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Register a new userThe warm molecular hydrogen of PKS B1718-649: feeding a newly born radio AGN
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Added by
Filippo Marcello Maccagni
Publication date
2016
fields
Physics
and research's language is
English
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We present new SINFONI VLT observations of molecular hydrogen (H2) in the central regions (< 2.5 kpc) of the youngest and closest radio source PKS B1718-649. We study the distribution of the H2 traced by the 1-0 S(1) ro-vibrational line, revealing a double disk structure with the kinematics of both disks characterised by rotation. An outer disk (r > 650 pc) is aligned with other components of the galaxy (atomic hydrogen, stars, dust), while the inner disk (r< 600 pc) is perpendicular to it and is polar with respect to the stellar distribution. However, in the innermost 75 pc, the data show the presence of H2 gas redshifted with respect to the rotating inner disk ($Delta v,$+150 km/s) which may trace gas falling into the super-massive black hole associated with the central radio source. Along the same line of sight, earlier observations had shown the presence in the central regions of PKS B1718-649 of clouds of atomic hydrogen with similar unsettled kinematics. The range of velocities and mass of these unsettled clouds of HI and H2 suggest they may be actively contributing in fuelling the central newly-born radio source.
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We present ALMA observations of the $^{12}$CO (2--1) line of the newly born ($t_mathrm{radio}sim10^2$ years) active galactic nucleus (AGN), PKS B1718-649. These observations reveal that the carbon monoxide in the innermost 15 kpc of the galaxy is distributed in a complex warped disk. In the outer parts of this disk, the CO gas follows the rotation of the dust lane and of the stellar body of the galaxy hosting the radio source. In the innermost kiloparsec, the gas abruptly changes orientation and forms a circumnuclear disk ($rlesssim700$ pc) with its major axis perpendicular to that of the outer disk. Against the compact radio emission of PKS B1718-649 ($rsim 2$ pc), we detect an absorption line at red-shifted velocities with respect to the systemic velocity ($Delta v = +365pm22$kms). This absorbing CO gas could trace molecular clouds falling onto the central super-massive black hole. A comparison with the near-infra red H$_{,2}$ 1-0 S(1) observations shows that the clouds must be close to the black hole ($rlesssim 75$ pc). The physical conditions of these clouds are different from the gas at larger radii, and are in good agreement with the predictions for the conditions of the gas when cold chaotic accretion triggers an active galactic nucleus. These observations on the centre of PKS B1718-649 provide one of the best indications that a population of cold clouds is falling towards a radio AGN, likely fuelling its activity.
We present neutral hydrogen (HI) and warm molecular hydrogen (H2) observations of the young (10^2 years) radio galaxy PKS B1718-649. We study the morphology and the kinematics of both gas components, focusing, in particular, on their properties in relation to the triggering of the radio activity. The regular kinematics of the large scale HI disk, seen in emission, suggests that an interaction event occurred too long ago to be responsible for the recent triggering of the radio activity. In absorption, we detect two absorption lines along the narrow line of sight of the compact (r<2 pc) radio source. The lines trace two clouds with opposite radial motions. These may represent a population of clouds in the very inner regions of the galaxy, which may be involved in triggering the radio activity. The warm molecular hydrogen (H2 1-0 S(1) ro-vibrational line) in the innermost kilo-parsec of the galaxy appears to be distributed in a circum-nuclear disk following the regular kinematics of the HI and of the stellar component. An exception to this behaviour arises only in the very centre, where a highly dispersed component is detected. These particular HI and H2 features suggest that a strong interplay between the radio source and the surrounding ISM is on-going. The physical properties of the cold gas in the proximity of the radio source may regulate the accretion recently triggered in this AGN.
We present new Australia Telescope Compact Array (ATCA) observations of the young (< 10^2 years) radio galaxy PKS B1718-649. We study the morphology and the kinematics of the neutral hydrogen (HI) disk (M(HI) = 1.1x 10^10 M(sun), radius ~ 30 kpc). In particular, we focus on the analysis of the cold gas in relation to the triggering of the nuclear activity. The asymmetries at the edges of the disk date the last interaction with a companion to more than 1 Gyr ago. The tilted-ring model of the HI disk shows that this event may have formed the disk as we see it now, but that it may have not been responsible for triggering the AGN. The long timescales of the interaction are incompatible with the short ones of the radio activity. In absorption, we identify two clouds with radial motions which may represent a population that could be involved in the triggering of the radio activity. We argue that PKS B1718-649 may belong to a family of young low-excitation radio AGN where, rather than through a gas rich merger, the active nuclei (AGN) are triggered by local mechanisms such as accretion of small gas clouds.
Warm absorbers are found in many AGN and consist of clouds moving at moderate radial velocities, showing complex ionization structures and having moderate to large column densities. Using 1D numerical calculations, we confirm earlier suggestions that the energy released by an AGN pushes the surrounding gas outward in a bubble until this reaches transparency. Typical AGN episode durations of $5times 10^4$ yr supply enough energy for this, except in very gas-rich and/or very compact galaxies, such as those in the early Universe. In those galaxies, the AGN might remain hidden for many periods of activity, hiding the black hole growth. The typical radii of $0.1-1$ kpc, velocities of $100-1000$ km s$^{-1}$ and resulting optical depths are consistent with observations of warm absorbers. The resulting structure is a natural outcome of outflows driven by AGN buried in an optically thick gas envelope, and has a total mass comparable to the final $M -sigma$ mass the central supermassive black hole will eventually reach.These results suggest that AGN can feed very efficiently by agitating this surrounding dense material. This may not be easy to observe, as this gas is Compton thick along many sightlines. The infall may produce episodic star formation in the centre, building up nuclear star clusters simultaneously with the growth of the central black hole.
Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spanning approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.
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