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Evidence for Galaxy Interactions in Powerful Radio Galaxies

Testing the idea that the powerful radio galaxies (PRGs) are triggered in galaxy interactions, Cristina Ramos Almeida, Clive Tadhunter (University of Sheffield, UK), and collaborators obtained deep imaging data using the Gemini Multi-Object Spectrograph (GMOS) at the Gemini South telescope to study the morphologies of a complete sample of 46 intermediate redshift (0.05<z<0.7) PRGs. In what represents the first systematic imaging study of a major sample of radio galaxies using an 8-meter-class telescope, the team searched for signs of nearby companions in the 2Jy southern radio galaxy sample to look for secondary nuclei, shells, and extended low surface brightness features such as tidal tails and bridges.
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Title: The optical morphologies of the 2Jy sample of radio galaxies: evidence for galaxy interactions
Authors: Cristina Ramos Almeida, Clive N. Tadhunter, K. J. Inskip, R. Morganti, J. Holt, D. Dicken

We present deep GMOS-S/Gemini optical broad-band images for a complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts (0.05<z<0.7). The high-quality observations show for the first time that the overall majority of PRGs at intermediate redshifts (78-85%) show peculiarities in their optical morphologies at relatively high levels of surface brightness (\mu v=23.6 and {\Delta}\mu v~[21,26] mag arcsec-2) including tails, fans, bridges, shells, dust lanes, irregular features, amorphous haloes, and multiple nuclei. While the results for many of the galaxies are consistent with them being observed at, or after, the time of coalescence of the nuclei in a galaxy merger, we find that more than 1/3 of the sample are observed in a pre-coalescence phase of the merger, or following a close encounter between galaxies. By dividing the sample into Weak-Line Radio Galaxies (WLRGs; 11 objects) and Strong-Line Radio Galaxies (SLRGs; 35 objects) we find that only 27% of the former show clear evidence for interactions in their optical morphologies, in contrast to the SLRGs, of which at least 94% appear interacting. This is consistent with the idea that many WLRGs are fuelled/triggered by Bondi accretion of hot gas. Of the 28% of the sample that display evidence for significant starburst activity, we find that 92% present disturbed morphologies, following the same general trend as the total and SLRG samples. By comparing our PRGs with various samples of quiescent ellipticals from the literature, we conclude that the percentage of morphological disturbance that we find here exceeds that found for quiescent ellipticals when similar surface brightnesses are considered. Overall, our study indicates that galaxy interactions are likely to play a key role in the triggering of AGN/jet activity.

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