Seyfert Galaxies: Nuclear Radio Structure and Unification

Abstract

A radio study of a carefully selected sample of 20 Seyfert galaxies that are matched in orientation-independent parameters, which are measures of intrinsic active galactic nucleus power and host galaxy properties, is presented to test the predictions of the unified scheme hypothesis. Our sample sources have core flux densities greater than 8 mJy at 5 GHz on arcsec scales due to the feasibility requirements. These simultaneous parsec-scale and kiloparsec-scale radio observations reveal (1) that Seyfert 1 and Seyfert 2 galaxies have an equal tendency to show compact radio structures on milliarcsecond scales, (2) the distributions of parsec-scale and kiloparsec-scale radio luminosities are similar for both Seyfert 1 and Seyfert 2 galaxies, (3) there is no evidence for relativistic beaming in Seyfert galaxies, (4) similar distributions of source spectral indices in spite of the fact that Seyferts show nuclear radio flux density variations, and (5) the distributions of the projected linear size for Seyfert 1 and Seyfert 2 galaxies are not significantly different as would be expected in the unified scheme. The latter could be mainly due to a relatively large spread in the intrinsic sizes. We also find that a starburst alone cannot power these radio sources. Finally, an analysis of the kiloparsec-scale radio properties of the CfA Seyfert galaxy sample shows results consistent with the predictions of the unified scheme.

Measuring the level of nuclear activity in Seyfert galaxies and the unification scheme

“…The sample is selected such that the two Seyfert subtypes have matched distributions in the orientation-independent properties. We refer readers to see Lal et al (2011) for greater details on the sample selection. The sample was formulated to study the nuclear radio properties of Seyferts and is constrained by the Very Large Baseline Interferometer (VLBI) observing feasibility.…”

“…Therefore, the samples that have large variations in the AGN and host galaxy properties are expected to show the effect of these variations in the observed properties other than the differences owing to the differing orientations of the obscuring torus. Lal et al (2011) Seyfert sample based on the matched distributions of the orientation-independent parameters, minimizes the impact of differences caused by the differing properties of the AGN as well as host galaxy.…”

“…Middleton et al (2008) emphasized that Seyfert type 1s and type 2s need to be matched in intrinsic properties such as Eddington ratio in order to explore the orientation based differences. In fact, we follow the similar approach by using a sample in which two Seyfert subtypes are matched in orientation-independent properties (Lal et al 2011). Seyfert galaxies of our sample have Eddington ratios much lower compared to luminous AGNs (i.e., quasars showing Eddington ratios >10 −1 , Shemmer et al 2006) suggesting that the low luminosity of Seyfert AGN is likely due to low accretion A128, page 6 of 7 rate.…”

“…It has been argued that the samples based on the properties that are independent to the orientation of the obscuring torus are more appropriate to test the unification scheme (Schmitt et al 2003;Lal et al 2011;Singh et al 2011). Such samples are less prone to the biases that are generally inherent in the samples derived from flux limited surveys.…”

From Nearby Low Luminosity AGN to High Redshift Radio Galaxies: Science Interests with Square Kilometre Array

“…Jet Speeds from Multi-epoch VLBI Using VLBI images of a rigorously selected sample of Seyfert galaxies, Lal et al (2004) have shown that the radio properties of the compact parsec-scale features are consistent with the unified scheme for Seyfert galaxies (see Section 1), with no significant evidence for relativistic beaming in their jets (see also Lal et al 2011). Some parsec-scale features in these sources could be termination points of radio jets.…”