Ylva M. Pihlstrom

EVN and MERLIN observations of III Zw 35 - A starburst continuum and an OH maser ring

EVN and MERLIN observations of the 18 cm continuum and OH maser emission in the luminous infrared galaxy III Zw 35 are presented. The continuum consists of two components. The rst component is made up of a number of compact sources, resembling the types of objects observed in Arp 220 and Mrk 273. In III Zw 35 the origin of these compact components is unclear, and we discuss the possibility they are individual young radio supernovae. Alternatively they could be clusters or clumps of young supernova remnants. The second continuum component is diuse and has an angular size of460 mas (230 pc) and Tb 10 5 K. We argue that this component is also powered by star formation. From the FIRflux we estimate a star formation rate of 19 M yr 1 ,w hich in turn implies a supernova rate of 0: 8y r 1 , sucient to power the diuse radio emission. Turning to the OH maser emission we nd, in agreement with earlier VLBI observations of III Zw 35, that the bright OH maser emission is concentrated in two compact clumps. However, in addition we observe diuse maser emission which lies in a clear ring structure of radius r 22 pc. The two compact maser features occur at the tangent points of the ring. This geometry and their similar 1667/1665 MHz line ratios suggests that the compact and diuse masers appear dierent because of path length eects and not pumping or other physical conditions. A simple model with many clouds within a ring seems able to t the main properties of both the diuse and compact maser emission. We nd a velocity gradient across the diuse maser emitting region which conrms rotation. From this gradient we estimate a dynamical mass of 7 10 6 M within r 22 pc. OH absorption is also present just outside the ring of maser emission. Both the OH maser emission and the continuum is weaker on the Eastern side of the source. We argue that this could be due to free-free absorption eects, which implies gas with an emission measure of 7 10 6 cm 6 pc.

COLA. III. Radio Detection of Active Galactic Nucleus in Compact Moderate Luminosity Infrared Galaxies

We present results from 4.8 GHz Very Large Array (VLA) and global very long baseline interferometry (VLBI) observations of the northern half of the moderate FIR luminosity (median L-IR = 10(11.01) L-circle dot) COLA sample of star-forming galaxies. VLBI sources are detected in a high fraction (20/90) of the galaxies observed. The radio luminosities of these cores (similar to 10(21) W Hz(-1)) are too large to be explained by radio supernovae or supernova remnants and we argue that they are instead powered by active galactic nuclei (AGNs). These sub-parsec scale radio cores are preferentially detected toward galaxies whose VLA maps show bright 100-500 parsec scale nuclear radio components. Since these latter structures tightly follow the FIR to radio-continuum correlation for star formation, we conclude that the AGN-powered VLBI sources are associated with compact nuclear starburst environments. The implications for possible starburst-AGN connections are discussed. The detected VLBI sources have a relatively narrow range of radio luminosity consistent with models in which intense compact Eddington-limited starbursts regulate the gas supply onto a central supermassive black hole. The high incidence of AGN radio cores in compact starbursts suggests little or no delay between the starburst phase and the onset of AGN activity.

H i Absorption in the Steep-Spectrum Superluminal Quasar 3C 216.

The search for H i absorption in strong compact steep-spectrum sources is a natural way to probe the neutral gas contents in young radio sources. In turn, this may provide information about the evolution of powerful radio sources. The recently improved capabilities of the Westerbork Synthesis Radio Telescope have made it possible to detect a 0.31% (19 mJy) deep neutral atomic hydrogen absorption line associated with the steep-spectrum superluminal quasar 3C 216. The redshift (z=0.67) of the source shifts the frequency of the 21 cm line down to the ultra-high-frequency (UHF) band (850 MHz). The exact location of the H i-absorbing gas remains to be determined by spectral line VLBI observations at 850 MHz. We cannot exclude that the gas might be extended on galactic scales, but we think it is more likely to be located in the central kiloparsec. Constraints from the lack of X-ray absorption probably rule out obscuration of the core region, and we argue that the most plausible site for the H i absorption is in the jet-cloud interaction observed in this source.

Continuum EVN and MERLIN Observations of Ultra Luminous Infrared Galaxies

Radio imaging of ULIR galaxies is ideal to explore the connection between the starburst and the AGN phenomenon since it is unaffected by dust obscuration, and provides the required high angular resolution to distinguish between an AGN and starburst emission. We have made combined 18 cm radio continuum, EVN and MERLIN observations of 13 ULIRGs that have the parsec and deci-parsec scale resolution necessary to distinguish between an AGN and supernovae remnants at the centres of these galaxies, and assess the contribution of each to the total energy distribution. Images of three galaxies are presented here.