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1.
Jet-driven shocks are responsible for an important fraction of the emission of the narrow-line regions (NLRs) in many classes of AGN. However, this cannot explain all observations. It is clear that the remaining sources are photoionised by the active nucleus. The 2-d hydrodynamic models from the RSAA group support an evolutionary scenario whereby the shock-excited NLRs are initially jet-driven but later, ionizing photons from the central engine replace shocks as the main excitation mechanism and shock induced star formation may also become important. In their photoionized phase, dusty and radiation-pressure dominated evolution produces a self-regulated NLR spectrum. This model aso explains the coronal emission lines and fast (3000 km s-1) outflows seen in some Seyferts. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

2.
We suggest a model that explains the stratification peculiarities of the [O III] and Hα line emission from some of the ring nebulae around Wolf-Rayet stars. These peculiarities lie in the fact that the [O III] line emission regions are farther from the central star than the Hα regions, with the distance between them reaching several tenths of a parsec. We show that the radiative shock produced by a Wolf-Rayet stellar wind and propagating with a velocity of ～100 km s?1 cannot explain such large distances between these regions due to the low velocity of the gas outflow from the shock front. The suggested model takes into account the fact that the shock produced by a Wolf-Rayet stellar wind propagates in a two-phase medium: a rarefied medium and dense compact clouds. The gas downstream of a fast shock traveling in a rarefied gas compresses the clouds. Slow radiative shocks are generated in the clouds; these shocks heat the latter to temperatures at which ions of doubly ionized oxygen are formed. The clouds cool down, radiating in the lines of this ion, to temperatures at which Balmer line emission begins. The distance between the [O III] and Hα line emission regions is determined by the cooling time of the clouds downstream of the slow shock and by the velocity of the fast shock. Using the ring nebula NGC 6888 as an example, we show that the gas downstream of the fast shock must be at the phase of adiabatic expansion rather than deceleration with radiative cooling, as assumed previously.  相似文献   

3.
We present the first C-shock and radiative transfer model that calculates the evolution of the line profiles of neutral and ion species like SiO, H13CO+ and HN13C for different flow times along the propagation of the shock through the unperturbed gas. We find that the line profiles of SiO characteristic of the magnetic precursor stage have very narrow linewidths and are centered at velocities close to the ambient cloud velocity, as observed toward the young shocks in the L1448-mm outflow. Consistently with previous works, our model also reproduces the broad SiO emission detected in the high velocity gas in this outflow, for the downstream postshock gas in the shock. This implies that the different velocity components observed in L1448-mm are due to the coexistence of different shocks at different evolutionary stages.  相似文献   

4.
Immediate consequences of nuclear explosions on the structure and physical state of a galactic disk are considered in this paper. Explosions in the nucleus of a Galaxy generate strong shock waves which, when propagating onward heat and condensing the gas, form thin dense ring-like gaseous features behind it. Such rings and dense gaseous complexes have been observed in the central region of the Galaxy. These features have been treated here as the remnants of galactic shocks generated by nuclear explosions. We have estimated the time elapsed since the corresponding explosion, the energy released by explosion and the initial temperature and the velocity of the shock wave thus generated. The cooling of the gas heated by strong shocks has also been considered. The time taken by shock-heated gas to cool to its original temperature has been estimated to be of the order of 105 to 106 yr, according to the initial shock temperature which is about 9×106 K or 6.4×107 K. The rate of emission of energy and the total amount of energy dissipated away in the form of radiation in the cooling process, have been calculated for different values of initial shocktemperatures and also for different field intensities. The high-energy radiation emitted in the cooling process is suggested here as a source for the heating of dust grains, which ultimately are radiated in the infrared spectrum. Thus, a part of the infrared radiation, as measured by many authors, in the central region of the Galaxy, may originate ultimately from the cooling of the shock-heated gas there.  相似文献   

5.
The assumption that radiative cooling of gas in the centres of galaxy clusters is approximately balanced by energy input from a central supermassive black hole implies that the observed X-ray luminosity of the cooling flow region sets a lower limit on active galactic nucleus (AGN) mechanical power. The conversion efficiency of the mechanical power of the AGN into gas heating is uncertain, but we argue that it can be high even in the absence of strong shocks. These arguments inevitably lead to the conclusion that the time-averaged mechanical power of AGNs in cooling flows is much higher than the bolometric luminosity of these objects observed currently.
The energy balance between cooling losses and AGN mechanical power requires some feedback mechanism. We consider a toy model in which the accretion rate on to a black hole is set by the classic Bondi formula. Application of this model to the best studied case of M87 suggests that accretion proceeds at approximately the Bondi rate down to a few gravitational radii with most of the power (at the level of a few per cent of the rest mass) being carried away by an outflow.  相似文献   

6.
We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disc and massive H  i gas content. Radio continuum observations of the galaxy were carried out at 320, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disc at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with active galactic nucleus (AGN) activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the Hα flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centres can host significant AGN activity. We also mapped the H  i gas disc and velocity field in PGC 045080. The H  i disc extends well beyond the optical disc and appears warped. In the H  i intensity maps, the disc appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disc but is fairly uniform in the other half. We derived the H  i rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of ∼190 km s−1.  相似文献   

7.
The dissipation of energy from sound waves and weak shocks is one of the most promising mechanisms for coupling active galactic nucleus (AGN) activity to the surrounding intracluster medium, and so offsetting cooling in cluster cores. We present a detailed analysis of the weak shock found in deep Chandra observations of the Perseus cluster core. A comparison of the spectra either side of the shock front shows that they are very similar. By performing a deprojection analysis of a sector containing the shock, we produce temperature and density profiles across the shock front. These show no evidence for a temperature jump coincident with the density jump. To understand this result, we model the shock formation using 1D hydrodynamic simulations including models with thermal conduction and  γ < 5/3  gas. These models do not agree well with the data, suggesting that further physics is needed to explain the shock structure. We suggest that an interaction between the shock and the Hα filaments could have a significant effect on cooling the post-shock gas.
We also calculate the thermal energy liberated by the weak shock. The total energy in the shocked region is about 3.5 times the work needed to inflate the bubbles adiabatically, and the power of the shock is around  6 × 1044 erg s−1  per bubble, just over  1045 erg s−1  in total.  相似文献   

8.
Long-slit spectra of the molecular outflow Herbig–Haro (HH) 46/47 have been taken in the J and K near-infrared bands. The observed H2 line emission confirms the existence of a bright and extended redshifted counter-jet outflow south-west of HH 46. In contrast with the optical appearance of this object, we show that this outflow seems to be composed of two different emission regions characterized by distinct heliocentric velocities. This implies an acceleration of the counter-jet.
The observed [Fe  ii ] emission suggests an average extinction of 7–9 visual magnitudes for the region associated with the counter-jet.
Through position–velocity diagrams, we show the existence of different morphologies for the H2 and [Fe  ii ] emission regions in the northern part of the HH 46/47 outflow. We have detected for the first time high-velocity (−250 km s−1) [Fe  ii ] emission in the region bridging HH 46 to HH 47A. The two strong peaks detected can be identified with the optical positions B8 and HH 47B.
The H2 excitation diagrams for the counter-jet shock suggest an excitation temperature for the gas of T ex≈2600 K . The lack of emission from the higher energy H2 lines, such as the 4–3 S(3) transition, suggests a thermal excitation scenario for the origin of the observed emission. Comparison of the H2 line ratios with various shock models yielded useful constraints about the geometry and type of these shocks. Planar shocks can be ruled out whereas curved or bow shocks (both J- and C-type) can be parametrized to fit our data.  相似文献   

9.
IR lines are calculated for AGN by SVMA code. IR line ratios, as temperature and density diagnostic of the emitting gas, show that the [S III] lines ratio increases with shock velocity, following compression and cooling of the gas downstream. The results are compared with observations and briefly discussed.  相似文献   

10.
Over the past three decades the reverberation mapping technique was used to measure the central regions of Active Galactic Nuclei (AGN), their size, velocity field, and the mass of the black hole in the center. This technique was used mainly in the optical with several studies in the UV. Reverberation mapping in the UV adds essential information to the AGN studies. This paper reviews these recent studies done in the UV, presents results from the recent HST campaign toward NGC?5548, and discuss two projects of reverberation mapping of UV emission lines in high-luminosity quasars. The advantages of reverberation mapping in the UV will be discussed as well as the needs from new UV missions in order to be able to advance UV reverberation mapping campaigns.  相似文献   

11.
We present a joint analysis of near-ultraviolet ( NUV ) data from the GALEX ( Galaxy Evolution Explorer ) mission and (optical) colour profiles for a sample of seven brightest cluster galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG, which has a blue rest-frame UV colour, also shows a blue core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. Furthermore, the UV enhancement cannot be from an active galactic nuclei (AGN) because the spatial extent of the blue cores is significantly larger than the possible contamination region due to a massive black hole. The recent star formation in the blue BCGs typically has an age less than 200 Myr and contributes mass fractions of less than a per cent. Although the sample studied here is small, we demonstrate, for the first time , a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV enhancement observed using GALEX . The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster's X-ray properties, strongly suggests that the star formation is fuelled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and once this flow starts, it is nearly always active. Collectively, these results suggest that AGN feedback in present-day BCGs, while important, cannot be as efficient as suggested by the recent theoretical model by proposed by De Lucia et al.  相似文献   

12.
The dust‐to‐gas ratios in three different samples of luminous, ultraluminous, and hyperluminous infrared galaxies are calculated by modelling their radio to soft X‐ray spectral energy distributions (SED) using composite models which account for the photoionizing radiation from H II regions, starbursts, or AGNs, and for shocks. The models are limited to a set which broadly reproduces the mid‐IR fine structure line ratios of local, IR bright, starburst galaxies. The results show that two types of clouds contribute to the IR emission. Those characterized by low shock velocities and low preshock densities explain the far‐IR dust emission, while those with higher velocities and densities contribute to the mid‐IR dust emission. Clouds with shock velocities of 500 km s–1 prevail in hyperluminous infrared galaxies. An AGN is found in nearly all of the ultraluminous infrared galaxies and in half of the luminous infrared galaxies of the sample. High IR luminosities depend on dust‐to‐gas ratios as high as ∼0.1 by mass, however most hyperluminous IR galaxies show dustto‐gas ratios much lower than those calculated for the luminous and ultraluminous IR galaxies. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
As part of a large spectroscopic survey of   z > 5  Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R -band dropout galaxies at   z ∼ 5  and above, only J104048.6−115550.2 at   z = 5.44  shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N  v is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope ( HST ) imaging. This, and the comparatively high Lyα/N  v flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux  ( I AB= 26.1)  is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.  相似文献   

14.
A double-double radio galaxy (DDRG) is defined as consisting of a pair of double radio sources with a common centre. In this paper we present an analytical model in which the peculiar radio structure of DDRGs is caused by an interruption of the jet flow in the central AGN. The new jets emerging from the restarted AGN give rise to an inner source structure within the region of the old, outer cocoon. Standard models of the evolution of FRII sources predict gas densities within the region of the old cocoon that are insufficient to explain the observed properties of the inner source structure. Therefore additional material must have passed from the environment of the source through the bow shock surrounding the outer source structure into the cocoon. We propose that this material is warm clouds (∼104 K) of gas embedded in the hot IGM which are eventually dispersed over the cocoon volume by surface instabilities induced by the passage of cocoon material. The derived lower limits for the volume filling factors of these clouds are in good agreement with results obtained from optical observations. The long time-scales for the dispersion of the clouds (∼107 yr) are consistent with the apparently exclusive occurrence of the DDRG phenomenon in large (≳700 kpc) radio sources, and with the observed correlation of the strength of the optical/UV alignment effect in z ∼1 FRII sources with their linear size.  相似文献   

15.
The origin of rovibrational H2 emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line-luminous such systems, presented in the companion paper by Edge et al.
We consider excitation by X-rays from the surrounding intracluster medium (ICM), ultra-violet (UV) radiation from young stars, and shocks. The   v = 1–0  K -band lines with upper levels within  104 K  of the ground state appear to be mostly thermalized (implying gas densities  ≳105 cm−3  ), with the excitation temperature typically exceeding 2000 K, as found earlier by Jaffe, Bremer & van der Werf. Together with the lack of strong   v = 2–0  lines in the H -band, this rules out UV radiative fluorescence.
Using the cloudy photoionization code, we deduce that the H2 lines can originate in a population of dense clouds, exposed to the same hot  ( T ∼ 50 000 K)  stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the Hα-luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters.  相似文献   

16.
GRB 990123 was a long, complex gamma-ray burst accompanied by an extremely bright optical flash. We find different constraints on the bulk Lorentz of this burst to be consistent with the speculation that the optical light is emission from the reverse shock component of the external shock. Motivated by this currently favoured idea, we compute the prompt reverse shock emission to be expected for bursts in which multiwavelength observations allow the physical parameters to be constrained. We find that for reasonable assumptions about the velocity of source expansion, a strong optical flash  mV≈9  was expected from the reverse shocks, which were usually found to be mildly relativistic. The best observational prospects for detecting these prompt flashes are highlighted, along with the possible reasons for the absence of optical prompt detections in ongoing observations.  相似文献   

17.
In this paper, recent results obtained on highly radiative shocks generated in a xenon filled gas cell using the GEKKO XII laser facility are presented. Data show extremely high shock velocity (??150 km/s) never achieved before in gas. Preliminary analyses based on theoretical dimensionless numbers and numerical simulations suggest that these radiative shocks reach a new radiative regime where the radiative pressure plays a role in the dynamics and structure of the shock. A major effect observed is a strong anisotropic emission in the downstream gas. This unexpected feature is discussed and compared to available 2D radiation hydrodynamic simulations.  相似文献   

18.
We review the basic shock properties and the origin and the geometry of Herbig-Haro (H-H) shock waves. We first discuss different aspects of “normal” H-H objects which are connected with working surfaces (including internal working surfaces) of jets from young stellar objects. The emphasis is on unsolved problems of the H-H shock waves and not on the problems of the jet. We study the line flux ratios of high excitation H-H objects (high velocity shocks) and low excitation HH objects (low velocity shocks) and carry out a comparison with theoretical predictions in both cases. We emphasize an unexplained deficit of higher ions (especially OIII and SIII, but also various other ions) in high excitation objects. This lets the line flux ratios of HH objects appear as if their shock velocities are almost never above 100 km s?1, while other shock diagnostics (position-velocity diagrams, integrated line profiles, distributions of fluxes along the axis of the bow shock, etc.) definitely indicate higher shock velocities. Some aspects of the spectrum interpretation of the very low velocity shocks (like HH7) are explained quite well by the theory. A basic unsolved problem is, however, the explanation of the CI lines whose flux is up to a factor 10 times stronger than predicted for any model. Obviously we are very far from correctly predicting the ionization of C in shock models. In the last chapter we discuss, as one example of a very unusual HH-object, HH255 (Burnham's nebula). Detailed line fluxes in the immediate environment of T Tauri (the source of HH255) have shown that HH255 has a shock wave spectrum and is definitely an HH object. In the very narrow region between 3″ and 4″ S of T Tauri we find a sharp peak of the velocity dispersion, the centroid velocity, and Ne. In the same region there is an almost discontinous increase in ionization. Between 4″ and 10″ S (corresponding to 600-1600 a.u.) of T Tauri (the source of HH255) the ionization remains high but the centroid velocity is zero (with respect to T Tauri) and the velocity dispersion is very small. This result is completely surprising for a shock wave which according to the flux ratios must have ～90 km s?1-1 shock velocity. Why should a cooling region of a shock have a centroid velocity of ～0 km s?1 over a large range of distance from the stellar source? At present the geometry of the HH255 is enigmatic.  相似文献   

19.
Supernovae launch spherical shocks into the circumstellar medium (CSM). These shocks have high Mach numbers and may be radiative. We have created similar shocks in the laboratory by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas; ablated material from the pin rapidly expands and launches a shock through the surrounding gas. Laser pulses were typically 5 ns in duration with ablative energies ranging from 1–150 J. Shocks in ambient gas pressures of ~1 kPa were observed at spatial scales of up to 5 cm using optical cameras with schlieren. Emission spectroscopy data were obtained to infer electron temperatures (< 10 eV). In this experiment we have observed a new phenomena; at the edge of the radiatively heated gas ahead of the shock, a second shock forms. The two expanding shocks are simultaneously visible for a time, until the original shock stalls from running into the heated gas. The second shock remains visible and continues to expand. A minimum condition for the formation of the second shock is that the original shock is super-critical, i.e., the temperature distribution ahead of the original shock has an inflexion point. In a non-radiative control experiment the second shock does not form. We hypothesize that a second shock could form in the astrophysical case, possibly in radiative supernova remnants such as SN1993J, or in shock-CSM interaction.  相似文献   

20.
We present the results of modelling of the H2 emission from molecular outflow sources, induced by shock waves propagating in the gas. We emphasize the importance of proper allowance for departures from equilibrium owing to the finite flow velocity of the hot, compressed gas, with special reference to the excitation, dissociation and reformation of H2. The salient features of our computer code are described. The code is applied to interpreting the spectra of the outflow sources Cepheus A West and HH43. Particular attention is paid to determining the cooling times in shocks whose speeds are sufficient for collisional dissociation of H2 to take place; the possible observational consequences of the subsequent reformation of H2 are also examined. Because molecular outflow sources are intrinsically young objects, J-type shocks may be present in conjunction with magnetic precursors, which have a C-type structure. We note that very different physical and dynamical conditions are implied by models of C- and J-type shocks which may appear to fit the same H2 excitation diagram.  相似文献   

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