Galactic Shock Waves Research Articles

Bow-shock structure of Sgr-B molecular-cloud complex in the Galactic Centre inferred from 3D CO-line kinematics

ABSTRACT Three-dimensional (3D) bubble structure of the Sgr-B molecular-cloud complex is derived by a kinematical analysis of CO-line archival cube data of the Galactic Centre (GC) observed with the Nobeyama 45-m telescope. The line-of-sight depth is estimated by applying the face-on transformation method of radial velocity to the projected distance on the Galactic plane considering the Galactic rotation of the central molecular zone (CMZ). The 3D complex exhibits a conical-horn structure with the Sgr-B2 cloud located in the farthest end on the line of sight at radial velocity $v_{\rm lsr} \sim 70$ km s$^{-1}$, and the entire complex composes a lopsided bubble opening toward the Sun at $v_{\rm lsr}\sim 50$ to 30 km s$^{-1}$. The line-of-sight extent of the complex is $\sim 100$ pc according to the large velocity extent for several tens of km s$^{-1}$ from Sgr-B2 to the outskirts. The entire complex exhibits a flattened conical bubble with full sizes $\sim 40 \ {\rm pc} \times 20 \ {\rm pc} \times 100 \ {\rm pc}$ in the l, b and line-of-sight directions, respectively. Based on the 3D analysis, we propose a formation scenario of the giant molecular bubble structure due to a galactic bow shock, and suggest that the star formation in Sgr-B2 was enhanced by dual-side compression (DSC) of the B2 cloud by the Galactic shock wave from up-stream and expanding H ii region from the down-stream side of the GC Arm I in Galactic rotation.

Газопылевые структуры в окрестности рукавов спиральных галактик

A two-dimensional model of the flow of the gas-dust interstellar medium in the vicinity of the spiral arm of the galaxy is constructed. The flow in a vertical plane transverse to the disk plane is considered. The effects of nonadiabatic flow (volumetric heating and cooling of the gas by radiation) are taken into account. The balance of heating and cooling ensures the coexistence of two phases, cold parsec-sized clouds of atomic hydrogen and warm intercloud gas. The consideration includes polydisperse dust, represented by three fractions of particles of different sizes and masses. Dust particles have finite inertia, their movements do not exactly repeat the movement of gas. Turbulence in the disk and in the spiral arm is also taken into account. Models are considered that use different combinations of the location of turbulence sources in the disk and/or in the arm. The main results obtained by the methods of computer hydrodynamic modeling are as follows. Clouds undergo significant transformations as they pass through the spiral arm. A significant part of the clouds is absorbed into a thin dense cloud layer, which extends in a spiral arm along the equatorial plane in the vicinity of the center of the arm and has a size of approximately half the width of the arm. A smaller part of the clouds passes without destruction or with partial destruction through the sleeve, experiencing strong deformations along the way. The small-scale cloud component is partially restored under the action of turbulence, which perturbs the extended cloud layer inside the arm and partially destroys it into separate fragments. A wedge-shaped galactic shock wave is formed on the rear side of the arm with respect to the incoming gas flow, attached to the rear edge of the extended cloud layer. A flow limited by a shock wave has the character of a jet that performs quasi-periodic transverse oscillations. The reason for the oscillations, apparently, is the instability of the shear flow, since tangential discontinuities are formed inside the jet along the flow and at a small angle to the shock fronts. Dust particles are dragged by turbulent eddies and carried to heights of 150–200 pc above the disk plane, which naturally explains the existence of chaotic filamentous dust structures extending above the galactic disk to heights of several hundred parsecs. The grains of dust are distributed differently inside the vortices. Dust grains with sizes of 0.01-0.1 μm cluster more easily than larger dust grains with a radius of 1 μm. Turbulence serves as a mechanism to effectively trap dust particles on the front side of the spiral arm. Modeling shows that dust lanes are more pronounced on the front side of the arm.

Highest-resolution rotation curve of the inner Milky Way proving the galactic shock wave

Abstract We present a rotation curve (RC) of the inner Galaxy of the first quadrant at 10° ≤ l ≤ 50° (R = 1.3–6.2 kpc) with the highest spatial (2 pc) and velocity (1.3 km s−1) resolutions. We used 12CO(J = 1–0)-line survey data observed with the Nobeyama 45 m telescope at an effective angular resolution of 20″ (originally 15″), and applied the tangent–velocity method to the longitude–velocity diagrams by employing the Gaussian deconvolution of the individual CO-line profiles. A number of RC bumps, or local variation of rotation velocity, with velocity amplitudes ∼±9 km s−1 and radial scale length ∼0.5–1 kpc are superposed on the mean rotation velocity. The prominent velocity bump and corresponding density variation around R ∼ 4 kpc in the tangential direction of the Scutum arm (4 kpc molecular arm) is naturally explained by an ordinary galactic shock wave in a spiral arm with small pitch angle, not necessarily requiring a bar-induced strong shock.

On Passing of Interstellar Clouds Through a Galactic Spiral Arm

It is believed that the taxonomy of interstellar clouds in their vicinity can serve as an indicator of the features of the geometry and intensity of galactic shock waves. In this paper, the authors present the results of a detailed two-dimensional hydrodynamic simulation of the passage of a cloud through the spiral arm of a galaxy and provide a brief analysis of the effects arising from this motion. The model of interstellar gas used assumes adiabatic flow in the spiral arm. The external gravitational field of the galactic disk and spiral arm is taken into account. The transverse dimensions of the arm in the calculations are taken as follows: the half-width of the arm is 1 kpc along the plane of the disk and 0.6 kpc in the vertical direction. A fragment of the flow is considered near and inside the spiral arm, the effects of the curvature of the arm and the influence of the Coriolis forces are neglected. It is shown that clouds passing through the arm are strongly deformed and lose a significant part of the mass or are completely destroyed in the case of low-mass clouds. The boundary value of the cloud mass at which complete destruction occurs lies in the interval between 3 000 and 6 000 M.

Giant elephant trunks from giant molecular clouds

Abstract We report the discovery of large elephant trunk (ET)-like objects, named giant elephant trunks (GETs), of molecular gas in star-forming complexes in the Scutum and Norma arms using the $^{12}$CO(J = 1–0)-line survey data with the Nobeyama 45 m telescope. In comparison with the CO maps of ETs in M$\, 16$ as derived from the same data, we discuss physical properties of the GETs. Their lengths are $\sim\!\! 20$ to $50\:$pc, an order of magnitude larger than ETs. GETs show a cometary structure coherently aligned parallel to the galactic plane, and emerge from the bow-shaped concave surface of giant molecular clouds (GMC) facing the H$\,$ ii regions, and point down-stream of the gas flow in the spiral arms. The molecular masses of the head clumps are $\sim 10^{3}$–$10^{4}\, M_{\odot}$, about three to four times the virial masses, indicating that the clumps are gravitationally stable. Jeans masses calculated for the derived density and assumed kinetic temperature are commonly sub-solar. We suggest that the GET heads are possible birth sites of stellar clusters, similarly to ET globules, but at much greater scale. We discuss the origin of the GETs by Rayleigh–Taylor instability due to deceleration of GMCs by low-density gas stagnated in the galactic shock waves as well as by pressure of the H$\,$ ii regions.

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45 m telescope (FUGIN). IV. Galactic shock wave and molecular bow shock in the 4 kpc arm of the Galaxy

AbstractThe FUGIN CO survey revealed the three-dimensional structure of a galactic shock wave in the tangential direction of the 4 kpc molecular arm. The shock front is located at G30.5+00.0 + 95 km s−1 on the upstream (lower longitude) side of the star-forming complex W 43 (G30.8−0.03), and comprises a molecular bow shock (MBS) concave to W 43, exhibiting an arc-shaped molecular ridge perpendicular to the galactic plane with width ∼0${^{\circ}_{.}}$1(10 pc) and vertical length ∼1° (100 pc). The MBS is coincident with the radio continuum bow of thermal origin, indicating association of ionized gas and similarity to a cometary bright-rimmed cloud. The upstream edge of the bow is sharp, with a growth width of ∼0.5 pc indicative of the shock front property. The velocity width is ∼10 km s−1, and the center velocity decreases by ∼15 km s−1 from the bottom to the top of the bow. The total mass of molecular gas in the MBS is estimated to be ∼1.2 × 106 M⊙, and ionized gas ∼2 × 104 M⊙. The vertical disk thickness has a step-like increase at the MBS by ∼2 times from lower to upper longitudes, which indicates hydraulic jump in the gaseous disk. We argue that the MBS was formed by the galactic shock compression of an accelerated flow in the spiral-arm potential encountering the W 43 molecular complex. A bow-shock theory can reproduce the bow morphology well. We argue that molecular bows are common in galactic shock waves, not only in the Galaxy but also in galaxies, where MBSs are associated with giant cometary H ii regions. We also analyzed the H i data in the same region to obtain a map of H i optical depth and molecular fraction. We found firm evidence of the H i to H2 transition in the galactic shock as revealed by a sharp molecular front at the MBS front.

Gas and stellar spiral arms and their offsets in the grand-design spiral galaxy M51

Theoretical studies on the response of interstellar gas to a gravitational potential disc with a quasi-stationary spiral arm pattern suggest that the gas experiences a sudden compression due to standing shock waves at spiral arms. This mechanism, called a galactic shock wave, predicts that gas spiral arms move from downstream to upstream of stellar arms with increasing radius inside a corotation radius. In order to investigate if this mechanism is at work in the grand-design spiral galaxy M51, we have measured azimuthal offsets between the peaks of stellar mass and gas mass distributions in its two spiral arms. The stellar mass distribution is created by the spatially resolved spectral energy distribution fitting to optical and near infrared images, while the gas mass distribution is obtained by high-resolution CO and HI data. For the inner region (r < 150"), we find that one arm is consistent with the galactic shock while the other is not. For the outer region, results are less certain due to the narrower range of offset values, the weakness of stellar arms, and the smaller number of successful offset measurements. The results suggest that the nature of two inner spiral arms are different, which is likely due to an interaction with the companion galaxy.

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.

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.

High speed flows and shock waves in astrophysical problems

Typical problems of high speed flows and shock waves in astrophysical environment are reviewed in the present paper. The emphases are especially to the solar wind acceleration, the jet structure of radio galaxies and Quasars, the galactic shock waves.

Structure of polar ring galaxies - Shock waves in the gas of polar rings

Polar ring galaxies are peculiar SO galaxies with gaseous rings rotating perpendicularly to the SO disks. The gas in the ring rushes into the potential well of the SO disk with rotational speeds of 130-250 km s −1 , exceeding the sound velocity of the ionized gas. Hence, if the stellar disk of an SO galaxy is dense enough, i.e., if the potential well is deep enough to severely disturb the motions of the gas, shock waves might form, like the galactic shock waves in spiral or barred galaxies. We examined the mass models of the SO disks of three well-observed polar ring galaxies, A0136-0801, MCG 5-29-86, and NGC 4650A, and found that their SO disks are dense enough to produce shock waves in the region of R/r 0 ≤4.5, where R and r 0 are the radius of the polar ring and the exponential scalelength of the SO disk, respectively

CO observations of Arp's interacting galaxies

A number of starburst galaxies are included in the Atlas of Peculiar Galaxies (Arp 1966) which is the collection of peculiar-morphology galaxies showing mutual tidal interaction. A scenario of starburst can be summarized as follows (e.g. Sofue 1987, Noguchi 1988). Tidal interaction between galaxies causes an oval disturbance in the gravitational potential of a galaxy. The oval disturbance causes a bar in the innermost region of the galaxy. By this bar a galactic shock wave is excited, which results in rapid accretion of gas toward the central region. The accreted gas produces a dense ring or a core of molecular gas in the central region, from which massive stars are born. Intense UV radiation from the stars heats dust in the dense molecular gas core/ring, and the dust emits strong FIR emission.

The modern view of the nature of the spiral structure of galaxies

The current state of the Lin-Shu density wave theory is discussed in the light of modern observational data. Much attention is paid to the problem of wave excitation and to the response of the interstellar gas to the wave gravitational potential. It is noted that the major predictions of the density wave theory—the galactic shock waves, the spiral velocity field of stars, and the age gradient across the spiral arms—have become fundamental observational facts at present, so that the density wave theory now has no competition from alternative theories. The nature of flocculent spirals is also discussed since, unlike regular spirals, they are probably not connected with density waves but with the effects of induced star formation in differentially rotating galactic disks.

On the spiral structure in the galactic distribution of co clouds

The CO distribution in the Galaxy is investigated through an analysis of longitude-velocity diagrams of CO emission lines for the two longitude ranges 20°<l<80° and 105°<l<140°. For the kinematics of the Galaxy we adopt the three typical models; the circular rotation, the linear density waves, and the galactic shock waves. It is shown that the distributions and kinematics of CO clouds are consistent with the predictions of the density wave model and the galactic shock model, and that the observed data of CO emissions do not contradict with the claim that the CO clouds form spiral arms.

A gaseous shock wave theory of the galactic spiral structure

In this paper, the galactic spiral structure is studied by the galactic shock wave of interstellar gas with self-gravitation. The perturbed gravitation of stars is not a necessary condition for the existence of such shock. It is proved first of all that there exists solution of local shock wave even if the perturbed gravitation is absent. The condition |ωη0|>α is required for such solution. The spiral structure can only be explained by the shock solution when the difference of density between the regions of arm and interarm is larger. The grand design of shock wave with self-gravitation is obtained by the iterative method. The features of shock wave can be analyzed qualitatively in the velocity plane for a special perturbed gravitation which is used to simulate the self-gravitation of interstellar gas. As the mass distribution in proto- galactic disk is irregular initially, the grand design of the galactic shock wave was developed by the processes of winding, growth of instability and overlapping of waves. Hence, it gives a complete figure about the origin, evolution and persistance. A lot of observed phenomena and classificational features of the galactic spiral structure can be explained by adopting these ideas.

A cloud/particle model of the interstellar medium - Galactic spiral structure

view Abstract Citations (48) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A cloud/particle model of the interstellar medium - Galactic spiral structure Levinson, F. H. ; Roberts, W. W., Jr. Abstract A cloud/particle model for gas flow in galaxies is developed that incorporates cloud-cloud collisions and supernovae as dominant local processes. Cloud-cloud collisions are the main means of dissipation. To counter this dissipation and maintain local dispersion, supernova explosions in the medium administer radial snowplow pushes to all nearby clouds. The causal link between these processes is that cloud-cloud collisions will form stars and that these stars will rapidly become supernovae. The cloud/particle model is tested and used to investigate the gas dynamics and spiral structures in galaxies where these assumptions may be reasonable. Particular attention is given to whether large-scale galactic shock waves, which are thought to underlie the regular well-delineated spiral structure in some galaxies, form and persist in a cloud-supernova dominated interstellar medium; this question is answered in the affirmative. Publication: The Astrophysical Journal Pub Date: April 1981 DOI: 10.1086/158823 Bibcode: 1981ApJ...245..465L Keywords: Galactic Structure; Interstellar Gas; Molecular Clouds; Shock Waves; Spiral Galaxies; Stellar Evolution; Supernovae; Astronomical Models; Disk Galaxies; Gas Density; Gas Flow; Gravitational Effects; Astrophysics full text sources ADS |

Galactic Shock Waves and Corrugation Waves

Galactic Shock Waves and Corrugation Waves

Dynamics of the Gas in Spiral Galaxies

AbstractThe dynamics of the gaseous component in disk-shaped galaxies is thought to play an important governing role in star formation, molecule formation, and the degree of development of spiral structure. The prospect that density waves and galactic shock waves are present on the large-scale has received support in recent years from a variety of observational studies. Large-scale shocks provide a most promising mechanism for driving star-forming and molecule-forming events on the scales of many kpc along spiral arms. Such shocks may also govern the kinematics and relative distribution of various galactic tracers. This is particularly apparent in M81 and other external spirals, because of our “bird’s-eye-view” perspective, and for the tracers of HI and CO in our own Galaxy. Comparison of the CO observations with model simulations based on the precepts of the density wave theory shows that these precepts are supported by several observational results.