Bright Knots Research Articles

Synchrotron nebulae created by anisotropic magnetized pulsar winds

In this paper, we give a detailed description of the first attempt to study the properties of the flow produced by a magnetized pulsar wind within a plerionic nebula via fully relativistic magnetohydrodynamic (MHD) simulations. Following the current theoretical models of pulsar winds, we assume that in the equatorial direction the magnetization of the wind drops to zero but its energy flux reaches a maximum. The results of our 2D axisymmetric simulations reveal complex dynamics of the post-shock flow, very different from the steady quasi-radial outflow assumed in earlier analytical models for plerions. The termination shock has the shape of a distorted torus and most of the downstream flow is initially confined to the equatorial plane. Provided the wind magnetization is higher than a certain value, the magnetic hoop stress stops the outflow in the surface layers of the equatorial disc and redirects it into magnetically confined polar jets. The outflow in the inner layers of the equatorial disc continues until it reaches the slowly expanding outer shell and then turns back and forms the vortex flow filling the nebular volume at intermediate latitudes. We simulated the synchrotron images of the nebula taking into account the relativistic beaming effect and the particle energy losses. These images are strikingly similar to the well-known images of the Crab and other pulsar wind nebulae obtained by Chandra and the Hubble Space Telescope. They exhibit both a system of rings, which makes an impression of an equatorial disc-like or even a toroidal structure, and well-collimated polar jets, which appear to originate from the pulsar. A number of fine details of the inner Crab nebula find natural explanation including the bright knot discovered by Hester et al. in 1995 very close to the Crab pulsar.

Relativistic models of two low-luminosity radio jets: B2 0326+39and B2 1553+24

We apply the intrinsically symmetrical, decelerating relativistic jet model developed by Laing & Bridle for 3C 31 to deep, full-synthesis 8.4-GHz VLA imaging of the two low-luminosity radio galaxies B2 0326+39 and B2 1553+24. After some modifications to the functional forms used to describe the geometry, velocity, emissivity and magnetic-field structure, these models can accurately fit our data in both total intensity and linear polarization. We conclude that the jets in B2 0326+39 and B2 1553+24 are at angles of 64° ± 5° and 7.°7 ± 1.°3 to the line of sight, respectively. In both objects, we find that the jets decelerate from 0.7-0.8c to < 0.2c over a distance of approximately 10 kpc, although in B2 1553+24 this transition occurs much further from the nucleus than in B2 0326+39 or 3C 31. The longitudinal emissivity profiles can be divided into sections, each fitted accurately by a power law; the indices of these power laws decrease with distance from the nucleus. B2 0326+39 also requires a discontinuity in emissivity to in order to fit a region with several bright knots of emission. In B2 1553+24, the sudden brightening of the jet can be explained by a combination of rapid expansion of the jet and a continuous variation of emissivity. The magnetic fields in both objects are dominated by the longitudinal component in the high-velocity regions close to the nucleus and by the toroidal component further out, but B2 0326+39 also has a significant radial component at large distances, whereas B2 1553+24 does not. Simple adiabatic models fail to fit the emissivity variations in the regions of high velocity but provide good descriptions of the emissivity after the jets have decelerated. Given the small angle to the line of sight inferred for B2 1553+24, there should be a significant population of similar sources at less extreme orientations. Such objects should have long (?200 kpc), straight, faint jets and we show that their true sizes are likely to have been underestimated in existing images.

Detection of Shocked Molecular Gas by Full-Extent Mapping of the Supernova Remnant W44

Molecular gas toward the supernova remnant (SNR) W44 (G34.7� 0.4) was extensively mapped in CO J ¼ 1 0 emission with the 17 0 beam of the Nobeyama 45 m radio telescope. We detected high-velocity (>25 km s � 1 ) CO line wings. They are confined to compact (� 1.5 pc) spots, and they are located adjacent to bright radio filaments or knots. The low 13 CO/ 12 CO intensity ratio of 0.03 and high HCO + / 12 CO intensity ratio of 0.3 suggest that the wing-emitting gas has a moderate 12 CO opacity of � 1 and a high density of n(H2 )>1 0 5 cm � 3 . This gas might be shocked molecular gas that has been accelerated and compressed by the expanding blast waves of W44. In addition, the high spatial resolution CO maps reveal several other features that may reveal the interaction between the SNR and the surrounding interstellar gas. The giant molecular cloud CO G34.8� 0.6 (vLSR ¼ 48 km s � 1 ) has a sharp edge coincident with the eastern radio continuum rim of W44, which may indicate that we observe the SNR/cloud interaction almost edge-on. The existence of the ‘‘edge’’ suggests that most of the molecular mass resides in smaller clumps that evaporate rapidly after the passage of the supernova blast wave. We also find spatially extended moderately broad emission (SEMBE) with a moderately large intensity of � 30 K km s � 1 in CO J ¼ 1 0 and a typical line width of � 10 km s � 1 (FWHM). Its extent coincides with the brighter region of the radio synchrotron emission. We discuss the SEMBE in terms of molecular clumps shocked and disturbed by the compressed shell of the SNR.

Jet-Gas Interactions in Markarian 78. I. Morphology and Kinematics

We present a detailed study of the Seyfert 2 galaxy Markarian 78, using continuum and emission-line images and multiaperture spectra from the Hubble Space Telescope (HST) and a deep 3.6 cm VLA image. Our overall aim is to study the interaction between the radio source and the emission-line gas, since ground-based data already indicate the presence of a strong bipolar jet-driven flow. First, in the wider context, Mrk 78 is probably a post-merger system, with a nuclear dust lane, approximate r1/4 continuum profile, and highly extended asymmetric gas distribution. The [O III] and radio images both show complex structures with many similarities but also important differences. A careful comparison shows convincing morphological evidence for jet-gas interaction: (1) the western inner radio jet terminates and flares at the position of a bright [O III] knot; (2) the weaker eastern radio jet changes direction as it encounters a large [O III] knot; (3) most [O III] features appear limb brightened on the upstream side, and flared on the downstream side; and (4) in the outer regions the radio components tend to lie between or adjacent to [O III] knots, indicating the radio and line emitting phases do not easily interpenetrate. In addition to evidence of jet-gas interaction, two features indicate the importance of a central ionizing radiation field: an inner fanlike structure to the east, with a straight kinematically quiet northern edge; and an approximately fanlike extended narrow-line region to the west, lying well outside the radio source. The [O III] line profiles from 10 Faint Object Spectrograph apertures provide further kinematic insight into the jet-gas interaction. (1) On the eastern side, where the radio source is deflected, the [O III] profile contains a highly redshifted component (~700 km s-1) that is also narrow (FWHM ≤ 200 km s-1), indicating significant coherent gas acceleration with almost no induced turbulence. (2) At this location, the [O III] profile is also double, suggesting lateral expansion away from a jet axis, as it burrows into the cloud complex. (3) In contrast, the bright western inner knot, which seems to disrupt the inner radio jet, has essentially undisturbed kinematics. (4) Across the outer complex western region, gas velocities are higher in the region center, decrease at the leading edge and may be highest where radio flows blow of the region. Overall, morphology and kinematics suggest the western side is best described as an initially disrupted jet that then fills, accelerates, and leaks out of a complex incomplete bubble of ionized gas. The eastern side is best described as a large centrally illuminated fanlike gas structure that is penetrated, accelerated, and ultimately deflects the radio source. We use these different regions to construct a plausible evolutionary sequence: Initially, a dense (molecular?) cloud enters the jet flow and disrupts it (inner western knot); as time passes, the jet begins to penetrate, accelerate, and ablate the cloud (eastern knot complex); continued jet influence further disperses the cloud fragments, sweeping out channels and gaps (western lobe region). This process may repeat itself many times over the lifetime of the jet. In companion papers, we study the ionization mechanisms and jet properties in more detail using recently acquired Space Telescope Imaging Spectrograph spectra, taking a more complete account of the energetics of the radio source and the ionized gas.

Chandra observations of SNR 1987A

We report on the results of our monitoring program of the X-ray remnant of supernova 1987A with the Chandra X-ray Observatory. We have performed two new observations during the Chandra Cycle 3 period, bringing the total to six monitoring observations over the past three years. These six observations provide a detailed time history of the birth of a new supernova remnant in X-rays. The high angular resolution images indicate that soft X-ray bright knots are associated with the optical spots, while hard X-ray features are better correlated with radio images. We interpret this in terms of a model in which fast shocks propagating through the circumstellar HII region produce the hard X-ray and radio emission, while the soft X-ray and optical emission arise in slower shocks entering into dense knots in the circumstellar inner ring. New observations begin to show changes in the morphology that may herald a new stage in the development of this incipient supernova remnant. The observed X-ray fluxes increase by nearly a factor of three over the last 30 months. The X-ray remnant is expanding at a velocity of ∼5000 km s −1.

60 Milliarcsecond Near‐Infrared Imaging of 3C 273 with Altair and Gemini11Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (US), the Particle Physics and Astronomy Research Council (UK), the National Research

ABSTRACTWe report data from the commissioning run of the Altair adaptive optics instrument on Gemini North. The bright QSO 3C 273 was observed at f/32 in the H and J bands, and at f/14 with J band, using the QSO itself as guide star. Resolution of approximately 60 mas full width at half‐maximum (FWHM) was achieved in H band. The data and point‐spread function are compared with those from the Hubble Space Telescope (HST). The Altair data allow a measurement of the spheroidal inner host galaxy to radii down to about 40 mas. A bright knot seen in visible wavelengths with the Advanced Camera for Surveys (ACS) instrument on HST is seen in the J but not the H band, and the dust lane seen with ACS is less visible, as expected. Other morphological features, and the jet, are discussed.

A $\mathsf{^{12}}$CO (J= 1 $\rightarrow$ 0) study towards the Ara OB1 region

Intermediate angular resolution (HPBW= 8: 0 7) carbon monoxide ( 12 CO J = 1!0) line observations towards the OB association Ara OB1 are reported. The molecular line observations cover an area of 2:75 3:00, and disclose a rich and complex distribution of the molecular line emission. The molecular features likely to be associated with the OB-association Ara OB1 span the velocity range from 28 to 20 km s 1 . The most negative radial velocities are observed along NGC 6188 (Rim nebula). This nebula marks the interface between the Hii region RCW 108 and the highly absorbing molecular material located westwards of the ionised region. The dominant CO structures, labeled E and F, have a radial velocity of about 23:5 and 21: 8k m s 1 , respectively. The former harbours a bright optical knot, the infrared cluster RCW-108 and the strong IRAS source 16362 4845. IRAS point sources are also seen in projection onto 12 CO concentrations F and H. This may indicate that the star forming process is a widespread phenomenon in this molecular complex. Extended continuum emission, having a high degree of spatial correlation with both the infrared and molecular emission, is observed in the region. We believe that this emission, thermal in nature, arises from the ionised surface layers of the molecular complex. The ionising agents are the high mass stars of NGC 6193, the nucleus of Ara OB1. The total amount of molecular gas related to Ara OB1 is about1:4 10 4 M.

Hubble Space Telescope imaging and ground-based spectroscopy of old nova shells - II. The bipolar shell of the slow nova HR Del

We present Hubble Space Telescope (HST) WFPC2 imaging of HR Del in filters centred on the lines of Hα, [N ii] and [O iii], with corresponding William Herschel Telescope (WHT) ISIS slit spectroscopy for the lines of Hα, Hβ, [N ii] and [O iii]. The data show that the ejected shell of HR Del is composed of many small bright knots. These are distributed in a bipolar structure with an equatorial ring, and polar rings or caps. The [O iii] emission is significantly more bipolar than that seen in Hα/[N ii]. Diffuse material which extends radially outward from these knots may be further evidence for a fast wind from the central binary system. There are also [N ii] and [O iii] knots at both higher and lower velocity than that of the main shell which follow the general bipolar symmetry and may be associated with secondary mass ejections at outburst or interactions with the fast wind. Three-dimensional models of the shell that reproduce the observed emission-line images and longslit spectra in both the [O iii] and Hα morphologies are presented. We use these models to estimate an inclination for the shell of 35°± 3°, a polar expansion velocity of 560 ± 50 km s−1, an axial ratio of 1.75 ± 0.15 and a distance of 970 ± 70 pc. We discuss how these parameters can be used to improve the shell shape and maximum magnitude versus rate of decline (SSRD and MMRD, respectively) relationships for novae.

Jets from subpc to kpc scale

The Chandra discovery of bright X-ray emission from kpc-scale jets provides us unprecedented insights into the physical state of the plasma in the flow. In particular, it is possible to get good constraints on the power and pressure in bright knots. For a group of selected sources with blazar-type cores it is also possible to constrain the physical quantities of the jet at sub-pc scale. We discuss how these results can help us to connect the properties of the jet at different scales.

Role of reconnection in AGN jets

We discuss the possible role of reconnection in electro-magnetically dominated cores of relativistic AGN jets. We suggest that reconnection may proceed in a two-fold fashion: initial explosive collapse on the Alfvén time-scale of a current-carrying jet (which is of the order of the light crossing time) and subsequent slow quasi-steady reconnection. Sites of explosive collapse are associated with bright knots, while steady-state reconnection re-energizes particles in the ‘bridges’ between the knots. Ohmic dissipation in reconnection layers leads to particle acceleration either by inductive electric fields or by stochastic particle acceleration in the ensuing electro-magnetic turbulence.

Relativistic parsec–scale jets: II. Synchrotron emission

We calculate the optically thin synchrotron emission of fast electrons and positrons in a spiral stationary magnetic field and a radial electric field of a rotating relativistic strongly magnetized force–free jet consisting of electron-positron pair plasma. The magnetic field has a helical structure with a uniform axial component and a toroidal component that is maximal inside the jet and decreasing to zero towards the boundary of the jet. Doppler boosting and swing of the polarization angle of synchrotron emission due to the relativistic motion of the emitting volume are calculated. The distribution of the plasma velocity in the jet is consistent with the electromagnetic field structure. Two spatial distributions of fast particles are considered: uniform, and concentrated in the vicinity of the Alfvén resonance surface. The latter distribution corresponds to the regular acceleration by an electromagnetic wave in the vicinity of its Alfvén resonance surface inside the jet. The polarization properties of the radiation have been obtained and compared with the existing VLBI polarization measurements of parsec–scale jets in BL Lac sources and quasars. Our results give a natural explanation of the observed bimodality in the alignment between the electric field vector of the polarized radiation and the projection of the jet axis on the plane of the sky. We interpret the motion of bright knots as a phase velocity of standing spiral eigenmodes of electromagnetic perturbations in a cylindrical jet. The degree of polarization and the velocity of the observed proper motion of bright knots depend upon the angular rotational velocity of the jet. The observed polarizations and velocities of knots indicate that the magnetic field lines are bent in the direction opposite to the direction of the jet rotation.

Clumps in large scale relativistic jets

We estimate the time dependent emission of the bright knots detected in large scale (tens to hundreds kpc) jets and discuss how the existing radio-to-X-ray observations, coupled with the correspondingly long cooling timescales, pose strong constraints on the structure and dynamics of knots. We suggest that the phenomenology of the jet in 3C 273 as observed by HST, Chandra and VLA, and in particular the fast decrease of the flux outside knots, can be satisfactorily accounted for if the emitting plasma is clumped. Clumping of the knots can also explain the month-year variability timescale reported for the large scale emission in M 87. We also point out that emission between the bright knots should be detected with the next generation of instruments (e.g. NGST). In fact the relatively intense X-ray emission discovered with Chandra suggests that jets (at least in powerful quasars) are still relativistic at large distances from the active nucleus. In such a case Compton scattering off the CMB photons is efficient even when electrons are cold or mildly relativistic in the comoving frame. This provides a powerful tool for testing the continuous vs discrete nature of the jet flow.

Arp 194: Evidence of Tidal Stripping of Gas and Cross-Fueling

We present new imaging and spectroscopic observations of the interacting system Arp 194 (≡UGC 6945 ≡ VV 126). The northern component (A194N) is a distorted spiral or ring galaxy likely disrupted by a collision or close encounter with a southern galaxy (A194S). There is evidence that a third galaxy with similar recession velocity is projected on A194N, but its role is likely secondary. A194S is connected to A194N by a string of emission knots, which motivates our interpretation that the former was the intruder. Three of the knots are easily discernible in B, R, and Hα images and are assumed to trace the path of the intruder following the encounter, which we estimate occurred a few times 108 yr ago. Both A194S and A194N are experiencing strong bursts of star formation: the Hα luminosity indicates a total star formation rate ~10 M⊙ yr-1. The lack of detectable J and K emission from the blobs, along with strong Hα emission, indicates that an evolved stellar population is not likely to be present. The brightest knot (closest to A194S) shows a star formation rate of ≈1.2 M⊙ yr-1, which, if sustained over a time ≈7 × 107 yr, could explain the spectral energy distribution. This suggests that the stripped matter was originally predominantly gaseous. The brightest knot is detected as a FIRST radio source, and this is likely the signature of supernova remnants related to enhanced star formation. Motions in the gas between the brightest knot and A194S, traced by an emission line link of increasing radial velocity, suggests infall toward the center of the intruder. Arp 194 is therefore one of the few galaxies where evidence of cross-fueling is observed.

Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe.

The impact of the Herbig--Haro object HH2 on local dust and gas

We present results from a study of molecular gas and dust in the vicinity of the Herbig Haro object HH2. Emission from the sub-mm continuum, 12CO and HCO+ was mapped with angular resolutions ranging from 14 arcsec to 5 arcsec (or 0.01pc at the distance of HH2). The continuum shows an extended dust clump of mass 3.8Msun and temperature 22K, located downstream of the bright optical HH knots. However, a compact emission peak lies within ~0.01pc of the low-excitation H2-prominent shocks, with a luminosity consistent with local heating by the outflow. The HCO+ emission shows two velocity components: firstly, ambient-velocity gas lying in a region roughly corresponding to the dust clump, with abundance enhanced by a factor of a few close to the H2-prominent knots. Secondly a component of high-velocity emission (20 km/s linewidth), found mainly in a collimated jet linking the low-excitation HH objects. In this high-velocity jet, the line wings show an abundance ratio HCO+/CO proportional to v^2, with an HCO+ enhancement compared with ambient gas of up to 10^3 at the most extreme velocities. Such high abundances are consistent with models of shock chemistry in turbulent mixing layers at the interaction boundaries of jets. Extrapolating this effect to low velocities suggests that the more modest HCO+ enhancement in the clump gas could be caused by low velocity shocks. A UV precursor may not, therefore be necessary to explain the elevated HCO+ abundance in this gas.

Radio/X-Ray Offsets of Large-Scale Jets Caused by Synchrotron Time Lags

In the internal shock scenario, we argue that electrons in most kiloparsec-scale (or even larger) jets can be accelerated to energies high enough to emit synchrotron X-rays, if shocks exist on these scales. These high-energy electrons emit synchrotron radiation at high frequencies and cool as they propagate downstream along the jet, emitting at progressively lower frequencies and resulting in time lags and hence radio/X-ray (and optical/ X-ray if the optical knot is detectable) offsets at bright knots, with the centroids of X-ray knots being closer to the core. Taking into account the strong effects of jet expansion, the behavior of radio/X-ray and optical/ X-ray offsets at bright knots in M87, Centaurus A, 3C 66B, 3C 31, 3C 273, and PKS 1127-145 is consistent with that of synchrotron time lags due to radiative losses. This suggests that the large-scale X-ray and optical jets in these sources are due to synchrotron emission.

Physical Conditions and Abundances in Abell 30

Abell 30 is one of a small number of PN known as born-again PN, which appear to be old nebulae whose central star has undergone a further ejection of highly processed material. It consists of a large, faint, spherical shell, in which several bright knots are embedded. The knots were first noted by Jacoby (1979), and analysis of their abundances by Hazard et al (1980) and Jacoby &amp; Ford (1983) showed that they are extremely hydrogen-deficient.

Detection of Optical Synchrotron Emission from the Radio Jet of 3C 279

We report the detection of optical and ultraviolet emission from the kiloparsec scale jet of the well-known quasar 3C~279. A bright knot, discovered in archival V and U band {\it Hubble Space Telescope} Faint Object Camera images, is coincident with a peak in the radio jet $\sim0.6\arcsec$ from the nucleus. The detection was also confirmed in Wide Field Planetary Camera-2 images. Archival Very Large Array and MERLIN radio data are also analyzed which help to show that the high-energy optical/UV continuum, and spectrum, are consistent with a synchrotron origin from the same population of relativistic electrons responsible for the radio emission.

Particle accelerators in the hot spots of radio galaxy 3C 445, imaged with the VLT.

Hot spots (HSs) are regions of enhanced radio emission produced by supersonic jets at the tip of the radio lobes of powerful radio sources. Obtained with the Very Large Telescope (VLT), images of the HSs in the radio galaxy 3C 445 show bright knots embedded in diffuse optical emission distributed along the post-shock region created by the impact of the jet into the intergalactic medium. The observations reported here confirm that relativistic electrons are accelerated by Fermi-I acceleration processes in HSs. Furthermore, both the diffuse emission tracing the rims of the front shock and the multiple knots demonstrate the presence of additional continuous re-acceleration processes of electrons (Fermi-II).

Spectrophotometric Observations of Blue Compact Dwarf Galaxies: Markarian 370

We present results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broadband and Hα narrowband observations, and on long-slit and two-dimensional spectroscopy of its brightest knots. The spectroscopic data are used to derive the internal extinction and to compute metallicities, electronic density, and temperature in the knots. By subtracting the contribution of the underlying older stellar population, modeled by an exponential function, removing the contribution from emission lines, and correcting for extinction, we can measure the true colors of the young star-forming knots. We show that the colors obtained this way differ significantly from those derived without the above corrections and lead to different estimates of the ages and star-forming history of the knots. Using predictions of evolutionary synthesis models, we estimate the ages of both the starburst regions and the underlying stellar component. We found that we can reproduce the colors of all the knots with an instantaneous burst of star formation and the Salpeter initial mass function (IMF) with an upper mass limit of 100 M☉. The resulting ages range between 3 and 6 Myr. The colors of the low surface brightness component are consistent with ages larger than 5 Gyr. The kinematic results suggest ordered motion around the major axis of the galaxy.