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1.
We have constructed a dynamo model for the magnetic field in spiral galaxies that takes into account the differences in star formation rates in different galaxies. The difficulty in constructing the model is that the star formation rate does not enter directly into the equations of magnetohydrodynamics, which include only the root-mean-square velocity of the interstellar gas, its density, and the half-thickness of the ionized gas disk. We propose a parametrization of these quantities that relates them to the star formation rate and investigate our model in terms of the so-called no-z approximation, which neglects the details of the magnetic field structure in a direction perpendicular to the galactic disk. The influence of the star formation rate on the galactic dynamo is a threshold one. This influence is small at moderate star formation rates and significant only at very high star formation rates. If the starburst intensity reaches some critical level (exceeding that in the Milky Way by an order of magnitude), then the large-scale magnetic field is destroyed and it is restored only after completion of the starburst. We provide a list of galaxies that exhibit a fairly high star formation rate and that can be interesting to study their magnetic fields.  相似文献   

2.
The problem of determining the pattern of gas motions in the central regions of disk spiral galaxies is considered. Two fundamentally different cases—noncircular motions in the triaxial bar potential and motions in circular orbits but with orientation parameters different from those of the main disk—are shown to have similar observational manifestations in the line-of-sight velocity field of the gas. A reliable criterion is needed for the observational data to be properly interpreted. To find such a criterion, we analyze two-dimensional nonlinear hydrodynamic models of gas motions in barred disk galaxies. The gas line-of-sight velocity and surface brightness distributions in the plane of the sky are constructed for various inclinations of the galactic plane to the line of sight and bar orientation angles. We show that using models of circular motions for inclinations i>60° to analyze the velocity field can lead to the erroneous conclusions of a “tilted (polar) disk” at the galaxy center. However, it is possible to distinguish bars from tilted disks by comparing the mutual orientations of the photometric and dynamical axes. As an example, we consider the velocity field of the ionized gas in the galaxy NGC 972.  相似文献   

3.
The far outer regions of galactic disks allow an important probe of both star formation and galaxy formation. I discuss how observations of HII regions in these low gas density, low metallicity environments can shed light on the physical processes which drive galactic star formation. The history of past star formation at large radii, as traced by observations of old and intermediate-age stars, constrains the epoch at which the highest angular momentum regions of disks were in place; first results for the M31 disk suggest this occured a significant (≳ 8 Gyr) time ago. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

4.
In order to study the state of gas in galaxies, diagrams of the relation of optical emission line fluxes are used allowing one to separate main ionization sources: young stars in the HII regions, active galactic nuclei, and shock waves. In the intermediate cases, for example, when the contributions of radiation from OB stars and from shock waves mix, identification becomes uncertain, and the issue remains unresolved on what determines the observed state of the diffuse ionized gas (DIG) including the one on large distances fromthe galactic plane. Adding of an extra parameter—the gas velocity dispersion in the line-of-sight—to classical diagnostic diagrams helps to find a solution. In the present paper, we analyze the observed data for several nearby galaxies: for UGC10043 with the galactic wind, for the dwarf galaxies VIIZw403 andMrk 35 with star formation, for the galaxy Arp 212 with a polar ring. The data on the velocity dispersion are obtained at the 6-m SAO RAS telescope with the Fabry-Perot scanning interferometer, the information on the relation of main emission-line fluxes–from the published results of the integral-field spectroscopy (the CALIFA survey and the MPFS spectrograph). A positive correlation between the radial velocity dispersion and the contribution of shock excitation to gas ionization are observed. In particular, in studying Arp 212, our approach allowed us to confirm the assumption on a direct collision of gaseous clouds on the inclined orbits with the main disk of the galaxy.  相似文献   

5.
While the importance of merging, accretion, and infall processesin determining galactic evolution is well established boththeoretically and observationally, details on how such processesare taking place nowadays even in our own Galaxy are stillrelatively poorly known, especially due to large remaininguncertainties on the location and origin of high velocity clouds.In this paper we focus on the possible role that galacticoutflows and gas infall may have on directly triggering starformation in the halo and in galactic disks. While compellingevidence has been accumulating in recent years suggesting thatsome level of star formation directly triggered by outflows isvery likely to exist in the halo of some galaxies, the evidencefor star formation dynamically triggered by infall is far moreelusive due to confusion with other, more efficient large-scalestar forming mechanisms operating in the galactic disk. Despite ofincreasingly realistic simulations of the gas circulation betwenthe gas and the halo and of high velocity cloud impacts ongalactic disks, the efficiency of star formation directlytriggered by such impacts remains an open question.  相似文献   

6.
We present our spectroscopic observations of the galaxy NGC 7468 performed at the 6-m Special Astrophysical Observatory telescope using the UAGS long-slit spectrograph, the MPFS multi-pupil fiber spectrograph, and the IFP scanning Fabry-Perot interferometer. We found no significant deviations from the circular rotation of the galactic disk in the velocity field in the regions of brightness excess along the major axis of the galaxy (the putative polar ring). Thus, these features are either tidal structures or weakly developed spiral arms. However, we detected a gaseous disk at the center of the galaxy whose rotation plane is almost perpendicular to the plane of the galactic disk. The central collision of NGC 7468 with a gas-rich dwarf galaxy and their subsequent merging seem to be responsible for the formation of this disk.  相似文献   

7.
We simulated collisions of High Velocity Clouds with the galactic disk with a simple hydrodynamical code. Main aspects of the morphology of nearby (d < 500 pc) cloud complexes, like the Oph, Orion and Taurus-Auriga-Perseus complexes, are reproduced. These aspects include total mass, distance from the galactic plane, orientation of elongated gas structures with respect to the plane, and relative position of clusters of O-B stars with respect to the main concentrations of molecular gas. The space distribution of stars of different ages, usually explained in terms of sequential star formation, is interpreted in a new way in our model.  相似文献   

8.
In order to simulate evolution of a large-scale magnetic field in a barred galaxy possessing a gaseous halo we apply a three-dimensional (3D) MHD numerical model. We solve a induction equation using a time-dependent velocity field of molecular gas resulting from self-consistent 3D N-body simulations of a galactic disk. The gaseous halo rotates differentially co-rotating with the disk. In our model we introduce the dynamo process causing the amplification of the magnetic field as well as the formation of field structures high above the galactic disk. The simulated magnetic fields are used to construct the models of a high-frequency (Faraday rotation-free) polarized radio emission that accounts for effects of projection and limited resolution, and is thus suitable for direct comparison with observations. We found that the resultant magnetic field correctly reproduces the observed structures of polarization B-vectors, forming coherent patterns well aligned with spiral arms and with the bar. The process initializing a wave-like behavior of the magnetic field, which efficiently forms magnetic maxima between the spiral arms, is demonstrated. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

9.
We propose a possible scenario of large-scale magnetic field evolution for galaxies with star formation. An important point affecting the results of our calculations is a parametrization of dynamo-governing quantities. In comparison with previous works, we have reconsidered the views of how star formation affects the stationary magnetic field strength, viscosity, and other parameters important for galactic dynamos. The calculations have been performed by taking into account the magnetic helicity fluxes, which introduce an additional nonlinearity into the model and change the regime of galactic dynamo action. We have confirmed the previously suggested idea that for weak star formation its influence on the magnetic field strength is minor and the relationship between them clearly manifests itself only when the star formation rate reaches a certain threshold value. In this case, on the one hand, the threshold lowers-this effect manifests itself at a star formation surface density greater than that in the Milky Way by a factor of 5. On the other hand, intense star formation can cause both a monotonic decay of the large-scale magnetic field and its oscillations near some value.  相似文献   

10.
Most astrophysical accretion disks are likely to be warped.In X-ray binaries,the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk.There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars,but relevant works on warped/tilted accretion disks are still lacking.In this paper we develop a simplified twocomponent model,in which the disk is comprised of an inner coplanar part and an outer,tilted part.Based on standard assumption on the formation and evolution of the toroidal magnetic field component,we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque.  相似文献   

11.
We have carried out 1.25 pc resolution MHD simulations of the ISM, on a Cartesian grid of 0 ≤ (x, y) ≤ 1 kpc size in the galactic plane and ?10 ≤ z ≤ 10 kpc into the halo, thus being able to fully trace the time-dependent evolution of the galactic fountain. The simulations show that large scale gas streams emerge, driven by SN explosions, which are responsible for the formation and destruction of shocked compressed layers. The shocked gas can have densities as high as 800 cm?3 and lifetimes up to 15 Myr. The cold gas is distributed into filaments which tend to show a preferred orientation due to the anisotropy of the flow induced by the galactic magnetic field. Ram pressure dominates the flow in the unstable branch 102 < T ≤ 103.9 K, whereas for T ≤ 100 K (stable branch) magnetic pressure takes over. Near supernovae thermal and ram pressures determine the dynamics of the flow. Up to 80% of the mass in the disk is concentrated in the thermally unstable regime 102 < T ≤ 103.9 K with ～30% of the disk mass enclosed in the T ≤ 103 K gas. The hot gas in contrast is controlled by the thermal pressure, since magnetic field lines are swept towards the dense compressed walls.  相似文献   

12.
Photoionizing feedback in star cluster formation   总被引：1，自引：0，他引：1  
We present the first ever hydrodynamic calculations of star cluster formation that incorporate the effect of feedback from ionizing radiation. In our simulations, the ionizing source forms in the cluster core at the intersection of several dense filaments of inflowing gas. We show that these filaments collimate ionized outflows and suggest such an environmental origin for at least some observed outflows in regions of massive star formation. Our simulations show both positive feedback (i.e. promotion of star formation in neutral gas compressed by expanding H  ii regions) and negative feedback (i.e. suppression of the accretion flow in to the central regions). We show that the volume filling factor of ionized gas is very different in our simulations from the result from the case where the central source interacted with an azimuthally smoothed gas density distribution. As expected, gas density is the key parameter in determining whether or not clusters are unbound by photoionizing radiation. Nevertheless, we find – on account of the acceleration of a small fraction of the gas to high velocities in the outflows – that the deposition in the gas of an energy that exceeds the binding energy of the cluster is not a sufficient criterion for unbinding the bulk of the cluster mass.  相似文献   

13.
A new mechanism of sweeping out of dust grains beyond galactic disks both in the radial direction along the galactic plane and in the vertical, cross-disk direction is proposed. The mechanism is driven by the interaction of dust grains with the bisymmetric nonstationary magnetic field of the galaxy, whose lines are curved and corotate with the stellar spiral density wave responsible for the arms. We attribute the radial transfer of interstellar dust grains in the plane of galactic disks to the fact that charged dust grains are “glued” to magnetic field lines and are therefore pushed outward because of the rotation of magnetic field lines and their tilt with respect to the radial direction parallel to the disk plane. In addition, dust is swept out vertically in the cross-disk direction because of the drift motion in crossed magnetic and gravitational fields (both are parallel to the galactic plane). Numerical computations of the motion of dust grains in real magneto-gravitational fields with the allowance for the drag force from interstellar gas show that the time scale of dust grain transport beyond galactic disks is on the order of 1 Gyr or shorter.  相似文献   

14.
The gravitational separation of iron atoms with respect to hydrogen is evaluated by simple models of globular clusters and spherical galaxies before star formation, as well as in a column of gas perpendicular to the galactic plane in the solar neighborhood. The changes of the iron abundance are small, but qualitatively consistent with the observed facts: (1) there is a positive correlation between the mass number of the element and the steepness of its abundance gradient; (2) there is a connection between the mass and the density concentration of the globular clusters and their chemical composition; (3) the changes of the chemical content in the galactic plane are very small during the life of the galactic disk, but are increasingly significant in the upper layers.  相似文献   

15.
We investigate in detail the kinematics and morphology of the Seyfert galaxy NGC 6104 in order to identify the mechanism of gas transportation to the active galactic nucleus (AGN). Our observational data were obtained at the 6-m Special Astrophysical Observatory telescope with the MPFS integral-field spectrograph and the SCORPIO universal device in three modes: direct imaging, a scanning Fabry—Perot interferometer, and long-slit spectroscopy. Images from the HST archive were invoked to study the structure of the circumnuclear region. An analysis of deep images has shown for the first time that NGC 6104 is in the phase of active merging with a companion galaxy. We have been able to study the detailed picture of ionized gas motions up to galactocentric distances of 14 kpc and to construct the stellar velocity field for the inner region. The radial gas motions toward the AGN along the central bar play a significant role at galactocentric distances of 1–5 kpc. In addition, we have detected an outflow of ionized gas from the nucleus that presumably resulted from the intrusion of a radio jet into the ambient interstellar medium. Using diagnostic diagrams, we estimate the contributions from the AGN and star formation to the galactic gas ionization. We estimate the bar pattern speed by the Tremaine-Weinberg method and show that the inner ring observed in the galaxy’s images has a resonant nature. Two possible ring formation scenarios, before and during the interaction with a companion, are discussed.  相似文献   

16.
The existence of partially ionized, diffuse gas and dust clouds at kiloparsec scale distances above the central planes of edge-on, galaxy discs was an unexpected discovery about 20 years ago. Subsequent observations showed that this extended or extraplanar diffuse interstellar gas (EDIG) has rotation velocities approximately 10–20 per cent lower than those in the central plane, and has been hard to account for. Here, we present results of hydrodynamic models, with radiative cooling and heating from star formation. We find that in models with star formation generated stochastically across the disc, an extraplanar gas layer is generated as long as the star formation is sufficiently strong. However, this gas rotates at nearly the same speed as the midplane gas. We then studied a range of models with imposed spiral or bar waves in the disc. EDIG layers were also generated in these models, but primarily over the wave regions, not over the entire disc. Because of this partial coverage, the EDIG clouds move radially, as well as vertically, with the result that observed kinematic anomalies are reproduced. The implication is that the kinematic anomalies are the result of three-dimensional motions when the cylindrical symmetry of the disc is broken. Thus, the kinematic anomalies are the result of bars or strong waves, and more face-on galaxies with such waves should have an asymmetric EDIG component. The models also indicate that the EDIG can contain a significant fraction of cool gas, and that some star formation can be triggered at considerable heights above the disc mid-plane. We expect all of these effects to be more prominent in young, forming discs, to play a role in rapidly smoothing disc asymmetries and in working to self-regulate disc structure.  相似文献   

17.
On the basis of 'sticky particle' calculations, it is argued that the gas features observed within 10 pc of the Galactic Centre — the circumnuclear disc (CND) and the ionized gas filaments — as well as the newly formed stars in the inner 1 pc can be understood in terms of tidal capture and disruption of gas clouds on low angular momentum orbits in a potential containing a point mass. The calculations demonstrate that a dissipative component forms a 'dispersion ring', an asymmetric elliptical torus precessing counter to the direction of rotation, and that this shape can be maintained for many orbital periods. For a range of plausible initial conditions, such a structure can explain the morphology and kinematics of the CND and of the most conspicuous ionized filament. While forming the dispersion ring, a small cloud with low specific angular momentum is drawn into a long filament which repeatedly collides with itself at high velocity. The compression in strong shocks is likely to lead to star formation even in the near tidal field of the point mass. This process may have general relevance to accretion on to massive black holes in normal and active galactic nuclei.  相似文献   

18.
We study the nature of the extended near-UV emission in the inner kiloparsec of a sample of 15 Seyfert (Sy) galaxies which have both near-UV (F330W) and narrow-band [O  iii ] high-resolution Hubble images. For the majority of the objects, we find a very similar morphology in both bands. From the [O  iii ] images, we construct synthetic images of the nebular continuum plus the emission-line contribution expected through the F330W filter, which can be subtracted from the F330W images. We find that the emission of the ionized gas dominates the near-UV extended emission in half of the objects. A further broad-band photometric study, in the bands F330W ( U ), F547M ( V ) and F160W ( H ), shows that the remaining emission is dominated by the underlying galactic bulge contribution. We also find a blue component whose nature is not clear in four out of 15 objects. This component may be attributed to scattered light from the active galactic nuclei, to a young stellar population in unresolved star clusters, or to early disrupted clusters. Star-forming regions and/or bright off nuclear star clusters are observed in 4/15 galaxies of the sample.  相似文献   

19.
The density distributions of the two main components in interstellar hydrogen are calculated using 21 cm line data from the Berkeley Survey and the Pulkovo Survey. The narrow, dense component (state I of neutral hydrogen) has a Gaussianz-distribution with a scale-height of 50 pc in the local zones (the galactic disk). For the wide, tenuous component (hydrogen in state II) we postulate a distribution valid in the zones where such a material predominates (70 pc?z? 350 pc the galactic stratum) i.e., $$n_H \left( z \right) = n_H \left( 0 \right)exp \left( { - \left( {z/300{\text{ }}pc} \right)^{3/2} } \right).$$ Similar components are found in the dust distribution and in the available stellar data reaching sufficiently highz-altitudes. The scale-heights depend on the stellar type: the stratum in M III stars is considerably wider than in A stars (500–700 pc against 300 pc). The gas to dust ratio is approximately the same in both components: 0.66 atom cm?3 mag?1 kpc in the galactic plane. A third state of the gas is postulated associating it the observed free electron stratum at a scale-height of 660 pc (hydrogen fully ionized at high temperatures). The ratio between the observed dispersions in neutral hydrogen (thermal width plus turbulence) and the total dispersions corresponding to the real inner energies in the medium is obtained by a comparison with the dispersion distribution σ(z) of the different stellar types associated with the disk and the stratum $$\sigma ^2 \left( {total} \right) = \sigma ^2 \left( {21{\text{ cm line}}} \right) \cdot {\text{ }}Q^2 ,$$ from which we graphically obtainedQ 2=2.9 ± 0.3, although that number could be lower in the densest parts of the spiral arms. Its dependence on magnetic field and cosmic rays is analysed, indicating equipartition of the different energy components in the interstellar medium and consistency with the observed values of the magnetic field: i.e., fluctuations with an average of ～ 3 μG (associated with the disk) in a homogeneous background of ～ 1 μG (associated with the stratum). A minimum and maximumK z-force are obtained assuming extreme conditions for the total density distribution (gas plus stars). TheK z-force obtained from the interstellar gas in its different states using approximations of the Boltzmann equation is a reasonable intermediate case between maximum and minimumK z. The mass density obtained in the galactic plane is 0.20±0.05M ⊙ pc?3, and the results indicate that the galactic disk is somewhat narrower and denser than has usually been believed. The effects of wave-like distributions of matter in thez-coordinate are analysed in relation with theK z-force, and comparisons with theoretical results are performed. A qualitative model for the galactic field of force is postulated together with a classification of the different zones of the Galaxy according to their observed ranges in velocity dispersions and the behaviour of the potential well at differentz-altitudes. The disk containing at least two-thirds of the total mass atz<100 pc, the stratum containing one-third or less of the total mass atz≤600–800 pc, and the halo at higherz-altitudes with a small fraction of such a mass which is difficult to evaluate.  相似文献   

20.
We construct an approximation for the magnetic field of galaxies that takes into account the magnetic helicity conservation law. In our calculations, we use the fact that the galactic disk is fairly thin and, therefore, the magnetic field component perpendicular to the galactic disk can be neglected (the so-called no-z approximation). However, an averaging of the magnetic field over the entire galaxy, as was done in previous works, is not performed. Our results are compared both with the approximation that disregards the helicity flux and with the results obtained in models with helicity fluxes but without averaging. We show that, compared to the classical model, there are a number of new effects (for example, magnetic field oscillations) and, compared to the model with averaging, the behavior of the magnetic field “softens”: its stationary value is reached more slowly and the oscillation amplitude decreases. This is because the dissipative processes changing the magnetic field growth rate are taken into account in our model. In contrast to the model with averaging, here it becomes possible to construct the dependence of the magnetic field and helicity on the distance from the galactic center.  相似文献   

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