The unfriendly ISM in the radio galaxy 4C 12.50 (PKS 1345+12)

¹ Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands e-mail: morganti@astron.nl
² Dep. Physics and Astronomy, University of Sheffield, Sheffield, S7 3RH, UK
³ National Radio Astronomy Observatory, Socorro, NM 87801, USA

Received: 9 April 2004
Accepted: 25 May 2004

Abstract

The radio source 4C 12.50 has often been suggested to be a prime candidate for the link between ultraluminous infrared galaxies and young radio galaxies. A VLBI study of the neutral hydrogen in the nuclear regions of this object shows that most of the gas detected close to the systemic velocity is associated with an off-nuclear cloud ( to 100 pc from the radio core) with a column density of ) cm^-2 and an  mass of a few times 10⁵ to 10⁶ . We consider a number of possibilities to explain the results. In particular, we discuss the possibility that this cloud indicates the presence of a rich and clumpy interstellar medium in the centre, likely left over from the merger that triggered the activity and that this medium influences the growth of the radio source. The location of the cloud – at the edge of the northern radio jet/lobe – suggests that the radio jet might be interacting with a gas cloud. This interaction could be responsible for bending the young radio jet. The velocity profile of the gas is relatively broad (  km s^-1) and we interpret this as kinematical evidence for interaction of the radio plasma with the cloud. We also consider the model where the cloud is part of a broader circumnuclear structure. Only a limited region of this structure would have sufficient background radio brightness and large enough column depth in neutral gas to obtain detectable absorption against the counterjet. The VLBI study of the neutral hydrogen in 4C 12.50 suggests that  detected near the systemic velocity (as it is often the case in radio galaxies) may not necessarily be connected with a circumnuclear disk or torus (as is very often assumed) but instead could be a tracer of the large-scale medium that surrounds the active nucleus and that may influence the growth of the young radio source.