Observations of the neutral gas and dust in the radio galaxy 3C305

Abstract

We present MERLIN and Hubble Space Telescope (HST) observations of the central region of the nearby radio galaxy 3C305 and use them to study the gas and dust in this object. TheMERLIN observations are of neutral hydrogen (Hi ) absorption against the strong non-thermal 20cm continuum seen towards the central 4 kpc of 3C305. Our 0.′′2 (160 pc) resolution observations show that the Hi absorption is highly localised against the south-western radio-emission with column densities 1.9×1021 cm−2. The absorption is broad (full width at half maximum, FWHM, of 145±26 km s−1) and red- shifted by 130 km s−1 relative to the systemic velocity. The HST images in multiple optical and infrared filters (430 nm, 702 nm, [Oiii] 500.7 nm, [Feii] 1.64µm and K-band polarization) are presented. Evidence is seen for coincidence of the [Feii] emission with the knot at the end of the radio jet, which is evidence for the presence of shocks. We compare the optical and radio images in order to investigate the relationship between the dust and neutral gas distributions. An unresolved (0.′′07) nucleus is detected in H and K and its properties are consistent with a quasar reddened by AV > 4. We propose that the absorption arises in a region of neutral gas and dust. Its structure is complex but is broadly consistent with an inclined disc of gas and dust encircling, but not covering, the active galactic nucleus. A comparison of the neutral gas observations and previous emission-line observations suggests that both the neutral and ionised gas are undergoing galactic rotation towards the observer in the north-east and away from the observer in the south-west. We propose that the outflow giving rise to the radio emission has a component towards the observer in the north-east and away from the observer in the south-west. Unfortunately as we do not detect radio emission from the compact nucleus we cannot set limits to neutral hydrogen absorption from a circumnuclear obscuring torus. (Refer to PDF file for exact formulas).