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