Other
Scientific paper
Apr 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........2l&link_type=abstract
Indian Institute of Science
Other
1
Galaxies, Active, Jets, Nuclei, Seyfert, Radio, Continuum
Scientific paper
Active Galactic Nuclei (AGN) are galaxies which harbour very powerful central regions whose energy output cannot be attributed directly to the stars alone. Seyfert galaxies are a class of AGN that are generally found in spiral-type host galaxies. Khachikian & Weedman categorised Seyfert galaxies into two subclasses, generally denoted as type 1 and type 2 which are distinguished by the presence or absence of extremely broad permitted emission lines. In recent years the notion that the two subclasses of Seyfert galaxies are intrinsically similar and differ only in their orientation with respect to our line of sight has gained considerable ground. This hypothesis is known as the unification scheme and requires the presence of an optically and physically thick obscuring torus which hides the central region when oriented edge-on. In this scheme, the Seyfert 1s are those where we have a direct view of the central region including the broad emission line clouds, and and the Doppler widths of the permitted emission lines observed are > 1000 km s-1. The Seyfert 2s are those where the line of sight to the central region is blocked by the torus and therefore only show permitted lines with Doppler widths < 1000 km s-1.
Seyfert galaxies have weak radio emission, but they do show radio emitting elongated jet-like structures on small-scales which appear to be the low-power analogues of jets seen in radio-powerful AGNs. The unified scheme predicts that the total radio emission should be similar in the two classes of Seyferts (since the radio emission is unattenuated by the obscuring torus), and their elongated radio structures should differ only by projection effects. However, this issue is controversial; e.g. in 1994 Roy et al. reported that compact radio emission is detected more in Seyfert 2s than in Seyfert 1s. This result is inconsistent with the predictions of the simple unified scheme.
The overall goal of the thesis is to rigorously test the predictions of the unified scheme for Seyfert galaxies. Specific objectives are
(a) to construct a sample of Seyfert 1s and Seyfert 2s that are matched in orientation-independent properties so that the two sub-samples are similar within the framework of the unification scheme,
(b) to obtain the nuclear (parsec-scale) radio structures for this sample,
(c) to test if the detection rate of compact parsec-scale radio structures is indeed higher in Seyfert 2 than in Seyfert 1 galaxies,
(d) to look for evidence of relativistic beaming in the nuclear regions of Seyfert galaxies, and
(e) to use data at other wavebands gleaned from literature for the sample to further test the predictions of the unified scheme.
A sample of Seyfert 1 and 2 galaxies, was constructed as follows: bona fide Seyfert galaxies were chosen, which were defined as those with line widths of [OIII]5007 > 300 km s-1, host galaxies that were confirmed spirals, and were radio-quiet, (i.e. (flux density at 5 GHz) / (flux density at B-Band) < 10). It was required that they have published arcsecond resolution data at 4-6 cm, and have a compact feature with flux density at λ4-6 cm > 8 mJy on these scales. Seyferts with edge-on host galaxies (observed ratio of minor and major isophotal diameter axes > 0.5) were avoided so as to minimise extinction effects of optical properties due to the host galaxy disc. 10 Seyfert 1s and 10 Seyfert 2s were then chosen from the set of sources meeting the above criteria such that the two subsamples were intrinsically similar in the unified scheme. This was done by matching them as far as possible in orientation-independent parameters viz., host galaxy properties and measures of intrinsic AGN power. The Seyfert 1s and Seyfert 2s have similar distributions of heliocentric redshift. The measures of intrinsic AGN power used were [OIII]5007 line widths, [OIII]5007 luminosity and galaxy bulge luminosity. The galaxy bulge luminosity correlates with the dispersion velocity of stars, and this latter parameter is a measure of the depth of the gravitational potential well, hence galaxy bulge luminosity was taken to be a measure of intrinsic AGN power. The orientation-independent host galaxy properties used were total stellar luminosity in the B-Band and the Hubble type of the galaxy.
In order to determine the parsec-scale radio structures for the sample, intercontinental Very Long Baseline Interferometry (VLBI) was required. An array consisting of the 10 stations of Very Long Baseline Array (VLBA), 3 stations of the European VLBI Network (EVN) and the phased-VLA was chosen. An observing frequency of 5 GHz was chosen to maximise the sensitivity of the array. The aforesaid array of 14 stations gave an angular resolution of 2-4 milli-arcseconds. The presence of two stations with large collecting area, viz. phased-VLA and Effelsberg, provided baselines with substantially lower detectable flux densities so that compact features with arcsec-scale flux density at λ4-6 cm > 8 mJy would be detectable. Due to the phased-VLA being one of the stations, we have simultaneous kiloparsec-scale data as well for all the objects observed, enabling a test of the relativistic beaming hypothesis.
15 of the 20 sample objects could be observed in the time allocated, all of which were detected. 4 of the remaining 5 sample objects have confirmed detection in the literature. Thus, nearly all objects from the sample are detected on parsec-scales and the detection rate of compact structures for Seyfert 1s and Seyfert 2s are similar. This result thus contradicts that of Roy et al. and is consistent with the predictions of the unified scheme. Further, the distribution of luminosities of the compact parsec-scale structure for the Seyfert 1s and Seyfert 2s are similar, as are the distribution of radio luminosities on the kiloparsec-scale.
Using the simultaneous VLBI and VLA measurements, distribution of the ratio of parsec to kiloparsec-scale radio flux densities were derived. The distributions for Seyfert 1s and Seyfert 2s do not differ significantly implying that evidence for relativistic beaming is at best marginal.
The distributions of far-infrared luminosity at 60 μ measured on scales of ˜ 5 arcminutes are similar for the two Seyfert sub-samples. There is weak evidence that the 2--10 keV X-ray luminosities are systematically higher for Seyfert 1s than Seyfert 2s.
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