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The Nearest GHz Peaked Spectrum Radio Galaxy, PKS 1718-649
No full textIn this paper we identify PKS 1718-649, at a distance of 56 Mpc (z=0.014; H0=75 km s-1 Mpc-1, q0=O), as the nearest GHz peaked-spectrum (GPS) radio galaxy, more than four times closer than any previously known. Extensive observations at radio wavelengths with the Australia Telescope Compact Array, the Southern Hemisphere VLBI Experiment array, and the Swedish-ESO Submillimetre Telescope have allowed us to determine the properties of the radio source: PKS 1718-649 consists of two compact sub-pc-scale components separated by approximately 2 pc, the overall radio polarization is low, and the radio spectrum is peaked near 3 GHz. Order-of-magnitude agreement between the quantitative model for GPS sources of Bicknell et al. [ApJ (1997) (in press)] and the radio data we present, as well as data at optical wavelengths from the literature, raises the interesting possibility that PKS 1718-649 may be frustrated in its development by the nuclear environment of its host galaxy, NGC 6328. The model of Bicknell et al. (1997) suggests free-free absorption as an explanation of the PKS 1718-649 radio spectrum. However, both free-free absorption and synchrotron self-absorption mechanisms are plausible for this source and both may contribute to the overall radio spectrum. PKS 1718-649 provides evidence to strengthen the speculative suggestion that GPS sources arise as a consequence of galaxy merger activity. © 1997 American Astronomical Society
Verlustliste Nr. 649. (Nr. 649. /1918)
No full textVERLUSTLISTE NR. 649.
Verlustliste (-)
Verlustliste Nr. 649. (Nr. 649. /1918) ([1])
Belehrung. (2)
Offiziere. (3)
A, B, C, D, E, F, G (3)
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I, J, K, L, M, N, P, R, S (4)
T, V, W, Z (5)
Einjährig-Freiwillige. (5)
Mannschaft. (6)
A (6)
B (6)
C (10)
D (11)
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Berichtigungen zu den Verlustlisten: (58)
Nr. 142. Nr. 493. Nr. 573. Nr. 609. Nr. 616. Nr. 617. Nr. 621. Nr. 624. Nr. 625. (58
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dowie, D A, 649
No full textThis record was harvested from a previous catalogue system and will be withdrawn in 2025. Information in this record may be superseded or incomplete. Visit this record in UMA's new catalogue at: https://archives.library.unimelb.edu.au/nodes/view/382457Surname: DOWIE. Given Name(s) or Initials: D A. Military Service Number or Last Known Location: 649. Missing, Wounded and Prisoner of War Enquiry Card Index Number: 10100.213688
Item: [2016.0049.14750] "Dowie, D A, 649
ALMA observations of AGN fuelling. The case of PKS1718-649
Get PDFActive galactic nuclei may play a crucial role in regulating the
balance between accretion and ejection of gas in a galaxy.
Absorption of neutral hydrogen (HI) in radio AGN is a powerful tool to identify
cold gas flowing in and out of the active nucleus and to determine the
effects of the interplay between the cold gas and the nuclear activity.
Recently, we observed 250 radio galaxies with the Westerbork Synthesis
Radio Telescope, detecting HI gas in 20% of them. In this talk, I will
show how different shapes, widths and optical depths of the HI
absorption lines that we identified trace different distributions of
gas, such as circumnuclear disks, and infalling and outflowing clouds.
In this talk, I will also present a multi-wavelength study of the cold
gas of PKSB 1718-649. In this young (~100 years) radio-AGN, we detected
two HI absorption lines tracing gas that may fuel the central black
hole. Follow-up SINFONI and ALMA observations allow us to detect
molecular gas that may fuel the nuclear activity. In particular, against
the compact radio emission of the source (r~2pc), we detect carbon
monoxide (CO) gas in absorption at red-shifted velocities with respect
to the systemic velocity (v = +365±22 km/s). This CO (2-1) absorption
line could trace molecular clouds falling onto the central super-massive
black hole. A comparison with the SINFONI observations of the H2 1-0
S(1) line shows that the clouds must be close to the black hole (r<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 indication that a population of cold clouds is
falling towards a radio AGN, likely fuelling its activity
An Investigation of Synchrotron Self-absorption and Free-Free Absorption Models in Explanation of the Gigahertz-peaked Spectrum of PKS 1718-649
No full textWe have undertaken the most detailed investigation yet of the shape and variability of the radio spectrum near the spectral peak of a gigahertz-peaked spectrum radio galaxy, using the Australia Telescope Compact Array. The radio spectrum of PKS 1718-649 was measured nearly simultaneously at 30-40 different frequencies over the range of 1-9 GHz at four epochs over a 14 month period, with the aim of constraining different physical models for the absorption process that causes the spectral peak at gigahertz frequencies. We find that the two most plausible absorption processes, synchrotron self-absorption and free-free absorption, can each explain most aspects of the data. However, each process also has difficulty explaining some aspects of the data, in particular the variability of the source at frequencies below the peak. In either case, models based on homogeneous absorbers are clearly inadequate and inhomogeneities must be introduced into either the synchrotron self-absorbed source or the free-free absorbing screen. Based on our analysis we cannot strongly prefer either synchrotron self-absorption or free-free absorption as the sole process responsible for the gigahertz-peaked spectrum of PKS 1718-649. However, the consistency of the measured source size, from VLBI observations, with the observed turnover frequency favors synchrotron self-absorption as a significant effect in PKS 1718-649
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