A GMRT Study of Seyfert Galaxies NGC4235 & NGC4594: Evidence of Episodic Activity ?

TL;DR

This study uses GMRT low-frequency radio observations to find evidence of episodic activity in Seyfert galaxies NGC4235 and NGC4594, revealing relic radio lobes and supporting AGN-powered jet activity.

Contribution

First detection of episodic radio activity in NGC4235, with multi-wavelength analysis confirming AGN origin and providing insights into Seyfert galaxy jet power and classification.

Findings

NGC4235 shows tentative relic radio lobes indicating episodic activity.

Infrared SED modeling suggests low star formation rates compared to those needed for radio lobes.

Radio jets are aligned with radio lobes, supporting AGN-powered origin.

Abstract

Low frequency observations at 325 and 610 MHz have been carried out for two "radio-loud" Seyfert galaxies, NGC4235 and NGC4594 (Sombrero galaxy), using the Giant Meterwave Radio Telescope (GMRT). The 610 MHz total intensity and 325-610 MHz spectral index images of NGC4235 tentatively suggest the presence of a "relic" radio lobe, most likely from a previous episode of AGN activity. This makes NGC4235 only the second known Seyfert galaxy after Mrk6 to show signatures of episodic activity. Spitzer and Herschel infrared spectral energy distribution (SED) modelling using the clumpyDREAM code predicts star formation rates (SFR) that are an order of magnitude lower than those required to power the radio lobes in these Seyferts (~0.13-0.23 M_sun/yr compared to the required SFR of ~2.0-2.7 M_sun/yr in NGC4594 and NGC4235, respectively). This finding along with the detection of parsec and sub-kpc radio jets in both Seyfert galaxies, that are roughly along the same position angles as the radio lobes, strongly support the suggestion that Seyfert lobes are AGN-powered. SED modelling supports the "true" type 2 classification of NGC4594: this galaxy lacks significant dust obscuration as well as a prominent broad-line region. Between the two Seyfert galaxies, there is an inverse relation between their radio-loudness and Eddington ratio and a direct relation between their Eddington-scaled jet power and bolometric power.