Jet-Cloud Interactions and the Brightening of the Narrow-Line Region in Seyfert Galaxies (original) (raw)
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Jet cocoons and the formation of narrow-line clouds in Seyfert galaxies
Monthly Notices of the Royal Astronomical Society, 1997
We present non-adiabatic hydrodynamic simulations of a supersonic light jet propagating into a fully ionized medium of uniform density on a scale representative of the narrow line region (NLR) in Seyfert galaxies with associated radio jets. In this regime the cooling distance of the swept up gas in the bowshock of the jet is of the same order as the transverse extent of the jet bowshock, as opposed to the more extreme regimes found for more powerful adiabatic large scale jets or the slow galactic jets which have been simulated previously. We calculate the emissivity for the Hα line and radio synchrotron emission. We find that the structure of the line emitting cold envelope of the jet cocoon is strongly dependent on the non-stationary dynamics of the jet head as it propagates through the ambient medium. We observe the formation of cloud-like high density regions which we associate with NLR clouds and filaments. We find that some of these clouds might be partially neutral and represent sites of jet induced star formation. The calculated Hα flux and the spectral line width are consistent with NLR observations. The simulation of the radio-optical emission with radiative cooling confirms the basic result of the geometric bowshock model developed by Taylor et al. (1989) that the start of noticeable optical line emission can be significantly offset from the hotspot of the radio emission. However, the time-dependent nature of the jet dynamics implies significant differences from their geometric bowshock model.
Jet cocoons in rotating Seyfert galaxies: adaptive three-dimensional hydrodynamics
Monthly Notices of the Royal Astronomical Society, 2001
The narrow-line regions of some Seyfert galaxies show evidence for nuclear jets interacting with the rotating interstellar gas; this is shown by point-symmetric emission-line structures in, for example, Mrk 573 and NGC 3393. We study this situation with numerical simulations of a jet in a sidewind of uniform density but linearly increasing velocity as one moves from the source. We use a new three-dimensional hydrodynamic code on a binary adaptive grid. We consider two different models, one with a cocoon expansion speed higher and one with expansion speed lower than the ISM speed. We find that the model with high cocoon expansion speed is similar to results from previous calculations without a sidewind, except for minor asymmetries. However, model B with the slow expansion speed and fast wind speed shows considerable qualitative differences. The jet hits and bounces off the dense cooling envelope, which is dragged by the sidewind into the straight path of the jet. The path of the jet within the cocoon is straight as long as the extended hot cocoon acts as a shield. Once the jet hits the cold envelope of the cocoon it is bent directly by the ram pressure of the ambient medium and follows a parabola of the third degree, which we derive as an analytical approximation for the path. The region where the jet hits the envelope is the start of strong radio emission. This point moves towards the source with age of the jet and its bending angle. We therefore find a possible observable correlation between the distance of the first strong radio knot and the overall bending of jets in Seyfert galaxies. A comparison of our results with observations of Mrk 573 shows that the essential structural and spectral features can be reproduced by choosing an appropriate viewing angle and evolutionary stage. Looking approximately along the original jet direction a structure is found which strongly resembles an ionization cone. Hence caution should prevail when interpreting these sorts of structure within the narrow-line region of Seyfert galaxies.
Shadowing of the Nascent Jet in NGC 4261 by a Line‐emitting Supersonic Accretion Disk
The Astrophysical Journal, 2003
NGC 4261 (3C 270) is a low-luminosity radio galaxy with two symmetric kiloparsec-scale jets. Earlier Hubble Space Telescope observations indicated the presence of a hundred-parsec scale disk of cool dust and gas surrounding a central, supermassive (∼ 4.9 × 10 8 M ⊙ ) black hole. The recent detection of free-free radio absorption by a small, geometrically-thin disk, combined with earlier studies of the disk's large scale properties, provide the strictest constraints to date on the nature of the accretion process in this system. We show here that a supersonic disk, illuminated by the active galactic nucleus (AGN), can not only account for the observed radio shadowing, but can also produce the optical broad lines emitted from this region. At large radii, the gas is optically-thin because the ram pressure due to turbulence is much larger than the thermal pressure of the gas. At smaller radii, but beyond a critical radius r c , line cooling dominates over gravitational dissipation and the gas is effectively cooled down to temperatures below 10 4 K. Within r c , however, heating due to 1 NSF Graduate Fellow.
Interaction of jets with the ISM of radio galaxies
Astrophysics and Space Science, 2007
We present three dimensional simulations of the interaction of a light hypersonic jet with an inhomogeneous thermal and turbulently supported disk in an elliptical galaxy. These simulations are applicable to the GPS/CSS phase of some extragalactic radio sources. We identify four generic phases in the evolution of such a jet with the interstellar medium. The first is a 'flood and channel" phase, dominated by complex jet interactions with the dense cloudy medium close to the nucleus. This is characterized by high pressure jet gas finding changing weak points in the ISM and flowing through channels that form and reform over time. A spherical, energy driven, bubble phase ensues, wherein the bubble is larger than the disk scale, but the jet remains fully disrupted close to the nucleus, so that the jet flux is thermalised and generates a smooth isotropic energy-driven bubble. In the subsequent, rapid, jet break-out phase the jet breaks free of the last obstructing dense clouds, becomes collimated and pierces the more or less spherical bubble. In the final classical phase, the jet propagates in a momentum-dominated fashion similar to jets in single component hot haloes, leading to the classical jet-cocoonbow-shock structure.
The Astrophysical Journal, 2003
Sensitive high angular and linear resolution radio images of the 240-pc radio jet in NGC 4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA and phased VLA at λ21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter ∼ <1.4 pc) underlying discrete components, seen in lower -2resolution MERLIN and VLA images, that appear to be shock-like features associated with changes in direction as the jet interacts with small gas clouds within the central ∼100 pc of the galaxy. In addition, λ21-cm spectral line imaging of the neutral hydrogen in the nuclear region reveals the spatial location, distribution and kinematics of the neutral gas detected previously in a lower resolution MER-LIN study. Neutral hydrogen absorption is detected against component C4W (E+F) as predicted by Mundell et al, but the absorption, extending over 3 pc, is spatially and kinematically complex on sub-parsec scales, suggesting the presence of small, dense gas clouds with a wide range of velocities and column densities. The main absorption component matches that detected in the MERLIN study, close to the systemic velocity (998 km s −1 ) of the galaxy, and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue-and red-shifted systems with narrow linewidths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 × 10 19 T S < N H < 1.7 × 10 20 T S cm −2 K −1 (T S is the spin temperature), with average radial velocities in the range 920 < V r < 1050 km s −1 . The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the AGN (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D (possibly subcomponent D3) is the most likely location for the AGN. We suggest that components C and E are shocks produced in the jet as the plasma encounters, and is deviated by, dense clouds with diameters smaller than ∼1.4 pc.
A relativistic mixing-layer model for jets in low-luminosity radio galaxies
Monthly Notices of the Royal Astronomical Society, 2009
We present an analytical model for jets in Fanaroff & Riley Class I (FR I) radio galaxies, in which an initially laminar, relativistic flow is surrounded by a shear layer. We apply the appropriate conservation laws to constrain the jet parameters, starting the model where the radio emission is observed to brighten abruptly. We assume that the laminar flow fills the jet there and that pressure balance with the surroundings is maintained from that point outwards. Entrainment continuously injects new material into the jet and forms a shear layer, which contains material from both the environment and the laminar core. The shear layer expands rapidly with distance until finally the core disappears, and all of the material is mixed into the shear layer. Beyond this point, the shear layer expands in a cone and decelerates smoothly. We apply our model to the well-observed FR I source 3C 31 and show that there is a self-consistent solution. We derive the jet power, together with the variations of mass flux and entrainment rate with distance from the nucleus. The predicted variation of bulk velocity with distance in the outer parts of the jets is in good agreement with model fits to Very Large Array observations. Our prediction for the shape of the laminar core can be tested with higher-resolution imaging.
The parsec-scale structure of jet-driven H I out ows in radio galaxies
Proceedings of the International Astronomical Union
Radio jets can play multiple roles in the feedback loop by regulating the accretion of the gas, by enhancing gas turbulence, and by driving gas outflows. Numerical simulations are beginning to make detailed predictions about these processes. Using high resolution VLBI observations we test these predictions by studying how radio jets of different power and in different phases of evolution affect the properties and kinematics of the surrounding H I gas. Consistent with predictions, we find that young (or recently restarted) radio jets have stronger impact as shown by the presence of H I outflows. The outflowing medium is clumpy with clouds of with sizes up to a few tens of pc and mass ∼ 104Mȯ) already in the region close to the nucleus (< 100 pc), making the jet interact strongly and shock the surrounding gas. We present a case of a low-power jet where, as suggested by the simulations, the injection of energy may produce an increase in the turbulence of the medium instead of an out...
Active galactic nuclei jet-induced feedback in galaxies - I. Suppression of star formation
Monthly Notices of the Royal Astronomical Society, 2008
Relativistic jets originating from Supermassive Black Holes (SBHs) can have a considerable impact on the Interstellar/Intergalactic Medium (ISM/IGM) within which they propagate. Here we study the interaction which a relativistic jet, and the cocoon associated with its penetration into the ISM, has on the evolution of a dense cloud, placed very near the cocoon's path, by analyzing a series of high-resolution numerical simulations, and studying the dependence on jet input power, between P jet = 10 41 − 10 47 erg/sec. The density Probability Distribution Function (PDF) within the cocoon can be described in terms of two distinct components, which are also spatially distinct: a low-and a high-density component. The former is associated with the shocked gas within the internal region of the cocoon, while the latter is associated with the outer, shocked region of the cocoon itself. The PDF of the post-shocked region is well approximated by a modified lognormal distribution, for all values of P jet . During the active phase, when the jet is fed by the AGN, the cloud is subject both to compression and stripping, which tend to increase its density and diminish its total mass. When the jet is switched off (i.e. during the passive phase) the shocked cloud cools further and tends to become more filamentary, under the action of a back-flow which develops within the cocoon. We study the evolution of the star formation rate within the cloud, assuming this is determined by a Schmidt-Kennicutt law, and we analyze the different physical factors which have an impact on the star formation rate. We show that, although the star formation rate can occasionally increase, on time scales of the order of 10 5 − 10 6 yrs, the star formation rate will be inhibited and the cloud fragments. The cooling time of the environment within which the cloud is embedded is however very long: thus, star formation from the fragmented cloud remains strongly inhibited.
Jet-Induced Emission-Line Nebulosity and Star Formation in the High-Redshift Radio Galaxy 4C41.17
1999
The high redshift radio galaxy 4C41.17 consists of a powerful radio source in which previous work has shown that there is strong evidence for jet-induced star formation along the radio axis. We argue that nuclear photoionization is not responsible for the excitation of the emission line clouds and we construct a jet-cloud interaction model to explain the major features revealed by the data. The interaction of a high-powered jet with a dense cloud in the halo of 4C41.17 produces shock-excited emission-line nebulosity through ~1000 km/s shocks and induces star formation. The CIII to CIV line ratio and the CIV luminosity emanating from the shock, imply that the pre-shock density in the line-emitting cloud is high enough (~1-10 cm^-3) that shock initiated star formation could proceed on a timescale of order a few x 10^6 yrs, well within the estimated dynamical age of the radio source. Broad (FWHM ~ 100 - 1400 km/s) emission lines are attributed to the disturbance of the gas cloud by a partial bow--shock and narrow emission lines (FWHM ~ 500 - 650 km/s) (in particular CIV) arise in precursor emission in relatively low metallicity gas. The implied baryonic mass ~ 8 \times 10^{10} solar masses of the cloud is high and implies that Milky Way size condensations existed in the environments of forming radio galaxies at a redshift of 3.8. Our interpretation of the data provides a physical basis for the alignment of the radio, emission-line and UV continuum images in some of the highest redshift radio galaxies and the analysis presented here may form a basis for the calculation of densities and cloud masses in other high redshift radio galaxies.
HIGH-ENERGY EMISSION FROM JET–CLOUD INTERACTIONS IN AGNs
International Journal of Modern Physics D, 2010
Active galactic nuclei present continuum and line emission. The emission lines are originated by gas located close to the central supermassive black hole. Some of these lines are broad, and would be produced in a small region called broad-line region. This region could be formed by clouds surrounding the central black hole. In this work, we study the interaction of such clouds with the base of the jets in active galactic nuclei, and we compute the produced high-energy emission. We focus on sources with low luminosities in the inner jet regions, to avoid strong gamma-ray absorption. We find that the resulting high-energy radiation may be significant in Centaurus A. Also, this phenomenon might be behind the variable gamma-ray emission detected in M87, if very large dark clouds are present. The detection of jet–cloud interactions in active galactic nuclei would give information on the properties of the jet base and the very central regions.