric effects of a nearby galactic gamma‐ray burst, the duration of the effects would be about 106 times greater. Current ozone depletion is a documented stress on the biosphere; it is not clear whether its consequences would be severe if of extended duration. We conclude that, for estimates at the upper end of the reasonable range of the cosmic‐ray variability over geologic time, the mechanism of atmospheric ozone depletion may provide a small additional stress, enhancing the impact of other events. However, in order to account for large fluctuations in biodiversity correlated with cosmic‐ray flux, other mechanisms should be investigated.

Instabilities of Spiral Shocks. II. A Quasi–Steady State in the Multiphase Inhomogeneous ISM

Galactic (Fujimoto; Roberts) are investigated using full three-dimensional hydrodynamic simulations that take into account the self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential improvement over previous numerical models, in which two-dimensional isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks are unstable and transient, and they shift to a globally quasi-steady, inhomogeneous pattern due to the nonlinear development of instabilities in the disk. The spiral patterns consist of many giant molecular cloud-like dense condensations, but those local structures are not steady, and they evolve into irregular spurs in the interarm regions. Energy feedback from supernovae does not destroy the quasi-steady spiral arms; rather, it mainly contributes to the vertical motion and the structures of the ISM. The results and methods presented here are a starting point for a more consistent treatment of the ISM in spiral galaxies, in which the effects of magnetic fields, radiative transfer, chemistry, and dynamical evolution of a stellar disk are taken into account.

Fine vertical structure of the galactic gaseous disk

The three-dimensional evolution of an ensemble of N particles (N = 8 × 105) in the external gravitational field of a galaxy perturbed by a spiral density wave is considered. The particles simulate clouds of interstellar gas, and inelastic two-body collisions between them are taken into account. The three-dimensional structure of the gaseous galactic layer and the vertical profile of the spiral arms are computed. It is shown that: (1) the local thickness of the gaseous galactic disk has a minimum where the volume gas density has a maximum (the maximum density of the interstellar medium is shifted downstream relative to the galactic shock front), (2) the configuration of the vertical profile of the spiral arms changes radically when the corotation region is crossed. Our first result explains the negative correlation between the thickness of the gas layer and the density derived from neutral-hydrogen observations. The second result can be used in the next generation of neutral-hydrogen observations to localize the corotation radius in the Galaxy.

Dense gas formation triggered by spiral density wave in M31

AbstractWe present the high-resolution 12CO(J = 1 − 0), 13CO(J = 1 − 0) and 12CO(J = 3 − 2) maps toward a GMA located on the southern arm region of M31 using Nobeyama 45 m and ASTE 10 m telescopes. The GMA consists of two velocity-components, i.e., red and blue. The blue component shows a strong and narrow peak, whereas the red one shows a weak and broad profile. The red component has a lower 12CO(J = 1 − 0)/13CO(J = 1 − 0) ratio (~ 5) than that of the blue one (~ 16), indicating that the red component is denser than the blue one. The red component could be the decelerated gas if we consider the galactic rotational velocity in this region. We suggest that the red component is “post shock” dense gas decelerated due to a spiral density wave. This could be observational evidence of dense molecular gas formation due to galactic shock by spiral density waves.We also present results from on-going observations toward NGC 604, which is the supergiant HII region of M33, using Nobeyama 45 m and ASTE 10 m telescopes. The ratio of 12CO(J = 3 − 2) to 12CO(J = 1 − 0) ranges from 0.3 to 1.2 in NGC 604. The 12CO(J = 1 − 0) map shows the clumpy structure while 12CO(J = 3 − 2) shows a strong peak near to the central star cluster of NGC 604. The high ratio gas is distributed on the arc-like or shell-like structure along with Hα emission and HII region detected by radio continuum. These suggest that the dense gas formation and second generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova in central star cluster.

Formation of galactic shocks and the vertical structure of the gaseous disk of the galaxy in a “turbulent” interstellar medium

The effects on the formation of Galactic shocks and the vertical structure of the Galactic disk due to thermal processes in a cloudy interstellar medium as it flows through a spiral density wave in the plane of the Galactic disk are considered. The evolution of the gas is fundamentally different, depending on the thermal properties of the medium. For example, if it is compressed in the horizontal direction (parallel to the Galactic plane) by the gravitational forces of the spiral density waves responsible for the formation of spiral arms, an isothermal and adiabatic medium is swept out in the vertical direction. However, on the contrary, a medium whose volume loss function increases fairly rapidly with density and temperature is further compressed under the action of the overall gravitational field of the galaxy. This effect is referred to as “self-focusing,” and may serve as an additional mechanism to explain the recently discovered anticorrelation between the width of the atomic hydrogen layer in the Galaxy and the gas density. The difference in the vertical behavior of media with different thermal properties can be used as an indicator of the thermal properties of a particular component of the interstellar gas (atomic or molecular). Attention is drawn to the fact that Galactic shocks themselves represent a mechanism that can heat the ensemble of clouds, i.e., increase the dispersion of cloud velocities. The vertical structure of a Galactic shock front is constructed, which is in qualitative agreement with the “bow shock” inferred from radio data.

Virgo High-Resolution CO Survey: IV. Spiral-Driven Gas Dynamics in the Non-Barred Seyfert Galaxy NGC 4501

Abstract We report on high-resolution interferometer observations of the ${{12\atop} \mathrm{CO}}$ ($J = 1 \hbox{--} 0$) emission in the central 5 kpc region of the Seyfert 2 galaxy NGC 4501. The observations were made using the Nobeyama Millimeter Array during a long-term CO line survey of Virgo spirals. The major features are: (1) a nuclear concentration with a radius of $r\sim 5^{\prime\prime}$ (390 pc), which is resolved into double peaks, and (2) spiral arms which extend out from the nuclear region. The nuclear component has a mass of $1.3 \times 10^8 \,{{{M}_{\odot}}}$, which corresponds to $\sim 3.5\%$ of the dynamical mass, and shows a slight non-circular motion. The double peaks are separated by $\sim {4\rlap {.}{}^{\mathrm {\prime \prime }}7}$ ($370 \:\mathrm{pc}$), and located on the root of optical spiral arms in a HST image. The gas arms are associated with the spiral dust lanes, and are linked to the central double peaks. The non-circular motions along the molecular arms indicate the fact that the gas is driven by the density wave, rather than the stochastic processes. We calculated the gas cloud orbits in a stellar spiral potential, and explained the observed CO spiral arms and non-circular motions. We suggest that the central gas condensation arises from spiral-driven gas transfer. We estimated and compared the effect of two possible mechanisms of angular-momentum transfer: galactic shock and gravitational torques. We discuss that the galactic shock is dominant.

First results from SPIFFI, II: The luminous infrared galaxy NGC 6240 and the luminous sub‐millimeter galaxy SMMJ 14011+0252

AbstractThis is the second of two papers (I: Horrobin et al. 2003) on the first scientific results from the SPIFFI integral field spectrometer at the VLT. Here we discuss the observations and properties of the prototypical luminous infrared galaxy NGC 6240 and the luminous sub‐millimeter galaxy SMMJ 14011+0252. Taking full advantage of the excellent seeing conditions of 0.27″, our integral field spectroscopy data allow us for the first time to study in detail the stellar and gas dynamics in NGC 6240 on scales of 125 pc, and to establish a galactic shock as the origin of the strong emission from molecular hydrogen. Our observations of SMMJ 14011+0252 provide us with deep, spatially resolved near infrared spectra of the SCUBA selected luminous submillimeter galaxy at a redshift of z = 2.565, revealing a remarkably old, massive and metal‐rich starburst galaxy for the early epoch at which it is observed. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

3D structure of the galactic shock waves

We present the results of 2- and 3-dimensional simulations of the vertical structure of the galactic shock wave. The aim of our study is to clarify whether galactic shocks may establish connection between disk and gaseous galactic halo. We find some curious properties of the structure of the steady state flow: multiple shocks formation and the effect of sharp bend of the shock front. We believe that galactic shock waves may serve as an effective mechanism for extended halo support.

Emission‐Line Properties of the Optical Filaments of NGC 1275

Extended nebular filaments are seen at optical wavelengths in NGC 1275, the central galaxy in the Perseus cluster. The agents responsible for the excitation of these filaments remain poorly understood. In this paper we investigate possible mechanisms for powering the filaments, using measurements from an extensive spectroscopic data set acquired at the Lick Observatory 3 m Shane telescope. The results show that the filaments are in an extremely low ionization and excitation state. The high signal-to-noise ratio of the spectra allows us to measure or place sensitive upper limits on weak but important diagnostic lines. We compare the observed line intensity ratios to the predictions of various ionization models, including photoionization by an active galactic nucleus, shock heating, stellar photoionization, and photoionization by the intracluster medium. We also investigate possible roles for cluster extreme-ultraviolet emission, and filtering of cluster soft X-ray emission by an ionized screen, in the energetics of the filaments. None of these mechanisms provides an entirely satisfactory explanation for the physical state of the nebulae. Heating and ionization by reconnection of the intracluster magnetic field remains a potentially viable alternative, which merits further investigation through Faraday rotation studies.

The Galactic Shock Pump: A Source of Supersonic Internal Motions in the Cool Interstellar Medium

We propose that galactic shocks propagating through interstellar density fluctuations provide a mechanism for the intermittent replenishment, or pumping, of the supersonic motions and internal density enhancements observed pervasively within cool atomic and molecular interstellar structures, without necessarily requiring the presence of self-gravity, magnetic fields, or young stars. The shocks are assumed to be due to a variety of galactic sources on a range of scales. An analytic result for the kinematic vorticity generated by a shock passing through a radially stratified two-dimensional isobaric model cloud is derived, assuming that the Mach number is not so large that the cloud is disrupted, and neglecting the shock curvature and cloud distortion. Two-dimensional lattice gas hydrodynamic simulations at modest Mach numbers were used to verify the analytic result. The induced internal velocities are initially a significant fraction of the shock speed divided by the square root of the density contrast, accounting for both the observed line width amplitudes and the apparent cloud-to-cloud line width-density scaling. The line width-size relation could then be interpreted in terms of the well-known power spectrum of a system of shocks. The induced vortical energy should quickly be converted to compressible and MHD modes and so would be difficult to observe directly, even though it would still be the power source for the other modes. The shock pump thus produces density structure without the necessity of any sort of instability. We argue that the shock pump should lead to nested shock-induced structures, providing a cascade mechanism for supersonic turbulence and a physical explanation for the fractal-like structure of the cool interstellar medium. The average time between shock exposures for an idealized cloud in our Galaxy is estimated and found to be small enough that the shock pump is capable of sustaining the supersonic motions against readjustment and dissipation, except for the smallest structures. This suggests an explanation of the roughly spatially uniform and nearly sonic line widths in small dense cores. We speculate that the avoidance of shock pumping may be necessary for a localized region to form stars and that the inverse dependence of probability of avoidance on region size may be an important factor in determining the stellar initial mass function.

A New High‐Resolution CO Map of the Inner 2.́5 of M51. I. Streaming Motions and Spiral Structure

The Owens Valley millimeter array has been used to map the CO 1-0 emission in the inner 25 of the grand-design spiral galaxy M51 at 2''-3'' resolution. These new images reveal the molecular spiral arms with unprecedented clarity; the emission in the two major arms (northeast and southwest) originates from supermassive cloud complexes, giant molecular associations (GMAs), which are for the first time resolved both along and perpendicular to the arms. The overall morphology of the CO emission is symmetric in reflection about the nucleus, with major complexes occurring opposite each other in the two major arms. On the other hand, the CO line flux in the area of the southwest arm closest to the nucleus is approximately twice as bright as that from the analogous location in the northeast arm. Streaming motions can be studied in detail and appear with great clarity along the major and minor axes of M51. The streaming velocities are very large, 60-150 km s-1. Our maps offer, for the first time, sufficient resolution to resolve the structure in the molecular streaming motions. Both the radial and tangential velocity components show steep gradients, in qualitative accordance with predictions of the density-wave models of Roberts & Stewart. Our data thus support the presence of galactic shocks in the arms of M51. In general, velocity gradients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep in between GMAs. The steep gradients cause conditions of strong reverse shear in several regions in the arms, and thus the notion that shear is generally reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the southwest arm, only one shows reduced shear. We find an unusual structure, an expansion in the northeast molecular arm at 25'' radius (1.2 kpc) southeast of the center. This broadening occurs right after the end of the northeast arm at the inner Lindblad resonance (ILR). Multiple-peak spectra, velocity twists, and structures with apparently high velocity dispersion are associated with this feature. Bifurcations in the molecular spiral arm structure, at a radius of ≈3.3 kpc (73''), may be evidence of a secondary compression of the gas caused by, and occurring near, the 4/1 ultraharmonic resonance. Several molecular spurs protrude from the main spiral arms, in particular on the western side of the map. Inside the radius of the ILR, we detect narrow (≈5'') molecular spiral arms, possibly related to the K-band arms found in the same region. We find evidence of noncircular motions in the inner 20'' of M51. The magnitude of these deviations is 20-30 km s-1, and they are consistent with gas on elliptical orbits in a bar.

A vertical structure of a galactic shock wave

Abstract We carry out a 2D numerical simulation for the interaction of interstellar gas flow with the potential well of the spiral arm within a vertical cross–section. Unexpected results were obtained. First, the arising shock wave does not settle down to steady state and always passes through the arm and away. Second, the shock wave changes the vertical gas density scale – the scale decreases in front of the arm and increases behind the arm. Third, the interaction of the flow with the well has a resonant dependence on the initial flow speed and reaches its maximum at M ∼ 2−3.

Over-reflection and instability of shock waves in an inhomogeneous medium

Abstract We present a resonant description of hydrodynamic instabilities of shock waves in inhomogeneous media. This approach (1) allows us to clarify the physical mechanisms of instabilities; (2) provides a natural classification of all hydrodynamic instabilities; (3) allows us to simplify the search and prediction of instabilities, reducing the analysis to studying the coefficients of transformation (reflection) of perturbations at the shock front. We apply the developed formalism to the analysis of the resonance characteristics of a model of an accelerating shock wave in an exponential atmosphere and a model of a galactic shock wave. Analysis shows that instability in these models is caused by the effect of spontaneous emission of waves by the shock front while the true resonant effects are insignificant. Finally, we predict that the standing shock wave in an accretion flow on to a point mass must be unstable for the same reasons.

Formation and stability of the galactic shock waves

Abstract The stability of the shocked and the shock-free gas flows in the gaseous galactic disc is discussed. It is shown that the periodic shock-free supersonic flow between the spiral gravitational potential wells is unstable due to the parametric instability and thus should be re-organized into the shocked flow. The steady-state shock at the rear side of the well (relative to the flow) is subject to the local instability. This instability is suppressed if the shock is at the front side of the well.

Distribution and Dynamics of Molecular Gas in the Galaxy M51. III. Kinematics of Molecular Gas

We present here a study of the dynamics of the molecular gas in the spiral galaxy M51 using CO data obtained with the Nobeyama 45-m telescope. From the distribution of the radial velocity of the CO gas, the velocity components parallel and perpendicular to the spiral arms were determined for each spiral phase. Although the variations in the velocity components across the arms are in qualitatively good agreement with density-wave theory, their magnitudes are much larger than those predicted by models with small-amplitude perturbations (5–10% of axisymmetric field). By tracing the velocity vectors at each spiral phase, the orbit of the CO gas was observationally determined. The orbits deviate largely from pure circular motions, forming an oval pattern, as expected from density-wave theory. The arm-to-interarm ratio of the surface density of the gas can be explained in terms of this oval motion. In addition, both the smooth deceleration across the arms and the symmetric density distribution with respect to the arms suggest the lack of the galactic shock expected in a continuous fluid model.