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Register a new userEvidence for a Circumnuclear Disk in 1946+708
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Authors
A. B. Peck
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A number of extragalactic radio sources which exhibit symmetric jets on parsec scales have now been found to have neutral hydrogen absorption at or near the systemic velocities of their host galaxies. Understanding the spatial distribution and kinematics of the HI detected toward the central parsecs of these sources provides an important test of unified schemes for AGN. We present results of Global VLBI Network observations of the redshifted 21 cm HI line toward the Compact Symmetric Object 1946+708 (z=0.101). We find significant structure in the gas on parsec scales. The peak column density of the HI (N_HI~3x10^23 cm^-2(Ts/8000K)) occurs near the center of activity of the source, as does the highest velocity dispersion (FWHM 350 to 400 km/s). There is also good evidence for a torus of ionized gas with column density 7x10^22 cm^-2}. The jets in 1946+708 exhibit bi-directional motion measurable on timescales of a few years. The resulting unique information about the geometry of the continuum source greatly assists in the interpretation of the gas distribution, which is strongly suggestive of a circumnuclear torus of neutral atomic and ionized material with one or more additional compact clumps of gas along the line of sight to the approaching jet.
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We report on a multi-frequency, multi-epoch campaign of Very Long Baseline Interferometry observations of the radio galaxy 1946+708 using the VLBA and a Global VLBI array. From these high-resolution observations we deduce the kinematic age of the radio source to be $sim$4000 years, comparable with the ages of other Compact Symmetric Objects (CSOs). Ejections of pairs of jet components appears to take place on time scales of 10 years and these components in the jet travel outward at intrinsic velocities between 0.6 and 0.9 c. From the constraint that jet components cannot have intrinsic velocities faster than light, we derive H_0 > 57 km s^-1 Mpc^-1 from the fastest pair of components launched from the core. We provide strong evidence for the ejection of a new pair of components in ~1997. From the trajectories of the jet components we deduce that the jet is most likely to be helically confined, rather than purely ballistic in nature.
We present Very Long Baseline Array (VLBA) HI absorption observations of the core region of the powerful radio galaxy Cygnus A. These data show both broad (FWHM = 231 pm 21 km/s) and narrow (FWHM <30 km/s) velocity width absorption components. The broad velocity absorption shows high opacity on the counter jet, low opacity against the core and no absorption on the jet side. We argue that these results are most naturally explained by a circumnuclear HI absorbing disk orientated roughly perpendicular to the jet axis. We estimate that the HI absorbing gas lies at a radius of ~80 pc has a scale height of about 20 pc, density n > 10^{4} cm^{-3} and total column density in the range 10^{23}-10^{24} cm^{-2}. Models in which the HI absorption is primarily from an atomic or a molecular gas phase can both fit our data. Modelling taking into account the effective beam shows that the broad HI absorbing gas component does not cover the radio core in Cygnus A and therefore does not contribute to the gas column that blocks our view of the hidden quasar nucleus. If however Cygnus A were observed from a different direction, disk gas on ~100 pc radius scales would contribute significantly to the nuclear column density, implying that in some radio galaxies gas on these scales may contribute to the obscuration of the central engine. We argue that the circumnuclear torus in Cygnus A contains too little mass to power the AGN over > 10^{7} yr but that material in the outer HI absorbing gas disk can provide a reservoir to fuel the AGN and replenish torus clouds. The second narrow HI absorption component is significantly redshifted (by 186km/s) with respect to the systemic velocity and probably traces infalling gas which will ultimately fuel the source. [abridged]
We present a first attempt to construct an analytic model for a clumped gas and dust disk and apply it to the Galactic Centre. The clumps are described as isothermal spheres partially ionized by the external UV radiation field. The disk structure formed by the clouds is described as a quasi standard continuous accretion disk using adequately averaged parameters of the discrete cloud model. The viscosity in the Circumnuclear Disk is due to partially inelastic cloud-cloud collisions. We find two different solutions for the set of equations corresponding to two stable cloud regimes: (i) the observed molecular clouds and (ii) much lighter and smaller clouds which correspond to the stripped cores of the observed clouds. It is shown that the resulting physical characteristics of the heavy clouds and the disk are in very good agreement with all comparable observations at multiple wavelengths. A mass accretion rate of approx. 10^-4 M_solar/yr for the isolated Circumnuclear Disk is inferred. We propose that the Circumnuclear Disk has a much longer lifetime (approx. 10^7 yr) than previously assumed.
We have found a photoevaporated disk in the Orion Nebula that includes a wide binary. HST/ACS observations of the proplyd 124-132 show two point-like sources separated by 0.15, or about 60 AU at the distance of Orion. The two sources have nearly identical I and z magnitudes. We analyze the brightest component, Source N, comparing the observed magnitudes with those predicted using a 1 Myr Baraffe/NEXTGEN isochrone with different accretion luminosities and extinctions. We find that a low mass (simeq 0.04 M_odot) brown dwarf ~1 Myr old with mass accretion rate logdot{M}simeq -10.3, typical for objects of this mass, and about 2 magnitudes of visual extinction provides the best fit to the data. This is the first observation of a circumbinary disk undergoing photoevaporation and, if confirmed by spectroscopic observations, the first direct detection of a wide substellar pair still accreting and enshrouded in its circumbinary disk.
Recent VLBI observations have identified several compact radio sources which have symmetric structures on parsec scales, and exhibit HI absorption which appears to be associated with the active nucleus. These sources are uniquely well suited to investigations into the physics of the central engines, in particular to studies of the kinematics of the gas within 100 pc of the core. In these compact sources, it is reasonable to assume that this circumnuclear material is accreting onto, and ``feeding, the central engine. We present results of HI imaging studies of 3 symmetric radio galaxies which show evidence of a circumnuclear torus.
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